High-intensity exercise and muscle glycogen availability in humans

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 (<10 min) and prolonged (>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) in m. vastus lateralis before both IEx_short and IEx_long was 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 IEx_short and IEx_long, significantly less work was performed following LOW-CHO compared with HIGH-CHO. In IEx_long, 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 IEx_long, 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 IEx_short and IEx_long), and FFA and glycerol (in IEx_long), increased several-fold above resting values with both experimental conditions. Oxygen uptake during the exercise periods in IEx_long approached 70% of V o_2max. 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.