ATP production and efficiency of human skeletal muscle during intense exercise: effect of previous exercise

The aim of the present study was to examine whether ATP production increase s and mechanical efficiency decreases during intense exercise and to evalua te how previous exercise affects ATP turnover during intense exercise. Six subjects performed two (EX1 and EX2) 3-min one-legged knee-extensor exercis e bouts [66.2 +/- 3.9 and 66.1 +/- 3.9 (+/-SE) W] separated by a 6-min rest period. Anaerobic ATP production, estimated from net changes in and releas e of metabolites from the active muscle, was 3.5 +/- 1.2, 2.4 +/- 0.6, and 1.4 +/- 0.2 mmol ATP.kg dry wt(-1).s(-1) during the first 5, next 10, and r emaining 165 s of EX1, respectively. The corresponding aerobic ATP producti on, determined from muscle oxygen uptake, was 0.7 +/- 0.1, 1.4 +/- 0.2, and 4.7 +/- 0.4 mmol ATP kg dry wt(-1).s(-1), respectively. The mean rate of A TP production during the first 5 s and next 10 s was lower (P < 0.05) than during the rest of the exercise (4.2 +/- 1.2 and 3.8 +/- 0.7 vs. 6.1 +/- 0. 3 mmol ATP kg dry wt(-1).s(-1)). Thus mechanical efficiency, expressed as w ork per ATP produced, was lowered (P < 0.05) in the last phase of exercise (39.6 +/- 6.1 and 40.7 +/- 5.8 vs. 25.0 +/- 1.3 J/mmol ATP). The anaerobic ATP production was lower (P < 0.05) in EX2 than in EX1, but the aerobic ATP turnover was higher (P < 0.05) in EX2 than in EX1, resulting in the same m uscle ATP production in EX1 and EX2. The present data suggest that the rate of ATP turnover increases during intense exercise at a constant work rate. Thus mechanical efficiency declines as intense exercise is continued. Furt hermore, when intense exercise is repeated, there is a shift toward greater aerobic energy contribution, but the total ATP turnover is not significant ly altered.