Non-Contact Respiration Measurement during Exercise Tolerance Test by Using Kinect Sensor - MDPI

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Non-Contact Respiration Measurement during Exercise Tolerance Test by Using Kinect Sensor - MDPI
sports
Article
Non-Contact Respiration Measurement during
Exercise Tolerance Test by Using Kinect Sensor
Hirooki Aoki 1, * and Hidetoshi Nakamura 2
 1    Chitose Institute of Technology, Chitose 066-8655, Japan
 2    Department of Respiratory Medicine, Saitama Medical University, Iruma-gun 350-0495, Japan;
      hnakamur@saitama-med.ac.jp
 *    Correspondence: h-aokil@photon.chitose.ac.jp; Tel.: +81-123-25-1005

 Received: 1 January 2018; Accepted: 7 March 2018; Published: 13 March 2018

 Abstract: This study aims to assess non-contact respiration measurement during the exercise stress
 test using an upright bicycle ergometer and to evaluate the ventilation threshold value. We propose
 the tracking of the chest and abdomen by applying the motion capture function of the Kinect V2
 sensor to cope with an increase in physical exercise accompanied by an increase in exercise intensity.
 In the proposed method, the region enclosed by the four joints corresponding to the left and right
 shoulders and the right and left hip extracted using the Kinect sensor is set as the region of interest.
 The region is updated in response to changes in body movements. By extracting the signal of
 the pedaling frequency component from the time series data of the volume in the region, only
 the volume change due to respiration was extracted. The point at which the increased rate of the
 volume change elevates is estimated as the ventilation threshold. The assessment of the efficacy
 of the proposed method by comparative analysis using an expiration gas analyzer confirmed that
 non-contact respiration evaluation is possible with an exercise intensity of about 160 W. Furthermore,
 the ventilation threshold estimated by the proposed method is ±10 W of the estimated value by
 expiratory gas analyzer.

 Keywords: non-contact respiration measurement; motion capture; Kinect sensor

1. Introduction
      In rehabilitation institutes, the anaerobic threshold is typically evaluated by the exercise tolerance
test to assess the systemic aerobic capacity [1]. Although an expiration gas analyzer is conventionally
used for the assessment of the anaerobic threshold (AT), it is expensive. In addition, convenient
assessment is difficult while using the expiration gas analyzer, because it requires mounting a gas mask
on the face of participants, which poses significant problems, particularly in children and the elderly
who usually have dfficulty wearing a mask. Thus, the development of a simple and cost-effective
method to assess the AT is imperative for performing safe and effective exercise.
      The VT, an equivalent index of the AT, is evaluated from the change in minute ventilation (VE)
obtained by the ramp load test using a bicycle ergometer [2]. The generation of anaerobic energy
stimulates the production of the lactic acid, which when buffered with bicarbonate ion increases the
ventilation volume by producing carbon dioxide [3]. Caiozzeo [4] demonstrated that the VT could
be evaluated not only from the increased amount of carbon dioxide in expiration but also from the
increased VE in the ramp load test.
      Previously, we investigated a non-contact measurement with respiration in the rest position by
using pattern light projection [5], which elucidated that the volume change in the chest significantly
correlated with the change in ventilation. In addition, this method established non-contact respiration
measurement during pedaling with a recumbent bicycle ergometer [2]. The application of Caiozzeo’s
VT calculation by VE elucidated that the VT could be calculated even by the non-contact respiration

Sports 2018, 6, 23; doi:10.3390/sports6010023                                   www.mdpi.com/journal/sports
Non-Contact Respiration Measurement during Exercise Tolerance Test by Using Kinect Sensor - MDPI
Sports 2018, 6, 23                                                                                 2 of 11

measurement [6]. In this method, the increase in the volume change because of the increase in the
exercise intensity in the ramp load test is evaluated in the non-contact measurement, and the point
at which the volume change rate increases is estimated as the VT, also referred as QVT. The result of
this method suggests the feasibility of the systemic aerobic capacity assessment using the non-contact
respiration measurement.
      In our previous study mentioned above, although we used a recumbent bicycle ergometer, upright
bicycle ergometers are more prevalent [7]. Hence, improving the versatility of the proposed method
necessitates non-contact respiration measurement using an upright bicycle ergometer. In upright
bicycle ergometers, the body motion by pedaling is higher than those in the recumbent types.
However, non-contact respiration measurement by pedaling using an upright bicycle ergometer
is considered challenging. Hence, one study proposed a method to facilitate stable non-contact
respiration measurement by reducing the impact of the pedaling motion [8].
      In our previous studies, we used a sensor based on the active stereo method, called fiber-grating
vision sensor (FG sensor), which was developed independently by us and is not sold in general [9].
In 2010, Microsoft launched the Kinect sensor as a non-contact controller of the game console.
The Kinect sensor is a three-dimensional vision sensor based on the active stereo in which the pattern
light—called ‘light coding’—is projected onto an object, and the three-dimensional shape of the object
is reconstructed by image analysis of the pattern light distribution. The Kinect sensor is cost-effective
and it facilitates real-time processing of three-dimensional data measured by our own program because
it supports USB connection to a PC. For this reason, Kinect sensors are used in a wide range of research
fields for various purposes.
      In another study of ours, we investigated the respiration measurement using the Kinect sensor
as an alternative to the FG sensor in the ramp load test. The findings revealed that the quantitative
respiration measurement in the resting posture is possible with the Kinect but not the FG sensor [10].
As mentioned above, since the change in the posture by pedaling is significant in the test using
the upright bicycle ergometer, reducing the impact of the body movement on the assessment of
signal is essential. Hence, we proposed a method to consider the part of the body (i.e., chest and
abdomen) where the respiratory motion was likely to appear as a region of interest (ROI). Consequently,
we illustrated that the accuracy of the quasi tidal volume (QTV) fluctuation can be enhanced by filtering
the three-dimensional shape change in the ROI [11]. However, even this method did not facilitate the
complete elimination of the effect of a large change in the posture and was, thus, considered difficult
to apply at a high exercise intensity of ≥100 W.
      In 2014, a new version of the Kinect sensor (Kinect v2 sensor) was released, in which the
measurement method was revised from active stereo to time-of-flight (TOF). The principle of TOF
states that the distance is calculated from the time until the light projected from the light source
is reflected by an object and reaches a light-receiving sensor. Typically, light-receiving sensors are
distributed in a two-dimensional array and the distance distribution, in which the distance image can
be acquired. Apparently, data obtained from the distance image facilitates three-dimensional shape
reconstruction of the object. Although the measurement accuracy of the Kinect v2 sensor is almost
equal to that of the previous version, its motion capture function has been enhanced for its easy use.
      By tracking the thoracoabdominal activity using the motion capture function of the Kinect
v2 sensor, this study aims to propose a non-contact respiration measurement during the exercise
tolerance test corresponding to an increase in the body movement with an increasing exercise intensity.
Additionally, it assesses the validity of the proposed method by comparative experiment using the
expiratory gas analyzer.
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2. Materials and Methods

2.1. Configuration of
2.1. Configuration of the
                      the Measurement
                          Measurement System
                                      System
      Figure
      Figure 1a
              1a shows
                  shows thethe measurement
                                measurement systemsystem in  in this
                                                                 this study.
                                                                      study. The
                                                                               The Kinect
                                                                                     Kinect sensor
                                                                                             sensor (Kinect
                                                                                                       (Kinect v2v2 sensor,
                                                                                                                     sensor,
Microsoft   Corporation,     Redmond,      WA,    USA)    is  placed   facing  the  test subject
Microsoft Corporation, Redmond, WA, USA) is placed facing the test subject sitting on the saddle ofsitting  on  the  saddle
of
thethe upright
     upright      bicycle
              bicycle      ergometer.
                        ergometer.       The
                                       The     testsubject
                                             test    subjectisisasked
                                                                  askedtotopedal
                                                                            pedalthethebicycle
                                                                                         bicycle at
                                                                                                  at aa constant
                                                                                                        constant rotation
                                                                                                                   rotation
frequency,   which   was   set at
frequency, which was set at 1 Hz. 1 Hz.
      As
      As described     earlier, the
          described earlier,    the Kinect
                                     Kinect v2v2 sensor    can acquire
                                                  sensor can      acquire the
                                                                           the distance
                                                                                distance image
                                                                                           image by by the
                                                                                                        the TOF
                                                                                                             TOF method,
                                                                                                                  method,
which
which isis more
            morerecently
                    recentlyknown
                               known   as as  a depth
                                          a depth         image.
                                                      image.          The Kinect
                                                               The Kinect            v2 sensor
                                                                             v2 sensor             is connected
                                                                                          is connected               to the
                                                                                                           to the general-
general-purpose      laptop    PC  with   a  USB    port.   Then,    a sequence    of  depth   image
purpose laptop PC with a USB port. Then, a sequence of depth image obtained by the Kinect v2 sensor      obtained    by  the
Kinect  v2  sensor   is processed    by  the  PC.   The   Kinect    v2 sensor   primarily   comprises
is processed by the PC. The Kinect v2 sensor primarily comprises a color camera (RGB camera), an           a color  camera
(RGB   camera),
infrared   cameraan(IRinfrared
                         camera),camera
                                     and (IR   camera), projector
                                           an infrared     and an infrared      projectoras(IR
                                                                        (IR projector),         projector),
                                                                                             shown            as shown
                                                                                                       in Figure          in
                                                                                                                   1b [12].
Figure  1b  [12]. Table  1  shows   the  specification    of  the  Kinect.  The  field of  view  is 60  ◦ vertical and 70◦
Table 1 shows the specification of the Kinect. The field of view is 60° vertical and 70° horizontal. In
horizontal.
addition, theInresolution
                  addition,ofthetheresolution
                                    RGB image    ofobtained
                                                     the RGBby   image    obtained
                                                                    the RGB   camera bywas
                                                                                         the1080
                                                                                              RGBpcamera       was 1080
                                                                                                      (1920 × 1080         p
                                                                                                                     pixels).
(1920  × 1080the
In contrast,   pixels).   In contrast,
                   resolution            the resolution
                                of the depth                 of the depth
                                                image obtained        by theimage
                                                                              infraredobtained   by the comprising
                                                                                         depth sensor,     infrared depththe
sensor,  comprising     the  IR  camera    and   the   IR projector,   was   512  ×  424   pixels.
IR camera and the IR projector, was 512 × 424 pixels. Notably, each pixel of the depth image        Notably,    eachhaspixel
                                                                                                                         13-
of
bitthe  depth image
    information,    and has   13-bit resolution
                         the depth    information,  is 1and
                                                         mm;  the
                                                                in depth   resolution
                                                                   the depth            is 1 mm; in the
                                                                                image measurement,           depth
                                                                                                           the limit image
                                                                                                                      depth
measurement,
range is 0.5–4.5them.limit depth range is 0.5–4.5 m.

                             (a)                                                     (b)
                             Figure
                             Figure 1.
                                    1. (a)
                                       (a) System
                                           System configuration
                                                  configuration and
                                                                and (b)
                                                                    (b) Kinect v2 sensor.
                                                                        Kinect v2 sensor.

                                    Table 1.
                                    Table 1. Specifications
                                             Specifications of
                                                            of the
                                                                the Kinect
                                                                    Kinect v2
                                                                           v2 sensor.
                                                                              sensor

                           Item                                         Specification
                                   Item                                 Specification
                 Resolution of RGB   camera                           1920   × 1080 pixel
                        Resolution  of RGB camera
                 Frame rate of RGB camera                             1920 ×30 1080
                                                                                  fpspixel
                        Frame rate of RGB camera                             30 fps
                  Resolution of IR camera                               512 × 422 pixel
                         Resolution of IR camera                       512 × 422 pixel
                  Frame rate of rate
                         Frame  IR camera
                                     of IR camera                             30fps
                                                                             30   fps
                       Depth range
                               Depth range                                 0.5–4.5mm
                                                                           0.5–4.5
                       Field ofField
                                viewof view                     70◦ (vertical),
                                                                70° (vertical), 60 ◦ (horizontal)
                                                                                 60°  (horizontal)

     The Kinect
     The  Kinectv2v2sensor
                      sensor   is characterized
                           is characterized  by aby a markerless
                                                 markerless motionmotion
                                                                   capturecapture
                                                                           functionfunction   (also
                                                                                     (also called   called
                                                                                                  skeleton
skeleton  tracking), which   can   estimate and  track the coordinates of 25 joints of  the human
tracking), which can estimate and track the coordinates of 25 joints of the human body (Figure 2).   body
(Figure 2).
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      Figure 2. The 25 joints
                  The         extracted  by the markerless motion  capture  function ofofthe Kinect v2  sensor.
        Figure
      Figure 2.2.
               The  2525 joints
                       joints   extracted
                              extracted byby the
                                           the   markerless
                                                markerless  motioncapture
                                                           motion   capturefunction
                                                                             functionof   theKinect
                                                                                        the   Kinectv2
                                                                                                     v2sensor.
                                                                                                         sensor.

2.2. Non-Contact
 2.2.Non-Contact
                   Respiration
     Non-ContactRespiration
                                Measurement
                    RespirationMeasurement
                                Measurement
2.2.
      In this study, we measured breathing of participants who pedaled on the upright bicycle
      In this study,
     In        study,wewe
                        measured
                           measured breathing of participants
                                        breathing             who pedaled
                                                   of participants        on the upright
                                                                   who pedaled    on the bicycle
                                                                                          uprightergometer
                                                                                                   bicycle
ergometer without contact in four steps as explained below.
without contact
ergometer   withoutin four steps
                      contact     as explained
                               in four steps as below.
                                                explained below.

 2.2.1.  Setting
 2.2.1.Setting
         Settingof  of  the
                     ofthe
                        theROIROI
                              ROI
2.2.1.
        In the
        Inthe    measurement
            themeasurement
                  measurementsystem,  system,
                                      system,thethe  horizontal
                                                thehorizontal        distance
                                                      horizontaldistance         between
                                                                     distancebetween
                                                                                 betweenthe   the  Kinect
                                                                                              theKinect
                                                                                                   Kinectv2v2   sensor
                                                                                                           v2sensor
                                                                                                                sensorandand
                                                                                                                         andthethe  saddle
                                                                                                                               thesaddle
                                                                                                                                    saddle
       In
 tip
 tip  of the
     of   thebicycle
                 bicycleergometer
                           ergometer     waswas
                                              0.70.7
                                                  m.   m.
                                                        In  In the
                                                           the  depth depth
                                                                         image image     acquired
                                                                                  acquired     by  theby the Kinect
                                                                                                       Kinect    v2      v2 sensor,
                                                                                                                    sensor,    we  imagedwe
tip of the bicycle ergometer was 0.7 m. In the depth image acquired by the Kinect v2 sensor, we
 imaged
 the upper   the  upper     body    of  subjects   (Figure    3a).  Then,   using    the   joint extraction    function     of the  Kinect
imaged      thebody
                  upper of body
                           subjects     (Figure 3a).
                                   of subjects          Then,
                                                  (Figure    3a).using
                                                                    Then,theusing
                                                                              joint the
                                                                                    extraction     function function
                                                                                          joint extraction    of the Kinect     v2Kinect
                                                                                                                           of the   sensor,
 v2
 we  sensor,
      extracted  we four
                      extracted
                             joints four
                                      as   joints asright
                                          follows:     follows:    right shoulder,
                                                              shoulder,     left        left shoulder,
                                                                                  shoulder,     right     right
                                                                                                       hip,   and  hip,
                                                                                                                    left and
                                                                                                                          hip. left
                                                                                                                                 A  hip. A
                                                                                                                                    region
v2 sensor, we extracted four joints as follows: right shoulder, left shoulder, right hip, and left hip. A
 region
 surroundedsurrounded
                    by these  by thesejoints
                                          four was
                                                joints   was   set as   the ROI (Figure        3b). We anticipated       changes     in  the
region    surrounded         by four
                                 these four joints    set  as set
                                                         was   the   ROI
                                                                    as the(Figure     3b). We
                                                                            ROI (Figure       3b).anticipated    changes
                                                                                                    We anticipated           in thein
                                                                                                                        changes       body
                                                                                                                                        the
 body
 surface surface
            because because     of  respiration
                         of respiration      becausebecause     the
                                                         the body    body
                                                                      surfacesurface   corresponding
                                                                                corresponding               to
                                                                                                     to thetoROIthe ROI
                                                                                                                    was waswas   the
                                                                                                                          the chest   chest
                                                                                                                                       wall.
body    surface     because     of respiration     because     the body     surface    corresponding           the ROI          the chest
 wall.
 Since   Since
         the       the coordinates
                coordinates      of  the   of the
                                          four      fourwere
                                               joints      joints    wereby
                                                                 shifted    shifted
                                                                               the  bodyby movement,
                                                                                             the body movement,
                                                                                                            the   ROI  was theupdated
                                                                                                                                ROI was   in
wall. Since the coordinates of the four joints were shifted by the body movement, the ROI was
 updated
 every frame.  in  every    frame.
                     By using     theByBy  using
                                        depth       the
                                               informationdepth    information      from     the  ROI,  we    performed       respiration
updated       in every     frame.          using   the depth from         the ROI,from
                                                                  information         we performed
                                                                                            the ROI, we  respiration
                                                                                                             performed   assessment
                                                                                                                             respiration  by
 assessment
 the  following   byprocedure.
                       the following procedure.
assessment by the following procedure.

                                   (a)                                                                 (b)
                                  (a)                                                                 (b)
                           Figure 3. (a) Depth image and (b) setting of ROI (region of interest).
                           Figure
                            Figure3.3.(a)
                                       (a)Depth
                                           Depthimage
                                                 imageand
                                                       and(b)
                                                           (b)setting
                                                               settingof
                                                                       ofROI
                                                                          ROI(region
                                                                              (regionof
                                                                                      ofinterest).
                                                                                         interest).
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2.2.2.Sports
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                   Calculation                                                                                                              5 of 11

      BasedVolume
      2.2.2.    on the principle
                          Calculation  of triangulation, the depth of each pixel of the depth image can be converted
to the three-dimensional coordinates of the vertex (X, Y, and Z) by the Equation (1)
             Based on the principle of triangulation, the depth of each pixel of the depth image can be
      converted to the three-dimensional coordinates of the vertex (X, Y, and Z) by the Equation (1)
                                                             ( x p − ph /2) tan(θh /2)
                                                     X=                                           z
                                                                     ( x −pp /2  / 2) tan(θ h / 2) p
                                                              X = p hh                              zp
                                                            ( pv /2 − y p ) ptan   h /(2θv /2)                                                     (1)
                                                     Y=                                          zp
                                                                    ( p / 2 v− y p ) tan(θ v / 2)
                                                                            p  /2                                                                 (1)
                                                             Y= v                                   zp
                                                     Z = zp                     pv / 2
                                                              Z = zp
where xp is the horizontal pixel coordinate on the depth image, yp is the vertical pixel coordinate on
      whereimage,
the depth       xp is thezphorizontal
                            is the depth   pixel    coordinate
                                                 value   at a pixel   oncoordinate
                                                                           the depth image, (xp andypyis p ),the  vertical
                                                                                                              ph is          pixel
                                                                                                                     the total      coordinate
                                                                                                                                 pixel  numberon    of
      the depth
horizontal          image,pzvp is the depth
               direction,                 total pixelvaluenumber
                                                              at a pixel      coordinate
                                                                         of vertical           (xp and θyhp),isphthe
                                                                                           direction,              is the  total pixel
                                                                                                                      horizontal        number
                                                                                                                                     angle  of viewof
of thehorizontal
        IR camera,   direction,
                        and θ v ispvthe
                                      is the    total pixelangle
                                           horizontal          number        of vertical
                                                                       of view      of the direction,
                                                                                              IR camera.   θhWeis the  horizontal
                                                                                                                    resampled     theangle of view
                                                                                                                                       coordinates
      of   the  IR   camera,     and    θ
of the vertices by applying the Delaunay triangulation with linear interpolation [13] and calculated the
                                          v   is  the   horizontal        angle     of   view     of   the   IR  camera.     We    resampled      the
volumecoordinates
            of the bodyof the  vertices
                            surface     in bytheapplying
                                                   ROI. the Delaunay triangulation with linear interpolation [13] and
      calculated the volume of the body surface in the ROI.
2.2.3. Extraction of the Respiration Component
      2.2.3. Extraction of the Respiration Component
      Next, we calculated the change in the volume in the ROI between frames. Changes in the volume
             Next, we
were primarily            calculated
                       caused     by thethe pedal
                                               changemotion
                                                          in the volume         in the ROI
                                                                       of subjects.              between frames.
                                                                                            In addition,                Changes
                                                                                                                the volume          in the volume
                                                                                                                                 change    slightly
      were primarily caused by the pedal motion of subjects. In addition, the volume change slightly
contained the component of the motion of the chest–abdominal wall caused by respiration. In the
      contained the component of the motion of the chest–abdominal wall caused by respiration. In the
exercise tolerance test, subjects were asked to pedal at a constant rotation frequency, which, in this
      exercise tolerance test, subjects were asked to pedal at a constant rotation frequency, which, in this
study, was set at 1 Hz (60 rpm). Figure 4a shows the frequency spectrum of the volume change.
      study, was set at 1 Hz (60 rpm). Figure 4a shows the frequency spectrum of the volume change. Two
Two peaks emerged at a 1-Hz neighborhood and at lower-frequency band than 1 Hz. The peak at
      peaks emerged at a 1-Hz neighborhood and at lower-frequency band than 1 Hz. The peak at about 1
about 1 Hz is attributed to the pedaling motion of subjects. Here, we term the frequency reflected by
      Hz is attributed to the pedaling motion of subjects. Here, we term the frequency reflected by the peak
the peak     as ‘pedaling
      as ‘pedaling      motion motion      frequency’.
                                   frequency’.        However, However, as theaspeak the peak      at a lower
                                                                                           at a lower              frequency
                                                                                                            frequency      band band
                                                                                                                                  than 1than
                                                                                                                                           Hz 1wasHz
was caused
      caused by  by the
                     therespiratory
                           respiratorymotion,motion,it itwas was     termed
                                                                  termed          ‘respiration
                                                                              ‘respiration           frequency’.
                                                                                                 frequency’.
      Figure Figure 4b shows that the respiration waveform was extractedby
                 4b   shows     that   the     respiration       waveform           was     extracted        byapplying
                                                                                                                 applying the the FFT
                                                                                                                                    FFT bandpass
                                                                                                                                         bandpass
filterfilter
       to the    waveform      of  the   volume        change      between         frames.      In
             to the waveform of the volume change between frames. In this study, the bandwidth of   this  study,    the  bandwidth      of the
                                                                                                                                           the FFT
                                                                                                                                                FFT
bandpass
      bandpassfilterfilter
                      waswasset between
                                 set between    0.1 0.1
                                                     andand 0.70.7Hz. Hz.Notably,
                                                                            Notably,    it it
                                                                                           isisessential
                                                                                                 essentialtotoapply
                                                                                                                  applythethe FFT
                                                                                                                               FFT filter after
                                                                                                                                          after the
                                                                                                                                                 the
end ofend the
            of measurement,
               the measurement,     andand  it isitnot  possible
                                                    is not  possible    to to
                                                                            extract
                                                                               extractthe thebreathing
                                                                                                breathingwaveform
                                                                                                              waveform in    in real time.
                                                                                                                                      time. Hence,
                                                                                                                                            Hence,
real-time     processing
      real-time    processing of the
                                  of thebreathing
                                              breathing  waveform
                                                             waveform       extraction
                                                                               extractionwas   wasattained
                                                                                                      attained by  by applying
                                                                                                                       applying aa digital
                                                                                                                                      digitalfilter.
                                                                                                                                               filter.
      The designed
The designed              digital
                    digital        filter
                              filter        comprised
                                      comprised              a least-squares
                                                         a least-squares              linear-phaseFIR
                                                                                   linear-phase          FIRfilter
                                                                                                               filterofofthe
                                                                                                                          the order
                                                                                                                               order 90. Figure
                                                                                                                                          Figure 55
shows shows     the characteristics
          the characteristics            of the
                                    of the          digital
                                                digital       filter.
                                                          filter.

                                 ↓   Component of respiration

                                      ↓    Component of pedaling motion
        Spectrum intensity

                             0         1       2       3        4     5   6

                                                Frequency [Hz]

                                                    (a)                                              (b)
         Figure 4. (a) Frequency spectrum of the measurement waveform and (b) extracted respiration waveform.
       Figure 4. (a) Frequency spectrum of the measurement waveform and (b) extracted respiration waveform.
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                                                                    振幅応答 (dB)                                                                              インパルス応答

                                                                     振幅応答 (dB)                                                                              インパルス応答
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                                  0             2       4       6        8       10        12        14                         0         0.5     1          1.5       2       2.5     3
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                                          0         2       4   Time
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                                                                                      10        12        14                          0     0.5       1   Time   [s]
                                                                                                                                                               1.5
                                                                                                                                                             時間 (秒)
                                                                                                                                                                           2     2.5       3

                                                                 Time [s]                                                                                  Time [s]
                                                                    (a)                                                                                    (b)
                                                                     (a)                                                                                    (b)
                                              Figure 5. (a) Amplitude response and (b) impulse response of the designed digital filter.
                                               Figure5.5.(a)
                                              Figure      (a)Amplitude
                                                             Amplituderesponse
                                                                       responseand
                                                                                and(b)
                                                                                    (b)impulse
                                                                                        impulseresponse
                                                                                                responseof
                                                                                                         ofthe
                                                                                                            thedesigned
                                                                                                                designeddigital
                                                                                                                         digitalfilter.
                                                                                                                                 filter.
2.2.4. Calculation of the QTV
 2.2.4. Calculation of the QTV
2.2.4. Calculation of the QTV
      The respiration waveform calculated above represents the volume change between frames of the
       The respiration waveform calculated above represents the volume change between frames of the
ROI. The
       In the   extracted
           respiration       respiratory
                         waveform          waveform,
                                      calculated  abovethe   positivethe
                                                          represents    and   negative
                                                                           volume       are between
                                                                                    change    reversed frames
                                                                                                         between of
 ROI. In the extracted respiratory waveform, the positive and negative are reversed between
exhalation
the  ROI. Inandtheinhalation,
                    extracted thereby    allowing
                                respiratory        the separation
                                             waveform,             of exhalation
                                                           the positive           and inspiration
                                                                          and negative   are reversedby detecting
                                                                                                         between
 exhalation and inhalation, thereby allowing the separation of exhalation and inspiration by detecting
the zero-crossing
exhalation            of the waveform.
             and inhalation,               Integrating
                                thereby allowing        the area ofofthe
                                                   the separation          waveform
                                                                       exhalation andofinspiration
                                                                                         one expiration    or one
                                                                                                     by detecting
 the zero-crossing of the waveform. Integrating the area of the waveform of one expiration or one
inspiration
the           facilitates
    zero-crossing          calculating
                      of the  waveform.the Integrating
                                            amount equivalent
                                                        the area to
                                                                  of the
                                                                      the TV, definedofasone
                                                                           waveform       QTV;    the QTVor
                                                                                               expiration   is one
                                                                                                               not
 inspiration facilitates calculating the amount equivalent to the TV, defined as QTV; the QTV is not
equal as thefacilitates
inspiration    absolute amount      of TV,
                           calculating  the but the rate
                                            amount       of change
                                                     equivalent   to in
                                                                      thethe
                                                                          TV,QTV  withas
                                                                              defined   time
                                                                                           QTV;is equal to that
                                                                                                  the QTV        of
                                                                                                            is not
 equal as the absolute amount of TV, but the rate of change in the QTV with time is equal to that of
the TV.
equal  as We
          the defined
               absoluteQTV     multiplied
                          amount            bythe
                                   of TV, but  breathing   rate (number
                                                  rate of change           of breaths
                                                                  in the QTV           per is
                                                                                with time  minute)   asthat
                                                                                              equal to  quasi   VE
                                                                                                            of the
 the TV. We defined QTV multiplied by breathing rate (number of breaths per minute) as quasi VE
(QVE).
TV. We defined QTV multiplied by breathing rate (number of breaths per minute) as quasi VE (QVE).
 (QVE).
2.3. Participants
2.3. Participants
 2.3. Participants
      The validity
      The   validity ofof the
                          the method
                                method explained
                                           explained above
                                                         above waswas determined
                                                                        determined through
                                                                                        through experiments
                                                                                                   experimentsin   inthis
                                                                                                                       thisstudy.
                                                                                                                            study.
We     The validity     of the method       explained     above was determined          through tolerance
                                                                                                   experiments      in this  study.
We conducted non-contact respiratory measurements during the exercise tolerance test on six male
     conducted      non-contact     respiratory      measurements        during  the    exercise               test  on  six  male
 We conducted non-contact
subjects                              respiratory     measurements        duringallthe   exercise   tolerance totest  onasix   male
subjects (Subject
           (Subject A, A,B,B,C,C,D,
                                  D,E,E,and
                                          andF).F).
                                                  During
                                                     During  thethe
                                                                  assessment,
                                                                     assessment,     subjects
                                                                                    all        were
                                                                                         subjects      asked
                                                                                                    were   asked  wear
                                                                                                                   to wear T-shirt
                                                                                                                               a T-
 subjects
that        (Subject    A,  B, C,  D,   E, and    F). During     the  assessment,     all subjects   were   asked    to wear    a T-
shirt remained
      that remained in ininclose  contact
                             in close         with
                                        contact      their
                                                  with      body.
                                                         their  body.Wrinkles
                                                                        Wrinklesandand twists
                                                                                          twistsofofthe
                                                                                                     thematerial
                                                                                                          materialcloth
                                                                                                                     cloth might
                                                                                                                            might
 shirt
affect that   remained
        the measured      in
             measuredvalue;   in close   contact
                            value;however,
                                     however,since with   their
                                                   sincethere    body.
                                                           thereare
                                                                  are   Wrinkles     and   twists  of the material    cloth  might
affect the                                                           nono  means
                                                                        means   to to    validate
                                                                                    validate   thisthis  assumption,
                                                                                                     assumption,          we did
                                                                                                                      we did    not
 affect
not      the
     consider measured
                  its impactvalue;  however,
                               in this            since
                                         experiment.      there
                                                           Figureare  no  means
                                                                    6 shows      to   validate
                                                                              a subject         this
                                                                                            pedaling  assumption,
                                                                                                        the bicycle    we  did not
                                                                                                                      ergometer.
consider    its impact   in this  experiment.       Figure    6 shows    a subject  pedaling    the bicycle    ergometer.     This
 consider its
This             impact    in thisbyexperiment.       FigureEthical
                                                               6 shows    a subject   pedaling    the  bicycle (approval
                                                                                                                ergometer.code:This
studystudy     was
        was approved approved          the Institutional
                            by the Institutional       Ethical Review  Review
                                                                           BoardBoard
                                                                                   of our of our  university
                                                                                             university   (approval code:       13-
 study
13-044    was
         (12    approved
             September       by  the
                            2013)),    Institutional
                                     and   all          Ethical
                                                experiments       Review
                                                                 were       Board
                                                                       conducted     of
                                                                                    in   our  university
                                                                                        accordance     with(approval
                                                                                                             the         code:
                                                                                                                  Declaration    13-
                                                                                                                                  of
044 (12 September 2013)), and all experiments were conducted in accordance with the Declaration of
 044  (12
Helsinki.  September     2013)),   and    all experiments       were  conducted     in  accordance     with  the  Declaration      of
Helsinki. We  Weobtained
                     obtained written   informed
                                  written            consent
                                               informed         from allfrom
                                                              consent     participants    after explaining
                                                                                all participants        after the  experimental
                                                                                                               explaining       the
 Helsinki.
procedures,    We     obtained     written      informed       consent    from   all   participants      after   explaining      the
experimentalrisks,      and benefits.
                   procedures,    risks, and benefits.
 experimental procedures, risks, and benefits.

                                                                                       (a)                                                                      (b)
                                                                                        (a)                                                                      (b)
                     Figure 6. A subject pedaling an upright bicycle ergometer. (a) Side direction;
                                                                                         direction; (b) Back
                                                                                                        Back direction.
                      Figure6.6.A
                     Figure     Asubject
                                  subjectpedaling
                                          pedalingan
                                                   anupright
                                                      uprightbicycle
                                                              bicycleergometer.
                                                                      ergometer.(a)
                                                                                 (a)Side
                                                                                     Side direction;(b)
                                                                                                     (b) Backdirection.
                                                                                                              direction.
Sports 2018, 6, 23                                                                                             7 of 11
Sports 2018, 6, x FOR PEER REVIEW                                                                              7 of 11

     Procedures
2.4. Procedures
      The validity
           validity of
                    of the
                        theassessment
                            assessmentusing
                                          usingananupright  bicycle
                                                      upright        ergometer
                                                               bicycle  ergometer(75XLII; Konami
                                                                                     (75XLII;      Sports
                                                                                              Konami      LifeLife
                                                                                                       Sports Co.,
Ltd., Ltd.,
Co.,  Tokyo,  Japan)Japan)
            Tokyo,    was examined     by comparing
                             was examined                with thewith
                                                by comparing      simultaneous    assessment
                                                                        the simultaneous       using an expiration
                                                                                            assessment   using an
expiration
gas analyzergas(Aeromonitor
                 analyzer (Aeromonitor       AE-280S;
                                AE-280S; Minato          Minato
                                                     Medical     Medical
                                                              Science  Co.,Science  Co., Ltd.,
                                                                             Ltd., Tokyo,      Tokyo,
                                                                                          Japan).  TheJapan). The
                                                                                                       ergometric
ergometric
intensity wasintensity   was thenautomatically
                then increased     increased automatically
                                                    to 20 W/min to ramp
                                                                   20 W/min
                                                                          loadramp
                                                                               from load   from
                                                                                      0 W. All   0 W. All
                                                                                               subjects   subjects
                                                                                                        continued
continued
pedaling forpedaling   for 8 min.ifHowever,
              8 min. However,       the fatigueifreached
                                                  the fatigue reached
                                                          the limit,    the limit,
                                                                     exercise was exercise  was stopped
                                                                                   stopped halfway.       halfway.
                                                                                                     Furthermore,
Furthermore,    the examinee
the examinee pedaling           pedaling
                           rate was   guidedrate
                                               bywas   guided by sound
                                                   a metronome    a metronome
                                                                          to keepsound
                                                                                   aroundto60keep around 60 rpm.
                                                                                               rpm.

3. Results
3. Results
      Figure 77 shows
     Figure     shows the
                        the results
                            results of
                                    of the
                                       the simultaneous
                                            simultaneous measurement
                                                             measurement as  as follows:
                                                                                follows: yellow
                                                                                          yellow squares,
                                                                                                    squares, the
                                                                                                             the QVE
                                                                                                                 QVE
obtained by
obtained  bythetheproposed
                    proposedmethod;
                                method;blueblue triangles,
                                              triangles,   thethe
                                                                VEVE   obtained
                                                                    obtained  by by
                                                                                  thethe expiration
                                                                                      expiration    gas gas analyzer;
                                                                                                        analyzer; the
horizontal axis, the normalized value of QVE/VE. With the minimum and the maximum as as
the  horizontal   axis,  the  normalized    value  of  QVE/VE.       With  the minimum      and    the maximum      a
a standard,
standard,  we we  calculated
                calculated     the
                             the   normalizedvalue
                                 normalized      valuesosothat
                                                            thatthe
                                                                  thevalue
                                                                      valuechanged
                                                                             changedfrom
                                                                                       from0 0toto1.1.

                           (a)                                                        (b)

                           (c)                                                        (d)

                           (e)                                                        (f)
     Figure
     Figure 7.
            7. Results
                Results of
                        of the
                           the simultaneous
                               simultaneous measurement
                                               measurement withwith the
                                                                    the proposed
                                                                        proposed method
                                                                                    method and
                                                                                             and expiratory
                                                                                                  expiratory gas
                                                                                                             gas
     analyzer. (a) Subject A; (b) Subject B; (c) Subject C; (d) Subject D; (e) Subject E; and (f) Subject F.
     analyzer. (a) Subject A; (b) Subject B; (c) Subject C; (d) Subject D; (e) Subject E; and (f) Subject F.
Sports 2018, 6, 23                                                                                                                                                                                           8 of 11
Sports 2018, 6, x FOR PEER REVIEW                                                                                                                                                                            8 of 11

      Among the six subjects, Subjects A, B, C, and D rode the bicycling ergometer up to an exercise
intensity of 160 W. W. In
                        In contrast,
                           contrast, Subjects
                                      Subjects E and
                                                   and FF were
                                                           were unable
                                                                   unable toto continue
                                                                               continue pedaling
                                                                                           pedaling with the exercise
intensity around 140 140 WW because
                             because ofof fatigue
                                          fatigue and
                                                   and stopped
                                                         stopped exercising
                                                                    exercising at at 140
                                                                                     140 W.
                                                                                          W.
      In all results, compared    to the VE,  the QVE    was  highly    variable
          all results, compared to the VE, the QVE was highly variable (Figure    (Figure   7). However,   we observed
                                                                                                      7). However,    we
that the  VE   and  QVE   changed   with  a similar  tendency    in each  subject.   Figure
observed that the VE and QVE changed with a similar tendency in each subject. Figure 8 shows 8 shows   scatter diagrams
                                                                                                                  scatter
of the VE and
diagrams          QVE.
             of the     Furthermore,
                    VE and              the linear regression
                             QVE. Furthermore,                    analysis revealed
                                                    the linear regression      analysisthat  the correlation
                                                                                         revealed             coefficient
                                                                                                    that the correlation
was  ≥  0.79  in any of our  subjects. For  each  subject,  the bias  and  the   95%  confidence   interval
coefficient was ≥0.79 in any of our subjects. For each subject, the bias and the 95% confidence interval     (95%  CI) of
the
(95%difference
       CI) of thebetween    VEbetween
                   difference   and QVE  VEasand
                                               well  as VT
                                                   QVE       estimated
                                                         as well   as VT by   the expiration
                                                                          estimated             gas analyzer
                                                                                       by the expiration   gasand   QVT
                                                                                                                analyzer
estimated    by the method
and QVT estimated        by theproposed    in our previous
                                method proposed        in ourworks
                                                               previous[6] are
                                                                           worksshown    in Table
                                                                                    [6] are  shown2. in Table 2.

                               1                                                                                                                                     1

                           0.8                                                                                                                                      0.8
           Normalized VE

                                                                                                                                                    Normalized VE
                           0.6                                                                                                                                      0.6

                           0.4                                                                                                                                      0.4
                                                             R=0.8981                                                                                                                          R=0.8361
                                                             p
Sports 2018, 6, 23                                                                                                                            9 of 11

        However, as the exercise intensity increased, we observed the tendency for the VE and QVE to
 Sports 2018, 6, x FOR PEER REVIEW                                                                                                             9 of 11
diverge, which may warrant further investigation. For example, with Subject D, the VE and QVE
started
 started diverging
            diverging at    at an
                               an exercise
                                    exerciseintensity
                                                 intensityof  of≥   120 W.
                                                                  ≥120   W. AtAt this
                                                                                   this time,
                                                                                         time, Subject
                                                                                                  Subject D   D was
                                                                                                                 was in in aa posture
                                                                                                                               posture in  in which
                                                                                                                                              which
the
 the left and right inclinations were significant (Figure 9a). Although the ROI was correctly set at
      left and    right    inclinations      were    significant     (Figure     9a).   Although        the  ROI    was    correctly      set at this
                                                                                                                                                  this
time,
 time, the large left and right movements presumably hindered the detection of respiration, which is
         the  large    left  and    right   movements         presumably         hindered       the   detection     of  respiration,       which     is
moving      in   the  depth     direction.     In  addition,     the  ROI    could     not   be  set
 moving in the depth direction. In addition, the ROI could not be set correctly because the subject wascorrectly    because      the  subject     was
too
 too close
       close toto the
                   the sensor,
                         sensor, which
                                     which restricted
                                               restricted the the precise
                                                                    precise respiration
                                                                               respiration measurement
                                                                                                 measurement (Figure   (Figure 9b).9b). This
                                                                                                                                          This may
                                                                                                                                                 may
be
 be attributed
     attributed to   to the
                         the fact
                              fact that
                                     that distance
                                           distance information
                                                         information is    is not
                                                                              not measured
                                                                                    measured because because the the upper
                                                                                                                      upper body body was was set
                                                                                                                                                set at
                                                                                                                                                     at
aa position     closer    than   the   lower    limit  (50  cm)    of the   sensing     range     of  the
   position closer than the lower limit (50 cm) of the sensing range of the Kinect v2. Thus, further study  Kinect   v2.   Thus,    further    study
is
 is necessary
    necessary to   to elucidate
                       elucidate the      conditions at
                                      the conditions          which the
                                                           at which     the accuracy
                                                                              accuracy of   of non-contact
                                                                                                non-contact respiration
                                                                                                                  respiration measurement
                                                                                                                                   measurement
deteriorates.
 deteriorates. As     As aa countermeasure,
                              countermeasure,two         twoapproaches
                                                                approachesare     areconceivable.
                                                                                        conceivable.The      Thefirst
                                                                                                                    firstapproach
                                                                                                                            approachis istotosetseta
alonger
   longerdistance
             distancebetween
                           betweenthe   thesubject
                                             subjectand andthe thesensor.
                                                                    sensor.Another
                                                                               Anotherapproach
                                                                                             approach is    is to
                                                                                                               to place
                                                                                                                   place thethe backrest
                                                                                                                                 backrest on  on the
                                                                                                                                                   the
ergometer’s        seat    chair.  When      subjects     pedal    with   their   back    in  contact
 ergometer’s seat chair. When subjects pedal with their back in contact with the backrest, it is possible with   the   backrest,     it is  possible
to
 to prevent
    prevent measurement
                  measurement from     from being
                                               being impossible
                                                        impossible due   dueto toaaforward-tilting
                                                                                      forward-tiltingposture. posture.
        The
        The correlation coefficient between the VE and QVE ininthis
               correlation        coefficient      between       the   VE     and     QVE           this   study
                                                                                                        study    was was     ≥0.79
                                                                                                                         ≥0.79          (Figure
                                                                                                                                  (Figure     8). An8).
An    investigation
 investigation        by by    optoelectronic
                            optoelectronic           plethysmography
                                                  plethysmography               (OEP)is isa awell-known
                                                                             (OEP)               well-knownstudy   study on  on the
                                                                                                                                  the correlation
                                                                                                                                        correlation
between
 between the volume change in the chest and exhalation flow during exercise. Apparently, OEP
             the    volume      change      in the   chest   and    exhalation       flow    during      exercise.   Apparently,          OEP is is an
                                                                                                                                                    an
application       of  motion     capture     using    markers      that  enables      strict   volume
 application of motion capture using markers that enables strict volume measurement than markerless        measurement          than    markerless
motion
 motion capture.
            capture. A      A previous
                               previous studystudy on on OEP
                                                           OEP revealed
                                                                   revealed thatthat the
                                                                                       the correlation
                                                                                             correlation coefficient
                                                                                                               coefficient with with spirometer
                                                                                                                                        spirometer
was
 was >0.75, which corroborates our results [14]. In our method, estimation of the thoracic region
       >0.75,    which      corroborates       our   results    [14].  In  our   method,        estimation      of  the   thoracic     region was was
not
 not strict;
       strict; however,
                however, we    we obtained
                                    obtainedaareasonable
                                                    reasonablecorrelation
                                                                     correlationby     bynoise
                                                                                            noisereduction
                                                                                                     reductionprocessing.
                                                                                                                    processing.
        In
        In addition,
            addition, aa difference
                               difference magnitude
                                               magnitude of      of±±1010 W W between
                                                                                 between the    the QVT
                                                                                                      QVT estimated
                                                                                                              estimated by    by the
                                                                                                                                   the proposed
                                                                                                                                          proposed
method      and    the   VT   estimated      by   the expiratory       gas  analyzer      suggests
 method and the VT estimated by the expiratory gas analyzer suggests that the proposed method           that  the   proposed       method     might
allow
 mightVT       calculation
           allow                 withoutwithout
                     VT calculation          contact in     the lamp
                                                          contact         load
                                                                     in the       test using
                                                                               lamp     load test a general
                                                                                                        using upright
                                                                                                                 a general   bicycle
                                                                                                                                 upright ergometer.
                                                                                                                                             bicycle
As   shown       in Table     2, the    difference    between       VE    and   QVE      is  rather
 ergometer. As shown in Table 2, the difference between VE and QVE is rather large, but the trends      large,  but   the    trends    of   changes
in
 of VE    and QVE
     changes      in VEare  andsimilar,
                                   QVE are as reflected
                                                 similar, asbyreflected
                                                                   the correlation        coefficient.coefficient.
                                                                              by the correlation             Consequently,          VT and QVT
                                                                                                                             Consequently,         VT
may     show     close   values.     In  this  experiment,       because      the   distribution
 and QVT may show close values. In this experiment, because the distribution of the subjects’ VT was   of  the  subjects’     VT   was     biased   to
approximately           100   W,   additional      tests   are  required     for   subjects
 biased to approximately 100 W, additional tests are required for subjects with more diverse VT. with    more    diverse     VT.   However,        the
primary
 However,    objective
                the primary of thisobjective
                                      researchof   was
                                                     thistoresearch
                                                             constructwas   a method
                                                                               to constructto conveniently
                                                                                                   a method to     calculate     the VTcalculate
                                                                                                                      conveniently          from an
engineering        perspective,        which     has  been    significantly      attained
 the VT from an engineering perspective, which has been significantly attained in this study.  in  this   study.
        This
        This study
               study has has some
                               some limitations.
                                        limitations. First,
                                                          First, the
                                                                  the sample
                                                                       sample size size of
                                                                                         of this
                                                                                              this study
                                                                                                     study isis small,
                                                                                                                 small, andand all
                                                                                                                                 all participants
                                                                                                                                      participants
were
 were males.        Thus, it
         males. Thus,         it is
                                 is necessary
                                     necessary to   to conduct
                                                         conduct extensive
                                                                      extensive studies
                                                                                     studies targeting
                                                                                                 targeting aa variety
                                                                                                                   variety of   of subjects
                                                                                                                                   subjects fromfrom
different      age    groups,       genders,       and    athletic    abilities.      In   addition,
 different age groups, genders, and athletic abilities. In addition, the structural and statistical        the   structural       and     statistical
examination
 examination is     is required
                        required for   formore
                                           moresubjects.
                                                    subjects.From  Fromthe theperspective
                                                                                 perspective     ofof    motion
                                                                                                      motion        analysis,
                                                                                                                 analysis,         a comparison
                                                                                                                               a comparison         by
by    introducing        OEP     is  considered       to   be   effective     for   assessing       the
 introducing OEP is considered to be effective for assessing the validity and reliability of the proposed  validity    and     reliability    of   the
proposed
 method. method.

                                            (a)                                                      (b)
                          Figure 9.
                          Figure 9. (a)
                                    (a) Lateral
                                        Lateral bending
                                                bending position
                                                        position and
                                                                 and (b)
                                                                     (b) forward-bending
                                                                         forward-bending position.
                                                                                         position.

5. Conclusions
     By tracking the thoracoabdominal region using the motion capture function of the Kinect v2
sensor, we proposed a non-contact respiration measurement during the exercise tolerance test
corresponding to an increased body movement with increasing exercise intensity. In this study, we
Sports 2018, 6, 23                                                                                              10 of 11

5. Conclusions
      By tracking the thoracoabdominal region using the motion capture function of the Kinect v2 sensor,
we proposed a non-contact respiration measurement during the exercise tolerance test corresponding
to an increased body movement with increasing exercise intensity. In this study, we designed a digital
filter for bandpass filter processing in real time. As a result of investigating the correlation between the
QVE and the VE by simultaneous measurement using an expiration gas analyzer, a high correlation
coefficient up to about 160 W of the exercise intensity was obtained. The study clarifies that posture
during the pedaling motion affects the measurement accuracy of the proposed method. Nevertheless,
elucidation of the condition that deteriorates the precise measurement warrants further study.
      QVT estimated from QVE showed a difference of ±10 W from VT calculated by expiratory gas
analyzer. In this experiment, the number of subjects, as well as difference in VT among these subjects,
was small. Thus, the examination of the correlation between QVT and VT was impossible. Therefore,
in this experiment, the feasibility of the proposed method could not be completely demonstrated.
However, QVT estimated by the proposed method shows a value close to VT. In addition, our VT
estimation method based on the knowledge of Caiozzeo shows a robust estimation accuracy in our
previous research. Taken together, these results suggest that VT can be estimated by non-contact
sensing for the unstable posture of the subject who pedals with an upright bicycle ergometer.

Acknowledgments: This work was supported by Grant-in-Aid for Scientific Research(C) from Japan Society for
the Promotion of Science, grant number JP17K09627.
Author Contributions: H.N. conceived, designed, and performed the experiments; H.A. developed the
measurement system, performed the experiments, and wrote the manuscript.
Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the
decision to publish the results.

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1.    Wasserman, K.; McIloy, M. Detecting the threshold of anaerobic metabolism in cardiac patients during
      exercise. Am. J. Cardiol. 1964, 14, 844–852. [CrossRef]
2.    Robergs, R.; Roberts, S. Fundamental Principles of Exercise Physiology: For Fitness, Performance, and Health;
      McGraw-Hill Higher Education: New York, NY, USA, 2000; pp. 176–177, ISBN 0801679079, 9780801679070.
3.    Hirakoba, K.; Maruyama, A.; Inaki, M.; Misaka, K. Effect of Endurance Training on Excessive CO2 Expiration
      due to Lactate Production in Exercise. Eur. J. Appl. Physiol. Occup. Physiol. 1992, 64, 73–77. [CrossRef]
      [PubMed]
4.    Caiozzo, V.J.; Davis, J.A.; Ellis, J.F.; Azus, J.L.; Vandagriff, R.; Prietto, C.A.; McMaster, W.C. A Comparison of
      gas exchange indices used to detect the anaerobic threshold. J. Appl. Physiol. 1982, 53, 1184–1189. [CrossRef]
      [PubMed]
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