Influence of muscle fiber type and pedal frequency on oxygen uptake kinetics of heavy exercise

We tested the hypothesis that the amplitude of the additional slow component of O-2 uptake (V over dot O-2) during heavy exercise is correlated with the percentage of type II (fast-twitch) fibers in the contracting muscles. Ten subjects performed transitions to a work rate calculated to require a V over dot O-2 equal to 50% between the estimated lactate (Lac) threshold and maximal V over dot O-2 (50%Delta). Nine subjects consented to a muscle biopsy of the vastus lateralis. To enhance the influence of differences in fiber type among subjects, transitions were made while subjects were pedaling at 45, 60, 75, and 90 rpm in different trials. Baseline V over dot O-2 was designed to be similar at the different pedal rates by adjusting baseline work rate while the absolute increase in work. rate above the baseline was the same. The V over dot O-2 response after the onset of exercise was described by a three-exponential model. The relative magnitude of the slow component at the end of 8-min exercise was significantly negatively correlated with %type I fibers at every pedal rate (r = 0.64 to 0.83, P < 0.05-0.01). Furthermore, the gain of the fast component for V over dot O-2 (as ml . min(-1). W-1) was positively correlated with the %type I fibers across pedal rates (r = 0.69-0.83). Increase in pedal rate was associated with decreased relative stress of the exercise but did not affect the relationships between %fiber type and V over dot O-2 parameters. The relative contribution of the slow component was also significantly negatively correlated with maximal V over dot O-2 (r = -0.65), whereas the gain for the fast component was positively associated (r = 0.68-0.71 across rpm). The amplitude of the slow component was significantly correlated with net end-exercise Lac at all four pedal rates (r = 0.64-0.84), but Lac was not correlated with %type I (P > 0.05). We conclude that fiber type distribution significantly affects both the fast and slow components of V over dot O-2 during heavy exercise and that fiber type and fitness may have both codependent and independent influences on the metabolic and gas-exchange responses to heavy exercise.