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 ((V) over dot O-2) is not achieved and, after the initial rapid increase, (V) over dot O-2 continues to increase slowly. The mechanism underlying the slow-component rise in (V) over dot O-2 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 (V) over dot O-2 slow component and muscle use assessed by contrast shifts in magnetic resonance images (magnetic resonance imaging (MRI)). Methods: The (V) over dot O-2 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 (V) over dot O-2 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, (V) over dot O-2, 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 (V) over dot O-2 and muscle T2 were related during high-intensity cycling (r = 0.63), bur 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 (V) over dot O-2 and lactate.