Muscle activation and the slow component rise in oxygen uptake during cycling

Purpose: During constant-rate high-intensity exercise, a steady state for oxygen uptake (VO₂) is not achieved and, after the initial rapid increase, VO₂ continues to increase slowly. The mechanism underlying the slow-component rise in VO₂ during high-intensity exercise is unknown. It has been hypothesized that increased muscle use may be a contributing factor, but only limited electromyograph (EMG) data are available supporting this hypothesis. The purpose of this study was to determine whether there is an association between the VO₂ slow component and muscle use assessed by contrast shifts in magnetic resonance images (magnetic resonance imaging (MRI)). Methods: The VO₂ slow component was measured in 16 subjects during two 15-min bouts of cycling performed at high and low intensities. EMG and MRI transverse relaxation times (T2) were obtained after 3 and 15 min to determine muscle activity at each intensity. Results: Low-intensity cycling produced no VO₂ slow component, and no increases in muscle activity, except for a small increase (P < 0.05) in the T2 of the vastus lateralis. During high-intensity cycling, VO₂, T2 of the vastus lateralis, rectus femoris and whole leg, and EMG activity and median power frequency of the vastus lateralis rose significantly (P < 0.05) from 3 to 15 min. Percent increases in VO₂ and muscle T2 were related during high-intensity cycling (r = 0.63), but not during low-intensity cycling (r = 0.00). Conclusion: We conclude that increased muscle use is in part responsible for the slow component rise in oxygen uptake. The results support the hypothesis that during constant-rate exercise at intensities above lactate threshold, progressively greater use of fast-twitch motor units increases energy demand and causes concomitant progressive increases in VO₂ and lactate.