An explanation of the upward drift in oxygen uptake during prolonged sub-maximal downhill running

The upward drift in oxygen uptake (UDO) that occurs during some prolonged sub-maximal exercises involving eccentric muscle action has received only scant attention in the literature. Those papers reporting UDO have generally used bicycling on an ergometer modified for eccentric exercise as the experimental activity. The purpose of this study was to quantify and propose an explanation for UDO during a different activity involving eccentric muscle action--downhill running. Oxygen consumption, quadriceps muscle electromyography, and stride length were collected from 10 experienced male runners (mean maximal oxygen consumption = 65.0 +/- 5.2 ml X kg X min-1) during two 40-min sub-maximal runs at 3.83 m X s-1. The first run was on a level grade and required 66% of the subjects' mean maximal oxygen consumption. The second run, completed 2 d later, was performed on a 10% downhill grade and elicited 44% of the mean maximal oxygen consumption. Results of the level run showed non-significant changes in oxygen consumption, rectified integrated electromyography, and stride length between minutes 10 and 40. Significant (P less than 0.05) increases in oxygen consumption (10%) and integrated electromyography (23%) were found across the same time period of the downhill run. No significant changes in downhill stride length were observed. Delayed-onset muscle soreness, a presumed indication of muscle damage, occurred only following the downhill run. It is hypothesized that UDO and increasing integrated electromyography during downhill running reflected increased motor unit recruitment within the eccentrically acting muscles caused by an ongoing combination of muscle damage, connective tissue damage, and local muscle fatigue.