Muscle glycogen depletion alters oxygen uptake kinetics during heavy exercise

PURPOSE: To test the hypothesis that muscle fiber recruitment patterns influence the oxygen uptake (VO2) kinetic response, constant-load exercise was performed after glycogen depletion of specific fiber pools.

METHODS: After validation of protocols for the selective depletion of Type I and II muscle fibers, 19 subjects performed square-wave exercise at 80% VT (moderate) and at 50% of the difference between VT and VO2max (heavy) without any prior depleting exercise (CON), after HIGH (10 x 1-min exercise bouts at 120% VO2max), and after LOW (3 h of exercise at 30% VO2max) exercise.

RESULTS: Differences in VO2 kinetic parameters were only observed in heavy exercise AFTER HIGH: the VO2 primary component was higher (1.75 +/- 0.12 L x min) compared with CON (1.65 +/- 0.11 L x min, P < 0.05), and the VO2 slow component was lower (0.18 +/- 0.03 L x min) compared with CON (0.24 +/- 0.04 L x min, P < 0.05).

CONCLUSIONS: The results indicate that the VO2 response to heavy constant-load exercise can be altered by depletion of glycogen in the Type II muscle fibers, lending support to the theory that muscle fiber recruitment influences both the VO2 primary and slow component amplitudes during heavy intensity exercise.