High‐intensity exercise and muscle glycogen availability in humans

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Abstract
This study investigated the effects of muscle glycogen availability on performance and selected physiological and metabolic responses during high‐intensity intermittent exercise. Seven male subjects completed a regimen of exercise and dietary intake (48 h) to either lower and keep low (LOW‐CHO) or lower and then increase (HIGH‐CHO) muscle glycogen stores, on two separate occasions at least a week apart. On each occasion the subjects completed a short‐term (30 min) intermittent exercise (IEX) protocol, 24 h apart, which consisted of 6‐s bouts of high‐intensity exercise performed at 30‐s intervals on a cycle ergometer. Glycogen concentration (mean ± SEM) inm. vastus lateralisbefore both IExshortand IExlongwas significantly lower following LOW‐CHO [180 (14), 181 (17) mmol kg (dw)–1] compared with HIGH‐CHO [397 (35), 540 (25) mmol kg (dw)–1]. In both IExshortand IExlong, significantly less work was performed following LOW‐CHO compared with HIGH‐CHO. In IExlong, the number of exercise bouts that could be completed at a pre‐determined target exercise intensity increased by 265% from 111 (14) following LOW‐CHO to 294 (29) following HIGH‐CHO (P < 0.05). At the point of fatigue in IExlong, glycogen concentration was significantly lower with the LOW‐CHO compared with HIGH‐CHO [58 (25) vs. 181 (46) mmol kg (dw)–1, respectively]. The plasma concentrations of adrenaline and nor‐adrenaline (in IExshortand IExlong), and FFA and glycerol (in IExlong), increased several‐fold above resting values with both experimental conditions. Oxygen uptake during the exercise periods in IExlongapproached 70% ofVo2max. These results suggest that muscle glycogen availability can affect performance during both short‐term and more prolonged high‐intensity intermittent exercise and that with repeated exercise periods as short as 6 s, there can be a relatively high aerobic contribution.