Glycogen breakdown in different human muscle fibre types during exhaustive exercise of short duration

The rates of glycogen breakdown during exhaustive intense exercise of three different intensities were determined in type I and subgroups of type II fibres. The exercise intensity corresponded to 122 +/- 2, 150 +/- 7 and 194 +/- 7% of VO2max. Muscle biopsies were taken from both legs before and immediately after exhaustion. Muscle lactate concentration increased by 27 +/- 1, 27 +/- 1 and 20 +/- 2 mmol kg-1 wet wt during the exercise at 122, 150 and 194% VO2max, respectively. The rates of glycogen depletion increased in all fibre types with increasing intensity, and the decline in type I fibres was 30-35% less than in type II fibres at all intensities. No differences were observed between the glycogen depletion rates in subgroups of type II fibres (IIA, IIAB and IIB). During the exercise at 194% VO2max, the rates of glycogen breakdown were 0.35 +/- 0.03 and 0.52 +/- 0.05 mmol s-1 kg-1 wet wt in type I and type II fibres, respectively. For both fibre types, the rates were 32 and 69% lower during the exercise at 150 and 122% VO2max. These data indicate that the glycolytic capacity of type I fibres is 30-35% lower than the capacity of type II fibres, in good agreement with the differences in phosphorylase and phosphofructokinase activities (Essén et al. 1975, Harris et al. 1976). The data also indicate that both fibre types contribute significantly to the anaerobic energy release at powers up till almost 200% VO2max.