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

Purpose: During constant-rate high-intensity exercise, a steady state for o xygen uptake ((V) over dot O-2) is not achieved and, after the initial rapi d increase, (V) over dot O-2 continues to increase slowly. The mechanism un derlying 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 dete rmine whether there is an association between the (V) over dot O-2 slow com ponent and muscle use assessed by contrast shifts in magnetic resonance ima ges (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 pe rformed at high and low intensities. EMG and MRI transverse relaxation time s (T2) were obtained after 3 and 15 min to determine muscle activity at eac h intensity. Results: Low-intensity cycling produced no (V) over dot O-2 sl ow component, and no increases in muscle activity, except for a small incre ase (P < 0.05) in the T2 of the vastus lateralis. During high-intensity cyc ling, (V) over dot O-2, T2 of the vastus lateralis, rectus femoris and whol e 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) ov er 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 conclu de that increased muscle use is in part responsible for the slow component rise in oxygen uptake. The results support the hypothesis that during const ant-rate exercise at intensities above lactate threshold, progressively gre ater use of fast-twitch motor units increases energy demand and causes conc omitant progressive increases in (V) over dot O-2 and lactate.