Basic Sciences: Original Investigations
During high intensity short-term exercise when energy demand exceeds that which can be met by oxidative delivery processes, adenosine triphosphate (ATP) must be resynthesized by the anaerobic breakdown of glycogen to lactate and the degradation of phosphocreatine (PCr). Because of the high maximum activity of the creatine phosphokinase reaction and the close proximity of this enzyme to its reactants in the cytoplasm(9), the rate at which ATP can be resynthesized from adenosine diphosphate (ADP) and PCr is considerably more rapid than that by glycogenolysis (10). Because of the relatively low resting muscle concentration of PCr and the fact that its depletion rate is closely related to decrements in force production and power output during short-term intense exercise (2,9,12), the question arises as to whether fatigue might be delayed with elevated muscle PCr.
Recent literature has demonstrated that oral creatine (Cr) supplementation can lead to increased muscle PCr and Cr content (6). The purpose of this study was to determine the effect of Cr supplementation on performance during a single 30-s bout of maximal cycle exercise. It was hypothesized that if Cr supplementation were to increase the PCr content of skeletal muscle this would also increase the duration of peak and average power output during the test.
Subjects. Nine healthy males (22-30 yr) participated in the study. Written informed consent was obtained in compliance with the Human Ethics Committee for McMaster University. Subjects were physically active, but not specifically trained for short-duration maximum cycling exercise.
Design. Subjects participated in 3 randomly ordered exercise test sessions following each of: creatine supplementation (CRE), placebo (PLA), and control (CON). In the CRE condition, creatine monohydrate(Cr·H2O) was consumed daily in 20-g doses (134.4 mmol) dissolved in 1200 ml of a heated flavored drink. The drink was cooled to +4° C and ingested in 4 equal portions (separated by 3-4 h). This procedure was repeated for 3 d prior to testing. CRE and PLA treatments were administered in a double-blind fashion, with PLA consisting of the drink only. Fourteen days separated each test session, and subjects were instructed not to perform strenuous exercise 24 h before testing. Subjects conformed to their habitual diets, and daily food records were kept for 2 d preceding each test session to ensure consistent food consumption prior to each test.
On each test occasion, subjects reported to the laboratory after an overnight fast. In the PLA and CRE conditions, the final 300-ml portion of the drink was consumed 2 h prior to arrival. On each occasion a pre-exercise muscle sample was obtained from the vastus lateralis using the percutaneous needle biopsy technique with the addition of manual suction. Biopsies were performed under local anesthesia, and approximately 50-150 mg of wet tissue was obtained per sample. Samples were rapidly removed from the needle and immediately frozen in liquid nitrogen. After the sampling procedure, pressure and ice were applied to the puncture site, and subjects rested quietly for 10 min.
Exercise consisted of a single 30-s Wingate anaerobic test(1) on a Monarch cycle ergometer which was modified for immediate resistance loading (0.075 kg·kg-1 body weight). This modification allowed resistance to be preloaded onto a weight pan and to be instantly applied at the beginning of the test, thus avoiding possible reliability problems associated with visually setting and adjusting the resistance. Prior to exercise, the feet were firmly strapped to the pedals, and the seat height was adjusted for optimal comfort and pedaling efficiency. Subjects attempted to reach maximal pedal frequency against only the ergometer's inertial resistance and within 2 s the full load was applied and the revolution counter activated. Subjects were instructed to pedal maximally for the duration of the task. Capillary blood samples were taken from a finger-tip puncture 3 min post-exercise for immediate analysis of blood lactate (YSI Lactate Analyzer). Prior to commencing the study, it was found that 2 or 3 pretrials were adequate to familiarize subjects with the exercise protocol and to establish reproducible power output measurements. Power output was computed for each second of exercise, as was peak power(W·kg-1), mean 10-s power (W·kg-1), mean 30-s power (W·kg-1), and percent fatigue.
Tissue samples were freeze dried and dissected free from visible blood and connective tissue. Metabolites were extracted with 0.5 M HCl04 (1.0 mM EDTA), neutralized, and analyzed enzymatically (11) for PCr, ATP, and TCr according to the method described by Harris et al.(5).
All data are means ± SE unless otherwise noted. Data were analyzed by one-factor repeated measures ANOVA, followed by Tukey A post-hoc analyses when significant F ratios (P < 0.05) were obtained.
Exercise data. Peak power scores were achieved in the initial few seconds of the exercise after which power declined rapidly(Fig. 1). Power outputs were remarkably consistent across the 3 tests, and no differences were observed between conditions for any of the recorded exercise measures (Table 1). Blood lactate concentrations also did not differ between conditions (Table 1).
Muscle metabolites. Muscle TCr and PCr concentrations are shown expressed relative to ATP concentration in Figure 2. The TCr/ATP ratio was significantly greater in the CRE condition than in the PLA and CON conditions, but no difference in the PCr/ATP ratio was detected between conditions (Fig. 2). The TCr and PCr contents were normalized relative to ATP content to control for possible differences that may have been a result of varying quantities of blood or connective tissue in the different samples.
Our data indicate that 3 d of Cr supplementation had no effect on power output during a single 30-s bout of maximal cycle exercise(Fig. 1). Biochemical analysis revealed that although muscle TCr was higher following the CRE condition, there was no difference in PCr content across the conditions (Fig. 2).
Our finding of no change in [PCr] is in contrast to that of Harris et al.(6) who reported a 6.4 mmol·kg-1 increase in mean PCr content following Cr feeding in 17 subjects. A possible explanation for this difference may relate to differences in the magnitude and/or the duration of Cr supplementation. The administration of Cr·H2O doses in the Harris et al. study ranged from 70 g given over 3.5 d to 330 g given over 21 d. Urinary excretion data reported in that study(6) revealed, however, that the greatest Cr uptake occurred within the first 2 d of supplementation. It was for this reason that 3 d of supplementation were chosen for the present study. Although we found that this was adequate to significantly increase TCr stores over this time period, it did not affect PCr concentration. The possibility that a longer duration of supplementation might have elevated muscle PCr concentration cannot be dismissed.
Factors affecting Cr metabolism and PCr synthesis are poorly understood. Cr can only be rephosphorylated by ATP, and the creatine phosphokinase reaction is thought to provide a buffer function for changes in the ATP to ADP ratio in muscle (8). Potentially, a relationship may exist between the concentration of free Cr available and the amount of Cr that will be phosphorylated to PCr. Harris et al. (6) did not express PCr content relative to TCr content in individual subjects, but he did report that despite the increase in mean PCr content which occurred subsequent to Cr supplementation the PCr percentage of mean TCr content decreased from 66.8% before Cr feeding to 61% following feeding. Therefore, there was a much greater increase in free Cr within the muscle than in PCr. Furthermore, the effect of Cr supplementation was greatest in those subjects who had the lowest initial TCr content (6). Whether these subjects also had the greatest increase in PCr content is unknown, but this information may provide insight into the mechanisms of PCr regulation.
The CRE supplementation protocol used in the present study did not result in an increase in PCr content, and thus the question still remains as to whether an increase in muscle PCr would effect performance during a single bout of maximal exercise. Studies involving repeated bouts of maximal exercise following Cr ingestion have shown minimal changes in performance during the initial bout of exercise (4,7). Greenhaff et al.(4) used a protocol of intermittent isokinetic contractions (before and after Cr feeding) and found no difference between conditions during the initial 30 s of the first bout of exercise.
Cr supplementation may play a greater role in delaying fatigue during repeated bouts of maximal exercise (4,7). Greenhaff et al. (4) used a repeated trial design of 5 bouts of 30 voluntary isokinetic leg extensions with 60 s rest following the administration of 4 × 5 g·d-1 for 5 d. Their results indicated significantly greater muscle peak torque following Cr supplementation during the final 10 contractions of bout 1, throughout the whole of bouts 2, 3, 4, and during contractions 11-20 in bout 5.
Similar findings were reported by Harris et al. (7) who found a significant reduction in running time over the final 300 m of a 4X300-m run (3-min rest intervals), the final 1000 m of a 4X1000 m run (4-min rest intervals), and the total 4X1000 m following CRE supplementation in trained middle distance runners, with running times unchanged in the PLA group. Although muscle was not analyzed for TCr or PCr in either of these studies, the Cr supplementation protocol spanned 5 or 6 d as in the study described by Harris et al. (6) which resulted in increased PCr concentration. It was suggested that the delay in fatigue reported following Cr ingestion was a consequence of accelerated PCr resynthesis owing to increased Cr availability (4). The subjects in these studies, however, were not matched for pretest scores and because a crossover protocol was not used, the difference in the post-test measures might be solely a result of the experimental design.
To clarify whether PCr resynthesis is accelerated following Cr feeding, muscle biopsies (vastus lateralis) or 31P-NMR (nuclear magnetic resonance) spectra (anterior tibialis muscle) were collected immediately following, 1 min-post, and 2 min-post intense electrical stimulation(3). Although no significant differences were found in the NMR data, after Cr ingestion PCr in biopsy samples was 20% higher 2 min post-exercise. Therefore, the possibility that oral Cr supplementation may accelerate the rate of muscle PCr resynthesis following intense contraction deserves further investigation.
The results of the present study indicated that 3 d of Cr ingestion did not increase the resting PCr/ATP ratio in spite of increasing the TCr/ATP ratio and had no effect on performance during a single short-term maximum cycling task. Further research is required to investigate the role of Cr supplementation and the effect of increased PCr on performance during maximal exercise.
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CREATINE LOADING; WINGATE TEST; NEEDLE BIOPSY
©1997The American College of Sports Medicine
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