The possible contribution of muscular work to the increase in oxygen uptake ( VO(2)) over time during running was investigated on 11 adult males who were asked to run until exhaustion at 90 (3)% [mean (SD)] of their maximal aerobic velocity on a treadmill ergometer. Ground reaction forces, expired gases and EMG from leg muscles were collected for 30 s at min 3 and during the last minute of the run. Subjects ran for 829 (165) s and showed an increase in VO(2 )of 179 (93) ml.min(-1) between min 3 and exhaustion. Increased ventilation explained 41 (27)% of the increase in VO(2). Stride frequency slightly decreased but no significant differences were found in the mechanical work or in integrated EMG. It was concluded that, in running, the increase in VO(2 )could not be related to a drift in muscle work.