Creatine is one of the most common supplements used to improve performance in anaerobic sporting activities. It is a naturally occurring compound that is found primarily in meat and fish and consists of the amino acids arginine, glycine, and methionine (2). A large percent of the creatine in the human body is stored in skeletal muscle (approximately 65%) in a phosphorylated form known as phosphocreatine (PCr) (5). PCr plays an important role in the energetics of muscle contraction during high-intensity exercise because it can be used to rapidly regenerate adenosine triphosphate (ATP). The ATP regeneration process occurs through a single reaction that is catalyzed by the near-equilibrium enzyme creatine phosphokinase (CPK) (13). The concentration of CPK in muscles is high, which allows for rapid regeneration of ATP as the intracellular adenosine diphosphate (ADP) concentration increases during prolonged or repeated muscle contractions (13). Thus, it is thought that large PCr concentrations within the muscle fibers enhance the ability to sustain fast ATP turnover rates (30), thereby improving performance and delaying fatigue during high-intensity exercise (3).
Oral supplementation with creatine monohydrate (CM) has been shown to increase muscle strength and performance during both acute and chronic supplementation protocols (10,15,17,18,22,31). Most previous investigations have examined the effects of chronic creatine supplementation in conjunction with a structured resistance training regimen (3,19,23,25,26,28,30). For example, Willoughby and Rosene (32) reported an increase in leg extension 1 repetition maximum (1RM) strength after 12 weeks of resistance training combined with CM supplementation (26 g·d−1 for 1 week and 6 g·d−1 for the remaining 11 weeks). In addition, many previous investigations have used loading periods at the beginning of the supplementation period to saturate the muscle with PCr (27), which usually involves ingestion of at least 20 g of CM per day for 5-7 days (27). However, it is unclear if loading periods or resistance training is required for CM to have an effect on muscular strength and performance during an extended period of time.
It is well known that ingestion of CM increases muscle PCr content, thereby improving muscular strength and performance. Creatine is usually ingested in the monohydrate form. However, the combination of creatine and polyethylene glycosylate (PEG) may help to increase the absorption and uptake efficiency of creatine into the muscle cell. Specifically, PEG is believed to enhance the gastrointestinal (GI) absorption of creatine by increasing permeability coefficients in the GI tract and across the sarcolemma (11). Thus, a smaller dose of PEG creatine would, theoretically, result in equal intramuscular PCr concentrations when compared with ingestion of a larger dose of CM. This may allow a smaller dose of PEG creatine to be just as beneficial for improving performance when compared to a larger dose of CM. No previous investigations, however, have examined the effects of PEG creatine on performance during activities that benefit from the use of CM (e.g., activities that require high levels of strength and power output). Therefore, the purpose of this study was to examine the effects of a moderate dose of CM and two smaller doses of PEG creatine on muscular strength, endurance, and power output.
Experimental Approach to the Problem
This study used a double-blind, placebo-controlled, cross-sectional design. Testing took place during the pre- and postsupplementation periods separated by 30 days. Each testing period required two visits to the laboratory separated by 48-72 hours. The first visit consisted of the countermovement vertical jump (CVJ) and Wingate testing trials. The second visit occurred at the same time of day (±2 hours) and consisted of the muscular strength and endurance tests. During the screening and enrollment period, subjects were randomly assigned to 1 of 4 groups: (a) placebo (PL; 3.6 g of microcrystalline cellulose per day; n = 15), (b) CM (5 g of creatine per day; n = 13), (c) small-dose PEG creatine hydrochloride (1.80 g) yielding 1.25 g of creatine per day (PEG1.25; n = 14), or (d) moderate-dose PEG creatine hydrochloride (3.60 g) yielding 2.50 g of creatine per day (PEG2.50; n = 16). The PL, CM, PEG1.25, and PEG2.50 were identical in appearance in tablet form and ingested during the morning hours for 30 consecutive days. Third-party laboratory testing (NUTRA, Greenville, S.C.) was randomly conducted on the CM, PEG1.25, and PEG2.50 tablets, and the contents were determined to be ±8% of the label claims. Subjects were instructed to consume their tablets with water daily. To ensure compliance, subjects were required to return the empty supplement packets to the laboratory every 7 ± 2 days. All subjects were compliant (i.e., 80% or more), which was calculated by dividing the number of total empty packets by the total number of packets distributed to the subject. Compliance was checked on a weekly basis. Thirty days after the start of the supplementation period, subjects returned to the laboratory to perform the postsupplementation testing in the same manner as described for the presupplementation testing and at the same time of day (±2 hours).
Fifty-eight healthy men (mean ± SD: age, 21 ± 2 years; height, 176 ± 6 cm; body mass [BM], 75 ± 14 kg) volunteered for this investigation. Each subject completed a pre-exercise health status questionnaire and signed a written informed consent document before testing. Forty-three of 58 subjects reported engaging in 1-8 hours of aerobic exercise per week, 42 of 58 subjects reported 1-10 hours of resistance exercise per week, and 41 of 58 subjects reported 1-10 hours of recreational exercise per week. None of the subjects were competitive athletes. However, because of their reported levels of aerobic exercise, resistance training, and recreational sports activities, the subjects can be classified as healthy, moderately active, and recreationally trained. None of the subjects reported: (a) a history of medical or surgical events that might have significantly affected the study outcome, including cardiovascular disease or metabolic, renal, hepatic, or musculoskeletal disorders; (b) use of any medication that might have significantly affected the study outcome; (c) use of nutritional supplements (i.e., creatine, protein drinks, amino acids, and vitamins) in the 9 weeks before the beginning of the study; or (d) participation in another clinical trial or ingestion of another investigational product within 30 days before screening and enrollment. All procedures were approved by the University Institutional Review Board for Human Subjects Research.
Countermovement Vertical Jump
Four maximal countermovement vertical jump (CVJ) trials were performed on a Just Jump™ mat (Probotics, Inc., Huntsville, Ala.) with a 1-minute rest period between trials. The Just Jump™ mat calculated CVJ height (cm) based on the flight time, which was the time that elapsed from the instant the feet left the mat until landing. To complete the CVJ trials, the participants stood on the mat with their feet shoulder width apart and the hands on the hips. A rapid descending quarter-squat countermovement was allowed before the ascending launch. However, the subjects were not allowed to take any steps before performing the CVJ. The participants launched with both feet at the same time and landed in the same position. The mean of the 3 highest CVJ trials was used to represent the CVJ score.
Subjects performed 2 consecutive Wingate tests on a calibrated Monarch cycle ergometer (Monarch 818 E; Quinton Instruments, Seattle, Wash.), which was modified for immediate resistance loading with a custom friction belt clip provided by SMI Power (SMI Power, Linear System Design, Delray Beach, Fla.). Seat height was adjusted to allow for 5 degrees of knee flexion at bottom dead center while pedaling. Toe clips with straps were used to prevent the feet from slipping off the pedals. Subjects warmed up on a treadmill for 3-5 minutes at 4-5 mph followed by a short 1- to 2-minute warm-up on the cycle ergometer while pedaling against a resistance of 0.5 kg. Before the start of the Wingate tests, subjects were instructed to pedal as fast as possible and maintain maximum speed throughout the 30-second test. The software program (SMI Power) counted down from 5 to 0 seconds, during which time the subject was instructed to reach maximum pedal speed by 1 second. At 0 seconds, the resistance was immediately loaded to the flywheel in the amount of 7.5% of the subject's BM in kilograms. As the resistance was applied, an optical sensor (OptoSensor 2000; Linear System Design) interfaced with a personal computer (Dell Inspiron 8200, Dell, Inc., Round Rock, Tex.) tracked the number of flywheel revolutions per second. Computer software (SMI Power) calculated peak power (PP; the highest power output in Watts during any 5-second period) and mean power (MP; the average power output in Watts during the 30-second test) (Bar-Or, the 21st World Congress in Sports Medicine, 1978). Immediately after the first Wingate test, subjects continued to pedal against a passive resistance (0.5 kg). Four minutes after the end of the first Wingate test, the subjects completed a second Wingate test. For each subject, the mean PP and MP values calculated across the 2 consecutive Wingate tests were used for the statistical analyses.
Muscular Strength and Endurance
Subjects performed tests to determine 1-repetition maximum (1RM) and repetitions to exhaustion using 80% of the 1RM for the incline leg press (1RMLP) and bench press (1RMBP) exercises. The LP exercises were performed using a plate-loaded hip sled device with a 45-degree incline (Paramount Fitness Corp., Los Angeles, Calif.). Subjects sat in the seat with their back flat against the backrest and were instructed to grasp the handles of the device tightly to avoid the buttocks losing contact with the seat during the exercise. Subjects placed their feet in the middle of the platform at shoulder's width apart, and this foot position remained constant for all the subsequent leg press tests. Subjects were instructed to lower the platform until the knee joints reached 90-degree of flexion and then fully extended the legs (i.e., 0 degrees of leg flexion). The BP exercise was performed on a standard free-weight bench (TuffStuff, Pomona, Calif.) with an Olympic bar. After receiving a lift off from a spotter, subjects lowered the bar to their chest, paused briefly, and then pressed the bar to full extension of the forearms. If a repetition for either the BP or LP exercises did not meet the aforementioned criteria, it was not counted, and another attempt was allowed after a 2-minute rest period. For both the LP and BP exercises, 1RM strength was determined by applying progressively heavier loads until the subject could not complete a repetition through the full range of motion. Additional trials were performed with lighter loads until the 1 RM was determined, which was usually achieved within 5 trials. Two minutes of rest were allowed between trials (6). Five minutes after the strength testing, muscular endurance tests were performed for both the LP and BP exercises by counting the number of full repetitions that each subject could perform with 80% of their predetermined 1RM. The same absolute load was used during the pre- and postsupplementation testing for the muscular endurance tests.
Adams and Beam (1) reported test-retest reliability statistics for the CVJ, PP, and MP from the Wingate tests and the 1 RM. The intraclass correlation coefficients were 0.98, 0.95, 0.98, and 0.98, respectively, with no significant (p > 0.05) differences between mean values for test versus retest.
Simple 1-way analysis of variance (ANOVA) between groups for the presupplementation scores indicated that the PL, CM, PEG1.25, and PEG2.50 groups were statistically equivalent (p > 0.05) at baseline for all dependent variables except for the BP repetitions to failure. Therefore, because only 1 of 11 dependent variables was (p ≤ 0.05) statistically different at baseline among the groups, these findings suggested that the group randomization was effective in providing equivalent baseline scores across groups.
Eight separate 2-way mixed factorial ANOVAs (time [pre- vs. postsupplementation] × group [PL vs. CM vs. PEG1.25 vs. PEG2.50]) were used to analyze BM, CVJ, PP, MP, 1RMLP, leg press repetitions to failure (REPLP), 1RMBP, and bench press repetitions to failure (REPBP) values. In addition, because the CVJ, PP, and MP values can be influenced by BM (i.e., launching the body during the vertical jump and load assignments for the Wingate Test that were based on BM), these variables were also expressed relative to BM and analyzed with 3 separate 2-way ANOVAs (time [pre- vs. postsupplementation] × group [PL vs. CM vs. PEG1.25 vs. PEG2.50]). When appropriate, follow-up analyses included paired-samples t-tests and one-way ANOVAs. An alpha level of p ≤ 0.05 was considered statistically significant for all comparisons. Statistical analyses were performed using SPSS v. 12.0 (SPSS Inc., Chicago, Ill.).
Table 1 provides the mean and SE values for all the dependent variables (BM, absolute and relative CVJ, absolute and relative anaerobic PP and MP, 1RMBP, REPBP, 1RMLP, and REPLP) from pre- to postsupplementation for all the groups (PL, CM, PEG1.25, and PEG2.50), and Table 2 provides the mean difference values for all the dependent values pre- to postsupplementation for CM, PEG1.25, and PEG2.50.
There was a significant 2-way interaction (time × group; p = 0.020). BM increased from pre- to postsupplementation for the CM group (p = 0.001) but was unchanged for the PL (p = 0.124), PEG1.25 (p = 0.374), and PEG2.50 (p = 0.173) groups.
Countermovement Vertical Jump
For the absolute CVJ data (cm), there were no significant 2-way interactions (time × group; p = 0.744) and no main effects among groups (p = 0.966), but there were main effects for time (p < 0.001). CVJ increased from pre- to postsupplementation (Figures 1A and 2A) for all groups (PL, CM, PEG1.25, and PEG2.50). For the relative CVJ data (cm·kg−1), there were no significant two-way interactions (time × group; p = 0.580) and no main effects among groups (p = 0.917), but there were main effects for time (p < 0.001). Relative CVJ increased from pre- to postsupplementation for all groups (PL, CM, PEG1.25, and PEG2.50).
Anaerobic Peak Power
For the absolute anaerobic PP data (W), there were no significant 2-way interactions (time × group; p = 0.209) and no main effects among groups (p = 0.989), but there were main effects for time (p < 0.001). Absolute PP increased from pre- to postsupplementation (Figures 1B and 2B) for all groups (PL, CM, PEG1.25, and PEG2.50). For the relative anaerobic PP data (W·kg−1), there were no significant two-way interaction (time ×group; p = 0.429) and no main effects among groups (p = 0.501), but there were main effects for time (p < 0.001). Relative PP increased from pre- to postsupplementation for all groups (PL, CM, PEG1.25, and PEG2.50).
Anaerobic Mean Power
For the absolute anaerobic MP data (W), there were significant 2-way interactions (time × group; p = 0.042). Absolute MP increased from pre- to postsupplementation for the CM group only (p < 0.001) but was unchanged for the PL (p = 0.265), PEG1.25 (p = 0.100), and PEG2.50 (p = 0.259) groups (Figures 1C and 2C). For the relative anaerobic MP data (W·kg−1), there were no significant 2-way interactions (time × group; p = 0.085) and no main effects among groups (p = 0.345), but there were main effects for time (p < 0.001). Relative MP increased from pre- to postsupplementation for all groups (PL, CM, PEG1.25, and PEG2.50).
For the 1RMBP data, there were significant 2-way interactions (time × group; p = 0.006). 1RMBP increased from pre- to postsupplementation for the CM (p < 0.001), PEG1.25 (p = 0.004), and PEG2.50 (p = 0.033) groups, but was unchanged for the PL (p = 0.800) group (Figures 1D and 2D). For the REPBP data, there were no significant 2-way interactions (time × group; p = 0.068), but there were main effects for time (p < 0.001). REPBP increased from pre- to postsupplementation (Figures 1E and 2E) for all groups (PL, CM, PEG1.25, and PEG2.50).
For the 1RMLP data, there were significant 2-way interactions (time × group; p = 0.029). 1RMLP increased from pre- to postsupplementation for the CM (p < 0.001), PEG1.25 (p = 0.001), and PEG2.50 (p = 0.011) groups but was unchanged for the PL (p = 0.699) group (Figures 1F and 2F). For the REPLP data, there were no significant 2-way interactions (time × group; p = 0.496), but there was a main effect for time (p < 0.001). REPLP increased from pre- to postsupplementation (Figures 1G and 2G) for all groups (PL, CM, PEG1.25, and PEG2.50).
The findings from this study demonstrated increases (p < 0.05) from pre- to postsupplementation for mean BM and anaerobic MP values for only the CM group. In addition, the mean 1RMBP and 1RMLP values increased (p < 0.05) from pre- to postsupplementation for the CM, PEG1.25, and PEG2.50 groups, but not for the PL group. Thus, all 3 forms of creatine (CM, PEG1.25, and PEG2.50) had the same effect on 1RMBP and 1RMLP strength, unlike BM and MP, where only CM demonstrated an effect. In addition, all groups (PL, CM, PEG1.25, and PEG2.50) demonstrated pre- to postsupplementation increases (p ≤ 0.05) in CVJ, PP, REPBP, and REPLP.
Several previous investigations (8,9,20,21,24) have demonstrated increases in BM after various doses and durations of CM supplementation in subjects that have different training backgrounds (8,9,20,21,24). For example, CM supplementation has resulted in increases in BM in healthy university athletes after 7 days of supplementation (21) and in American football players after 50 weeks of supplementation (28). In addition, Kelly and Jenkins (20) reported an increase in BM for experienced power lifters after 26 days of CM supplementation. Unlike the present study, however, the authors (20) used a 5-day loading period (20 g·d−1) followed by a 21-day period where the dose was reduced to 5 g. In the present investigation, there was no loading period, and the subjects were healthy, recreationally trained college men, rather than experienced power lifters. It is also important to note that in the present study, supplementation with PEG1.25 and PEG2.50 did not result in the same increase in BM, as demonstrated by the CM group (Table 1). A possible explanation for the increase in BM in the CM group is that creatine may have caused an increase in cellular volume (30). This, in turn, may have caused an increase in water retention at the cellular level that may have increased BM. The BM discrepancy between the CM and PEG groups may be a result of the smaller dose of creatine in the PEG groups (1.25 and 2.50 g) versus the CM group (5 g). Future studies should examine the effects of a larger dose of PEG creatine on BM and the subsequent influences on athletic performances that are dependent on changes in BM (e.g., running, jumping, etc).
Previous investigations have demonstrated increases in anaerobic MP during repeated Wingate tests after 5 (22), 6 (23), and 28 (8) days of CM supplementation. Unlike the present study, these previous investigations used a CM supplementation loading period that consisted of 20 g·d−1 for at least a portion of the total duration of the protocol (8,18,22,24). Similar to the present study, Earnest et al. (8) reported increases in MP after 28 days of supplementation; however, subjects were experienced in weight training, rather than healthy recreationally trained college men. In contrast, the PEG1.25 and PEG2.50 groups did not demonstrate the same increase in absolute MP as seen in the CM group in the present study. However, this interaction was not observed when the changes in BM were accounted for, because all groups (PL, CM, PEG1.25, and PEG2.50) increased from pre- to postsupplementation for the relative MP values. MP is calculated using the resistance set on the cycle ergometer multiplied by the total number of flywheel revolutions (16), and the cycle ergometer resistance is a function of BM (resistance = BM × 0.075). Therefore, increases in BM without increases in the total number of revolutions performed during the Wingate test may have accounted for our observations for absolute MP in the present study. Thus, our findings suggested that the increases in absolute MP for the CM group were BM-dependent and may not have reflected substantive improvements in power output compared with the PL, PEG1.25, and PEG2.50 groups. Therefore, similar to the improvements in CVJ, PP, REPBP, and REPLP observed for all groups (PL, CM, PEG1.25, and PEG2.50), MP may not have been influenced by the relatively small doses of creatine in the present study. Future studies should further investigate the potential dose-response nature of creatine (CM or PEG) for improving Wingate-based estimates of anaerobic power output.
Several previous investigations (23,25,26,28,29,32) have demonstrated increases in muscular strength (i.e., 1RM) after CM supplementation. In the present study, 1RMBP and 1RMLP increased for all creatine groups (CM, PEG1.25, and PEG2.50) after supplementation. A unique aspect of this study was that subjects were allowed to maintain their current exercise schedule and did not perform a structured resistance training regimen like previous investigations (3,4,19,23,26). In addition, the daily dose of creatine consumed by the PEG groups (1.25 g and 2.50 g) was considerably less than that used in previous investigations that reported similar increases (26,28,29,32) in muscular strength. For example, several studies (25,26,29) have reported increases in muscular strength after a CM supplementation period that consisted of an initial loading period of 20 g·d−1, followed by a maintenance period of 5-10 g·d−1 for the remainder of the supplementation period. Thus, the results from this study indicated that supplementation with CM (5 g) or PEG creatine (PEG1.25 and PEG2.50) may result in increases in muscular strength that are similar to those reported by studies that have used larger daily doses (5-20 g) of CM (3,4,19).
The results from the present investigation also indicated that all groups (PL, CM, PEG1.25, and PEG2.50) demonstrated significant increases in CVJ, absolute and relative PP, relative MP, REPBP, and REPLP from pre- to postsupplementation. These findings differ from those for BM, absolute MP, 1RMBP, and 1RMLP, which increased from pre- to postsupplementation for only the creatine groups. Although the exact cause for this discrepancy is unclear, it is not uncommon for CM supplementation to have different effects for different exercises (4,10). For example, Brenner et al. (4) reported that CM supplementation (20 g·d−1 for 1 week and 2 g·d−1 for 4 weeks) increased 1RMBP strength significantly when compared with a PL. However, in the same study, it was also reported that both the CM and PL resulted in significant increases in 1RM leg extension strength. In addition, Green et al. (12) reported similar increases in MP and PP after CM and PL supplementation. Furthermore, as stated previously, the subjects in the present study were allowed to continue their current exercise schedule, which could have contributed to the increase in CVJ, PP, MP, REPBP, and REPLP from pre- to postsupplementation for the PL group. Overall, our findings suggested that certain performance variables, such as vertical jump, Wingate power outputs, and REPs may be creatine dose-dependent and may require larger doses than 1.25 - 2.50 g of PEG creatine for 30 days to observe improvements. An increase in PCr concentrations from creatine supplementation is thought to enhance the ability to sustain fast ATP turnover rates (30), thereby improving performance and delaying fatigue during high-intensity exercise (3). However, the PEG-creating doses of 1.25 and 2.50 g may have not been enough of a dose to increase PCr concentrations to the extent that a dosage of 5 g of CM may have been able too. In contrast, 1RM strength measurements may be improved by relatively small doses of creatine used in the present study. However, it has also been suggested that resistance training exercise can improve creatine uptake (14). Therefore, future investigations should examine the effects of small-dose CM and PEG creatine on performance after a period of supplementation combined with resistance training.
The observed safe limit (OSL) has been proposed by Shao and Hathcock (27) as a guideline for the amount of creatine that can be safely consumed throughout a long-term supplementation period. Shao and Hathcock (27) have recommended that, based on the findings of Derave et al. (7), the OSL for creatine should be 5 g·d−1. The present investigation's supplementation doses were equal to (CM) or less than (PEG1.25 and PEG2.50) the suggested OSL and, therefore, demonstrated that increases in 1RM performance can be achieved when adhering to Shao and Hathcock's (27) OSL recommendations. However, a limitation of the present investigation is that the increases in CVJ, PP, MP, REPLP, and REPBP for the creatine groups (CM, PEG1.25, and PEG2.50) were not significantly greater than those for the PL group. Thus, it is possible that larger doses of creatine may have been required to result in significant improvements in these performance measures when compared with the PL group.
In summary, the results from this study showed that CM supplementation resulted in increases in BM and absolute MP that were significantly greater than those for the PL, PEG1.25, and PEG2.50 groups. However, for relative MP, all groups (PL, CM, PEG1.25, and PEG2.50) improved over time. In addition, CM, PEG1.25 and PEG2.50 supplementation resulted in increases in 1RMBP and 1RMLP that were significantly greater than those for the PL group. These results supported those from previous investigations (3,4,19,23,26,28,29,32) that have examined the effects of creatine on BM and 1RM strength. However, our findings did not support previous studies that have observed creatine-induced improvements in muscular power (15,18,22,24) and repetitions to exhaustion (17,31). Furthermore, these findings are important from a practical standpoint because they indicated that PEG creatine may result in similar increases in strength when compared with CM but with a dose reduction of as much as 75%. It is important to note, however, that there were increases from pre- to postsupplementation for CVJ, PP, relative MP, REPBP, and REPLP for all groups (PL, CM, PEG1.25 and PEG2.50). Thus, additional studies are required to determine if PEG creatine supplementation has ergogenic effects that are comparable to those of CM. Future investigations should also examine the potential ergogenic effects of PEG creatine combined with a strength/power training program.
One of the unique aspects of the present investigation was that CM and PEG creatine supplementation improved 1RM strength without the use of a loading period. In addition, the daily doses of creatine consumed by the PEG creatine groups (1.25 g and 2.50 g) were considerably less than those used in many previous investigations (3,4,19,23,25,26,29) but still resulted in increases in 1RMBP and 1RMLP that were the same as those for a 5-g dose of CM. Thus, the use of PEG creatine may result in improvements in strength that are comparable to those with the OSL dose for CM (5 g·d−1) recently proposed by Shao and Hathcock (27). More research is required, however, to examine the effects of PEG creatine combined with training.
This study was funded by a research grant from the General Nutrition Corporation (Pittsburgh, Pa.).
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