The Effect of Caffeine Supplementation on Muscular Endurance in Recreationally Active College Age Males

Objective: Caffeine is a drug consumed regularly by approximately 90% of adults worldwide, primarily due to its ability to reduce fatigue and increase wakefulness. The benefit of caffeine consumption on athletic performance in large doses (3-9 mg/kg body weight or BW) is frequently documented in aerobic athletes. The benefits of caffeine supplementation in resistance training variables, such as muscular endurance, has shown mixed results, partially due to the inconsistency of testing variables. Furthermore, while caffeine supplementation shows promising ergogenic effects in muscular endurance in elite athletes, it is unknown if this effect translates to the recreational athlete. Therefore, the purpose of this study is to observe the potential ergogenic effect caffeine supplementation may have in recreational athletes and to consider how caffeine habituation may influence individuals’ response to a high dosage of 7 mg/kg BW. Design: This study used a randomized, double-blind crossover design. Subjects performed bench press and Smith machine squat repetitions to failure using 60% of their respective one repetition maximum (1RM), vertical jump, and isometric squat tests. Subjects consumed either caffeine equivalent to 7 mg/kg BW or placebo 60 minutes prior to testing. Test sessions were separated by 7 days. Number of complete bench press and Smith machine squat repetitions, vertical jump height, and isometric power were evaluated. Rating of perceived exertion (RPE) was also recorded and assessed. A repeated measures analysis of variance (ANOVA) was used to determine differences between treatments. Subjects: Subjects were healthy college age males with at least 6 months of prior strength training experience (n=23, 22.0±2.2 years). Results: There was no effect of treatment order. There was a significant increase in bench press repetitions to failure between caffeine (18.9±3.7) and placebo (17.3±3.7, p=0.002). There was a significant increase in Smith machine squat repetitions to failure between caffeine (17.2±4.7) and placebo (15.3±4.5, p=0.018). No significant difference was found in vertical jump or isometric force plate tests between treatments. RPE was not statistically different between treatments. Conclusions: This study suggests that acute caffeine supplementation equivalent to 7 mg/kg BW has an ergogenic effect in recreationally trained males in resistance training exercises. RPE was not statistically different between treatments, indicating that caffeine supplementation may also reduce perception of exertion relative to the amount of work performed immediately following a bout of high-intensity resistance exercise to failure.

Thank you to Linda Sebelia and Cathy English for the many opportunities that have been provided to me over the years. I am fortunate enough to have had the pleasure to be both a student and a worker for you both, and the experiences I have gained are incomparable to any other.
To the dietetic interns -Chelsea, Greg, Jacqueline, Laura, and Kelsi -working and learning alongside all of you has been the greatest experience, and I believe we have grown together immensely in such a short time. To Mike Macarthur, I hope to one day be able to help another with statistics the way you have helped me. To Eric Nelson, our competition as undergraduates is partially the reason why I am here today,     Male subjects (n=23, 22.1±2.2 years) were recruited for this randomized, double-blind crossover trial. Subjects performed bench press and Smith machine squat repetitions to failure using 60% 1RM, vertical jump, and isometric squat tests. Subjects consumed either caffeine equivalent to 7 mg/kg BW or placebo 60 minutes prior to testing.
Number of complete bench press and Smith machine squat repetitions, vertical jump height, isometric power, and rating of perceived exertion (RPE) were assessed.
Repeated measures ANOVA was used to determine differences between treatments.
Significance was set at p≤0.05. There was no effect of treatment order. There was a significant increase in bench press repetitions (p=0.002) and Smith machine squat (p=0.018). This study suggests that acute caffeine supplementation equivalent to 7 mg/kg BW has an ergogenic effect in recreationally trained males in these two resistance training exercises.

INTRODUCTION
Caffeine, a drug sought for its ability to increase wakefulness and reduce fatigue, is one of the most widely consumed drugs in the world (29). Caffeine is found in products such as coffee and tea, soda and energy drinks, ice cream, chocolate, and nutrition supplements. Evidence suggests caffeine consumption may have an ergogenic effect in a wide variety of athletic performance, including cardiorespiratory endurance events and high-intensity, short-duration activities (2,13,14,22,27). While the effects of caffeine on aerobic performance have been investigated extensively in exercises such as running, cycling, and rowing, the effect of caffeine in resistance training variables such as muscular endurance are inconclusive (5,13).
A common method of measuring muscular endurance in caffeine trials is by having subjects perform repetitions to failure using a percentage of their maximum lifting ability, or 1-repetition maximum (1RM) (10). A study by Woolf, et al. (34) found no significance between treatments in bench press to fatigue in caffeine-naïve collegiate football athletes using 5 mg/kg, while Duncan, et al. (11) found significance in bench press repetitions to failure using the same dosage in University-level rugby, football, and basketball players. A study by Astorino, et al. (2) found no differences between treatments in resistance-trained men who ingested caffeine equivalent to 6 mg/kg. While a limited amount of literature exists evaluating lower doses of caffeine, the consensus suggests there is not an ergogenic effect in muscular endurance at 5-6 mg/kg body weight (BW) (2,10).
The majority of existing research focuses on subjects of elite athletic status, often with several years of experience in resistance training exercise. Less research has been performed on the impact of acute caffeine ingestion on strength and endurance in the average individual who participates in light to moderate consistent physical activity. Therefore, in our study, we had recreationally trained athletes perform a combination of resistance exercises incorporating large muscle groups in both upper and lower body while ingesting a dose of caffeine equivalent to 7 mg/kg of body weight. Our primary hypothesis is that acute caffeine ingestion in the amount of 7 mg/kg of body weight will increase the number of bench press repetitions to failure compared to placebo ingestion in college age, recreational male athletes. Our secondary hypotheses is that acute ingestion of caffeine will also increase the number of squat repetitions to failure, increase the amount of force generated from a vertical jump and isometric squat exercise, and decrease rating of perceived exertion (RPE) at the time of testing, when compared to placebo ingestion.
Furthermore, previous studies have not taken body composition into consideration for caffeine dosing (2,13,22). Considering caffeine metabolism does not occur in adipose tissue (1), our exploratory hypothesis is that subjects with lower body fat percentage will demonstrate a significant increase in repetitions to failure in bench press and squat exercises when ingesting caffeine when compared to subjects with a higher body fat percentage.  (31). Total body estimates of percent fat were computed from the measured body volume using the Siri equation (28). During the initial visit, 1RM strength testing for the bench press and squat exercises was conducted using National Strength and Conditioning Association (NSCA) guidelines in the HFL (3). In addition, subjects completed a health history questionnaire and caffeine frequency questionnaire for screening and descriptive purposes, during the initial visit.

Muscular Strength
During the pre-test visit, subjects were asked to estimate their 1RM for the bench press and Smith machine squat. For each exercise, fifty percent of their stated 1RM was calculated, and subjects were asked to perform 5-10 complete repetitions.
After 3 minutes of rest, subjects were asked to perform 3-5 complete repetitions based on 70% of their stated 1RM. Subjects then would perform 1-2 repetitions of gradually increasing weights, with three minute rests in between, until they were no longer able to complete a full repetition. This was performed to estimate the maximum amount of weight the subject can lift one time and served as the value for calculating the amount of weight to be used during testing visits (9).

Muscular Force and Calculated Vertical Jump Height
Muscular power was assessed by an isometric squat and vertical jump exercise.
Force and power were measured using a force plate and Accupower software (AMTI, Watertown, MA). Following subject familiarization of the isometric squat protocol, subjects were asked to stand on the force plate in a quarter-squat position under a Smith machine squat bar. Once in position, subjects were asked to push against the stationary bar maximally for 10 seconds. Knee angle was measured with a goniometer.
Angle of the knee can be within a range of 100-135 degrees; this number was the same for both test trials (18).
Subjects were also familiarized with the vertical jump protocol and asked to perform three consecutive, maximal effort jumps. Procedure for the vertical jump required subjects to have hands placed on their hips and feet shoulder-length apart during the exercise. Subjects performed this exercises an additional two times, with 2-3 minutes of rest between each set. The highest force, power, and jump height in the three sets were recorded and averaged.

Muscular Endurance
Muscular endurance for both the bench press and Smith machine squat were assessed using 60% of the subjects' measured 1RM. Following familiarization of the exercise protocol, subjects were asked to perform a single set of repetitions of each exercise until failure. A trained tester was used to count the number of repetitions completed and to ensure each repetition was completed with proper form. A second trained tester was present to assist with counting complete repetitions performed.

Rating of Perceived Exertion
RPE was assessed before and after each exercise using the Borg CR-10 scale (8). Prior to the start of exercises, instructions on how to use the Borg CR-10 scale were read to each subject. The scale is ranked from 0 (resting state) to 10 (maximal effort) and assesses how strenuously the subject perceived he worked, based on the self-reported number chosen.

Caffeine and Placebo Administration
Subjects were supplied caffeine in the form of encapsulated powder. Seven mg/kg BW were used to determine the total dose for each subject. Placebo capsules were filled with microcrystalline cellulose free of the eight major allergens as well as gluten. Caffeine or placebo was ingested one hour prior to experimental trials to achieve maximum plasma concentration (16). Subjects were provided 12 fluid ounces of water to aid in capsule ingestion. Once the subject had ingested caffeine or placebo capsules, he remained stationary for 45 minutes, viewing TED Talks© or reading magazines. At 45 minutes, subjects were tested for hydration status by using a refractometer. Inadequate hydration was determined by a specific gravity of 1.024 or greater (25). Subjects who were found to be inadequately hydrated were provided an additional 12 fluid ounces of water to consume.

Statistical Analysis
G-power (G*Power, Version 3.1.9.2) was used to determine sample size using results from a similar study analyzing bench press repetitions in collegiate athletes ingesting 5 mg/kg body weight of caffeine versus placebo (12). Significance for all analyses were set at p≤0.05.

RESULTS
Subject demographics are presented in Table 1 Results indicated that subjects completed significantly more repetitions to failure with the caffeine treatment compared to placebo in both the bench press (mean difference of 1.4 repetitions, p=0.006) and Smith machine squat (mean difference of 1.5 repetitions, p=0.032) ( Figure 1). Treatment order was not significant in either test.
There were no significant differences in average vertical jump height ( Figure 2) or isometric force ( Figure 3) between treatments.
With respect to lean body mass, no correlations were found with performance in the caffeinated condition in the bench press or Smith machine squat tests. No correlations were found in bench press or Smith machine squat tests with self-reported caffeine habituation under the caffeinated condition. Data not shown.
Rating of perceived exertion was not significantly influenced by condition bench press or squat tests, as determined by 2 (time) x 2 (treatment) repeated measures ANOVA. Rating of perceived exertion was significant in the isometric squat test between treatment and time; however, significance was lost when controlling for treatment order.

DISCUSSION
Both the bench press and Smith machine squat tests resulted in a significant In that study, all subjects were highly experienced in the respective sport (rugby, basketball, football) at the University level and have been competing in their sport for a mean time of 10.4±2.3 years (11). Therefore, the present study suggests that caffeine supplementation in recreational athletes has similar ergogenic effects to that which has been observed in athletes trained in their respective sports for longer durations.
This study examined the acute effect of a high dose of caffeine on muscular endurance and sought to address gaps in the literature by employing a design where multiple resistance exercises were utilized and a population of recreationally trained athletes was analyzed. Employing the bench press, Smith machine squat, and isometric force plate arguably create a greater level of fatigue than studies examining performance in a single exercise. The bench press test in particular is an exercise frequently used in studies evaluating the impact of caffeine in muscular endurance; employing this test in our study allows us to compare efficacy in our population with groups that have been previously evaluated (2,5,32). The present study also sought to recruit subjects who participate in resistance training activity on a recreational basis, which was defined as a minimum of twice a week for a period of at least six months.
Other researchers have suggested that caffeine may have different effects for upper-versus lower-body exercise (5, 10). Davis et al reported that the ergogenic effect of caffeine may not elicit effects for leg musculature until later into an exercise, when fatigue plays a more prominent role, compared to earlier sets of repetitions (10).
In contrast, tests using upper-body musculature have shown greater improvements in earlier sets, such as a study by Beck et al which found significant increases in bench press repetitions to failure, but not in bilateral leg extension (5).
The increases in both upper and lower endurance exercises observed in the present study are not consistent with previous literature, and may be explained due to the population recruited. Previous literature has questioned whether the ergogenic properties of caffeine are limited by the amount of muscle mass recruited and the total number of repetitions performed (10). This question is plausible given the absorptive properties of caffeine: when absorbed, caffeine is able to distribute freely into intracellular tissue water, allowing the transport to metabolic tissue such as the muscle and brain (1). Therefore, our exploratory hypothesis sought to determine whether subjects with a lower body fat percentage (and thus a higher percentage of metabolically active tissue) would benefit from the ergogenic effects of caffeine than those with a higher body fat percentage. To the researchers' knowledge, this is the first study examining the dosage each subject received based on lean body mass. The range of body fat percentages recorded was 5.8-27.1%, and the caffeine dosage averaged 8-9 mg per kg lean tissue. Despite this, there were no significant correlations between calculated body fat percentage and performance in any of the four tests when subjects received the caffeine treatment. This suggests that the ergogenic effect of caffeine was not greater in individuals with a lower body fat percentage.
Additionally, there was no correlation in performance in the bench press or Smith machine squat tests with caffeine habituation. An early review article by Graham concluded that any differences caused by caffeine habituation do not appear to be significant (16). Moreover, a study by Astorino et al. evaluated the efficacy of 6 mg/kg BW caffeine versus placebo on bench press, leg press, lat row, and shoulder press; results found that 66% of subjects who demonstrated increases in the caffeine condition were relatively heavy caffeine users, while subjects who had reduced performance in the caffeine condition consumed less than 150 mg/day (2). Of note, the study had a sample size of 14 men with resistance training experiences >2 days/week for 7.5+1.2 years, who all identified as daily caffeine consumers; as a result, it cannot be concluded that caffeine habituation has a significant impact on resistance training variables (2).

Bloms et al. reported significantly higher vertical jump performance in
Division I collegiate athletes who consumed 5 mg/kg BW of caffeine (7). They concluded the ergogenic of caffeine in vertical jump performance is likely to only be observed in subjects who are frequently exposed to repeated ballistic tasks, such as basketball and volleyball players (7). The current study did not recruit individuals based on experience in sports or other activities that may predispose individuals to activities requiring frequent jumping ( Figure 2). Furthermore, subjects were not familiarized with the vertical jump protocol prior to the test visits. To our knowledge, there has not been a caffeine trial in vertical jump exercises in recreationally active athletes naïve to ballistic activity. Therefore, is it plausible that the proposed conclusion by Bloms et al. may explain the results witnessed in the present study, which found no significant difference in calculated vertical jump height (7).
Previous literature has hypothesized that caffeine does not alter maximal forcegenerating capacity of a muscle, but may extend time to fatigue by altering pain perception (10). This would explain the results observed from the isometric force test, which had no significance between the caffeine treatment and placebo ( Figure 3). To the researchers' knowledge, this may be the first study to use the isometric force test in order to measure maximal isometric force generation in recreational athletes.
Results from the present study support the hypothesis that caffeine does not significantly alter maximal force-generating capacity.
While there are several mechanisms that may play a role in the observed ergogenic effects of caffeine, the most prominent mechanism of action involves caffeine's ability to inhibit adenosine receptors (20). Adenosine, a molecule similar in structure to caffeine has been shown to enhance pain perception, induce sleep, and reduce arousal, among other functions (6,23,30). Caffeine, which has a nonselective affinity to adenosine receptors, can bind to adenosine receptors in the brain and peripheral tissues (15). The resulting inability of adenosine to bind to receptor sites prevents the adenosine-induced suppression of dopamine release (10). This contributes to the reported increase in arousal and alertness frequently associated with caffeine intake (26). As a result, it is believed that the primary mechanism of action is inhibitory effects on adenosine modifying pain perception while sustaining motor unit firing rates, resulting in an overall ergogenic effect (10). The resulting inhibition of adenosine in the presence of caffeine may justify why a significant improvement was found in the two tests that utilized muscular endurance, but not in the tests that evaluated a short-duration (<10 second) bout of force, such as the isometric force test, or the vertical jump test, which arguably did not drastically increase subjects' perceived exertion (7,10).
Rating of perceived exertion was not significantly different in the bench press or Smith machine squat test between treatments. These results are consistent with a range of studies that have also found no difference in RPE with resistance exercise (4,11,17,33). A proposed reason for the lack of a dampening effect on RPE following caffeine ingestion is the short duration in exercise to failure in a given exercise (such as bench press or squats) is insufficient to elicit a perceived difference in exertion between treatments (11). Evaluating caffeine supplementation at later time points following resistance training exercise has shown decreased perception of exertion in caffeine treatments. For example, Hurley et al (21) found decreased perception of exertion following bench press exercises at 72 hours post-exercise.
Despite the non-significant findings in this current study, subjects had improved performance in the bench press and squat tests without having a significant change in RPE between treatments. This is in opposition of the results of a study by Duncan et al, where subjects had no significant difference in number of bench press repetitions between 5 mg/kg caffeine and placebo, but did have a lower RPE in the caffeine condition (13). As a result, it can be argued that RPE was not reduced as a result of the caffeine treatment in the present study, but perception of exertion was maintained in the caffeine treatment while subjects completed an additional 1.4 repetitions, on average.

Figure 1: Number of Bench Press and Squat Repetitions by Treatment
BP=Bench press, S=Squat. (n=23) Data analyzed using repeated measures ANOVA. There was a significant increase in repetitions to failure between caffeine and placebo treatments in both the bench press and squat tests. *p<0.05, **p<0.01.

Overview
This literature review will discuss different types of resistance training variables -specifically, muscular strength and muscular endurance -and provide a synopsis of the previous literature on cardio-respiratory endurance and resistance training trials utilizing caffeine as an ergogenic aid. First, we will define strength and endurance and provide methods of measuring muscular strength, muscular endurance, and vertical jump height. Then, we will discuss the availability of caffeine in the diet along with its absorption, metabolism, and several potential mechanisms that may explain its ergogenic effects in athletic exercises. Finally, we will provide an overview of the previous literature describing caffeine as an ergogenic aid in both cardiorespiratory endurance and resistance training.

Muscular Strength
Muscular strength is defined as the maximum force or torque produced by a muscle group in an isometric action at a specific joint angle (42). The 1-repetition maximum (1RM) is currently the gold standard for determining isotonic strength (15).

Muscular Endurance
In most laboratory studies, endurance performance is measured as the time taken to reach exhaustion at a given power output (70). Resistance training programs that emphasize muscular endurance typically involve many repetitions -typically 12 or more -per set (4). Despite this high number of repetitions, loads lifted are lighter than in exercises evaluating muscular strength, and fewer repetitions (usually 2-3) are performed (4). This is in contrast to strength training exercises, where loads used are typically higher and the number of repetitions are lower (6 or less) (4). A common method of measuring muscular endurance performance is by using repetitions to failure (17). Repetitions to failure involve performing sub-maximal force production in several repetitions until fatigue, and is usually performed with a percentage of 1RM (17).

Vertical Jump Height
The vertical jump (VJ) test is the primary test used to asses muscular power in the legs (15). There are two forms of the VJ test utilized: the squat jump (SJ) and the counter-movement jump (CMJ) (15). Both the SJ and CMJ can be performed with or without the use of arm motions (15). When arm motions are not allowed, subjects are required to place hands on their hips (15). While the CMJ generally results in higher jump heights than the SJ, Sayers, et al has argued that SJ is a preferred testing method due to the variability in CMJ technique as well as the accuracy in calculating peak power (68).

Intake & Metabolism
Caffeine ( Caffeine binds to plasma proteins and is able to distribute freely into intracellular tissue water, accounting for 10-30 percent of the total plasma pool; caffeine is also lipophilic and is able to cross the blood-brain barrier (1,71).
Metabolism of caffeine occurs in the liver through processes of demethylation and oxidation (33). The primary route of caffeine metabolism is 3-ethyl demethylation to paraxanthine; this step makes up approximately 75-80 percent of caffeine metabolism and involves cytochrome P4501A2 (1). Caffeine is also metabolized to theophylline and theobromine, however metabolism to paraxanthine is the primary metabolic pathway (1). Caffeine is also reabsorbed by the renal tubules, however only a small amount of caffeine is excreted in urine unchanged (1). Repeated ingestion of caffeine does not alter absorption or metabolism of caffeine (28). Research does suggest menstrual cycles or use of oral contraceptives may alter caffeine clearance (43).

Physiology
Caffeine is both water and fat soluble, which allows distribution to all tissues of the body (1, 2, 54, 71, 73). As a result, a specific mechanism of action in regards to exercise performance has yet to be chosen (73). There are several principle mechanisms that have been proposed to explain the ergogenic potential of caffeine during exercise: 1) increased myofilament affinity for calcium and/or the increased release of calcium from the sarcoplasmic reticulum (SR) in skeletal muscle; 2) cellular action caused by the accumulation of cyclic-3'-5'-adenosine monophosphate (cAMP) in tissues such as skeletal muscle and adipocytes; 3) cellular actions mediated by the competitive inhibition of adenosine receptors in somatic cells and the central nervous system (19). Additionally, early research by Powers et al. suggest that the ergogenic effects of caffeine in aerobic exercise is related to an increase in fatty acid oxidation, leading to the sparing of muscle glycogen (62). Increased oxidation of fatty acids inhibits glycogen phosphorylase activity, switching the preference from glycogen to fat (60,67). This resulting increase in free fatty acids is hypothesized to decrease cellular lactic acid production, a pathway that has been linked to fatigue during heavy exercise (62). Recent research, however, has found little evidence to support the hypothesis that caffeine has ergogenic effects due to enhanced fat oxidation (31 Another proposed role of the ergogenic effect of caffeine involves calcium and phosphodiesterase inhibition (17). In vitro studies have shown that caffeine inhibits phosphodiesterase enzymes, allowing an increase in cAMP (17,25). An increase in cAMP, along with an increase in blood catecholamines (such as epinephrine), results in the activation of hormone sensitive lipase (34). The resulting free fatty acids are mobilized from the cell membrane of the adipocyte and are transported to tissues and are oxidized for energy (34). However, this mechanism is unlikely to explain the ergogenic effect of caffeine observed during athletic activity; while in vitro studies have demonstrated inhibitory effects on phosphodiesterase, in vivo studies would require toxic doses of caffeine to observe a physiological benefit (17).
Arguably the most favored mechanism of action involves caffeine's ability to inhibit adenosine receptors (36). Adenosine, a molecule similar in structure to caffeine has been shown to enhance pain perception, induce sleep, and reduce arousal, among other functions (12,41,72). Caffeine, which has a nonselective affinity to adenosine receptors, can bind to adenosine receptors in the brain and peripheral tissues (26). The resulting inability of adenosine to bind to receptor sites prevents the adenosineinduced suppression of dopamine release (17). This contributes to the reported increase in arousal and alertness frequently associated with caffeine intake (55). As a result, it is believed that the main mechanism of action is inhibitory effects on adenosine modifying pain perception while sustaining motor unit firing rates, resulting in an ergogenic effect (17).
Caffeine ingestion before exercise may cause the undesired effect of an increase in the inflammatory response, demonstrated by increases in markers of muscle damage and leukocyte cells (6,75). As a result, an additional mechanism that may aid in the ergogenic effect of caffeine involves creatine kinase (CK), a physiological marker that indicates muscle damage and is associated with higher levels of pain perception after acute episodes of resistance exercise (48). Creatine kinase de-phophorylates creatine phosphate to enable rapid phosphorylation of ADP to ATP for quick, intense muscle contractions (24). Previous literature suggests resistance exercise results in an increase in CK concentrations (37,48). Additionally, other researchers have found that caffeine causes an increase in circulating catecholamines, such as epinephrine and norepinephrine, which are responsible for the increase in leukocytes frequently observed post-exercise (11). Bassini-Cameron et al.
hypothesized the fatigue delaying effect of caffeine may even enhance the extent of muscle damage occurring during intense exercise, as subjects can potentially perform a higher volume of work following acute caffeine ingestion (6). However, this does not explain the potential ergogenic effect during exercise, but instead addresses muscle injury, and related muscle soreness, post-exercise. A study employing caffeine equivalent to 4.5 mg/kg BW found that an acute ingestion prior to resistance exercise does not appear to cause greater muscle cell injury, as CK and leukocytes observed were not above levels that occurred in resistance exercise alone (48).

Additional Effects of Caffeine
Caffeine has been noted to have multiple effects in the body. Caffeine acutely raises blood pressure as a result of sympathetic system stimulation and the antagonistic effect on adenosine (26,69). These effects on the cardiovascular system generally return to baseline after 10-60 hours, depending on the amount of caffeine ingested (33). Both mood and cognitive ability improve following both acute and chronic caffeine consumption (26). Furthermore, caffeine has been shown to increase alertness and ability to concentrate, and has long been used to treat headaches due to its synergistic effects with analgesics; as a result, caffeine is an ingredient used both alone or in conjunction with other medications, such as acetaminophen (9,26,47,69).
Persons who abstain from caffeine overnight (8-12 hours) have a significant depletion of caffeine by early morning; as a result, subjects are more sensitive to the stimulant effects upon reintroduction into the body (66).
Caffeine's impact on athletic performance has been investigated in a range of athletic exercises, including endurance events, team sports, and high-intensity, shortduration activities (3,23,24,39,57). Due to the observed effects of caffeine, the World Anti-Doping Agency has caffeine placed on the 2015 monitoring program (79).
While there is no restriction set to the amount of caffeine to be consumed prior to an athletic event, caffeine concentration is monitored for potential repetitive misuse (10,40,79). Previously, the International Olympic Committee (IOC) prohibited urinary caffeine concentrations in excess of 12 mcg/mL (52). This currently unrestricted limit of caffeine can allow athletes to consume amounts of caffeine associated with ergogenic benefits prior to athletic events. In a meta-analysis of caffeine studies examining various types of physical activity performance, the amount of caffeine commonly shown to improve endurance is between 3 and 6 mg/kg of body mass, consumed no more than 60 minutes before activity (27).
Considering the multiple proposed mechanisms of caffeine, the remaining sections of the literature review will review the effects of caffeine in aerobic and anaerobic athletic performance.

Aerobic Performance
The effects of caffeine on aerobic performance have been investigated extensively in aerobic exercises, particularly in running, cycling, and rowing. Several meta-analysis report that caffeine has an ergogenic effect on aerobic performance (20,27 Paton, et al utilized a dose of 6 mg/kg caffeine or placebo in a randomized, double-blind, crossover experiment with 16 male team-sport athletes (59). Subjects performed 10 sets of 10-second sprints, with each sprint followed by 10 seconds of rest (59). The observed effect of caffeine was not significant in sprint performance and on fatigue; in fact, the caffeine treatment was found to have a slight decrease in agility (59).

Anaerobic Performance
Similarly to aerobic performance, the effects of caffeine supplementation in anaerobic exercise have been reviewed at length. However, testing methods chosen in anaerobic testing have been less consistent, partially because anaerobic performance can be more difficult to quantify (30). A review of the literature indicates uncertainty towards whether the perception of athletic improvement is related to maximum strength, power, or rate of fatigue (30).
Conflicting results have been found in the literature regarding caffeine and 1RM. Beck et al examined 1RM for bench press and leg extension exercises in 37 resistance-trained males (7). A significant improvement was found in bench press 1RM but not in the leg extension (7). However, Williams et al and Astorino et al both failed to find and effect for 1RM in the bench press and leg press in 9 resistancetrained men with a mean of 4.2 years experience and in 22 resistance-trained males, respectively (3,77). This inconsistency in results suggests that further research is required before a definitive conclusion can be made.
Duncan, et al (23) conducted a double-blind, randomized crossover study involving 9 males and 2 females with specific experience in performing resistance exercise and were actively participating in greater than ten hours per week of programmed strength and conditioning activities. Each subject was provided placebo or 5 mg/kg of caffeine and tested in randomized order for number of repetitions to failure, rating of perceived exertion (RPE) and perception of muscle pain during resistance exercise (23). All subjects were competent in techniques performed in the study, including bench press, deadlift, prone row, and back squat exercises (23).
Subjects were asked to refrain from vigorous exercise and to maintain normal dietary patterns for the 48 hours prior to testing, and were asked to cease caffeine use from 6:00 pm the night before testing (23). In the caffeinated condition, subjects had a lower RPE and muscle pain perception compared to the placebo condition. This study determined that caffeine ingestion did not enhance performance in number of repetitions, but did reduce perception of exertion and muscle pain (23).
A power trial performed by Doherty et al evaluated the effect of moderate-dose caffeine on performance during high-intensity cycling (21). Eleven trained male cyclists recruited from local cycling clubs were recruited for this double-blind, randomized, crossover study where they received caffeine equivalent to 5 mg/kg BW or placebo and participated in a ramp test designed to exhaust participants in 10-12 minutes (21). Mean power output was significantly greater in the caffeine treated group compared to placebo. Additionally, blood lactate was significantly higher in the caffeine treatment group compared to placebo (21). This was hypothesized to be one of the mechanisms that allowed the caffeine treatment group to perform at a higher intensity than the placebo group (21). press and leg press using 60% of their determined maximal lifting ability (1RM) (3).
Subjects refrained from caffeine intake for 48 hours and strenuous exercise for 24 hours before each visit. There was no significant effect of caffeine on muscular strength or endurance, determined as complete number of repetitions to failure, in subjects when consuming caffeine when compared to placebo when a dosage of 6 mg/kg BW was used (3).
In another crossover study, twenty elite male athletes performed knee extensor and flexor exercises (39). Subjects recruited were intercollegiate Division I varsity American football team members. Exclusion criteria included high daily caffeine consumption (defined as >100 mg/day) or lacking sufficient weight training experience (defined as less than two years). Subjects were required to abstain from exercise for 48 hours and from caffeine for one week prior to testing (39). A significant increase in muscular power was noted in subjects when they ingested capsules containing 7 mg/kg BW, compared to placebo (39).
Woolf, et al (78) performed a randomized crossover study examining the effect of 5 mg/kg BW of caffeine in 17 collegiate football athletes. All participants recruited were considered low caffeine users, with a reported average intake of 16+20 mg/day (78). Participants ingested either caffeine or placebo beverage with a small meal and completed three exercise tests: a 40-yard dash, 20-yard shuttle, and bench press until fatigue using either 185 or 225 pounds, with the lower weight used for participants who were unable to bench 225 pounds (78). No differences were found between treatments for any of the three exercise tests; however, 59% of the participants improved in performance with caffeine with the bench press and 40-yard dash (78).
Unlike other studies, which use 60% of participant's calculated 1RM for testing purposes, this study chose a standardized weight, regardless of each subject's individual ability (3,78).
In a study by Bloms et al, 25 male and female NCAA Division I collegiate athletes participating in 8-20 hours of training per week were recruited to asses squat jump (SJ) height following ingestion of caffeine equivalent to 6 mg/kg BW (13).
Caffeine ingestion had a positive significant effect (p=0.001) in SJ height, with an improvement of 5.4+6.5% (13). Of the 16 males enrolled, 9 were identified as responders during the SJ; 78% (7/9) of these subjects who responded to caffeine were identified as habitual consumers (13) (61). Results of the study concluded that caffeine increased muscular endurance in repeated submaximal isometric contractions in the quadriceps (61). In this study, all subjects were non-habitual caffeine users, defined as those who reportedly consumed less than 200 mg of caffeine/wk (61).
Furthermore, this study did not define the current resistance training status of its participants (61).
Duncan, et al evaluated bench press repetitions to failure in 13 moderately resistance trained men (22). Participants in his study consumed 5 mg/kg caffeine or placebo and performed bench press repetitions to failure using 60% 1RM (22).
Participants completed significantly more repetitions to failure and lifted significantly greater weight with the caffeine treatment compared to placebo (22). However, RPE was not significantly different between groups (22). Subjects recruited were all active participants in University team sports, including rugby, football, and basketball, and

Conclusions
As previously stated, muscle endurance is commonly measured using repetitions to failure with weights equivalent to a percentage of an individual's 1RM (17). Currently, information published in the literature on resistance training variables is insufficient in terms of concluding whether or not caffeine has an ergogenic effect on resistance training variables, such as muscle endurance, in recreationally trained athletes, as a majority of the literature recruits participants at the collegiate athletic or above level. Additionally, to our knowledge, there is limited research comparing caffeine's effects for resistance training between habitual and non-habitual caffeine users. Therefore, in our study, we ask recreationally trained athletes to perform a combination of resistance exercises incorporating large muscle groups in both upper and lower body -bench press repetition to failure, squat repetitions to failure, isometric force plate, and vertical jump -while ingesting a dose of caffeine equivalent to 7 mg/kg BW.
Currently, research of the potential effect of caffeine on muscular endurance has been performed on subjects demonstrating elite athletic ability (3,39). Less research has been performed on the impact of acute caffeine ingestion on strength and endurance in the average individual who participates in light to moderate consistent physical activity. Our primary hypothesis is that acute caffeine ingestion in the amount of 7 mg/kg BW will increase the number of bench press repetitions to failure compared to placebo ingestion in college age, recreational male athletes. Our secondary hypothesis is that acute ingestion of caffeine will also increase the number of squat repetitions to failure, increase the amount of force generated from a vertical jump and isometric squat exercise, and decrease rating of perceived exertion at the time of testing, when compared to placebo ingestion.
Furthermore, previous studies have not taken body composition into consideration (3,23,39). Our exploratory hypothesis is that subjects with lower body fat percentage will demonstrate a significant increase in repetitions to failure in bench press and squat exercises when ingesting caffeine when compared to subjects with a higher body fat percentage. To determine this, body fat percentage will be collected prior to testing. As an additional exploratory hypothesis, we believe rating of perceived exertion will be decreased in subjects when ingesting caffeine supplementation compared to placebo.

Description of the project:
You have been asked to take part in the study that tests the potential effect of a high caffeine dosage on muscular endurance and power.
What will be done: 1. Height, weight, and 1-repetition maximum (the maximum amount of weight that can be moved with one repetition) estimates will be taken. 2. The study will consist of two test days, one week apart, where you will perform repetitions with weights equal to approximately 60% of your respective 1-repetition maximum until failure in two exercises (Smith machine squat and bench press). 3. 24-hours prior to the test day, subjects are asked to abstain from consuming caffeine-containing products. 4. On the test day, a capsule(s) containing either a placebo or a pre-made caffeine supplement equal to 7 milligrams per kilogram of body weight will be provided to the subject for consumption (for example, if a subject weighs 75 kilograms, they will ingest capsules equivalent to 525 milligrams of caffeine). Twelve fluid ounces of water will be provided to aid in pill ingestion. 5. Subjects will remain stationary to allow absorption for one hour after consuming the pill(s). 6. A brief questionnaire will be provided to be completed throughout the testing process. 7. The following tests will be performed: • Bench press to failure using weight equivalent to 60% of the 1-repetition maximum weight (calculated from the bench press value obtained during the first visit) • Smith machine squat to failure using weight equivalent to 60% of the 1repetition maximum weight (calculated from the leg press value obtained during the first visit) • Force plate test • Vertical jump test 8. Subjects are to consistently keep a log for three days following the test procedure.
No dietary restrictions will be in place at this time; however, 24-hours prior to the second test day, subjects will be asked to abstain from caffeine-containing products. 9. One week later, subjects will return to perform the same procedure, consuming the alternative capsule(s). Throughout the study, both the subject and the researchers will be unaware as to whether you have consumed the caffeine capsule(s) or the placebo until after all testing has been completed.

Risks or discomfort:
Caffeine is a stimulant, and this test involves the consumption of a significant dosage of caffeine. While the amount consumed is well within the safe limit, there is a risk of: increased blood pressure, reduced control of fine motor movements, and risk of insomnia. Risk is greater in non-habitual consumers. Caffeine withdrawal can also produce headache, fatigue, and decreased alertness. In addition, caffeine has been used as a diuretic, which can be detrimental to athletes performing in long-term endurance events.
In addition to caffeine use, there is risk of injury in performing any form of strength training exercises. This study requires testing for 1-repetition maximum and performing repetitions to failure in different muscle groups.
The amount of caffeine used in this study is well within the safe limits of consumption for healthy, adult males. In addition, many previous studies testing the effect of caffeine on healthy adults during physical activity have incorporated caffeine with doses at and exceeding the dosage used in this study (7 milligrams of caffeine per kilogram of body weight). In order to maintain safety of all subjects, the following criteria warrants exclusion from the study: those with diagnosed high blood pressure, known or suspected allergies/negative reactions to caffeine, and/or known or suspected heart conditions.

Benefits of this study:
Although there will be no direct benefit to you for taking part in this study, the researcher may learn more about caffeine supplementation in regards to strength athletes. Currently, there is significant data to demonstrate the benefit of caffeine consumption prior to cardiorespiratory endurance activities (running, cycling). However, little data is currently available in regards to muscular strength/endurance.

Confidentiality:
Your participation in this study is strictly confidential. None of the results or collected data will identify you by name. All records will be stored in a locked cabinet and viewed solely within the Energy Balance Lab located in Fogarty Hall. Data entered in any computer programs will not contain information identifiable back to you. Please note, all data is subject to inspection by federal, state, and local agencies, such as the Food and Drug Administration (FDA).

In case there is any injury to the subject: (If applicable)
In the event of an injury during the testing process, the URI emergency medical services will be contacted at (401) Decision to quit at any time: Participation in this study is up to you. You are in no way required to participate. If you decide to take part in the study, you may quit at any time. Whatever you decide will in no way be recorded, penalize you, affect enrollment status and/or grades. If you wish to quit, you simply inform the lab (Fogarty 205, phone 401-874-2067) of your decision.