Caffeine has become a popular ergogenic aid among recreational and competitive athletes. The proposed benefits of caffeine include increased secretion of catecholamines (epinephrine and norepinephrine) (15), greater use of fats as an energy source and sparing of muscle glycogen (7), and increased motor unit recruitment and firing rates (17). Most studies have tested caffeine's effects on measures of endurance (i.e., time to exhaustion [TTE] at fixed power outputs or speeds during cycling or running tasks) (7,13,14,21,26,27) or anaerobic performance (peak power and mean power output during Wingate Anaerobic Tests, swimming velocity during 100-m swimming sprints, or time required to complete a 2000-m rowing trial) (4-6). Generally speaking, these investigations have reported that caffeine improved performance during endurance-based activities (7,13,14), but the results during anaerobic activities have been less consistent (4,6). For example, Beck et al. (4) recently found that a 201 mg dose of caffeine taken 45 minutes before exercise had no effect on peak power or mean power output during 2 consecutive Wingate Anaerobic Tests (separated by 7 min of rest) in college-age, resistance-trained men. Collomp et al. (6), however, reported that a slightly higher dose of caffeine (250 mg) ingested 1 hour before exercise resulted in a significant increase in average velocity for trained swimmers during 2 100-m sprints separated by 20 minutes of rest. Although the discrepancies between the results from these studies (4,6) may have been caused by the use of slightly different doses of caffeine, it is also possible that they reflected differences in the types of activities that were performed (cycling vs. swimming). Another popular ergogenic aid is capsaicin (active component of capsicum extract), which has been shown to increase fatty acid use (18,20,31). Furthermore, a combination of caffeine and capsaicin has been shown to increase energy expenditure and reduce energy intake (30). In addition, bioperine (black pepper extract) has been reported to increase the bioavailability of other nutrients such as coenzyme Q10 and curcumin (2,24). Moreover, tetrahydrobiopterin (the active component in bioperine) is a cofactor to endothelial nitric oxide synthase (eNOS), a producer of nitric oxide (a vasodilator) and a key moderator of vascular homeostasis (23,28). Niacin is a vitamin that is essential in energy metabolism, and its bioavailability may be affected by bioperine. Therefore, a combination of caffeine, capsaicin, bioperine, and niacin may act synergistically to increase exercise performance.
The mechanisms by which caffeine enhances performance during aerobic activities have been examined in many studies, and most investigations have suggested that caffeine's ergogenic effects on endurance are caused, in part, by increased use of fats as an energy source and sparing of muscle glycogen (7,12,25). Very few investigations have examined the mechanism(s) by which caffeine could enhance performance during maximal strength tasks. Kalmar and Cafarelli (17) suggested that caffeine's action as an adenosine receptor antagonist could help enhance motor unit recruitment or firing rates, both of which would contribute to increased force production. This hypothesis, however, has only been tested for the leg extensors during a unilateral isometric muscle action (17). Thus, there is very little information regarding caffeine's effects on strength, particularly for activities that are commonly performed by both recreational and competitive athletes (i.e., bench presses, power cleans, squats, leg extensions, etc.). Therefore, the purpose of the present study was to examine the acute effects of a caffeine-containing supplement (SUPP) on 1 repetition maximum (1RM) bench press and leg extension strength, as well as TTE, during cycle ergometry at a power output that corresponded to 80% of o2peak. On the basis of the results of previous studies (4,7,13,14,17), we hypothesized that the SUPP would result in an increase in 1RM bench press and leg extension strength, as well as TTE, during cycle ergometry at a power output that corresponded to 80% of o2peak.
Experimental Approach to the Problem
This study used a randomized, double-blinded, placebo-controlled, within-subjects crossover design. During the first laboratory visit, each subject performed an incremental test to exhaustion on an electronically braked cycle ergometer to determine o2peak. After a 1-week rest period, the subjects were randomly assigned to ingest either the SUPP or placebo (PLAC). The SUPP contained 400 mg of caffeine, 66.7 mg of capsicum extract, 10 mg of bioperine, and 40 mg of niacin. The PLAC (microcrystalline cellulose) was designed by the manufacturer (General Nutrition Corporation, Pittsburgh, PA, USA) such that each dose (2 tablets = 1 dose) had the same volume, taste, and color as the SUPP. After randomization, the subjects ingested one dose of either the SUPP or PLAC and sat quietly in the laboratory for 60 minutes. The subjects were then tested for 1RM bench press and leg extension strength. Approximately 15 minutes after the 1RM bench press and leg extension strength tests, the subjects were tested for TTE on a cycle ergometer at a power output that corresponded to 80% of their o2peak. After the TTE test, the subjects were allowed to rest for 1 week, during which they did not ingest either the SUPP or PLAC. After the 1-week rest period, the subjects ingested the opposite substance (SUPP or PLAC) and were retested for 1RM bench press and leg extension strength, as well as TTE.
Twenty-one men (mean ± SD age = 23.0 ± 2.6 yr; bodyweight = 81.0 ± 12.1 kg; height = 180.2 ± 4.8 cm) volunteered to participate in the investigation. The subjects were untrained in both resistance and aerobic exercise and engaged in no more than 4 hours of recreational activity per week. In addition, the subjects did not report or exhibit (a) a history of medical or surgical events that may significantly affect the study outcome, including cardiovascular disease, metabolic, renal, hepatic, or musculoskeletal disorders; (b) use of any medication that may significantly affect the study outcome; (c) use of nutritional supplements (such as creatine, protein drinks, amino acids, and vitamins) in the 6 weeks before the start of the study; and (d) participation in another clinical trial or ingestion of another investigational product within 30 days before screening/enrollment. The study was approved by the University Institutional Review Board for Human Subjects, and all subjects completed a health history questionnaire and signed a written informed consent document before testing. Furthermore, all subjects were encouraged to maintain their normal dietary habits and exercise routines during the study.
Determination of o2peak
Each subject performed an incremental test to exhaustion on a Calibrated Quinton (Corval 400) electronically braked cycle ergometer (Quinton Instruments, Inc., Seattle, WA, USA) at a pedal cadence of 70 rev·min−1. Seat height was adjusted so that the subject's legs were at near full extension during each pedal revolution. In addition, toe clips were used to ensure that each subject maintained pedal contact throughout the ride. All subjects wore a nose clip and breathed through a 2-way valve (2700; Hans Rudolph, Kansas City, MO, USA). Expired gas samples were collected and analyzed using a calibrated TrueMax 2400 metabolic cart (Parvo Medics, Sandy, UT, USA) with O2,, CO2, and ventilatory parameters expressed as 30-second averages. The metabolic cart was calibrated before each test. Each subject was fitted with a Polar Heart Watch system (Polar Electro, Inc., Lake Success, NY, USA) to monitor heart rate throughout the test. The test began at 50 W, and the power output was increased by 30 W every 2 minutes until voluntary exhaustion or the subject could no longer maintain a pedal cadence of 70 rev·min−1 despite strong verbal encouragement. o2peak was the highest o2 value in the last 30 seconds of the exercise test that met at least 2 of the following 3 criteria (1,8): (a) 90% of age-predicted heart rate; (b) respiratory exchange ratio greater than 1.1; and (c) a plateauing of oxygen uptake (less than 150 ml·min−1 in o2 over the last 30 s of the test). The test-retest reliability data for o2peak testing from our laboratory indicated the intraclass correlation coefficient (ICC) was R = 0.95, with no significant mean difference between test and retest values.
1RM Bench Press Strength Test
The 1RM bench press strength test was performed on a standard free-weight bench (Body Power, Williamsburg, VA, USA) with an Olympic bar. After receiving a lift-off from a spotter, the subject lowered the bar to his chest, paused briefly, and then pressed the bar to full extension of the forearms. The 1RM was determined by applying progressively heavier loads until the subject could not complete a repetition through the full range of motion (full extension of the forearms). Additional trials were performed with addition of lighter loads until the 1RM was determined within 2.27 kg, and this was usually achieved within 5 trials. Two minutes of rest were allowed between all trials (16). The ICC for 1RM bench press strength for our laboratory is R = 0.99, with no significant mean difference between test and retest values.
1RM Leg Extension Strength Test
The 1RM leg extension strength test was performed on a Body-Solid plate-loaded leg extension machine (Model CEC340; Forest Park, IL, USA). Each subject sat with his torso against the backrest and was instructed to hold tightly to the handles at the sides of the device. The backrest was adjusted to align the anatomic axes of the knees with the mechanical axis of the machine. Shin pads, attached to the machine's lever arm, were placed against the subject's legs. The shin pads were a fixed distance from the axis of rotation of the lever arm and thus were not adjustable. Positioning, however, was consistent for each subject across all tests. The 1RM was determined by applying progressively heavier loads until the subject could not complete a repetition through the full range of motion (full extension of the legs). Additional trials were performed with lighter loads until the 1RM was determined within 2.27 kg, and this was usually achieved within 5 trials. Two minutes of rest were allowed between all trials. Test-retest reliability data for 1RM leg extension strength testing indicated the ICC was R = 0.98, with no significant mean difference between test and retest values.
Time to Exhaustion Test
One week after the o2peak test, each subject performed a constant power output ride on the cycle ergometer to determine TTE. Each subject rode at 70 rev·min−1 at a power output that corresponded to 80% of the power output at o2peak as determined during the o2peak test. The seat height, toe clips, and warm-up procedures were the same as for the incremental test. Each subject was instructed to ride until voluntary exhaustion, and strong verbal encouragement was provided.
One repetition maximum bench press and leg extension strength, as well as TTE values, were compared between the SUPP vs. PLAC using 3 separate paired-samples t-tests. An alpha of p ≤ 0.05 was considered statistically significant for all comparisons. An a priori power analysis indicated that, for a repeated measures design, a sample size of 21 subjects resulted in statistical power values of 0.75 or greater for all of the dependent variables.
Figure 1 shows the mean (±SD) 1RM bench press strength values for the SUPP (82.9 ± 12.4 kg) and PLAC (82.9 ± 12.3 kg). The mean (±SD) 1RM leg extension strength values for the SUPP and PLAC were 121.7 ± 22.4 kg and 119.2 ± 17.6 kg, respectively (Figure 1). The mean (±SD) TTE values for the SUPP and PLAC were 692.3 ± 214.8 s and 668.5 ± 195.7 seconds, respectively (Figure 1). There were no significant mean differences for the SUPP vs. PLAC for 1RM bench press strength, 1RM leg extension strength, or TTE values. Figure 2 shows the individual 1RM bench press, 1RM leg extension, and TTE values for the PLAC and SUPP. For the bench press, 6 subjects had greater 1RM values for the SUPP than the PLAC, 3 subjects had greater values for the PLAC than the SUPP, and for 12 subjects the 1RM values were the same for the SUPP and PLAC. For leg extension, 12 subjects had greater 1RM values for the SUPP than the PLAC, 5 subjects had greater values for the PLAC than the SUPP, and for 7 subjects the 1RM values were the same for the SUPP and PLAC. For TTE, 11 subjects had greater TTE values for the SUPP than the PLAC, whereas 10 subjects had greater TTE values for the PLAC than the SUPP.
The results of this investigation showed that the SUPP containing 400 mg of caffeine (approximately 4.9 mg·kg−1 of bodyweight) had no effect on mean 1RM bench press or leg extension strength. There were, however, individual differences in responses for 1RM bench press and leg extension strength. For 1RM bench press strength, 28.6% of the subjects had greater 1RM values (2.3 kg) for the SUPP, 14.3% of the subjects had greater 1RM values (range = 2.3-6.8 kg) for the PLAC, and 57.1% of the subjects' 1RM values were the same for the SUPP and PLAC. The SUPP resulted in greater 1RM leg extension values (range = 2.3-18.1 kg) for 57.1% of the subjects, the PLAC resulted in greater 1RM leg extension values (range = 9.1-18.1 kg) for 23.8% of the subjects, and 33.3% of the subjects' 1RM leg extension values were the same for the SUPP and PLAC. Recent studies (3,4,6,12,13,17) have suggested that the conflicting evidence regarding the effect of caffeine on muscular performance may be caused by the training status of the subjects, doses used, type of muscle action performed (isometric vs. dynamic), or the muscle group tested. For example, Kalmar and Cafarelli (17) reported that a 6 mg·kg−1 of bodyweight dose (mean ± SD = 428 ± 54 mg) of caffeine significantly increased maximal voluntary unilateral, isometric leg extension strength by a mean of approximately 5.8 ± 3%. Beck et al. (4) reported that a 201 mg dose of caffeine resulted in a significant increase (2.1 kg = 2.1%) in 1RM bench press but not leg extension strength in resistance trained subjects (regularly participating in at least 4 resistance training sessions per week). The same 201 mg dose of caffeine, however, had no effect on 1RM bench press strength in untrained subjects (3). Thus, the findings of Beck et al. (3), as well as the present study, indicated that, in untrained subjects, 201 and 400 mg of caffeine had no effect on 1RM bench press or leg extension strength. In resistance trained subjects, however, doses of caffeine as low as 201 mg increased 1RM strength for upper-body (bench press), but not lower-body (leg extension), exercises (4).
The specific mechanism by which caffeine affects performance during maximal strength activities is unknown. Kalmar and Cafarelli (17) reported that caffeine (6 mg·kg−1 of bodyweight ingested 1 hr before exercise) resulted in significant increases in both isometric leg extension strength (approximately a 5.8% increase) and maximal voluntary muscle activation (approximately a 3% increase) during a unilateral isometric maximum voluntary contraction of the leg extensors. It was hypothesized (17) that caffeine, which is similar in structure to adenosine, may have acted as an antagonist to adenosine receptors of the central nervous system (CNS). Specifically, adenosine binding to its receptor in the CNS inhibits neurotransmitter release and decreases neuronal firing rates, both of which can result in reduced muscle activation and force production (17). Binding of caffeine with adenosine receptors in the CNS could increase maximal voluntary activation and force production (i.e., because of competitive inhibition of adenosine) by allowing for greater motor unit recruitment or firing rates (17). The results from the present study, in conjunction with previous studies (3,4), however, indicated that caffeine supplementation may produce different results with regard to the expression of maximal strength in trained vs. untrained subjects. Additional research is needed to investigate the acute effects of various doses of caffeine on muscular strength during a variety of exercise activities. In addition, on the basis of the findings of Beck et al. (3,4), as well as the current study, future studies should examine the acute effects of caffeine on maximal strength for upper- and lower-body movements in trained vs. untrained subjects.
The results from this investigation also showed that the 400 mg caffeine SUPP (approximately 4.9 mg·kg−1 of bodyweight) had no effect on TTE (mean = 11.4 and 11.2 min for SUPP and PLAC, respectively) (Figure 1) during cycle ergometry at 80% o2peak. There were, however, individual differences for TTE. That is, the TTE values (range = 8-702 s) for 52.4% of the subjects were greater for the SUPP, whereas 47.6% of the subjects' TTE values (range = 4-402 s) were greater for the PLAC. These findings were consistent with those of Beck et al. (3), who reported that a 201 mg dose of caffeine had no effect on TTE at 85% o2peak during treadmill running. These findings, however, were not consistent with previous studies that reported caffeine improved endurance performance (i.e., TTE) during activities that lasted 30 to 60 minutes (7,15,21,25-27). The primary hypotheses regarding increased TTE during aerobic activities (7,11,12,25,27) were increased use of fatty acids and a glycogen sparing effect. For example, Essig et al. (11) examined the effects of caffeine (5 mg·kg−1 of bodyweight ingested 60 min before exercise) on substrate use (assessed by way of respiratory exchange ratio) during 30 minutes of cycle ergometry at 65-75% o2peak and reported that fatty acid use was significantly higher and carbohydrate use was significantly lower for caffeine supplementation compared with placebo. Furthermore, Greer et al. (15) examined the effects of caffeine (6 mg·kg−1 of bodyweight ingested 90 min before exercise) on muscle glycogen content (assessed by way of muscle biopsies) during a 45-minute cycle ergometer workbout at 65-70% o2peak and reported that muscle glycogen decreased at similar rates for caffeine supplementation and a dextrose placebo. Thus, it was suggested that, during aerobic exercise, caffeine may promote increased use of fatty acids but may not decrease the rate of muscle glycogen use (15).
It should be noted that the key component determining performance during the TTE workbouts in the present study may not have been muscle glycogen stores. Although muscle glycogen and blood lactate were not measured in this investigation, it is possible that TTE at 80% o2peak was influenced more by the accumulation of metabolites (i.e., lactate, inorganic phosphate, ammonia) than depletion of glycogen. This hypothesis is supported by the results from cycle ergometry studies that have examined the acute effects of caffeine on performance during exercise tasks that elicit fatigue within 10 to 20 minutes (9,22). For example, Powers et al. (22) reported that, during an incremental cycle ergometer test (beginning workload of 30 W, with 30 W increases every 3 min until exhaustion), caffeine supplementation (5 mg·kg−1 of bodyweight ingested 1 hr before exercise) had no effect on TTE or the time course of blood lactate accumulation. Dodd et al. (9) used an incremental cycle ergometry exercise protocol identical to the present study (a beginning workload of 50 W, with 30 W increases every 2 min until exhaustion) and reported that 2 different caffeine doses (3 or 5 mg·kg−1 of bodyweight) resulted in a significant increase in plasma-free fatty acid concentration but had no effect on the lactate threshold or TTE. Thus, the findings of the present study, in conjunction with those of Powers et al. (22) and Dodd et al. (9), indicated that caffeine did not affect TTE during activities designed to elicit fatigue within 10 to 20 minutes. Future studies should test this hypothesis with various caffeine doses during both cycling and running activities.
In addition to caffeine, the SUPP also contained capsicum extract (active component is capsaicin), bioperine (black pepper extract), and niacin. Capsicum is a genus of plants including various peppers (i.e., chili, jalapeño, habanero), which contain capsaicin. Previous studies (10,18,20,29-31) have suggested that intake of thermogenic ingredients (i.e., caffeine, capsaicin, and black pepper) have the potential to increase fatty acid or carbohydrate use. Furthermore, Lim et al. (19) reported that respiratory quotient and blood lactate levels of trained runners, both at rest and during exercise, increased after ingesting a meal containing 10 g of hot red pepper (2.5 hr before cycling at 60% o2peak). The authors (19) also reported increased catecholamine levels (epinephrine and norepinephrine) 30 minutes after ingestion and suggested that hot red peppers stimulated carbohydrate oxidation both at rest and during exercise. Bioperine (piperine) is a standardized extract from the fruits of Piper nigrum (black pepper) and has been reported to increase the absorption and bioavailability of other nutrients (2,24). Specifically, Badmaev et al. (2) reported that 5 mg of piperine co-administered with 120 mg coenzyme Q10 for 21 days significantly increased plasma levels of coenzyme Q10 when compared with a control group (120 mg of coenzyme Q10 with placebo). Furthermore, Shoba et al. (24) reported that 20 mg of piperine plus 2 g of curcumin significantly increased blood serum levels of curcumin compared with administration of curcumin alone (2 g of curcumin in capsules identical to combined piperine and curcumin capsules). Furthermore, the active component in bioperine, tetrahydrobiopterin (BH4), is a cofactor to eNOS, a producer of nitric oxide (a vasodilator) and a key moderator of vascular homeostasis (23,28). Thus, theoretically, capsicum extract and bioperine may act synergistically with caffeine to increase substrate oxidation. Future studies should examine the possible synergistic effects of caffeine, capsaicin, and black pepper extract to determine their metabolic effects during both exercise and rest (pre- and postexercise).
In summary, the results from this study showed that the SUPP containing caffeine, capsicum extract, bioperine, and niacin had no effect on 1RM strength (bench press or leg extension) or TTE at 80% o2peak. It is possible that these findings were influenced by the training status of the subjects (untrained) or the relative intensity of the exercise task. Future studies should examine these issues in addition to testing the acute effects of various doses of caffeine on performance during strength, power, and endurance activities.
The results from this study indicated that ingestion of a SUPP that contained caffeine, capsicum extract, bioperine, and niacin had no effect on upper- and lower-body strength or endurance cycling performance in untrained men. Thus, these findings did not support the use of caffeine, at the dosage examined in the present investigation, as an ergogenic aid for untrained individuals. The results from previous studies, however, demonstrated that the acute effects of caffeine on upper-body strength (4) and sprint swimming performance (6) may be greater for trained than untrained individuals. Therefore, additional research is needed to determine if the acute effects of various doses of caffeine are influenced by training status.
This study was funded by a research grant from General Nutrition Corporation. The results of the present study do not constitute endorsement of the product by the authors or the NSCA.
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