Glycogen and lactate metabolism during low-intensity exercise in man

The influence of high lactate concentration on glycogen metabolism in active type I and inactive type II fibres was investigated. High muscle lactate concentration (26.7 +/- 1.4 mmol kg-1 wet wt) was achieved by three bouts (2 min duration) of bicycle exercise at 112% Vo2 max. Exercise was continued at 40% Vo2 max for 1 h. Serial venous blood samples and biopsies from the vastus lateralis muscle were taken. Over the first 20 min of this low-intensity exercise muscle lactate concentration decreased by 22.9 +/- 0.7 mmol kg-1 wet wt, while glycogen remained unchanged in type I fibres and increased by 20 mmol kg-1 wet wt in type II fibres. During the next 40 min of low-intensity exercise lactate decreased by 1.6 +/- 1.2 mmol kg-1 wet wt, while glycogen concentration decreased by 21 +/- 7 mmol kg-1 wet wt in type I fibres but remained stable in type II fibres. In a second series of experiments, in which lactate was allowed to disappear before the light exercise was started, no changes in glycogen concentration were seen in type II fibres during the 1 h of 40% Vo2 max exercise, while a continuous reduction in glycogen of 28 +/- 8 mmol kg-1 wet wt was found in type I fibres. The results indicate that in the presence of high lactate levels muscle glycogen was resynthesized in inactive type II muscle fibres, while lactate was oxidized in preference to glycogen in type I fibres.