High-intensity exercise and muscle glycogen availability in humans

This study investigated the effects of muscle glycogen availability on perf ormance and selected physiological and metabolic responses during high-inte nsity intermittent exercise. Seven male subjects completed a regimen of exe rcise and dietary intake (48 h) to either lower and keep low (LOW-CHO) or l ower and then increase (HIGH-CHO) muscle glycogen stores, on two separate o ccasions at least a week apart. On each occasion the subjects completed a s hort-term (<10 min) and prolonged (>30 min) intermittent exercise (IEX) pro tocol, 24 h apart, which consisted of 6-s bouts of high-intensity exercise performed at 30-s intervals on a cycle ergometer. Glycogen concentration (m ean +/- SEM) in m, vastus lateralis before both IEx(short) and IEx(long) wa s 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 IE x(short) and IEx(long), significantly less work was performed following LOW -CHO compared with HIGH-CHO. In IEx(long), the number of exercise bouts tha t could be completed at a pre-determined target exercise intensity increase d by 265% from 111 (14) following LOW-CHO to 294 (29) following HIGH-CHO (P < 0.05). At the point of fatigue in IEx(long), glycogen concentration was significantly lower with the LOW-CHO compared with HIGH-CHO [58 (25) vs. 18 1 (46) mmol kg (dw)(-1), respectively]. The plasma concentrations of adrena line and nor-adrenaline (in IEx(short) and IEx(long)), and FFA and glycerol (in IEx(long)), increased several-fold above resting values with both expe rimental conditions. Oxygen uptake during the exercise periods in IEx(long) approached 70% of Vo(2max) These results suggest that muscle glycogen avai lability can affect performance during both short-term and more prolonged h igh-intensity intermittent exercise and that with repeated exercise periods as short as 6 s, there can be a relatively high aerobic contribution.