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Article title: The Effect of Caffeine on Athletic Performance
Authors: Nabeel Hussain[1]
Affiliations: Saba University School of Medicine, Saba[1]
Orcid ids: 0000-0002-2776-0398[1]
Contact e-mail: hussain.nabeel@outlook.com
License information: This work has been published open access under Creative Commons Attribution License
http://creativecommons.org/licenses/by/4.0/, which permits unrestricted use, distribution, and reproduction in any
medium, provided the original work is properly cited. Conditions, terms of use and publishing policy can be found at
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Preprint statement: This article is a preprint and has not been peer-reviewed, under consideration and submitted to
ScienceOpen Preprints for open peer review.
DOI: 10.14293/S2199-1006.1.SOR-.PPAGBLN.v1
Preprint first posted online: 31 January 2021
Keywords: caffeine, athletic performance, exercise, physical performance, sports
Running Head: EFFECT OF CAFFEINE ON ATHLETIC PERFORMANCE
1
The Effect of Caffeine on Athletic Performance
Nabeel Hussain
SABA University School of Medicine
2 Napanee Street, Brampton ON, L6S 4X8, Canada
(647) 606-7173, n.hussain@saba.edu
Advisor: Dr. DeLaCruz
Article Word Count: 8280
Hypothesis: The intake of caffeine can increase physical performance during athletic activity
EFFECT OF CAFFEINE ON ATHLETIC PERFORMANCE
2
ABSTRACT
Hypothesis: The intake of caffeine can increase physical performance during athletic activity
Methods: A search for primary sources was done using PubMed with MeSH terms. The search
was limited to randomized controlled trials that were published between 2015 and 2020. After
application of inclusion and exclusion criteria, seven articles were selected for this literature
review.
Results: Of the seven randomized controlled trials selected, six demonstrated caffeine ingestion
led to a statistically significant increase in physical performance. One of the randomized
controlled trials found no statistically significant relationship between caffeine and run timings.
The level set for statistical significance for this literature review was set to p < 0.05.
Conclusion: With regards to the results of the selected studies, caffeine was shown to have
ergogenic activity and was able to increase physical performance during exercise and sporting
competition through multiple mechanisms. Further research should be done with greater sample
sizes to determine the effect of rate of metabolism on caffeine activity and to compare caffeine
responders and non-responders.
Word count: 176
Keywords: Caffeine, athletic performance, exercise, physical performance, sports
EFFECT OF CAFFEINE ON ATHLETIC PERFORMANCE
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INTRODUCTION
Caffeine is the most widely utilized psychoactive substance in the world and may be
ingested in various forms such as capsules and beverages. Caffeine use is completely unregulated
throughout the world and many individuals consume it daily for its stimulant effects on both the
central and autonomic nervous system. Caffeine has an ability to induce alertness, increase
ability to sustain intellectual activity and enhance body reactivity to stimulus (Vanderveen,
2001). A widely studied benefit of caffeine is its effect on physical performance especially when
utilized before athletic activity. Dietary supplements are consumed by individuals at all
competitive levels and competitive level rises in a sport, athletes seek any advantage to enhance
their performance. In comparison to anabolic steroids and other performance-enhancing drugs,
improvements by caffeine are not as exaggerated but may still be greatly beneficial for any type
of athlete. Athletic performance is categorized into physical output and the neurocognitive
abilities of an athlete. Therefore, enhancing either of these factors can benefit the performance of
athletes in a sport. Caffeine emerges as a potential performance enhancer due to its stimulatory
effect on the nervous system with direct and hormonal effects on skeletal muscle function. The
use of caffeine can easily be implemented due to the Olympic committee and all major sporting
organizations have already permitted its use.
The effects of caffeine are carried out through multiple biochemical reactions. Caffeine
acts as an adenosine receptor antagonist to decrease the impairment of attentional processes and
counteract the consequences sleep deprivation, to improve focus and reduce fatigue (Urry, 2014).
Caffeine is a non-selective antagonist to both the A1 and A2 adenosine receptors and indirectly
effects the release of other neurotransmitters such dopamine, GABA and acetylcholine. Caffeine
has the ability to reduce myocyte glycogen consumption by increasing lipolysis of triglycerides
EFFECT OF CAFFEINE ON ATHLETIC PERFORMANCE
4
which can lead to reduced lactic acid production by working muscles. This effect occurs due to
the methylxanthine function of caffeine, which allows it to inhibit phosphodiesterase in order to
increases cAMP concentration within skeletal and adipose tissue. Elevated cAMP levels induce
release of norepinephrine and epinephrine to stimulates the cardiovascular system leading to
increased coronary and skeletal blood flow. As a result, there is enhanced endurance through the
reduction of pain and discomfort associated with lactic acid accumulation and lengthening of the
time muscle fibers take to fatigue. Caffeine has been shown to reduce exertion perceived by
muscles, resulting in increased power output due to muscle failure points being heightened so
muscle fibres can output more work over time (Grgic, 2017).
Caffeine is readily absorbed in the digestive tract within an hour of ingestion
(Vanderveen, 2001) and redistributes throughout the body by its lipophilic nature. It serves as an
ideal performance enhancer because it can be ingested in various forms, is widely available,
inexpensive and is relatively safe. The half-life of caffeine ranges from 2.5 to 10 hours allowing
its effects to last through the entire competition. Caffeine has been shown to enhance skeletal
muscle endurance and strength leading to elevated sprinting speed as well as increasing the speed
and number of high-intensity movement performed by the athlete (Puente, 2017). Use of caffeine
would likely be most beneficial to sports that require short bursts of explosive movements.
The study of caffeine and its relation to athletic performance is important to many sports.
Caffeine has been thought to have a positive relationship to improvements of muscular
performance, therefore athletes should be encouraged to utilize caffeine to maximize their
probability of succeeding during athletic competition.
EFFECT OF CAFFEINE ON ATHLETIC PERFORMANCE
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METHODS
The major source of information for both the results and introduction of this study was
PubMed which served as a database providing meta-analyses and systemic reviews for general
information and randomized controlled trials for the results. Using Medical Subject Headings
(MeSH), a wide-ranging search was conducted on caffeine and its relation to athletic
performance using terms, ("Caffeine"[Mesh]) AND "Athletic Performance"[Mesh] which
yielded 464 results. To narrow the search further, search term “AND "Exercise"[Mesh]” was
added to locate studied using exercises in a quantitative manner to measure the effect of caffeine,
which minimized the search to 219 results. The search was limited to studies published within 5
years from the initiation of this literature review, to center the search results on more recent
publications. Inclusion criteria for the studies was limited to randomized control trials. Including
all inclusion criteria, the search results generated 45 possible articles.
Articles were screened based on their methods and abstract sections to determine if they
most clearly displayed the effect caffeine on performance in a sport, sports related exercise,
general exercise or sport-specific training techniques. Studies that did not measure dependent
variables in any of these forms or included multiple independent variables were excluded.
Studies were also excluded if they focused solely on the neurological effects of caffeine in
sports. This exclusion was included due to the difficulty in separating caffeine’s physiological
activity and neuronal actions as well as the inability to remove the confounding bias from muscle
memory and timing had on physical performance. Caffeine ingested in energy drink form was
excluded to minimize the confounding effects carbohydrates and electrolytes ingestion had on
caffeine. Other criteria for exclusion, were if the study parameters were too specific and not
reflective of conditions a normal athlete would find themselves in. Studies not included in the
EFFECT OF CAFFEINE ON ATHLETIC PERFORMANCE
6
study were used to provide background information to better understand the results of the chosen
studies and provide insight into the multi-faceted influence caffeine on physical performance.
After the exclusion process, of seven journal articles were selected to be studied in this paper. A
PRIMSA flow diagram in Appendix A was included to demonstrate how papers were excluded
and selected.
RESULTS
The contents of this literature review focused on seven randomized controlled trials that
were analyzed to provide a comprehensive view of how caffeine may alter athletic performance.
The studies were based on various sports and general exercises to maximize the generalizability
of the results (See Table 1 on Appendix B). The exercises used to measure the effects of caffeine
could be categorized into maximum performance-based and endurance-based physical activity.
Maximum performance exercises consisted of strength-training, explosive movements and speed
tests which demonstrated the effects of caffeine on maximal skeletal muscle output during a
short time period. Examples of these exercises include one repetition maximum and sprint
timings. On the other hand, endurance-based exercises demonstrated the caffeinated effect on
sustained skeletal muscle activity and alterations on the cardiopulmonary system. These
exercises include long duration runs or output over a full game of a specific sport.
The first article analyzed was performed by Puente et al (2017), to determine if caffeine
could improve basketball performance on twenty experienced male and female basketball
players. The participants were ideal for the study because they were composed of both female
professional basketball players and male semi-professional players with extensive previous
physical training and sport-specific experience. Age range of the participants were similar with
EFFECT OF CAFFEINE ON ATHLETIC PERFORMANCE
7
averages for females being 27.9 years and males 27.1 years. All participants had over 10 years of
experience playing basketball to reduce confounding from discrepancies in basketball
experience. Participants were advised to refrain from caffeine use or use less than 100mg per day
during the course of the study. Female participants were only tested during the luteal phase of
their menstrual cycle to avoid performance changes that may occur through their ovulatory cycle.
The study was designed as a randomized controlled trial, participates ingested either a placebo or
3mg of caffeine per kilogram of body mass 1 hour before the experimental trial began to provide
ample time for caffeine to be absorbed. Performance was tested by ten repetitions of the
Abalakov jump, which tests for vertical reach, a change of direction and acceleration test
(CODAT) and two free throws. For the CODAT, the first five repetitions were performed
without the basketball and the last five were performed with the ball, to measure any differences
in sprinting speed with and without dribbling. Following performance testing, two 10-minute
stimulated games were recorded and analyzed by two basketball statistical specialists to measure
the performance index rating of each individual using FIBA standards, which used the number of
movements and actions conducted by a subject through the game. Afterwards participants
completed a survey regarding perception of their strength, stamina and overall exertion.
The caffeine group was found to have an increase in the average height reached during
the ten repetitions of the Abalakov jump test (37.3 ± 6.8 for placebo vs. 38.2 ± 7.4 cm for
caffeine; 95% CI = 0.3 to 1.6cm). In reference to the placebo, participants who had ingested
caffeine during the game had an increase to their average jump height, number of free throws,
offensive rebounds, total rebounds and quantity of assists. Caffeine was found to have no effect
on the accuracy of free throws indicating it did not impact sport-specific accuracy. The increased
number of assists in caffeinated individuals (1.1 ± 0.9 vs 2.1 ± 1.6 assists) demonstrates
EFFECT OF CAFFEINE ON ATHLETIC PERFORMANCE
8
improvement in player coordination and timing. For the CODAT, a select number of repetitions
shows improvement but there was no change in overall average time to completion or maximum
running speed between the caffeine and placebo groups. In comparison to the placebo, the
caffeine group had increased performance index ratings (8.4 ± 8.3 vs 11.6 ± 7.3), number of
impacts per minute between players (396 ± 43 vs. 410 ± 41 impacts/min; p < 0.001). Overall
caffeine was able to act as an ergogenic agent to increase physical and overall performance
during basketball but was found to have negative effects on sleeping patterns, with over half of
participants reported insomnia. Long term changes to sleep may negatively impacts performance
by depriving athletes of adequate rest. Statistical analysis for this study was done with the
Shapiro-Wilk test to determine the normality of each variable, followed by a Student’s t-test on
each of the normally distributed dependent variables to compare the caffeinated group to the
placebo. Afterwards for the Abalakov jump and CODAT, the caffeine and placebo groups were
compared using a two-way analysis of variance (ANOVA). Statistical significance level was set
to p < 0.05 and 95% confidence intervals for the difference of means between the placebo and
caffeine groups were determined.
A study conducted by Marques et al. (2018) aimed to investigate whether caffeine
consumption could reduce the time needed to complete an 800-meter run in twelve overnight
fasted male runners. Twelve individuals were selected for the study as it was found only ten
subjects were required to obtain statistically significant results when a pilot study was conducted
by the same research group. The study utilized a double-blind format and randomized subject
into either the decaffeinated coffee placebo group or 5.5mg per kilogram body mass of caffeine
group for the first trial and for the second trial subjects were crossed over to the other group.
Both the caffeine and the placebo were dissolved in 200ml of hot water and participants had 2
EFFECT OF CAFFEINE ON ATHLETIC PERFORMANCE
9
minutes to consume the beverage, this was followed by an hour wait period for caffeine
absorptions. Study participants were required to fast overnight for a period of 8 to 10 hours prior
to the trial to reduce available levels of carbohydrates and were told to refrain from caffeine for
48 hours. This prevented the long-term effects of caffeine from interfering with the study results.
Blood pressure, serum lactate and glucose levels were measured before and after each trial.
Following the 8-10 hour fast and consumption of their respective beverages, the participants
were required to run two laps around a 400-meter track. Both sets of trials were conducted on the
during the morning, with subjects being adequately hydrated and instructed to exert maximal
effort. Following the trials, the researchers found serum glucose, serum lactate and systolic blood
pressure were elevated almost immediately after the trial began in the caffeine and placebo
groups but there was no statistically significant difference between the groups. Diastolic pressure
was found to have no changes during the trials compared to baseline in either group. These
findings are thought to be independent of caffeine and were exercise-induced physiological
reactions. No differences were found in trial timings (Decaffeinated: 2.38 + 0.10 vs. Caffeine:
2.39 + 0.09 minutes, p = 0.336), and ratings of perceived exertion (Decaffeinated: 16.50 + 2.68
vs. Caffeine: 17.00 + 2.66, p = 0.326) for both groups. Furthermore, the researchers concluded
that consumption of caffeine was unable to improve running times for 800-meter races in fasted
runners. Statistical analysis was conducted using ANOVA to determine differences between the
caffeine and placebo groups for serum glucose and lactic acid levels, followed by a t-test to
determine differences in trial times and rating of perceived exertion. Level of statistical
significance was set at p < 0.05 for tests conducted.
A double-blind randomized control trial was conducted by Grgic et al. (2017) to
investigate the short-term effects of caffeine on athletic performance through changes in
EFFECT OF CAFFEINE ON ATHLETIC PERFORMANCE
10
muscular strength, power and endurance. The study also aimed to determine if perceived
exertion and pain perception were impacted by caffeine. Seventeen male participants were
selected that had over twelve months of prior resistance-based training with resistance training
done atleast three times a week for the last six months prior to the induction of the study.
Participants with neuromuscular or musculoskeletal disorders were excluded. Participants were
given 6 mg per kilogram body mass of anhydrous caffeine to maximize serum caffeine levels
(Graham, 1995) or a placebo. Three study trials were conducted, each separated by a from each
other. The first session served as a control and the following two session had subjects
randomized into the caffeine or placebo groups. The placebo and caffeine were diluted into 250
mL of water with 20g of orange beverage powder. Following ingestion, participants waited one
hour to allow for absorption of the caffeine. Before performing any of the exercises, a timed 5-
minute stationary bike session was conducted, followed by multiple push-ups and “walk-outs” to
serve as warmups. The researchers utilized different exercises to measure power, strength and
endurance. To measure muscular power, a vertical jump exercise and seated medicine ball
throws were used, whereas for muscular strength, a one repetition maximum for the barbell back
squat and bench press exercises were utilized. The following equation 1 repetition maximum =
Weight x (36/ (37 – Repetitions) (Bryzcki, 1993) was used in order to calculate participants’ one
repetition maximum and the same weight was repeated through the sessions. When measuring
for changes in muscular endurance, number of repetitions for the barbell back squat and bench
press until muscle failure were used by the study group. Muscular power was measured first,
followed by strength then muscular endurance with each being measured with two exercise, one
for lower and one for upper body. Rating of perceived exertion and pain perception were
EFFECT OF CAFFEINE ON ATHLETIC PERFORMANCE
11
measured using the Borg scale (Borg, 1970) with participants being familiarized with the scales
during the first session.
Using an ANOVA analysis, the caffeine group had improvement to the one repetition
maximum for the barbell back squat, reduction in ratings of perceived exertion for the back squat
and reduced pain perception for the bench press one repetition maximum. The researchers found
improvement to the distance the medicine ball was thrown for the caffeine group. Acute
enhancements to lower body strength performance were observed with caffeine intake, whereas
upper body muscular strength was unaffected by the ingestion of caffeine. A significant
reduction in pain perception was found during lower body muscular strength testing.
Additionally, upper body muscular power was positively impacted by caffeine intake while
lower body muscular power remained unaffected. Some variations in muscular strength and
power were observed in individuals of the caffeine group which may suggest differences in
motor unit recruitment. There was no difference in rating of perceived exertion observed for the
bench press, possibly due to its simpler nature and shorter range of motion. Since the bench press
was performed at the end of each sessions, the results for it could be inaccurate due the buildup
of fatigue from previous exercises. The results of this study indicate a reduction in perceived
pain may allow subjects to perform at a higher level. For muscular endurance, there were no
significant differences found for either upper or lower body endurance between the caffeine and
placebo groups. Additionally, no improvements to pain perception or rating of perceived exertion
were found during muscular endurance testing. Lack of significant changes observed for
muscular endurance may be due to fatigue from intensive muscular power and strength testing
done earlier in the session.
EFFECT OF CAFFEINE ON ATHLETIC PERFORMANCE
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Statistical analysis for this study was done using the Shapiro-Wilk test to
determine normality of each variable. Afterwards ANOVA was used to determine if there were
statistically significant differences in muscular power, strength and endurance between the
caffeinated and placebo groups. Level of statistical significance was set to p < 0.05 and each
exercise had a 95% confidence interval calculated for it using Microsoft Excel.
Cheng et al. (2016) conducted a double-blind, randomized crossover study to
investigate the effect of caffeine on the performance of athletes in a 3 minute all out test (3MT)
and changes to their plasma electrolytes. Fifteen male college basketball players with an average
caffeine intake of 50 to 100 mg per day were selected. Participants were given either 6 mg per
kilogram mass of caffeine or a placebo in capsular form, followed by an hour where they laid
supine in a dark, quiet room to allow for adequate absorption of caffeine. Following a 5-minute
warmup, study subjects conducted the 3MT, which consisted of 3 minutes of baseline peddling
on a stationary bike, followed immediately by 3 minutes of all out effort pedaling. Testing was
done in 4 session over 3 weeks, with the first session participants completed an incremental
cycling test in order to measure their maximal oxygen uptake (VO2 max) and gas exchange
threshold (GET). The cycling test involved subjects cycling at a constant revolutions per minute
on a stationary bike until exhaustion, during which the resistance of the bike was increased by
100W every thirty minutes. Gas exchange threshold was determined during the cycling test,
using a face mask attached to a gas analysis system. The second visit, the athletes performed the
3MT to familiarize and train themselves for 3MT. In the third and fourth visit, the trials were
spaced one week apart for washout and participants were randomized into either the caffeine
group or placebo group. The results of the 3MT were used to determine critical power and the
curvature constant, which have direct associations with aerobic and anaerobic capabilities,
EFFECT OF CAFFEINE ON ATHLETIC PERFORMANCE
13
respectively. Both were measured by exercising until exhaustion and calculation relations
between the work done during this period. Critical power represents the upper limit of work that
can be done at a constant state through metabolism. Curvature constant is work done past critical
power that uses only available energy resources to power the muscles. Researchers believe
caffeine was able to prolong the time to exhaustion by increasing both critical power and
curvature constant leading to improve overall endurance. Caffeine’s benefits are likely to be on
observed better in trained athletes due to the subtle enhancement motor unit recruitment and
reduction to perceived effort because less confound would arise proper diet, rest and training.
Serum potassium was measured because high levels can reduce membrane potential leading to
muscle fatigue and decline in force production. The study team believed caffeine could
counteract these effects by reducing serum potassium buildup by promoting Na+/K+ ATPase
pump activity and shunting potassium back inside muscle cells.
After compiling the results, researchers found subjects in the caffeine group had a higher
work end power than the placebo group (increase of 10.7% p < 0.05). End power is a
measurement that can be used to calculate critical power through critical power models
(Vanhatalo et al., 2007). These results may be due central nervous system stimulation by caffeine
and lowered sensitivity to pain which may be responsible for enhanced fatigue resistance. The
study team found no differences in end-test power output between placebo and caffeinated
groups. When compared to the placebo group, the mean power output for time intervals 0
seconds to 60, 90 and 120 seconds were all increased in the caffeinated group by 2.0%, 2.5%,
and 2.2% respectively. In all time intervals, the caffeinated group showed a mean power greater
than the placebo group. In the placebo group, there was a faster decrease in average power output
observed at 0, 60 and 120 seconds, while the caffeine group was more resistant to fatigue and
EFFECT OF CAFFEINE ON ATHLETIC PERFORMANCE
14
average power decreased at a slower pace. In the caffeinated group, ratings of perceived exertion
did not change, but power output was increased, indicating caffeine intake could improve the
tolerance to higher intensity exercises. Blood lactate concentration were elevated after exercise
in both the caffeine and placebo group which was expected, however in the caffeinated group
serum lactate post exercise (11.25 +/- 2.58 mmol/L) was significantly higher than in the placebo
group (9.80 +/- 1.76 mmol/L). Plasma potassium levels were measured after the trials and one
hour after ingestion of caffeine or the placebo, although both groups had increase in potassium
levels, the caffeinated group (3.87 +/- 0.22 mmol/L) had a significantly lower potassium plasma
concentration than the placebo group (3.99 +/- 0.30 mmol/L) after the 3MT. Researchers in this
study utilized the Shapiro-Wilk test to determine normality of the dependent variables, with a
significance of p < 0.05. A 95% confidence interval and effect size were used to compare overall
differences in the groups. A t-test was used to compare the means of caffeinated and placebo
groups for VO2 max, heart rate and exercise outcomes. A within-participants repeated measures
ANOVA was used to compare differences in serum lactate, sodium concentration, potassium
concentration and pH. If any changes were found to be statistically significant, a one-way
ANOVA with Bonferroni post hoc comparisons were used to determine how much the
differences between the two groups change over time.
A double-blind randomized crossover study conducted by Zbinden-Foncea et al. (2018)
was done to determine the impact 5 mg of caffeine per kilogram body mass had on performance
of a countermovement jump (CMJ) by ten elite male volleyball players. Study subjects were
selected if they had atleast six years of prior volleyball experience and trained at minimum 4 to 5
days a week or 14 hours in a week total for the past year. Testing was done over three separate
occasions, during the first session individual caffeine and food intake were determined and
EFFECT OF CAFFEINE ON ATHLETIC PERFORMANCE
15
participants were introduced to CMJ testing to ensure good technique. During the second
session, subjects were randomized into a placebo or caffeinated groups with the third session
being used as a crossover. Participants were given either 5mg per kilogram body mass anhydrous
caffeine or a placebo in identical capsules to be ingested with 200mL water, afterwards there was
a one-hour period to allow for absorption. After ingestion subjects warmed up at 50W on a
stationary bike followed by additional three repetitions of CMJ at submaximal levels. CMJ
involves two motions, the first being a downward eccentric countermovement following by a
concentric upward jump. In between the two movements there was a slight pause to allow for
power transfer.
Zbiden-Foncea et al. found the caffeinated group had a increase in flight time by 5.3% +/-
3.4%, (p < 0.1) when compared to the placebo group. It was found the caffeine group had
increased concentric phase peak force by 6.5% ± 6.4%; (p = .01) and peak power was increased
by 16.2% ± 8.3%; (p < .01) in comparison to the placebo. Jump velocity and force produced
during peak power were increased in the caffeinated group by 10.6% ± 8.0% (p < .01) and 6.0%
± 4.0% (p < .01) respectively. Caffeine ingestion lead to an increase of 10.8% ± 6.5%; (p < .01)
in the height of the jump when compared to placebo. Specifically during the concentric phase,
caffeine increased peak velocity (12.6% ± 7.4%; p < .01), peak power (6.0% ± 4.0%; p < .01),
peak acceleration (13.5% ± 8.5%; p < .01), velocity at peak power (10.6% ± 8.0%; p < .01), peak
displacement (10.8% ± 6.5%; p < .01) and force developed at peak power (6.0% ± 4.0%; p < .01)
Other tested variable had no significant changes, but researchers found the caffeinated group had
a greater elevation to diastolic blood pressure after the trials (71.4 ± 5.0 before vs 81.2 ± 11.3
mm Hg; p < .05).
EFFECT OF CAFFEINE ON ATHLETIC PERFORMANCE
16
The study team used a Shapiro-Wilk test to determine normality of CMJ, followed by a
paired-sample t-test to compare the mean and maximum jump heights of CMJ for the placebo
and caffeine group. Afterwards, a two-way repeated-measures ANOVA with Bonferroni post
hoc test were used to measure the difference between caffeine and placebo group for heart rate
and blood pressure. The side effects of caffeine were measured using the McNemar
nonparametric test and p < 0.05 was the level set for statistical significance.
A double-blind, randomized, crossover study conducted by Potgieter et al. (2018) was
done to determine the effect caffeine had on triathlon performance in both males and female
subjects. The study subjects consisted of fourteen male and twelve female registered triathlon
athletes and were instructed to stop caffeine usage two weeks and limit exercise to 48 hours prior
to testing. Trials consisted of two triathlons conducted two weeks apart, with each consisting of
1.5km swimming portion, 40km bike race and a 10km run. Subjects were randomized into either
the caffeinated or placebo group during the first test and then given the remaining treatment in
the second. Both races were conducted at similar times of day with comparable weather
conditions. Participants were given capsules containing 6 mg of caffeine per kilogram of body
pass or a placebo to be taken orally followed by an hour wait period for absorption. Average
caffeine consumed was 652 +/- 72 mg for males and 540 +/- 68 mg for females. Researchers
measured serum caffeine, leukocyte count, cortisol, testosterone, lactate and hematocrit before
and after completion of the race and used the Borg Scale to determined subjects’ rating of
perceived exertion. A shortened profile of mood states (POMS) was used to determine mood
state differences pre and post-triathlon in both groups.
Before initiation of the trials, average plasma caffeine levels were below 2 mg/L all
subjects. After ingestion of the capsule, caffeine levels were maintained in the ergogenic range
EFFECT OF CAFFEINE ON ATHLETIC PERFORMANCE
17
between 7 to 10 mg/L serum caffeine. Caffeine levels continued to rise through the triathlon due
to the capsule used by the study group. The study findings demonstrate caffeine supplementation
had a statistical significance improvement to overall performance in male subjects only, with a
1.7% reduction in time for males and 0.9% for females. Swimming time was shown to have the
most reduction within the caffeinated group, with 4.5% and 2.8% reduction in males and females
respectively. In 71% (10 of 14) of male subjects, caffeine supplementation improved the 10km
run portion of the triathlon, while 64 % (9 of 14) of males had reduction to the 40km cycling
portion of the race. In female subjects, 67% (8 of 12) had an improvement to swim and cycling
portions and 50% (6 of 12) demonstrated better running performance. No differences were
observed in rating of perceived exertion or in POMS scores in the caffeinated and placebo
groups. Both serum cortisol (p < 0.001) and testosterone ( p< 0.001) levels were elevated, with
cortisol levels being significantly greater in the caffeine group. Testosterone levels were found to
be unrelated to caffeine ingestion, with the only differences found were between male and
female subjects. Male subjects in the caffeinated group had statistically increased serum lactate
levels (p < 0.05) indicated caffeine slowed clearance of lactate. Leukocyte counts post-race were
higher in caffeine group, with greater leukocytosis in male subjects. Analysis of the raw data in
this study was done with ANOVA with restricted maximum likelihood (REML) to compare the
caffeine and placebo groups. This allowed the researchers to compare an individual to their
treatment group and gender as well as compare individual with themselves for placebo or
caffeine ingestion. Maximum likelihood chi-squared test was utilized to compare non-qualitative
data obtained by the study.
Fett et al. (2017) conducted a single-blind crossover study to determine the effect of
caffeine on muscular strength and fatigue tolerance. The participants of study were eight women
EFFECT OF CAFFEINE ON ATHLETIC PERFORMANCE
18
with ages ranging from 20 to 30 years old, with each having atleast one year of continuous
resistance training. Average body mass index (BMI) for the subjects was restricted between 20
and 25 kg/m2. During testing fifteen individuals were excluded from the results; eight did not
perform any of the tests, two did not restrict themselves from caffeine and five had difficulty
performing all the tests. After review of literature, the researchers decided to use 6 mg per
kilogram body mass of caffeine to ensure consistent ergogenic effects of caffeine which
normally occur between 3 and 9 mg per kilogram body mass (Mora- Rodriguez, 2014). Both the
caffeine and placebo groups were given identical capsules 30 minutes before testing began. The
study was separated into four sessions done over four weeks. Each session consisted of four
different movements, the first three tested muscular strength and consisted of a pull-down
movement testing back and elbow flexor strength, hack squat to test lower limb strength and
bench press to test chest, shoulder and elbow extensor strength. Every exercise was down three
to five times to determine the one repetition maximum for each individual. To test caffeine’s
effects on muscle fatigue, a knee extension exercise was done for three sets in a drop set format
of 100, 80 and 60 kg. Each set was done until exhaustion and total repetitions done were added
together. The first session was used to determine baseline results for each individual with
restriction from caffeine for 48 hours prior to the test. This session also allowed subjects to
familiarize themselves with testing and the exercises. The second session subjects were given 6
mg per kilogram body mass caffeine with blinding before testing was conducted. The third
session, subjects received the placebo and fourth session subjects again ingested caffeine. Each
session had over 168 hours separating them to reduce the accumulation of fatigue and wear on
the subjects. Researchers used analysis of variance (ANOVA) with a Tukey-Kramer Multiple
comparison test to compare the means of each of the exercises for the final three sessions.
EFFECT OF CAFFEINE ON ATHLETIC PERFORMANCE
19
Afterward, the mean of the two caffeine groups was compared to the means of the basal and
placebo groups together, to examine the mean effects of caffeine. Delta was used to determine
the level of effect caffeine had on the exercises, this was achieved by subtracting the mean of the
two caffeine groups from the mean of the placebo and baseline groups. A paired t-test was then
used to compare the delta values of each exercise.
Researchers found caffeine did not have an effect of the pull-down test, while it did show
to increase hack squat strength in the second caffeine trial significantly (p < 0.001). Bench press
strength for the caffeine groups was elevated compared to the basal group (p < 0.01 and p <
0.001). Both caffeine groups had improvements to drop-set performance in relation to the
placebo and basal groups (p < 0.001 and p < 0.001). The mean of the caffeinated groups and
placebo were shown to increase strength of the hack squat (mean of caffeine: p < 0.001, placebo:
p < 0.05) and bench press (mean of caffeine: p < 0.001, placebo: p < 0.01) when compared to the
baseline. Resistance to exhaustion had a statistically significant increase for mean of the caffeine
groups (p < 0.001) when compared to both the basal and placebo. Researchers found caffeine
had a significant positive effect of the delta of the hack squat and bench press but no effect to the
pull-down exercise.
DISCUSSION
After examination of the seven randomized controlled trials (Punete et al. 2017, Marques
et al. 2018, Grgic et al. 2017, Cheng et al. 2016, Zbinden-Foncea et al. 2018, Potgieter et al.
2017, and Fett et al. 2017), this literature review sought to determine if caffeine was able to
enhance athletic performance through physiological and neuromuscular effects. The end goal of
this paper was to prove caffeine could be used as a performance enhancing agent prior to an
athletic event to give athletes an advantage during competition.
EFFECT OF CAFFEINE ON ATHLETIC PERFORMANCE
20
In the study conducted by Puente et al. (2017), researchers found caffeine was able to
increase height reached in Abalakov jumps, amount of body impacts during a game, number of
free throws attempted and made, total assists, total rebounds and offensive rebounds. A major
finding in this study, not previously well researched was caffeine had a positive impact in overall
basketball-specific performance. Caffeine did not demonstrate any effect on the accuracy of free
throws, two-point or three-point shots during stimulated games indicating it only acts as
ergogenic substance, but lacked net effects on sport-specific accuracy or skill-based actions. The
increased physical performance in caffeinated group improved their positioning during
stimulated games or allowing them to out-maneuver of their opponents putting them in more
advantageous situations. This likely had a direct impact on number of assists, total and offensive
rebounds during a game, which rely heavily on player coordination, position, and strength
indicating caffeine could improve sport-specific skills. Rebounding was improved due to
caffeine increasing jump height, which allowed players to reach the basketball before opponents.
Another possible way caffeine may have improved sport-specific skills was by central nervous
system stimulation through adenosine receptor antagonism. This may improve decision making
and focus, which are essential during team sports. Lack of improvement to acceleration during
CODAT testing contraindicates the findings of previous studies that show caffeine can improve
acceleration. The researchers believe this may be due to CODAT testing multiple bouts of
acceleration and deceleration, commonly performed in team-based sports while other studies had
tested linear, continuous acceleration. The increase to total impacts in the caffeine group during
the stimulated basketball games was an indicator caffeine influences player movement to be
more physical. Increased physicality during basketball games allows players to use their strength
to draw fouls and overpower opponents, giving themselves and their team an advantage.
EFFECT OF CAFFEINE ON ATHLETIC PERFORMANCE
21
Caffeine may have also improved athletic performance by increasing self-perceived muscular
strength and endurance, which may allow these athletes to performance at more intensive levels
by avoiding symptoms of over-exertion and fatigue.
Negative effects of caffeine were centered insomnia due to the short six-hour window
between the trials and bedtime. Other concerns the researchers had were possible under
absorption of caffeine due to a pre-exercise meal. Possible confounding in this study could be
caffeine’s ergogenic effects relying on adequate glycogen stores in the liver and muscle which
may not have been replenished. Limitations of the study were the experimental design of the
games played. These did not accurately reflect real basketball games due to lack of substitutions
and time-outs. Other limitations could be non-responders to caffeine could have shown no affect
which might have interfered with the results of the study. Multiple trials were not conducted for
both caffeine and non-caffeine groups putting too much emphasis on the two games played.
Performance during team sports may have been enhanced or impaired by team cohesion and
influence other players have on each other.
Marques et al. (2018) concluded caffeine did not improve times in overnight fasted
athletes during an 800m run. No differences were found in serum glucose, lactate and blood
pressure between the caffeine and placebo groups. These results may be explained due to coffee,
the mode of caffeine delivery in this study, might have contained various forms of caffeine such
as caffeic acid and chlorogenic acid that may not be ergogenic. Previous studies conducted by
Graham et al. (1998) found chlorogenic acid may interfere with the ergogenic activity of
caffeine. Another possible explanation could be from theophylline found in coffee and
decaffeinated coffee (placebo) which acts similar to caffeine, to antagonize adenosine receptors
and stimulate carbohydrate oxidation to increase available ATP levels. The variable effects of
EFFECT OF CAFFEINE ON ATHLETIC PERFORMANCE
22
caffeine within the study population may be due to polymorphisms in the CYP1A2 gene and
ADORA2A adenosine receptor gene. Polymorphisms in the ADORA2A adenosine receptor may
change the ability caffeine has to antagonize the adenosine receptor and could reduce its effects.
CYP1A2 encodes for an enzyme of the p450 monooxygenase family involved in caffeine
metabolism. CYP1A2 genotype AA are fast caffeine metabolizers and receive greater effects
from caffeine than genotypes CC and AC. Genotype AA was found to have a 4.8% reduced time
during a race with 2mg per kilogram body mass caffeine consumed verses genotype CC
individuals that had a 13.7% longer time to complete the run (Guest et al., 2018). The CYP1A2
gene was found to influence perceived exertion in individuals taking caffeine (Guest et al, 2018)
but caffeine non-responders had no changes to perceived exertion rating. It was also found
environmental temperature and humidity did not play a role in caffeine’s ergogenic effects.
Possible limitations for this study were the small sample size of twelve participants, lack
of repetition of the trials and lack of separation of responders and non-responders to caffeine
which might have influenced the results. Prior to conducting the study, participants were not
genotyped for their CYP1A2 and ADORA2A genes, therefore genetic differences in either might
have led to lower mean values. Another limitation of this study might have been overnight
fasting which could have depleted glycogen stores utilized by caffeine to produce its ergogenic
effects.
A study conducted by Grgic et al. (2017) concluded caffeine ingestion enhanced lower
body strength performance and reduce perceived exertion. No changes to upper body strength,
endurance, pain perception or perceived exertion were found. Caffeine had an overall 2.8%
positive increase to lower body physical performance. A study conducted by Meur et al. (2009)
found increases of atleast 3% in performance may be enough to propel athletes past their
EFFECT OF CAFFEINE ON ATHLETIC PERFORMANCE
23
competition and grant them victory. The improvements to lower body strength opposed findings
of previous studies that found caffeine had no effect on lower body strength, these contrasting
findings are likely explained by differences in the form caffeine was ingested, exercises
performed or time for absorption. One way to measure the caffeine absorption is using salivary
caffeine levels. Caffeine ingested as coffee or powder dissolved in liquid reaches ergogenic
levels much quicker than caffeine ingested in capsular form. Other differences seen in the
subjects could be due to differential response of an individual to caffeine. In caffeine responders,
caffeine increased muscular strength by enhancing motor unit recruitment through central
nervous stimulation. The only exercise without reduction in perceived exertion was the bench
press due to its simple nature and being less physically taxing. The lack of changes in bench
press strength may be due to the smaller muscles in the upper body having less motor unit
recruitment after caffeine ingestion (Warren et al. 2010). The increases in muscular strength in
the lower body was likely due to reduced perception of exertion allowing athletes to output more
force and work. Caffeine influence on muscular endurance can reduce fatigue accumulation or
reducing perception of fatigue, permitting athletes to perform the same exercises for longer
periods.
The study design of this study could be a possible limitation due the bench press being
performed at the end of each session. The accumulated fatigue and energy expenditure could
have reduced the ability for athletes to perform the bench press and possibly limited their ability
to perform as well. Another limitation the sample size of seventeen which might have impacted
the results due to outliers in the subjects having a greater impact on the results than if a much
greater sample size was utilized. The study was unable to determine how the placebo effect could
EFFECT OF CAFFEINE ON ATHLETIC PERFORMANCE
24
affect physical performance because the study lacked a control or ensuring blinding was
maintained.
Cheng et al. (2016) concluded the caffeine group had increased work above the end-test
power (WEP), increased fatigue resistance and lowered serum potassium levels during 3MT. The
researchers found the caffeine group had increased serum lactate without an effect on pH after
3MT. Caffeine ingestion was shown to have no effect on aerobic capacity due to no changes in
end power (EP) of 3MT. EP can be used to measure critical power in an exercise which is
predictive of aerobic capacities (Moritani et al. 1981). Researcher believe caffeine is able to
increases WEP by reducing pain levels in late stages of 3MT exercise where anaerobic capacities
are utilized. Reduction in pain perception was achieved through central nervous stimulation that
decreases pain sensitivity, thereby allowing caffeinated individuals to have increased time for
muscular activity before muscule pain and fatigue sets in and they must rest. WEP can be used to
determine the curvature constant, a measurement of the abilities of an individual to perform
actions to exhaustion. The accumulated oxygen deficit is the difference between the oxygen
required by the body and oxygen taken up, this deficit can be widened by caffeine to enhance
anaerobic capacity requiring less oxygen for the same actions. Caffeine was able to improve the
muscle power of an athlete and slow the decline in power output by allowing athletes to exert
more force while perceiving the same levels of exertion and be more tolerant of discomfort from
exercise. Caffeine’s adenosine receptor antagonism in the CNS can improve individual tolerance
towards low and moderate intensity exercise by inducing hypoalgesia (Black et al. 2015). At
higher intensities the caffeine-induced hypoalgesia was ineffective at opposing the excessive
nociceptive activation. Studies found caffeine improved muscle strength by increasing motor unit
activation and the total force produced during contraction of muscles (Bazzucchi, 2011).
EFFECT OF CAFFEINE ON ATHLETIC PERFORMANCE
25
Caffeine use was thought to be effective in sports due to the majority of actions and movements
performed are at submaximal levels. The caffeinated group was found to have lower interstitial
potassium levels than placebo which may enhance performance by maintaining membrane
potential and permitting rapid and continuous muscular stimulation. These effects are achieved
through caffeine’s enhancement of the sodium potassium ATPase pump activity.
Limitations for this study were the small sample size of fifteen and lack of control group.
Other limitations could be the relationship between serum potassium and interstitial muscle
potassium levels due to caffeine showing no changes to serum potassium levels during in the
study.
Zbinden-Foncea et al. (2018) determined 5mg per kilogram body mass caffeine ingested
by volleyball athletes led to increased jump height, velocity of jump and force produced during
jump in countermovement-jump (CMJ) performance. The caffeine group had increased
maximum velocity, maximum acceleration, maximum power, and maximum force generated
during concentric contraction. These results mirrored those of Del Coso et al. (2014), who found
caffeine increased height and power of CMJ by 5% and 2.5%. The effects achieved by caffeine
ingestion in athletes unfamiliar to CMJ mimic the results of athletes with previous CMJ training
(Del Coso et al. 2014). Zbinden-Focea et al. (2018) determined caffeine did not affect jump
mechanics with similar results being found in athletes of other sports, such as soccer and
basketball players when they were tested with CMJ. The study team believed the ergogenic
effects of caffeine were explained by its central stimulation of the nervous system through
adenosine receptor antagonism. Adenosine has inhibitory action of neural excitability and
synaptic transmission. Caffeine improved intermuscular and intramuscular coordination by
increasing overall motor unit recruitment and led to elevated calcium release from the myocyte
EFFECT OF CAFFEINE ON ATHLETIC PERFORMANCE
26
sarcoplasmic reticulum leading to increased myocyte contractility. Researchers found single
nucleotide polymorphisms within intron 1 of CYP1A2 enhanced its breakdown of caffeine which
blunted caffeine’s ergogenic effects in individuals with this mutation. This served as a major
limitation of the study due to lack of separation of participants based of their CYP1A2 genotype.
Another limitation was the small sample size of ten and the lack of placebo which may lead to
confounding from the placebo effect. Lack of repetition of trials is another limitation, due to
outliers having a more profound effect on the data.
Potgier et al. (2018) concluded usage of caffeine enhanced triathlon performance in
almost all individuals tested. Males subjects improved by 1.7% while females subject by 0.9%.
Improvement were less pronounced in this study compared to others due to the realistic nature of
the trials and participants actively competing against each other. Variations in caffeine
habituation between the genders was likely explained by females consuming caffeine on average
at 501 ± 649 mg per day than males at 337 ± 345 mg per day (Ganio et al., 2009). The
researchers believed caffeine ingestion led to increased glucose levels, changes in serum ion
levels and decreased perceived exertion ultimately causing its ergogenic effects. Caffeine might
have been especially beneficial for low to moderate intensity exercise by boosting force
production of skeletal muscles. Caffeine might have been less effective for intense exercise
because the majority of motor units were already in use and caffeine had no direct effect on
muscle. This might have led to swimming timings to decrease by 3.7% on average while overall
time on decreased by 1.3% due to the moderate intensity of swimming. The presence on non-
responders and responders in the study group functioned as a limitation to the study due to lack
of stratification of the groups. Sport-specific training and adaptation serve as other limiting
factors in the study due to top-forming participants benefiting less from the ergogenic effects of
EFFECT OF CAFFEINE ON ATHLETIC PERFORMANCE
27
caffeine than lower performing individuals. The central effects of caffeine allowed athletes to
separate perceived effort during physical activity from the symptoms of physical activity such as
pain and fatigue and allow them to exert higher levels of effort. Another source of limitation in
this study was the differences in males and females exercise-induced cortisol levels which may
change overall bodily response to exercise. Sample size, lack of control and lack of trial
repetition were other sources of limitation.
The final study analysed for this literature review was performed by Fett et al. (2017) and
concluded caffeine improved strength of the lower limbs and increased tolerance to fatigue. The
research team believed caffeine’s central nervous stimulation increased muscular contraction and
improve total output by reducing experience of fatigue. Caffeine was thought to be beneficial in
movements requiring recruitment of large muscle groups because it can reduce CNS fatigue that
occurs through excessive muscular stimulation. Due to caffeine use, muscles were able contract
more often and at higher intensities, which was demonstrated by an increase of 1kg and 2kg for
the hack squat in the caffeinated trials when compared to the placebo group. The second caffeine
trial had improvements in the drop set exercise and bench press while the first trial lacked
improvement which may point to a learning period the body must go through to work efficiently
with caffeine supplementation. The negative effects of caffeine included gastrointestinal distress,
anxiety and insomnia and were seen in a few of the subjects. Those who infrequently ingested
caffeine experienced more symptoms and at higher levels. Limitations of this study were its
small sample size and the absence of measurement to changes in glucose, lactate and other
physiological variables before and after the exercises were performed.
The studies analyzed had many sources of limitation, primarily all of them had small
sample sizes with many lacking control groups. There was lack of repetition of trials in both
EFFECT OF CAFFEINE ON ATHLETIC PERFORMANCE
28
placebo and caffeinated group which would have accounted for any gradual improvements in the
trials from experience. No stratification was done to separate caffeine responders and non-
responders, and none of the studies considered the effects of polymorphism in the CYP1A2 and
ADORA2A genes.
Future Directions and Conclusion
Possible avenues for future research would be quantifying the effect each genotype of the
CYP1A2 gene would have on the ergogenic effects of caffeine and determining the ideal dosing
to achieve ergogenic activity while avoiding side effects. More research should be done with
greater sample sizes and additional repetition of trials. Caffeine research should also be done
separating caffeine responders and non-responders as well as research for its use during training
for athletic competition. There also should be research conducted on the ideal timing for caffeine
ingestion to ensure ergogenic activity was reached and maintained through competition.
After the analysis of the seven primary articles, six out of the seven found caffeine to
have a statistically significant impact on improving the physical performance of athletes. Two of
the studies tested caffeine in a realistic sporting setting, and found a statistically significantly
improvement in both for sport-specific performance. Based on these findings, caffeine can be
used prior to competitions by athletes to utilize its ergogenic properties to boost athletic
performance.
ACKNOWLEDGEMENTS
The author declares there were no conflicts of interest or funding in regard to this paper.
EFFECT OF CAFFEINE ON ATHLETIC PERFORMANCE
29
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33
APPENDIX A
Figure 1. PRISMA (Moher, 2009) flow diagram displaying exclusion and selection of articles
Records identified through
database searching
(n = 46)
Screening
Included
Eligibility
Identification
Additional records identified through
other sources
(n = 1 )
Records after duplicates removed
(n = 47 )
Records screened
(n = 47 )
Full-text articles
assessed for eligibility
(n = 7 )
Excluded articles (n= 40 )
- Solely neurological (n= 5 )
- Multiple variables tested
(n= 15 )
- Study parameters too
specific (n= 6 )
- Caffeine given in energy
drink form (n= 8 )
- Methods to measurement of
the effects of caffeine too
nonspecific (n= 6 )
Studies included in
systematic review
(n = 7 )
EFFECT OF CAFFEINE ON ATHLETIC PERFORMANCE
34
APPENDIX B
Table 1. Evidence table of primary sources used in results
First Author
Date of publication
Study
Design
Level
of
Evide
nce
Study Population
Therapy/Exposure
Outcome/results
1. Puente,
Carlos
June 2017
RCT
1
Twenty professional and semi-
professional male and female
basketball players
3mg/kg body
mass
anhydrous
caffeine or a
placebo
Caffeine
supplementation
increased average
Abalakov jump
height, number of
body impacts, and
basketball
performance
index rating
2. Marques,
Alexandre C.
April 2018
RCT
1
Twelve males with amateur
running experience that had
fasted overnight (8 -12 hours)
5.5 mg /kg
caffeine
ingested as
coffee or a
decaffeinated
coffee placebo
No differences
were found
between the
caffeine and
placebo group in
time to complete
the 800m race and
no changes in
rating of
perceived
exertion, blood
glucose or blood
lactate levels
3. Grgic, Jozo
May 2017
RCT
1
Seventeen male participants with
over one year of resistance
training and had been training
atleast 3 time a week for the last
six months
Caffeine
supplementati
on 6mg/kg
body mass or
a placebo one
hour before
testing
The caffeinated
group had
increased one
repetition
maximum for
back squat
strength,
increased power
in the seated
medicine ball
EFFECT OF CAFFEINE ON ATHLETIC PERFORMANCE
35
First Author
Date of publication
Study
Design
Level
of
Evide
nce
Study Population
Therapy/Exposure
Outcome/results
throw, increased
overall lower
body strength,
and reduction in
perceived
exertion and pain
perception
4. Cheng,
Ching-Feng
January 2016
RCT
1
Fifteen Division 1 college male
basketball players
6mg/kg body
mass of either
caffeine or a
placebo
It was found
caffeine increased
work above end-
power (WEP) and
power output
between 60 to 150
seconds
5. Zbiden-
Foncea,
Hermann
May 2017
RCT
1
Ten male volleyball players from
the Chilean national team
5mg/kg
anhydrous
caffeine or a
placebo
Caffeine intake
increased
countermovement
-jump peak
concentric force,
peak power and
increased the
maximum height,
velocity, power
and acceleration
achieved during
the jump
6. Potgieter,
Sunita
Decemeber
2017
RCT
1
Twenty-six male and female
triathlon athletes
6mg/ kg body
mass caffeine
or a placebo
Caffeine led to a
mean 3.7%
decrease in
swimming times
and mean 1.3%
decrease in total
time to complete
the triathlon.
7. Fett, Carlos
April 2017
RCT
1
Eight women between ages 20 to
30 years old and BMI between
20-25 with atleast one year of
continuous resistance training
6mg/kg body
mass of
caffeine or a
placebo 30
minutes
before
Caffeine
increased hack
squat strength and
for the second
caffeine trial,
there was an
EFFECT OF CAFFEINE ON ATHLETIC PERFORMANCE
36
First Author
Date of publication
Study
Design
Level
of
Evide
nce
Study Population
Therapy/Exposure
Outcome/results
performance
tests
increase to bench
press strength,
and increased
time until
exhaustion
