Repeated-measures ANOVA for BW showed no 3-way interaction (time × gender × treatment), but a main effect for time was found (p = 0.013). Repeated-measures ANOVA for each gender group indicated no significant 2-way interaction (time × treatment) for either the male or female groups, but a significant main effect for time was found in the male group (p = 0.022). Follow-up dependent t-tests showed only nonsignificant increases in BW for male (∼0.6 kg, p = 0.096) and female (∼0.3 kg, p = 0.296) Cr groups and the PL groups (p > 0.05).
Repeated-measures ANOVA for ARC showed a significant 3-way interaction (time × gender × treatment) (p = 0.042, η 2 = 0.087). Repeated-measures ANOVA for each gender group indicated a significant main effect for time (p = 0.006). A significant 2-way interaction (time × treatment) was shown for the male group (p = 0.025, η 2 = 0.209) with no interaction for the female group (p > 0.05, η 2 = 0.007). Follow-up dependent t-tests highlighted a significant increase in ARC for the male Cr group (p = 0.003), with no change in the male PL, female PL, or female Cr groups (p > 0.05) (Figure 1). A 1-way ANOVA showed postsupplementation values for the male Cr group that were significantly higher than those of the male PL group (p = 0.019).
The primary finding of this investigation demonstrated that ARC increased in men, but not in women. Body weight changes as a result of Cr loading have been suggested to impact running performance, but only nonsignificant increases in weight were present in the current study for both men and women supplementing with Cr. Gender differences with Cr loading have previously been compared using the relationship between power output and time to exhaustion on a cycle ergometer (8,9). However, the current study is the first to examine the effects of any nutritional intervention on ARC. Previous research incorporating the use of nuclear magnetic resonance has shown increases of up to 20% in intramuscular PCr as a result of Cr loading (10). Although the current investigation did not incorporate sophisticated substrate tracking techniques, a precedent has been set in which the effectiveness of Cr loading in increasing muscle PCr content can be assumed.
As mentioned, individual fluctuations in BW and the discrepancy between responders and nonresponders may influence mean changes in weight. Individual percent change scores for each gender and treatment group (Figure 2) further highlight the relatively neutral nature of the changes elicited during this study. Sixty-seven percent of men and 58% of women in their respective Cr groups demonstrated nonsignificant increases in BW in the current study. Interestingly, 75% of the male Cr group that showed increases in BW also improved in ARC. Comparably, Green et al. (11) noted increases in mean power and peak power from repeated Wingate testing after Cr loading in only subjects that had corresponding increases in BW.
Anaerobic working capacity was originally described by Monod and Scherrer to quantify the total amount of energy available solely from anaerobic energy stores, specifically ATP and PCr, as provided by the critical power test, which quantifies the relationship between work rate and time to exhaustion. (26) Anaerobic running capacity is the treadmill analog of AWC and represents the total distance an individual could theoretically run using only stored anaerobic energy because of the inability to use the aerobic energy system (3,15,16). This measure is provided as a variable from the CV test conducted on a treadmill, which examines the relationship between time to exhaustion and total distance. Anaerobic running capacity is a useful tool that may be used in tracking changes because of specific interventions, such as training or supplementation (8,9,19,25,31,34). Creatine loading significantly increases the amount of available substrates used in the Cr kinase reaction (13,14), which are the same constituents measured as anaerobic energy stores in both AWC and ARC and therefore may positively influence these performance measures (6).
The results of the current investigation revealed a significant 23% increase in ARC for the male group after Cr loading compared to a nonsignificant 3% change in the PL group. Additionally, ARC and BW were unchanged in the female group postsupplementation. These findings are in contrast to the initial hypotheses of the current investigation, suggesting no improvement in ARC for men because of increased BW and increased ARC in women with no change in BW. Although this is the first study to examine the effects of Cr loading and ARC, previous data examining the relationship between Cr and AWC may provide some explanation for the current study. Increased AWC during cycle ergometry as a result of Cr loading has previously been demonstrated. Stout et al. (32) showed a significant 9.4% increase in AWC after Cr loading in men. Smith et al. (31) and Miura et al. (25) reported similar increases, ranging from 10 to 25%, in AWC after Cr supplementation. Despite the initial concern outlined in this investigation regarding the translation of AWC improvements during non-weight-bearing exercise on a cycle ergometer to ARC during treadmill running, the current findings suggest that the mechanisms triggered by Cr loading, including minimal changes in BW, work in a similar manner when examining weight-bearing exercise. Further research regarding the nature of the differences between AWC and ARC by mode of exercise is warranted.
Within the current study, increases in ARC were observed over time for all groups (significant for the male Cr group and nonsignificant for all others). This finding may be explained by the intense nature of the CV test and the subsequent physiological and mental adaptations that developed over the course of the study. Regardless of the nature of these adaptations, this development further supports the finding of a significant increase in ARC for the male Cr group, because any significant increase would have to overcome the pre-existing increases that appear to have occurred during the testing protocol.
The results from this investigation provide direction with regard to athletes and the use of Cr loading to influence short-duration, high-intensity performance. Notably, the 23% increase in male ARC can be compared to an increase in treadmill running of approximately 60 m, a distance that could lead to enhanced performance in a variety of exercise settings. The current investigation revealed only nonsignificant increases in BW from Cr supplementation for both men and women. Also, ARC via the CV test may be a more sensitive measure in men than in women when Cr loading.
Creatine loading may be used before competition in anaerobic athletes or mixed aerobic and anaerobic athletes in an effort to provide improvements in anaerobic running activities. This study also provides further research concerning possible weight gain as a result of Cr loading and the impact on performance. Decrements in weight-bearing performance, as a result of an increase in BW, are a concern for many athletes. However, the current results show that runners, or athletes who require running as part of their sport, may supplement with Cr without a hindrance in performance because of increased BW.
The author(s) declare that they have no competing interests. The results of the present study do not constitute endorsement of the product by the authors or the National Strength and Conditioning Association. The authors would like to thank FSI Nutrition Fortress Systems, LLC, for supplying the supplements for this study.
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