Assurance of creatine loading is difficult without muscle biopsies. Increases in body weight have, however, implicated water retention during initial creatine loading. Harris et al. (14) demonstrated phosphocreatine loading in the CRE group, reporting a 6.4 mmol·kg−1 increase in mean PCr content with creatine uptake greatest during the first 2 d of supplementation. Significant increases in body weight averaging 1 kg during 6 d of creatine supplementation have been demonstrated (1). Hultman et al. (17) demonstrated weight gain was from water retention due to a marked decline in urine output during creatine supplementation. In our study, the magnitude of the body weight increase was significant for the creatine group which was similar to other studies (8,10,17).
Our data indicate that 5 d of creatine supplementation had no effect on maintaining peak isokinetic torque. The modest increase in torque (∼3%) from PRE to POST supplementation can possibly be attributed to a learning effect, because no difference was demonstrated between the CRE and PLA groups.
Our finding that creatine did not reduce the loss of torque across sets when comparing CRE and PLA are in contrast to those of Greenhaff et al. (12). Greenhaff et al. (12) found that creatine significantly reduced the torque loss at the second (P < 0.01) and third (P < 0.05) sets of exercise, but not the first, fourth, or fifth sets. They also indicated that portions of sets 1, 4, and 5 were significantly different by separating each set of 30 repetitions into sections corresponding to contractions 1–10, 11–20, and 21–30. Greenhaff et al. (12) analyzed each section by performing multiple Student’s t-tests on the data. The use of multiple t-tests inflates the overall rate of type I errors. Interestingly, investigations that have demonstrated differences have used multiple t-tests to analyze their data (5,15), whereas studies supporting the null hypothesis used an ANOVA to analyze their data (6,22–24).
The use of five sets of 30 repetitions with one 60-s rest period between sets was similar to the Greenhaff et al. (12) study. However, subjects in our study were instructed to maximally extend and flex their knee with no pause between contractions to more closely duplicate functional athletic performance. Greenhaff et al. had subjects passively flex to 90° after each knee extension. An increase in time used to passively flex the extremity over the course of five sets may have allowed for subjects to replenish CrP and ATP, which may be less typical of athletic endeavors.
Previous studies have indicated that creatine phosphate (CrP) stores are 98% depleted in 20 s of intense muscle contraction (18,27,28,30). Consequently, studies that examine the effect of creatine on single explosive bouts of exercise of 1–10 s in length may not be of sufficient duration to deplete normal levels of CrP (21). Studies in which subjects exercise for greater than 45 s may be too long in duration and, consequently, dilute any affect that may be present (24). An increase in phosphocreatine resynthesis has been demonstrated with supplementation (9,22) and has been implicated in the delay of fatigue after multiple sets of exercise (12,15); however, this was not found in our study.
Several studies on creatine supplementation with exercise durations of sufficient length, have contradicted one another. In swimming, Burke et al. (6) found no resistance to fatigue from creatine supplementation for sprint swimming of 25, 50, and 100 m (approximately 13- to 60-s duration). Balsom et al. (1) found creatine supplementation enhanced fatigue resistance during five repeated bouts of cycling lasting 6 s, and a 10-s bout of cycling. Yet in a similar study, Barnett et al. (3) found that creatine supplementation did not reduce the loss in power output during seven bouts of 10-s sprint cycling. In separate studies, Odland et al. (22) and Cooke et al. (7) found no benefit from creatine for 30 s, and 15 s × 2 sprint cycling, respectively. Birch et al. (5) found a positive effect from Cr ingestion during the initial two bouts of 3 × 30 s isokinetic sprint cycling tests. A study on running by Harris et al. (15) demonstrated an enhancement of running times with Cr supplementation during the final set of a 4 × 300 m and a 4 × 1000-m run. Terrillion et al. (29) did not find that Cr supplementation improved running times for 2 × 700-m runs in competitive male runners. Although variations in the methodology of these studies may account for the divergent findings, no clear pattern can be identified.
One factor that appears to separate contradictory findings of previous studies examining the affects of creatine supplementation were the data analyses. Several studies that have used appropriate statistical analyses were negative, and those with more liberal analyses demonstrated positive results. This study followed a preplanned statistical analysis. The results of this study indicated that 5 d at 20 g·d−1 of creatine did not attenuate quadricep muscle fatigue during five sets of 30 repetitions. Because of the numerous contradictory studies on creatine’s role on increasing exercise/athletic performance, further research is needed to confirm any potentially positive effects. Although this study did not confirm that acute creatine loading enhances athletic performance, studies examining chronic creatine loading during training may prove otherwise.
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