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sports
Article
The Effects of Acute Caffeine Supplementation on
Performance in Trained CrossFit Athletes
Jesse A. Stein * , Melitza Ramirez and Katie M. Heinrich
Department of Kinesiology, College of Human Ecology, Kansas State University, Manhattan, KS 66506, USA;
melitzar@ksu.edu (M.R.); kmhphd@ksu.edu (K.M.H.)
* Correspondence: jastein@ksu.edu; Tel.: +1-218-469-1837
Received: 20 March 2019; Accepted: 24 April 2019; Published: 25 April 2019
Abstract: Caffeine’s ergogenic effects persist during various exercise modalities; however, information
establishing its efficacy during CrossFit protocols is limited. Our study aimed to determine the effects of
caffeine supplementation on CrossFit performance. Thirteen CrossFit-trained men (age = 28.5 ± 6.6 years,
experience = 49.2 ± 36.3 months) were randomized in a double-blind, crossover design. Participants
completed two sessions separated by a seven-day washout period, 60 min after consuming 5 mg/kg
body mass of caffeine or a placebo. In each session, participants completed as many rounds as possible
in 20 min of 5 pull-ups, 10 push-ups, and 15 air squats. CrossFit performance was the total number of
repetitions completed in 20 min. Paired samples t-tests were used to compare CrossFit performance
between caffeine and placebo conditions and to test for a potential learning effect between the first and
second sessions. CrossFit performance was significantly higher during the caffeine condition compared to
the placebo (461.4 ± 103 vs. 425.0 ± 93.5 repetitions, p < 0.05). No significant learning effect was identified
between the first and second sessions (445.6 ± 95.0 vs. 440.8 ± 105.0 repetitions, p = 0.73) nor was there
a significant treatment order effect (p = 0.40). Caffeine’s ergogenic effect is present during CrossFit;
however, future investigations should establish caffeine’s efficacy during other CrossFit protocols and
among female athletes.
Keywords: high intensity functional training; exercise; males; muscular endurance; ergogenic aids;
sports nutrition
1. Introduction
Caffeine supplementation is pervasive in sporting disciplines with 74% of elite athletes consuming
caffeine prior to competition for its ergogenic effects [1]. Support for this strategy is recognized
by the International Olympic Committee and the International Society of Sports Nutrition who
both acknowledge caffeine as a dietary supplement with ‘good evidence’ for its ergogenic effects,
benefiting endurance and strength/power in athletes [2–5]. A majority of this evidence has been
established utilizing singular exercise modalities when assessing muscle function (muscular strength
and endurance) or exhaustive protocols (time-to-exhaustion and repeated-sprint ability) [3,4,6–9].
However, some athletes are not easily classified into physical demands that are strictly endurance
or strength/power in nature and require multiple facets of health- (e.g., aerobic capacity, muscular
strength) and skill-related (e.g., agility, speed, power) physical fitness [10]. Recently, Mielgo-Ayuso
addressed this concern within soccer players; however, a majority of the reviewed investigations
evaluated aspects of physical performance (i.e., speed, power, agility, time-to-exhaustion) with limited
data reported regarding simulated performance [11]. Reasonably, instructing athletes with diverse
physical demands to consume caffeine for performance benefits has limited evidence to support its
efficacy [11,12]. Since preserving muscle function during competition is important for preventing
Sports 2019, 7, 95; doi:10.3390/sports7040095 www.mdpi.com/journal/sportsSports 2019, 7, 95 2 of 9
premature fatigue, investigating the role of acute caffeine supplementation during combined exercise
modalities is warranted.
In the most recent meta-analysis of the effects of caffeine supplementation on muscle function,
the authors reported significant improvements (+6–7%) in muscular endurance after caffeine
supplementation [13]. A majority of the 17 investigations cited in the review assessed large muscle
group(s) muscular endurance via repetitions to failure over the course of multiple weightlifting sets
separated by recovery periods [14–17]. These investigations utilized isotonic exercise machines and
reported relatively low repetitions (Sports 2019, 7, 95 3 of 9 body fat were determined using a bioelectrical impedance analysis in standard mode (TBF-300A; Tanita, Japan). Following anthropometric measures, participants (n = 13, age = 28.5 ± 6.6 years; height = 178.9 ± 5.1 cm; mass = 84.3 ± 9.9 kg; percent body fat = 20.1 ± 2.9%; experience = 49.2 ± 36.3 months) were randomized to consume either caffeine or the placebo. To determine treatment order (placebo then caffeine vs. caffeine then placebo), participants were randomized using a random number generator (0–99) with odd numbers being placed into the placebo condition and even numbers being placed into the caffeine condition for the first visit (placebo then caffeine: n = 7; caffeine then placebo: n = 6). The caffeine pill(s) (Prolab) were provided at 5 mg·kg−1 body mass [28]. A 300 µg biotin pill(s) was used as a placebo to match for color and texture of caffeine pills. The same amount of pill(s) of biotin were administered as caffeine pills to ensure blinding. Participants began a self-selected warm-up 50-min after consumption of the pill(s). Participants began the CrossFit workout—‘Cindy’—60-min after consumption of the pill(s). The workout was performed indoors in a gym with the temperature set to 22 ◦ C. Each participant performed the workout alone with no clock or timing device visible to them and without music. ‘Cindy’ was chosen since it is a standardized CrossFit workout, and it has been previously described in the literature [21,22,29]. Briefly, participants completed as many rounds as possible of 5 pull-ups, 10 push-ups, and 15 air squats in 20 min. CrossFit movement standards were followed; kipping was allowed for the pull-ups; push-ups were performed on the toes, with the subject lowering himself with a straight body until his chest touched the floor; and air squats required subjects to reach full knee and hip extension at the top of each repetition and have their hip crease below their knee at the bottom of each repetition [29]. Judges, with CrossFit Level 1 or 2 Certificates, verbally counted repetitions. Repetitions that did not meet movement standards were not counted, and participants were provided feedback to meet the movement standards. CrossFit performance was the total number of repetitions completed in 20 min. Participants were given a post-exercise survey to determine their rate of perceived exertion (RPE) achieved during the workout on a scale of 1–10 [30]. Participants returned to the laboratory after a 7-day washout period, consumed the opposite pill(s) from their first visit (placebo n = 6; caffeine n = 7), and identical procedures were followed for the workout and post-workout survey. For each participant, the testing sessions were scheduled within a 2 h window. Percent change in the performance (i.e., repetitions completed) was calculated to assess the individual effects of caffeine supplementation [(caffeine − placebo)/(placebo) × 100)]. Individuals were classified as ‘non-responders’ if the percent change was
Sports 2019, 7, 95 4 of 9
Sports
Sports 2019,
2019, 7,
7, xx FOR
FOR PEER
PEER REVIEW
REVIEW 44 of
of 99
exercise responses (i.e., RPE) between the caffeine (8.5 ± 1.3) and the placebo conditions (8.3 ± 1.3),
(8.3=± −0.562,
t(12) p=
1.3), t(12) 0.584 (Figure
= −0.562, p = 0.5842).(Figure
No significant learning effect
2). No significant learningwas identified
effect betweenbetween
was identified the firstthe
and
second sessions
first and second(445.6 ± 95.0(445.6
sessions vs. 440.8 ± 105.0
± 95.0 vs. 440.8 ± 105.0t(12)
repetitions, = 0.348, t(12)
repetitions, p = 0.73). After
= 0.348, p =controlling
0.73). Afterfor
the total number
controlling of repetitions
for the total number performed during
of repetitions the placebo
performed condition,
during no significant
the placebo condition,treatment order
no significant
effect was observed
treatment for the
order effect wastotal number
observed forof repetitions
the performed
total number duringperformed
of repetitions the caffeineduring
condition between
the caffeine
treatment
conditionorder groups
between treatment = 0.760,
(F(1,13)order p = 0.40).
groups Figure
(F(1,13) 3 shows
= 0.760, the percent
p = 0.40). Figurechange
3 showsin performance
the percent
change
(i.e., total in performance
repetitions) (i.e., total
[(caffeine repetitions) [(caffeine
− placebo)/(placebo) − placebo)/(placebo)
× 100)] performed across×all 100)] performed
subjects (8.9%, across
95% CI
all subjects (8.9%, 95% CI (4.0–13.7%)), three of whom were
(4.0–13.7%)), three of whom were ‘non-responders’ (−0.5%, −0.4%, 0.27%). ‘non-responders’ (−0.5%, −0.4%, 0.27%).
*
Figure
Figure 1. 1.
Figure 1. Performance
Performance
Performance (i.e.,(i.e.,
(i.e., repetitions
repetitions
repetitions performed)
in thein
performed)
performed) in the
the caffeine
caffeine
caffeine and
and placebo
and placebo placebo condition.
condition.
condition. Data
Data
Data represent
represent
mean ± SD.mean
represent mean ±± SD.
SD. ** Significantly
* SignificantlySignificantly different
different
different from from
from performance
performance
performance during theduring
during the placebo
placebo condition
thecondition
placebo p < 0.05.ppSports 2019, 7, 95 5 of 9
Sports 2019, 7, x FOR PEER REVIEW 5 of 9
Figure 3.
Figure 3. Percent
Percent change
change in
in performance
performance [(repetitions completed with
[(repetitions completed with caffeine
caffeine −
− repetitions
repetitions completed
completed
with placebo)/(repetitions
with placebo)/(repetitions completed
completed with
with placebo)
placebo) ×× 100)]
100)] in
in the
the caffeine and placebo
caffeine and placebo condition
condition for
for
each participant.
each participant.
4. Discussion
The purpose of our investigation was to determine the effects of acute caffeine supplementation
on CrossFit performance
performance and and perceptual
perceptualresponses
responsestotoexercise
exerciseininCrossFit-trained
CrossFit-trainedmales.
males. Because
Because of
of the well-documented ergogenic effects of caffeine that are likely due to central
the well-documented ergogenic effects of caffeine that are likely due to central mechanisms [2–5], we mechanisms [2–5],
we hypothesized
hypothesized thatthat caffeine
caffeine supplementationwould
supplementation wouldimprove
improveCrossFit
CrossFitperformance.
performance. OurOur hypothesis
was partially supported with significant improvements in CrossFit performance with no changes in
perceptual responses after caffeine supplementation. Additionally, we
supplementation. Additionally, we tested
tested for a learning effect
between the thefirst
firstand
andsecond
second session
session of of ‘Cindy’
‘Cindy’ andandfound found no significant
no significant differences.
differences. Lastly,Lastly, our
our study
study aimed to provide a novel contribution to the literature regarding
aimed to provide a novel contribution to the literature regarding caffeine supplementation caffeine supplementation and
muscular
muscular endurance
endurance by providing a unique high-volume muscular endurance challenge. On average, average,
participants
participants ininthethecurrent
current investigation
investigation performed
performed overover400 repetitions
400 repetitionsfor thefor
20the
min20
workout during
min workout
the caffeine and placebo conditions, which we believe adequately addressed
during the caffeine and placebo conditions, which we believe adequately addressed this challenge. this challenge. To the
best
To theof our
bestknowledge, this is the first
of our knowledge, this study
is theto first
determine
study theto effects
determineof caffeine supplementation
the effects of caffeine
on performance during
supplementation any CrossFit
on performance workout,
during which taxed
any CrossFit multiple
workout, whichmuscle groups for
taxed multiple an extended
muscle groups
period of time with minimal rest.
for an extended period of time with minimal rest.
In the current
current study,
study,caffeine
caffeinesupplementation
supplementation(5 mg·kg−1
(5mg·kg −1 body mass) increased mean CrossFit
performance by by 8.9%.
8.9%.Caffeine’s
Caffeine’sergogenic
ergogenic effect has been
effect has been reported reported to improve
to improve performance
performance by
by 5.5–
5.5–8.5%
8.5% during during other
other repeated-high-intensity
repeated-high-intensity efforts
efforts in in teamsports
team sportsathletes,
athletes,andand by
by 6–7% during
muscular endurance exercise [6,13,32]. Previous Previous investigations
investigations determining
determining the the effects of caffeine
supplementation on muscular endurance had participants perform a comparatively low number of
repetitions (Sports 2019, 7, 95 6 of 9
results [3]. Thus, researchers might consider additional measurements of during-workout exertion
since RPE is usually taken after the exercise bout given the nature of resistance exercise [35].
Our study assessed CrossFit performance via ‘Cindy’, which has been the most studied CrossFit
workout to date [21,22,29]. Butcher and colleagues reported ‘Cindy’ performance between competitive,
experienced, and novice CrossFit athletes completing 698, 469, 389 repetitions, respectively [21,23].
The current study, which aimed to recruit CrossFit-trained (i.e., experienced) participants, seems to
follow those trends. Moreover, Crawford [24] measured work capacity derived from performance
during a similar CrossFit workout and reported ~16% increase in work capacity after 6 weeks of
a high-intensity functional training intervention which followed a CrossFit template for novice healthy
adult participants. This is promising for researchers and practitioners alike as the use of benchmark
workouts, such as ‘Cindy’, may be sensitive enough to detect changes in CrossFit performance from
ergogenic aids (e.g., exercise training interventions, nutrient modification) [21].
Strengths of the current investigation include a robust study design with subjects serving as their
own controls, the recruitment of trained male CrossFit participants who were able to complete ‘Cindy’
as prescribed as a high-volume muscular endurance workout, and it is the first study to document
an ergogenic effect during a CrossFit workout. However, our study does not go without limitation.
Although 13 CrossFit-trained men completed our study, our sample size is reasonably small. However,
the average sample size reported in a recently published systematic review on the effects of caffeine in
trained soccer players was 15 participants; therefore, our investigation reflects similar sample sizes
compared to other studies regarding caffeine’s ergogenic effect in trained populations [11]. Participant
training volume was not reported leading up or during the investigation. Although participants were
instructed not to exercise vigorously for 24 h prior to each testing session or change their training
regimen during the study period, fatigue and/or delayed onset muscle soreness from other training
sessions could impact our results. Additionally, our study lacked more invasive measures to determine
blood caffeine concentration and caffeine metabolism. Recently identified, caffeine supplementation
may have a ‘responder’ vs. ‘non-responder’ nature, which limits the translation of our investigation to
‘non-responder’ populations [27]. In our investigation, we had three participants (−0.5%, −0.4%, 0.27%)
who were ‘non-responders’ to caffeine supplementation (Figure 2). The inter-individual differences
in the ergogenicity of caffeine are thought to be related to genetic polymorphisms associated with
the CYP1A2 and ADORA2A genes, which discern fast and slow caffeine metabolism/clearance [36].
Although the current investigation did not characterize the genetic differences among our participants,
these differences may explain the ‘responder’ vs. ‘non-responder’ nature of our findings and present
an avenue for future investigations. However, to truly elucidate ‘responders’ vs. ‘non-responders’,
a baseline control condition where no supplements are provided to the participants prior to the
exercise bout is necessary. Additionally, our study investigated the effects of caffeine supplementation
on CrossFit performance for males and may not be generalizable to female participants. Caffeine
supplementation in females is complicated by the effects of estrogen and oral contraceptive steroids
on caffeine metabolism, both of which appear to prolong the effects of caffeine in the body [3,37].
To increase internal validity, participants performed the workout alone, with no clock visible, and no
music was playing. Results may differ when ‘Cindy’ is performed in a group setting with a visible
clock and music playing [38–40]. Lastly, our investigation utilized a 10-point Likert scale for RPE
and may not be sensitive enough to capture perceptual changes during CrossFit protocols. Although
mixed results exist for 10-point and 15-point scales for RPE during caffeine supplementation utilizing
resistance- and endurance-based protocols, a recent publication by Crawford and colleagues highlight
that a 15-point scale may be more appropriate in CrossFit athletes [3,34,35].
5. Conclusions
In conclusion, we have provided evidence that caffeine supplementation at 5 mg·kg−1 body mass
ingested 60 min prior to the CrossFit workout ‘Cindy’ increased the number of repetitions performed
in male CrossFit-trained individuals. However, despite the central effects of caffeine, we found noSports 2019, 7, 95 7 of 9
significant changes in perceptual responses during the exercise bout. Caffeine’s ergogenic effects are
well documented in the literature and further supported by our investigation, yet practitioners and
athletes should be aware that some ‘non-responders’ may exist; thus, evaluating caffeine’s effectiveness
prior to competition for a specific athlete is encouraged.
Author Contributions: Conceptualization, J.A.S. and M.R.; methodology, J.A.S. and K.M.H.; software, J.A.S.;
formal analysis, J.A.S.; investigation, J.A.S., M.R. and K.M.H.; resources, J.A.S. and M.R.; data curation, M.R.
and J.A.S.; writing—original draft preparation, J.A.S.; writing—review and editing, J.A.S., M.R. and K.M.H.;
visualization, J.A.S.; supervision, K.M.H.; project administration, J.A.S. and M.R.; funding acquisition, M.R.
Funding: This research was funded by the Kansas State University Office of Undergraduate Research and Creative
Inquiry awarded to M.R. Time spent preparing the manuscript was funded by the National Institutes of Diabetes
and Digestive and Kidney Diseases (R01DK099516) awarded to K.M.H.
Acknowledgments: We would like to acknowledge the lab assistants from the Kansas State University Functional
Intensity Training lab Timothy C. Hepler, Brady K. Kurtz, Bryan S. Heinrich, Jason Sartor, and Victor Andrews
during data collection.
Conflicts of Interest: The authors declare no conflict of interest. The funders 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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