Muscle glycogen depletion during exercise at 9? C and 21? C

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Abstract
This study compared glycogen depletion in active skeletal muscle after light and moderate exercise in both cold and comfortable ambient conditions. Twelve male subjects (Ss) were divided into two groups equally matched for the submaximal exercise intensity corresponding to a blood lactate concentration of 4 mM (W4) during cycle exercise. On two separate days Ss rested for 30 min at ambient temperatures of either 9‡ C or 21‡ C, with the order of temperature exposure being counter-balanced among Ss. Following rest a tissue specimen was obtained from the m. vastus lateralis with the needle biopsy technique. Six Ss then exercised on a cycle ergometer for 30 min at 30% W4 (range=50–65 W) while the remaining group exercised at 60% W4 (range=85–120 W). Another biopsy was taken immediately after exercise and both samples were assayed for glycogen content. Identical procedures were repeated for the second environmental exposure. No significant glycogen depletion was observed in the Ss exercising at 30% W4 in 21‡ C, but a 23% decrease (p=0.04) was observed when the same exercise was performed at 9‡ C. A 22% decrease (p=0.002) in glycogen occurred in the 60% W4 group at 21‡ C, which was not significantly different from that observed during the same exercise at 9‡ C. The results suggest that muscle substrate utilization is increased during light exercise in a cold environment as compared to similar exercise at a comfortable temperature, probably due to shivering thermogenesis. Heat produced with higher exercise intensities seems to be sufficient to prevent shivering and the accompanying glycogenolysis.