Introduction

As much as 90% of the population ingests caffeine on a regular basis, and caffeine is the most popular stimulant used by athletes to enhance sport performance (3,14). The endurance performance enhancing effects of caffeine have been well documented (25,32). Several potential mechanisms exist that may explain this increase in performance, including the release of cortisol and beta endorphins (12,15,32). Caffeine ingestion also increases lipid oxidation, thus resulting in muscle glycogen sparing (20,28,29,45). In addition to these physiological effects, these hormonal and endorphin mechanisms may account for decreased perception of fatigue during physical activity (15).

Less is known about the effect of caffeine ingestion on muscular strength and endurance. To date, the results from studies that examine caffeine's effect on muscle strength are equivocal. Some studies have found a significant gain in muscle strength with caffeine ingestion (26,28), whereas other studies report no significance (5). Because resistance training and other short-term high-intensity activities do not require such high concentrations of glycogen, further research on additional mechanisms in which caffeine affects the muscle function has emerged (5,20). There has been some speculation on potential mechanisms in which caffeine affects the resistance training performance (5,20). Caffeine may increase the ability of muscles to recruit motor units, increasing the force of a muscle contraction (28,29). In addition to the benefits on performance, caffeine is an adenosine antagonist. By blocking adenosine receptors, caffeine may enhance the response of sympathetic nervous system, including reduced feelings of pain and soreness (2,13,17,36). In turn, these reduced feelings of pain and soreness may play a role in enhanced performance through increased ability to do work. However, some investigators report no effect of caffeine ingestion on perceived effort (4). Differences in dosage, timing of caffeine ingestion, mode of exercise, subject population, and acute nature of individual studies may account for the varied results on performance and perceptual indices.

Resistance Training

In addition to pain and soreness scores, activity of muscle enzymes also acts as an indirect marker of muscle damage (9,11,33,41). Further, to date, only 1 study has examined the effects of caffeine on physiological markers of muscle damage such as creatine kinase (CK) that are associated with higher levels of pain perception after an acute bout of resistance exercise (35). Investigators in that study reported that caffeine does not attenuate CK levels immediately after a resistance training session. However, peak level of blood CK activity and corresponding peak of delayed onset muscle soreness (DOMS) and pain occurs between 24 and 48 hours after exercise (10,40). Currently, no study has investigated how caffeine affects either physiological markers like CK or perception of pain in the days after an acute bout of resistance exercise, when muscular soreness is at the highest levels. Without this information, the practicality of ingesting caffeine to decrease feelings of pain caused by DOMS is unknown.

Therefore, the primary purpose of this investigation was to examine both perceptual and physiological markers of muscle damage and soreness for several days after resistance exercise with and without ingestion of caffeine. Given the conflicting research, a secondary purpose was to examine the effect of caffeine ingestion on upper-body muscular strength performance. Our hypothesis is that caffeine ingestion will increase upper-body resistance training performance and, through the role of caffeine as an adenosine antagonist, will attenuate the perception of DOMS.

Methods

Experimental Approach to the Problem

To test this hypothesis, healthy, college-age, recreationally trained males were recruited to participate in this double-blind, within-subject, crossover design, placebo-controlled study during the spring semester at the University of Rhode Island (URI).

Subjects were required to complete 3 testing sessions. The first testing session consisted of informed consent, anthropometric measures, and a 1 repetition maximum (1RM) bicep curl test. After this, subjects participated in 2 experimental exercise trials in which they ingested either caffeine or placebo and then underwent a strenuous upper-body exercise protocol. Each subject also completed 2 follow-up periods of 5 days after each experimental trial in which they evaluated soreness and fatigue using scales. The experimental trials were completed 2 weeks apart at approximately the same time of the day.

Subjects

Twelve healthy resistance-trained males aged 18–25 years who were familiar with the biceps curl exercise volunteered for this study. Resistance trained was defined as taking part in a regular (at least twice a week) resistance exercise regime for the past 6 months (31). Previous training experience in the bicep curl exercise was thought to be important to ensure that learning effects did not influence the results. Subjects were asked to refrain from caffeine 7 days before the start and for the entire duration of the study. This eliminated any possible tolerance to caffeine and its effect on pain and soreness after exercise. Subjects maintained their normal dietary habits but were advised to refrain from exercise, alcohol, and nicotine consumption at least 48 hours before exercise testing sessions. For the duration of the study, subjects were to refrain from upper-body resistance training and the use of pain relievers and analgesics. Subjects kept a dietary log of all items ingested during the 48-hour time period before each exercise session.

Before participation, each subject gave written informed consent and completed a medical history, an exercise history, and a nutritional questionnaire. Procedures were approved by the Institutional Review Board at the URI. Subjects were of age: 20 ± 1 years, body mass: 76.68 ± 8.13 kg, and height: 179.18 ± 9.35 cm. Percent body fat as measured by skinfold calipers was 9.64 ± 4.86%.

Procedures

During the familiarization session, anthropometric measurements were taken, body mass index was calculated, percent body fat was measured, and 1RM for the bicep curl exercise was determined. The 1RM test was performed on a preacher bench (Yukon Fitness Equipment, Aurora, OH, USA). The 1RM was measured using a standard 1RM protocol from Kraemer and Fry (31). After testing, subjects were carefully instructed on how to use the muscle soreness scales.

After this preliminary testing, subjects underwent 2 sessions of testing. The testing sessions were separated by 7 days, and each took place at the same time of the day. Previous studies have suggested that a 7-day washout period is long enough to ensure that caffeine is no longer in one's system (45). Subjects were instructed to eat the same or similar meals 2 days before each testing session. This was done to help eliminate the chance of an increase in carbohydrate intake and potential effect on muscle glycogen stores and to check that they did not ingest caffeine-containing foods or beverages. All testing sessions took place in the same location, which were supervised by the same investigators. Care was taken to ensure that all subjects received the same verbal encouragement and instructions for all exercises to negate potential differences in state of arousal.

All experimental trials were performed in the morning, after a 12-hour fast, excluding water. Time of the day was standardized (±1 hour) to avoid confounding influences of diurnal hormonal variations, and subjects were asked how they got sleep the night before to ensure the same amount of rest was had before each trial. Additionally, subjects were instructed to drink ∼1 L of water the night before and the morning of the experimental trials to ensure adequate hydration. Hydration state was determined before exercise and caffeine or placebo ingestion via urine refractometry. After a resting blood draw, subjects ingested a capsule containing 5 mg·kg−1 body weight of caffeine or a placebo with approximately 500 ml of water. Each subject then sat quietly for 1 hour, a time frame that allows for a peak plasma caffeine concentration (39). After the rest period, subjects completed 5 sets of a bicep curl performed on a preacher bench with the nondominant arm using a weight equal to 75% of each subject's predetermined 1RM. Subjects were instructed to complete 10 repetitions during the first 4 sets and as many repetitions as possible during the last set. A metronome cadence of 30 was used to control the motion to ensure proper speed of the movement. The Borg CR10 scale was used after completion of each set to score perception of effort and soreness (8). Subjects were provided with soreness scales to take home and use to report levels of soreness during the 5-day follow-up by marking the proper line on the visual analog scale. Scales were returned to investigators at the completion of follow-up. Subjects reported back to the exercise science laboratory 48 hours after the experimental trials for a second blood draw.

Follow-up

Days 1–5

On days 1–5 (24–120 hours) after experimental trials, subjects evaluated level of muscle soreness using the specific instructions received. Subjects also ingested the appropriate capsule at the same time of the day on these follow-up days. Phone call reminders were sent to subjects during follow-up to remind subjects to evaluate muscle soreness and to ingest the appropriate capsule. Completed scales were collected at completion of each follow-up period. On day 2, subjects also reported to laboratory for a second blood draw.

Caffeine and Placebo Administration

Caffeine (USP grade; Wilkem Scientific, Pawtucket, RI, USA) and placebo were prepared in capsules by the Pharmacy Department at URI. Body weight of 5 mg·kg−1 was used to determine the total dose for each subject. This dosage is equal to approximately two and one-half cups of brewed coffee and is consistent with several previous studies examining the effect of caffeine on exercise (27,36,39). Placebo capsules were filled with flour, which resembles caffeine in color and texture. Caffeine or placebo was ingested 1 hour before experimental trials, 24 hours after the experimental trials, and every day for 4 days thereafter. Subjects ingested the caffeine or placebo at the same time of the day for the entire study. An individual who was not involved in data collection or data analysis was responsible for the assignment of caffeine and placebo to protect against the expectancy effect by the subjects and investigators. Subjects were provided with soreness scales to take home.

Study Measures

Anthropometric Measurements

Body weight was measured to the nearest 0.1 kg using a digital read scale (Tanita BF-556; Arlington Heights, IL, USA). Height was measured to the closest 0.5 cm using a stadiometer (Seca, Hamburg, Germany). Body mass index was calculated as weight (kg) divided by height (m) squared. Body fat percentage was determined from skinfold measurements of the chest, abdomen, and thigh (Lange Skinfold Caliper, Cambridge, MD, USA). The 3-site skinfold equation was used to determine body density (24). Body fat was converted from body density using a population-specific formula for white males aged 20–80 years (43).

Muscle Soreness

Instructions on the use of 3 scales were read to all subjects: overall soreness scale, overall fatigue scale, and a soreness on palpation scale. Subjects rated their level of muscle soreness by selecting a number of the continuous scales. Ratings for each scale were as follows: 0—no soreness/fatigue; 1—very light soreness/fatigue; 2—moderate soreness/fatigue; 3—light (weak) feelings of soreness/fatigue; 5—(heavy) strong feelings of soreness; 7—very heavy feelings of soreness; 10—maximal soreness. This type of scale is a valid and reliable measure that has been used in multiple studies regarding muscle soreness and pain (22,37).

Blood Profile

All blood draws were obtained by venipuncture on subject's dominant arm because of the fact that each subject performed the bicep curl on their nondominant arm. A resting blood draw was taken before both experimental trials and during the second day of the follow-up of each trial. Whole blood was collected and transferred into appropriate tubes to obtain serum and plasma. Blood was centrifuged at 1500g for 15 minutes at 4° C. Resulting serum and plasmas were aliqouted and stored at −80° C for analysis. Serum CK was measured using colorimetric procedures at 340 nm.

Statistical Analyses

All results are reported as mean ± SD. A 2 × 2 (trial × time) repeated-measures analysis of variance (ANOVA) was used to analyze the CK data, a 2 × 6 (trial × time) was used to analyze the soreness and fatigue resting data from the exercise trial day to day 5, and 2 × 5 (trial × sets) was used to analyze the Borg CR10 scale ratings collected after each set of bicep curls. A paired-sample t-test was used to analyze the repetition number for the last set of bicep curls in the 2 trials. Linear assumptions were tested for and met. In the case of a significant F score in the ANOVAs, a Bonferroni post hoc test was performed to determine where significant differences lay. Significance for all analysis was set a p ≤ 0.05. Based on previous studies, it was determined that an n of 10 was adequate to defend the 0.05 alpha level of significance with a Cohen probability level of 0.8 (G-Power software, version 3.1.2; Kiel University, Kiel, Germany).

Results

In general, this study demonstrated that caffeine ingestion has beneficial effects on ratings of muscle soreness, perceived exertion, and performance surrounding an acute bout of resistance exercise.

Soreness

Soreness values at day 1 and day 2 were significantly different than pre-exercise values with caffeine and placebo ingestion. Caffeine produced a significantly lower soreness value on day 2 and day 3 after exercise when compared with placebo. Values for overall soreness can be seen in Figure 1.

Figure 1: Overall soreness values (mean ±SD) pre-exercise through day 5 of follow-up. * Significantly (p ≤ 0.05) different from corresponding placebo value. ^ Significantly (p ≤ 0.05) different from corresponding prevalue.

Change in Soreness

Change in soreness values is shown in Figure 2. Increase in soreness from pre-exercise to day 2 of follow-up was significantly higher under both conditions; however, there were no between-group differences.

Figure 2: Changes in soreness (mean ±SD) from pre-exercise values. ^ Significantly (p ≤ 0.05) different from corresponding prevalue.

Soreness on Palpation

A significant difference was demonstrated in soreness on palpation values with caffeine ingestion on day 2 of follow-up compared with soreness with placebo ingestion. Soreness on palpation values on days 1, 2, 3, and 4 for caffeine and days 1, 2, and 3 for placebo were significantly different than the pre-exercise soreness on palpation value. These values are shown in Figure 3.

Figure 3: Soreness on palpation values (mean ±SD) pre-exercise through day 5 of follow-up. * Significantly (p ≤ 0.05) different from corresponding placebo value. ^ Significantly (p ≤ 0.05) different from corresponding prevalue.

Change in Soreness on Palpation

Figure 4 shows change in soreness on palpation values. A significant increase was observed in changes in soreness on palpation values from pre-exercise to day 1 and day 2 of follow-up with both conditions in addition to day 3 with caffeine ingestion. There was also a significant difference in soreness on palpation values on day 4 of follow-up between conditions.

Figure 4: Change in soreness on palpation (mean ±SD). ^ Significantly (p ≤ 0.05) different from corresponding prevalue. * Significantly (p ≤ 0.05) different from corresponding placebo value.

Performance (Repetition Number)

There was a significant increase (p ≤ 0.05) in total repetitions of bicep curls in the final set with caffeine ingestion compared with placebo. These values are represented in Figure 5.

Figure 5: Total number of repetitions (mean ±SD) in the fifth set of exercise. * Significantly (p ≤ 0.05) different from corresponding placebo value.

CR10 Rating of Perceived Exertion

The Borg CR10 Scale of Perceived Exertion (Figure 6) was significantly lower with caffeine ingestion compared with placebo in sets 3, 4, and 5.

Figure 6: Borg CR10 rating of perceived exertion (mean ±SD). # Significantly (p ≤ 0.05) different from corresponding preceding set. * Significantly (p ≤ 0.05) different from corresponding placebo value.

Perceived exertion was significantly higher after sets 2, 3, 4, and 5 compared with set 1 under both conditions. However, the 2 conditions exhibited different within-group increases in perception from set to set. There was a significant increase in CR10 ratings from set 1 to 2, 2 to 3, set 3 to 4, and set 4 to 5 with placebo ingestion. However, with caffeine ingestion, there was a significant increase in CR10 ratings from sets 1 to 2 and sets 4 to 5 only.

There was no significant differences found in the response of CK between the caffeine and placebo conditions, but expected differences in pre- to postexercise were observed (Figure 7).

Figure 7: Creatine kinase (µ·L−1) response (mean ± SD). * Significantly (p ≤ 0.05) different from corresponding prevalue.

Discussion

The primary findings of this study demonstrate that a caffeine dosage of 5 mg·kg−1 body weight elicits beneficial responses on perception of muscle soreness, perceived exertion, and performance after maximal resistance exercise.

Subjects experienced increased soreness levels on day 2 after exercise with both caffeine and placebo ingestion. Given that DOMS peaks 24–48 hours after exercise (10,18,22,42), this suggests that the protocol used to induce soreness was efficient. Soreness level on day 3 after caffeine ingestion returned to the pre-exercise value, whereas it remained elevated after placebo ingestion, suggesting that caffeine augmented the recovery process.

Although both conditions resulted in increased soreness levels at day 2 of follow-up, caffeine produced significantly less soreness when compared with placebo. Subjects' change in soreness on day 3 was also less with caffeine ingestion compared with placebo, but not significant. Adenosine concentrations tend to increase in the working muscle and blood after high-intensity exercise (13,38,45). Caffeine is an adenosine antagonist and affects the activity of central nervous system (CNS) by blocking adenosine receptors, thus resulting in decreased levels of soreness. Reduced feelings of pain and fatigue resulting from the adenosine antagonist action of caffeine have been demonstrated in multiple studies (36,38,39). However, this is the first study that reports reduced perception of soreness beyond an acute exercise bout.

To our knowledge, this is the first study that has measured perception of soreness and fatigue of the upper body in relationship to caffeine ingestion. Maridakis et al. (36) investigated delayed onset muscle pain intensity in response to repeated electrically induced eccentric muscle actions of the quadriceps muscle. Subjects ingested 5 mg·kg−1 body weight of caffeine either 24 or 48 hours after exercise, and placebo on the opposite day. Given that DOMS peaks 24–48 hours after exercise, it is problematic to administer both treatments during this time period. A moderate dose (300 mg) of caffeine, such as that used in the previous study, has a half-life of approximately 4–6 hours (34). This suggests that small amounts of caffeine may remain in the body for the past 24 hours. The present study better controlled the complete “washout” of caffeine, allowing a full week between trials.

The present study also sought to examine whether caffeine ingestion during a 5-day follow-up period attenuated soreness. There was an advantage to ingesting caffeine before the peak of DOMS, but there was no significant difference in soreness levels between treatments past the third day of follow-up. This suggests that short-term caffeine ingestion after a strenuous workout may decrease overall soreness levels. Subjects reported continued feelings of soreness on palpation while ingesting caffeine on the fourth day of follow-up; however, this may be related to the fact that they did more work during the exercise bout than when ingesting the placebo. Furthermore, the overall perception of soreness may be more important in application, as this may directly affect a person's motivation to perform subsequent workout bouts as opposed to spot palpation of a muscle group.

In the present study, subjects were also able to perform more repetitions in the final set of bicep curls after ingesting caffeine when compared with placebo. This is consistent with the findings of Beck et al. (6) that exhibited an increase in bench press 1RM for a caffeine-containing supplement. However, the caffeine dosage in the previous study was approximately 201.0 mg or 2.4 mg·kg−1 of body weight, half the dosage used in the present study. This suggests that caffeine dosages as small as 2.4 mg·kg−1 body weight may elicit benefits in muscular performance. A similar investigation reported an increase in total repetitions of an arm curl exercise (p = 0.051) with caffeine ingestion (23). Increase in muscular performance in all studies could be attributed to the ability of caffeine to act on the neuromuscular system. The failure of the E-C coupling process and the accompanying decline of Ca2+ release from the sarcoplasmic reticulum is reported to cause the final phase of fatigue in the muscle (1,42). The ability to overcome the fatigue and more specifically the decline of muscle ability may be reversible with caffeine (1). Caffeine increases the susceptibility of Ca2+ release channels in the sarcoplasmic reticulum, thus increasing muscular performance.

Decreases in rating of perceived exertion (RPE), effort, and pain perception are consistent with other investigations of exercise and caffeine ingestion across all modes, intensities, and duration (15,16,19). It can be speculated that the enhanced performance reported in this study may also be because of the influence of caffeine on perceptual fatigue and pain (16,38), leading the subjects to feel that they can do more work.

In addition to the increase in performance, subjects' CR10 RPE was significantly lower with caffeine in the final 3 sets of exercise. This response is attributed to the role of caffeine as a CNS stimulant and inhibiting adenosine receptor activity. Caffeine stimulates the CNS by secreting serotonin into the cerebral cortex, which results in mood improvements, increased mental awareness, and decreased fatigue and tiredness (13) This is all a result of inhibited adenosine activity thus reducing perception of pain, which could increase ability to perform more repetitions. An increase in acute volume will lead to chronic increases in strength.

During the placebo trials, perceived exertion increased between sets 2 and 3, sets 3 and 4, and sets 4 and 5, whereas caffeine only produced a significantly higher CR10 rating from set 4 to 5. Because subjects also completed more repetitions in set 5 with caffeine, it seems plausible that with more work, one's perceived exertion would increase. These findings suggest that caffeine attenuated the increase of perceived exertion from set to set when compared with placebo. Previous investigations have reported attenuated RPEs in response to an acute bout of aerobic exercise and reduced perception of exertion during recovery (14,15,30). Thus caffeine ingestion may increase performance via physiological and perceptual mechanisms. Practically speaking, individuals who are engaging in upper-body resistance exercise may perceive that they are doing less work and are willing to increase the intensity. The coupled affect of this perception and physiological effects of caffeine such as increased motor unit recruitment and force output may in turn lead to chronic increases in strength and power.

This study demonstrated no significant differences in CK levels with caffeine ingestion compared with placebo. Although this could be a possible indication that caffeine mediates changes in soreness and performance strictly through perceptual measures, it is also likely that the lack of substantial findings is because of the physiologically small increase in CK and extremely high SDs. Although all subjects were recreationally trained individuals, there may have been enough training variance between the subjects that led to the large SD. An increase in CK activity after exercise is expected given that eccentric exercise results in damage and thus higher CK levels. This peak level of CK activity typically corresponds to the peak of DOMS (Clarkson and Hubal (10)). However, larger CK increases are expected after maximal eccentric exercise or exercises (11,33). These investigations used a similar number of repetitions to induce soreness; however, the exercise consisted of strictly the eccentric component of a bicep curl. This technique results in a greater eccentric resistance and more concomitant muscle damage than using a bicep curl with a full range of motion including a concentric component. Had the present study used the maximal eccentric action only, CK activity may have been more pronounced, resulting in a greater effect size and possible differences between the 2 conditions. In addition, given that the biceps is a relatively small muscle group comparatively to others, the single-joint exercise bout used in the present investigation may not have caused enough a great amount of damage as measured by CK concentrations. However, it is important to note that all subjects reported some level of muscular soreness within the 48 hours after the exercise, and average soreness levels were significantly greater after the workouts along with increased CK levels after the workout. Thus, it is likely that some muscle damage did occur.

This study demonstrated a high level of control to minimize limitations. Subject exercised 1 hour after caffeine ingestion in the present study to allow peak plasma concentration, as reported by the previous research (44). The timing of caffeine ingestion in relation to enhanced performance was reported by Bell and McLellan (7). An increased time to exhaustion was observed 1 hour after exercise and 3 hours after caffeine ingestion. The exercise trial 6 hours after caffeine ingestion (5 mg·kg−1) did not demonstrate an increased time to exhaustion or any additional increases in performance when compared with a placebo. This suggests that the optimal time period between caffeine ingestion and exercise should be less than 6 hours for improvements in performance (7), which was done in the present study.

Considerable increases in performance have been reported in studies where caffeine is administered in the form of capsules compared with coffee (21). It has also been reported in studies using coffee to examine performance that it may mask the possible effectiveness of the caffeine (21). Although further research is needed to compare how effective different forms of caffeine deliverance are, the present study used capsules because they were easy to ingest and the dosage could be specifically controlled.

Although there is a possibility that the repeated bout effect may have played a small role in the outcomes of this study, the randomization of trials counterbalanced this possibility. In addition, the subjects recruited for this study did not habitually use caffeine. This population was chosen to reduce possible bias of increased exercise performance that normal habitual users may have. However, Astorinio (5) and Tarnopolsky (45) suggest that caffeine habits do not affect the degree to which caffeine enhances performance, so we hypothesize that habitual users may experience the same performance benefits. Further, the present study used a washout period of 7 days in which subjects were to refrain from all caffeine-containing food and beverages. Given the half-life of caffeine previously discussed, this ensured that caffeine consumption throughout the study was controlled, and that the outcomes of this study are because of acute caffeine ingestion.

The findings of this study greatly add to the existing literature concerning the effects of caffeine. Literature is sparse concerning the effects of caffeine on upper-body resistance training performance; the results of the present study suggest that caffeine ingestion can acutely increase volume. In addition, this study suggests that caffeine's positive effects on pain and soreness perception can be carried over for 48 hours after exercise with continued daily use. The results of this study are also generalizable to other populations. Although somewhat conflicting, the majority of previous research suggests that both sexes, untrained, and athletic populations experience performance benefits from caffeine use (3).

Practical Applications

The primary findings of this study demonstrate that a dosage of 5 mg·kg−1 body weight of caffeine has a beneficial effect on perception of muscle soreness, perceived exertion, and performance in the days after maximal resistance exercise. This has practical implications in that daily caffeine consumption can enhance resistance training performance and yet decrease soreness after resistance training despite doing more work. For an athletic population, this may translate to the ability to perform subsequent exercise sessions with less perceived soreness and possibly increase total work. In addition, the findings of this study suggest that caffeine does act as an ergogenic aid during upper-body resistance training exercise.