Effects of prolonged cycle ergometer exercise on maximal muscle power and oxygen uptake in humans

The mechanical power (W(tot), W . kg-1) developed during ten revolutions of all-out periods of cycle ergometer exercise (4-9 s) was measured every 5-6 min in six subjects from rest or from a baseline of constant aerobic exercise [50%-80% of maximal oxygen uptake (VO2max)] of 20-40 min duration. The oxygen uptake [VO2 (W . kg-1, 1 ml O2 = 20.9 J)] and venous blood lactate concentration ([1a]b, mM) were also measured every 15 s and 2 min, respectively. During the first all-out period, W(tot) decreased linearly with the intensity of the priming exercise (W(tot) = 11.9-0.25 . VO2). After the first all-out period (t greater than 5-6 min), and if the exercise intensity was less than 60% VO2max, W(tot), VO2 and [la]b remained constant until the end of the exercise. For exercise intensities greater than 60% VO2max, VO2 and [la]b showed continuous upward drifts and W(tot) continued decreasing. Under these conditions, the rate of decrease of W(tot) was linearly related to the rate of increase of VO2 [(d W(tot)/dt) (W . kg-1 . s-1) = 5.0 . 10(-5) -0.20 . (d VO2/dt) (W . kg-1 . s-1)] and this was linearly related to the rate of increase of [1a]b [(d VO2/dt) (W . kg-1 . s-1) = 2.3 . 10(-4) + 5.9 . 10(-5) . (d[1a]b/dt) (mM . s-1)]. These findings would suggest that the decrease of W(tot) during the first all-out period was due to the decay of phosphocreatine concentration in the exercising muscles occurring at the onset of exercise and the slow drifts of VO2 (upwards) and of W(tot) (downwards) during intense exercise at constant W(tot) could be attributed to the continuous accumulation of lactate in the blood (and in the working muscles)