Effect of Glycogen Depletion on the Oxygen Uptake Slow Component in Humans

Abstract Previous studies have indicated that the V·O 2 slow component is related to the recruitment of type II muscle fibres. We therefore hypothesised that an exercise and dietary regimen designed to deplete type I muscle fibres of glycogen would result in a greater contribution of type II muscle fibres to the exercise power output and therefore a larger amplitude of the V·O 2 slow component. Eight male subjects took part in this study. On day 1, the subjects reported to the laboratory at 8 a.m., and completed a 9 min constant-load cycling test at a work rate equivalent to 85 % V·O 2 peak. On day 2 at 12 p.m., the subjects were fed a 4200 kJ meal (60 % protein, 40 % fat); at 6 p.m. they completed a 2 h cycling test at 60 % V·O 2 peak. On day 3 at 8 a.m., the subjects performed an exercise test identical to that of day 1. Metabolic and respiratory measurements indicated that our experimental design was effective in reducing the muscle glycogen content. V·O 2 was significantly higher (by approximately 140 ml · min -1 ) throughout exercise following glycogen depletion but no appreciable changes in V·O 2 kinetics were found: neither the time constant of the primary response (from 35.4 ± 2.5 to 33.2 ± 4.4 s) nor the amplitude of the slow component (from 404 ± 95 to 376 ± 81 ml · min -1 ) was significantly altered. Therefore, we suggest that the increased V·O 2 throughout exercise and the unaltered V·O 2 slow component following glycogen depletion might be explained by a shift towards a greater reliance on fat metabolism in type I muscle fibres with no appreciable change in fibre type recruitment patterns.