Effects of prolonged cycle ergometer exercise on maximal muscle power and oxygen uptake in humans
Summary 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 (VO 2max )] of 20–40 min duration. The oxygen uptake [VO 2 (W·kg −1 , 1 ml O 2 = 20.9 J)] and venous blood lactate concentration ([la] 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 = 1 11.9−0.25·VO 2 ). After the first all-out period ( i greater than 5–6 min), and if the exercise intensity was less than 60% VO 2max , W tot , VO 2 and [la] b remained constant until the end of the exercise. For exercise intensities greater than 60% VO 2max , VO 2 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 V [(d W tot /dt) (W·kg −1 ·s −1 ) = 5.0·10 −5 −0.20·(d VO 2 /dt) (W·kg −1 ·s −1 )] and this was linearly related to the rate of increase of [la] b [(d VO 2 /dt) (W·kg −1 ·s −1 ) = 2.310 −4 + 5.910 −5 ·(d [la] 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 VO 2 (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).
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