Muscle oxygen kinetics at onset of intense dynamic exercise in humans

The present study examined the onset and the rate of rise of muscle oxidati on during intense exercise in humans and whether oxygen availability limits muscle oxygen uptake in the initial phase of intense exercise. Six subject s performed 3 min of intense one-legged knee-extensor exercise [65.3 +/- 3. 7 (means +/- SE) W]. The femoral arteriovenous blood mean transit time (MTT ) and time from femoral artery to muscle microcirculation was determined to allow for an examination of the oxygen uptake at capillary level. MTT was 15.3 +/- 1.8 s immediately before exercise, 10.4 +/- 0.7 s after 6 s of exe rcise, and 4.7 +/- 0.5 s at the end of exercise. Arterial venous O-2 differ ence (a-v(diff) O-2) of 186 +/- 5 ml/l before the exercise was unchanged af ter 2 s, but it increased (P < 0.05) after 6 s of exercise to 43 +/- 10 ml/ l and reached 146 +/- 4 ml/l at the end of exercise. Thigh oxygen uptake in creased (P < 0.05) from 32 +/- 8 to 102 +/- 28 ml/min after 6 s of exercise and to 789 +/- 88 ml/min at the end of exercise. The time to reach half-pe ak a-v(diff) O-2 and thigh oxygen uptake was 13 +/- 2 and 25 +/- 3 s, respe ctively. The difference between thigh oxygen delivery (blood flow 3 arteria l oxygen content) and thigh oxygen uptake increased (P < 0.05) after 6 s an d returned to preexercise level after 14 s. The present data suggest that, at the onset of exercise, oxygen uptake of the exercising muscles increases after a delay of only a few seconds, and oxygen extraction peaks after sim ilar to 50 s of exercise. The limited oxygen utilization in the initial pha se of intense exercise is not caused by insufficient oxygen availability.