THE SLOW COMPONENT OF O2 UPTAKE KINETICS DURING HEAVY EXERCISE IN YOUNG AND OLDER MEN

Abstract

Previous studies examining the effect of aging on O2 uptake kinetics have focused on the transition to light- or moderate-intensity exercise. Age-related differences in O2 uptake kinetics during heavy exercise (above anaerobic threshold) have yet to be examined. PURPOSE To examine the effect of aging on on-transient kinetics, and the relationship between muscle electromyography (EMG) activity and the slow component of O2 uptake kinetics during heavy exercise. METHODS Nine young (21.7±0.9 yr) and 9 older (71.6±0.8 yr) men performed a 7 min square-wave bout of exercise on a cycle ergometer at an intensity corresponding to 50% of the difference between the power output achieved at the anaerobic threshold and that attained at peak O2 uptake. Oxygen uptake and surface EMG activity of the right vastus lateralis muscle were measured during constant-load exercise. The O2 uptake data were fit with a 2-term exponential model to determine O2 uptake kinetics and the slow component. RESULTS The time constant (τ) of the primary component was significantly longer in the older (42±4 s) compared to the young (27±4 s) group. In contrast, the τ for the slow component was not different between the two groups, although the onset of the slow component occurred ∼ 60 s earlier in the young than in the older group. The absolute amplitude of the primary and slow components were greater in the young compared to older subjects. The slow component expressed relative to the total O2 uptake response was not different between the two groups (young: 24±2%; older: 20±3%). The mean power frequency (MPF) of the EMG increased significantly during the slow component phase of exercise by 6.4±1.0% in the young and by 5.4±0.7% in the older group. The increase in MPF was not significantly different between the two groups. CONCLUSION On-transient O2 uptake kinetics during heavy exercise are slowed, whereas the relative slow component is unaltered with increasing age. The EMG results are consistent with the hypothesis that the slow component of O2 uptake kinetics is caused in part by increased recruitment of fast-twitch muscle fibers.