Gradual increase in leg oxygen uptake during repeated submaximal contractions in humans

We examine whether muscle oxygen consumption (VO2) increases gradually during repeated submaximal isometric contractions. Six subjects made two-legged isometric quadriceps contractions at 30% maximal voluntary contraction for 6 s with 4 s of rest between until exhaustion (58 +/- 8 min). Blood samples were taken from the femoral vein and artery, and blood velocity was recorded by ultrasound-Doppler technique in the femoral artery. Blood flow was calculated from velocity and artery diameter values. Leg VO2 increased sixfold within the 1st min of exercise. A further doubling of the VO2 was seen during the remainder of the exercise, reaching 307 +/- 22 ml/min at exhaustion. This latter increase was due to a 54% increase in blood flow and a 34% increase in oxygen extraction. After 20 min of recovery VO2 was still 75% higher than preexercise values. The results show a twofold increase in energy demand of the working muscle during repeated constant-force isometric contractions. The increased energy cost of contraction is probably localized at the cellular level, and it parallels fatigue determined as decreased force-generating capacity.