Journal of Strength and Conditioning Research, 1999, 13(4), 394–399 q 1999 National Strength & Conditioning Association Creatine Supplementation Does Not Increase Peak Power Production and Work Capacity During Repetitive Wingate Testing in Women ASHLEIGH LEDFORD AND JOHN DAVID BRANCH Human Performance Laboratory, Wellness Institute and Research Center, and Department of Exercise Science, Physical Education, and Recreation, Old Dominion University, Norfolk, Virginia 23529. ABSTRACT The effect of creatine monohydrate (CM) supplementation on performance of high-intensity, short-duration tasks has been studied extensively in men, but few studies have investigated this issue in women. To test the hypothesis that CM improves Wingate test (WT) peak power production and work capacity in women, 9 well-trained subjects (baseline age mean 6 SD 5 27 6 6 years; body mass 5 56.9 6 5.7 kg; peak power 5 538 6 105 W; work capacity 5 12.27 6 1.20 kJ) received, in a double-blind manner with counterbalanced treatment order, 2 supplementation regimens (20 g·d21 for 5 days) of both CM and Polycose placebo (PL). Both CM and PL were separated by a 95 6 3-day washout period. Immediately following each supplementation period, subjects completed 3 WTs, interspersed with a 5-minute rest interval. No significant treatment (p 5 0.30) or treatment-by-test interaction effects (p 5 0.39) were observed in WT peak power production (CM mean 6 SE 5 540 6 30, 484 6 23, 435 6 19 W vs. PL 5 522 6 30, 481 6 22, 403 6 32 W). The WT work capacity was also unchanged by CM supplementation (CM mean 6 SE 5 12.39 6 0.67, 10.60 6 0.50, 9.56 6 0.59 vs. PL 5 12.09 6 0.32, 10.60 6 0.54, 9.39 6 0.67 kJ) with no treatment (p 5 0.76) or treatment-by-test interaction effects (p 5 0.66). The practical application of this study is that 5 days of CM supplementation did not increase WT peak power production and work capacity in women. Key Words: ergogenic, phosphocreatine, glycolysis, ergometer, anaerobic Reference Data: Ledford, A., and J.D. Branch. Creatine supplementation does not increase peak power production and work capacity during repetitive Wingate testing in women. J. Strength Cond. Res. 13(4):394–399. 1999. Introduction C reatine, an endogenous nitrogenous amine capable of rapid and reversible phosphorylation, is an important source of phosphate for anaerobic adenosine triphosphate (ATP) synthesis by the ATP-phosphocre394 atine (PCr) energy system. As early as the 1920s, it was hypothesized that an increase in free creatine through exogenous supplementation could result in rapid replenishment of PCr and ATP synthesis during highintensity, short-duration activity (9). Since 1990, the ergogenicity of creatine supplementation has been most extensively investigated in single-bout and repetitive high-intensity performance tasks of #30 seconds (i.e., tasks that rely primarily on the ATP-PCr system) and has been the subject of several recent reviews (2, 16, 17, 26, 40). Many studies have reported enhanced performance in laboratory tasks, such as cycle ergometry (1, 3, 6, 8, 11, 12, 29, 33), isotonic force production (12, 39), and isokinetic (18, 36) force production following creatine supplementation. Other studies, however, reported no improvement in cycle ergometer tasks #30 seconds (4, 10, 11, 28) and 25-m swim (7, 27) and 60m sprint (30) performance tasks following creatine supplementation, indicating that this issue remains equivocal. The literature is replete with studies of enhanced performance in various tasks by men following creatine supplementation, but there are few investigations involving women. Although long-term (10-week) creatine supplementation combined with resistance training has recently been reported to enhance muscle strength in women (38), no improvement in upperbody isokinetic torque (20, 37, 38) or isotonic strength (20) has been observed in women following short-term (#7 days) supplementation. Furthermore, the effect of creatine supplementation on lower-body anaerobic power and work capacity as measured by cycle ergometer testing has been studied in men (6, 8, 12, 28) but not in women. Therefore, the purpose of this study was to investigate the effect of creatine supplementation on power production and work capacity during repetitive Wingate cycle ergometer tests in physically active women using a double-blind, placebo-control, crossover design. Creatine Supplementation in Women 395 Methods Subjects Subjects (N 5 10) were physically active women who were exercise science college students (mean 6 SD, age 5 27 6 6 years, body mass 5 56.6 6 5.8 kg) who denied currently taking any nutritional supplement. Subjects were instructed to maintain both normal nutritional intake and level of physical activity for the entire study. Before participation, subjects were appraised of potential risks and provided informed consent in accordance with the Institutional Review Board of Old Dominion University. Testing Protocol The Wingate test of anaerobic power production and work capacity (5) was administered using a Monark 818E cycle ergometer equipped with a digital revolution counter and a resistance pendulum. Before each test, revolution counter calibration was confirmed by observing the digital display in response to incremental pedal revolutions (i.e., right leg extension to next right leg extension 5 1 unit). The resistance pendulum was calibrated at both 39.2 N (using a 4.0-kg mass) and 0 N of force. Before testing, subjects completed a standardized 5-minute warm-up (50 rev·min21 pedal rate against 33% of the resistance force to be used in the test), which also included 4 maximal ‘‘sprint’’ bouts lasting 5 seconds. Subjects then rested passively for 2 minutes, followed by static stretching (10 seconds each) of hamstring, hip flexor, quadricep, and calf muscles. Immediately preceding testing, subjects pedaled for 10 seconds against 33% of the test resistance force and were then instructed to achieve a maximal pedal revolution rate. The pendulum was quickly adjusted to apply a resistance force to the ergometer flywheel, which was calculated to the nearest 0.25 kg as follows: Ergometer Resistance Mass (kg) 5 Body Mass (kg) · 0.075 (1) Ergometer Resistance Force (N) 5 Resistance (kg) 4 0.102 (1A) When the resistance was set, the test commenced (T0) with simultaneous resetting of the revolution counter to 0. Subjects were verbally encouraged to maintain the maximal possible pedal rate for the duration of the 30-second test. Cumulative revolutions were recorded every 5 seconds (i.e., T5, T10, T15, T20, T25, T30). Power production for each 5-second interval was calculated in watts as follows: Power (W) 5 Resistance (kg) · (6 m·rev21 ) · (rev·min21 rate) 4 6.12 (2) Peak power production was the highest rate of power production during any 5-second interval and was ob- served in the first or second 5-second interval (i.e., from T0–T5 or from T6–T10). Work capacity was calculated in kilojoules as follows: Work (kJ) 5 Resistance (kg) · (6 m·rev21 ) · (total rev) · 9.8 4 1000 (3) Design The study commenced in the summer of 1997. Following familiarization with the test protocol, subjects were administered a minimum of 2 Wingate tests separated by at least 3 days for measurement of baseline peak power production and work capacity. For statistical purposes, baseline peak power production and work capacity were reported as the mean of the best 2 tests with a reliability coefficient of variation of #5% (mean 5 2.3%). The test-retest correlation coefficient (r 5 0.91) for baseline work capacity was similar to previously reported data on Wingate test reliability (5). Subjects were then matched according to baseline values for age, body mass, peak power, and work capacity. In a double-blind, counterbalanced treatment order manner, subjects in each pair were randomly assigned to creatine monohydrate (CM, Universal Labs, New Brunswick, NJ) or glucose polymer placebo (PL, Polycose) supplementation regimens (20 g·d21 for 5 days). The CM or PL was provided to subjects in weighed (;120 g) unlabeled vials. Subjects refrained from ingesting caffeine, which has been reported to eliminate the ergogenic effect of creatine supplementation (36). Since it has also been reported that creatine uptake is augmented when combined with carbohydrate (15), subjects were instructed to ingest supplements (4 separate doses of ;5 g [i.e., ;1 heaping teaspoon]) dissolved in ;8 oz of orange juice. Vials were returned to confirm compliance with the supplementation regimen. On the day following completion of the initial supplementation (day 6), subjects were administered 3 Wingate tests interspersed by a 5-minute rest interval, during which peak power and work capacity were measured. A 3-month (95 6 3 day) washout period followed the initial supplementation and testing. Subjects were administered a minimum of 2 postwashout Wingate tests for the purpose of comparing baseline and postwashout peak power and work capacity. The other supplementation regimen was then administered in a double-blind, counterbalanced treatment order manner, followed by repetitive Wingate testing. For each subject, data were collected at approximately the same time of day throughout the study to minimize any circadian effect on performance. Statistical Analyses The effects of creatine supplementation on peak power production, work capacity, and body mass were analyzed by repeated-measures analysis of variance using SAS version 6.09 PROC GLM (32). The criterion for 396 Ledford and Branch Table 1. Characteristics of the subjects at baseline (BL) and following the washout (WO) period (mean† 6 SE). Variable Age (yr) Body mass (kg) Peak power (W) Work capacity (kJ) Group 1‡ BL WO BL WO BL* WO 26 54.4 54.3 526 540 12.24 10.94 6 6 6 6 6 6 6 4 2.9 2.7 77 53 0.86 0.84 Group 2§ 28 58.9 58.0 547 600 12.30 11.69 6 6 6 6 6 6 6 7 2.8 2.9 28 39 0.35 0.39 * Time main effect, BL work capacity . WO work capacity (p 5 0.0004). † Peak power and work capacity values are the mean for the best 2 tests at BL and post-WO, respectively. ‡ Placebo → creatine monohydrate, n 5 4. § Creatine monohydrate → placebo, n 5 5. significant treatment (CM vs. PL), test (postsupplement Wingate test 1 vs. test 2 vs. test 3), and treatment-by-test interaction effects was a 5 0.05, with a Bonferroni adjustment for post hoc comparisons. Results Subject Characteristics at Baseline and Following Washout One subject became ill during repetitive Wingate testing following both supplementation regimens and failed to complete the testing protocol. The following results are based on the remaining (N 5 9) subjects. Baseline and postwashout values for age, body mass, power, and work capacity are presented in Table 1. According to study design, there were no significant differences between groups 1 (PL → CM) and 2 (CM → PL) at baseline or following washout. For all subjects, there were no differences in body mass (p 5 0.66) or peak power production (p 5 0.54) at baseline compared with postwashout, but work capacity was greater at baseline than at postwashout (p , 0.0004). Subjects returned empty vials following each supplementation period and reported the ingestion of CM and PL according to the prescribed regimen. Furthermore, a McNemar x2 test for dependent samples (31) revealed no association between supplementation regimen and ability to discern the treatment (p 5 0.37), suggesting that subjects were adequately blinded to treatments. Peak Power Production Following CM Supplementation The effect of CM on peak power production during repetitive Wingate testing is presented in Table 2. As expected, a significant Wingate test main effect was observed since peak power production declined during repetitive testing following both CM and PL (p , 0.002). However, CM supplementation did not improve peak power production compared with PL (p 5 0.30). Furthermore, the interaction between treatment and test was not significant (p 5 0.52), suggesting that the decline in peak power production during repetitive Wingate testing was not attenuated by CM supplementation. Work Capacity Following CM Supplementation The significant Wingate test main effect (p , 0.0005) observed following both CM and PL documented the expected decline in work capacity during repetitive bouts. Work capacity was not changed following CM compared with PL (p 5 0.76), nor was the decline in work performed in repetitive bouts attenuated by CM supplementation (p 5 0.76 for test-by-treatment interaction). Effect of CM Supplementation on Body Mass Body mass was unchanged (p 5 0.56) across 6 trials (mean 6 SE; pre-PL1 5 56.8 6 1.8; pre-PL2 5 56.7 6 1.8; post-PL 5 56.4 6 1.8; pre-CM1 5 57.0 6 1.8; preCM2 5 56.8 6 1.9; post-CM 5 56.1 6 1.8 kg). Discussion In recent years, the efficacy of CM as an ergogenic aid has received considerable but not unanimous research support. As a result, the issue remains somewhat equivocal. In addition, disproportionate research attention has been placed on men. Examination of mixed-sex studies (7, 18, 29, 30) and abstracts (22, 23, 24, 41) of creatine ergogenicity reveals most subjects to be men, with only 2 of these studies (18, 29) reporting an ergogenic effect. Few crossover designs exist in the literature (14, 25, 27, 36). To our knowledge, the present study is the only double-blind, crossover design of creatine supplementation in women. The key Table 2. Effect of creatine supplementation on peak power output and work capacity during repetitive Wingate tests (mean 6 SE). Variable Peak power (W) Work capacity (kJ) Treatment PL CR PL CR Test 1 523 540 12.09 12.39 6 6 6 6 30 30 0.32 0.67 Test 2 481 484 10.60 10.60 6 6 6 6 22 23 0.54 0.50 Test 3 403 435 9.39 9.56 6 6 6 6 32 19 0.67 0.59 Creatine Supplementation in Women 397 finding of our study was that creatine supplementation (20 g·d21 for 5 days) did not increase Wingate test peak power production or work capacity in physically active women. These results are in agreement with previous reports of no improvement in isotonic strength or endurance (20), middle distance interval running (34), or isokinetic torque production (20, 37, 38) in women following short-term (#7 days) creatine supplementation. This small but unanimous group of null findings suggests that short-term supplementation may not be effective in active women. The ergogenicity of long-term creatine supplementation in women has been the focus of 2 studies (35, 38). Thompson et al. (35) reported that 42 days of creatine supplementation (2 g·d21) failed to improve 100and 400-m swim performance. Muscle PCr, PCr:b-ATP ratio, and adenosine diphosphate, measured using phosphorus-31 nuclear magnetic resonance spectroscopy and near-infrared spectroscopy at rest and during exercise (plantar flexion) both before and after supplementation remained unchanged. In contrast, Vandenberghe et al. (38) reported that ‘‘high-dose’’ creatine supplementation (20 g·d21 for 4 days), followed by 10 weeks of ‘‘low-dose’’ creatine supplementation (5 g·d21), combined with resistance training significantly increased muscle strength and isokinetic arm torque compared with a training group that ingested a placebo. Creatine supplementation also increased muscle PCr, PCr:b-ATP ratio, and fat-free mass. One possible explanation for these discordant findings is the 5-fold difference in total creatine ingested. There appears to be better research support for improvement in high-intensity, short-duration (#30 seconds) work performance following short-term creatine supplementation in men compared with women, which begs the elucidation of a possible explanatory mechanism. In a sex comparison of muscle composition, Forsberg et al. (13) reported a 10% greater concentration of total muscle creatine (TCr) in women (145 6 10 mmol·kg21 dry mass) compared with men (132 6 10 mmol·kg21 dry mass). It is plausible that women, with higher endogenous TCr than men, are less responsive to creatine supplementation. Although this explanation is consistent with our finding, muscle TCr was not measured in the present study. Therefore, further commentary would be speculative. Considerable interindividual variation appears to exist in response to creatine uptake in men. It has been reported that human muscle has an upper TCr of 150– 160 mmol·kg21 dry mass (16). Greenhaff et al. (19) reported a 25% increase in muscle TCr in 5 (63%) of 8 men but no increase in the other 3 subjects following a creatine supplementation regimen similar to that used in this study (20 g·d21 for 5 days). In their study (19), increased muscle TCr was observed in individuals with low-normal endogenous TCr levels (;120 mmol·kg21 dry mass) following creatine supplementation, whereas subjects with higher endogenous TCr were somewhat less responsive. It has been suggested by Casey et al. (8) that enhanced performance may be associated with creatine uptake by type IIb (fast glycolytic) fibers. Interindividual differences in muscle fiber types may affect response to supplementation, with a lower muscle creatine uptake observed in individuals with a greater percentage of type I (slow oxidative) fibers. Although the subjects in the present study participated primarily in aerobic activities, such as swimming, bicycling, and running, the muscle fiber distribution of these subjects is unknown since fiber typing was not included in our methods. Increases in body mass of 0.7–2.0 kg have been reported in men following short-term (20–25 g·d21 for 5– 7 days) creatine supplementation (1, 3, 11, 12, 15, 19, 39). According to Hultman et al., urinary volume is decreased during short-term supplementation (21), a finding that suggests that water retention may explain the early increase in body mass. Our finding of no change in body mass in women following a 20 g·d21 for 5 days supplementation regimen is in accord with other studies of women athletes (20, 34, 35). However, Vandenberghe et al. (38) reported a 60% greater increase in fat-free mass following 10 weeks of creatine supplementation combined with resistance training compared with a resistance training group that ingested a placebo. A sex effect, if real, may possibly be explained by the previously discussed sex differences in baseline TCr and the resulting decreased responsiveness to creatine uptake. Limitations to the present study include no measurement of muscle TCr, which is essential to investigate sex differences in endogenous TCr, muscle creatine uptake, and effectiveness of creatine supplementation. Although the study was well designed, with counterbalanced treatment order and a washout period exceeding the 4 weeks required for muscle TCr to return to baseline (38), the 3-month washout may have increased the chance of a maturation or history threat to internal validity. The decreased work capacity following washout compared with baseline was probably related to the fact that the postwashout treatment was administered during midsemester examinations, a time of decreased physical activity for the student subjects. Although the treatment main effect and treatment-by-test interaction did not approach statistical significance, the sample size was small and the observed statistical power may have been low. Finally, the Wingate and other 30-second cycling tests have been used to assess the efficacy of creatine supplementation (6, 8, 12, 28), but work capacity, which requires contribution of ATP by fast glycolysis, is a more reliable Wingate test measurement than peak power production, which is more closely linked to the ATPPCr energy system (5). 398 Ledford and Branch It is recommended that future investigations of this issue include direct sex comparison of baseline TCr and subsequent creatine uptake in a design that controls for such possible confounding influences as muscle fiber type distribution and baseline muscle TCr. Previously reported mixed-sex designs have lacked adequate statistical power to investigate sex as a main effect. In summary, 5 days of CM supplementation failed to increase peak power and work capacity during repetitive Wingate testing in active women. Creatine supplementation appears to be more effective in men, and the result of this study is in agreement with the null findings of other researchers with regard to the ergogenicity of short-term creatine supplementation in women. Although a direct sex comparison awaits investigation, it is possible that differences in baseline muscle TCr between men and women may affect subsequent responsiveness of women to creatine supplementation. 8. 9. 10. 11. 12. 13. 14. Practical Applications Creatine monohydrate is an increasingly popular supplement used by many physically active individuals. Reported outcomes associated with creatine supplementation include increased body mass and enhanced performance in high-intensity, short-duration tasks. There is considerable research support for the ergogenicity of creatine in men. 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