Effects Of Low Muscle Glycogen On High-intensity Exercise Performance: A Placebo-controlled Study

Abstract

Muscle glycogen is the major fuel during high-intensity exercise (HIE) and large reductions can occur after relatively short durations; however, the relationship between low muscle glycogen and HIE performance has not been investigated in a placebo-controlled design. PURPOSE: To investigate the effects of low muscle glycogen content on repeated sprint ability (RSA), ratings of perceived exertion (RPE) during HIE and neuromuscular function using a double-blinded design. METHODS: Eighteen well-trained subjects (VO2max; 57 ± 5 mL·kg-1·min-1) performed glycogen-depleting cycling exercise consisting of three periods of 10x45 s at ~110% VO2max interspersed by 135 s of passive recovery between bouts and 15 min between periods. After the glycogen-depleting exercise subjects were randomly assigned to a low (LOW) or high (HIGH) carbohydrate dietary intake for 5 hours (n = 9 in each group). The diets consisted of fluid and small amounts of bars and was blinded to assessors and participants. At baseline, after each period and following the diet manipulation RSA (5x6 s sprints separated by 24 s of rest) was evaluated. Moreover, isometric knee-extensor function (maximal voluntary contractions, voluntary activation level and electrically induced low- (20 Hz) and high-frequency (50 Hz) force production) was assessed at baseline, after the exercise intervention and following the diet administration. At the same time points a 2-min fixed intensity (~90% VO2max) cycling protocol was performed with subsequent RPE measures. Finally, muscle biopsies and blood samples were obtained. RESULTS: The glycogen-depleting exercise followed by recovery in LOW and HIGH resulted in distinct glycogen concentrations (176 ± 99 vs. 292 ± 78 mmol·kg-1 dw, P < 0.01), but indifferent blood glucose concentrations (P > 0.05). This was accompanied by an impaired RSA only in LOW (7.8 ± 5.9% reduction relative to baseline, P < 0.05) coupled with a high rating of perceived exertion during HIE in LOW compared to HIGH (7.7 ± 1.2 vs. 5.6 ± 1.2 AU, P < 0.01). In contrast, no between-group difference was observed for any aspects of neuromuscular function (P > 0.05). CONCLUSIONS: Low muscle glycogen results in impaired RSA and increased RPE during HIE, whereas voluntary and electrically induced isometric knee-extensor force remains unaffected.