In this experiment we studied the effect of different pedalling rates during cycling at a constant power output (PO) 132 +/- 31 W (mean +/- S.D.), corresponding to 50% Vo(2) max, on the oxygen uptake and the magnitude of the slow component of Vo(2) kinetics in humans. The PO corresponded to 50% of Vo(2) max, established during incremental cycling at a pedalling rate of 70 rev.min(-1). Six healthy men aged 22.2 +/- 2.0 years with Vo(2) max 3.89 +/- 0.92 l.min(-1), performed on separate days constant PO cycling exercise lasting 6 min at pedalling rates 40, 60, 80, 100 and 120 rev.min(-1), in random order. Antecubital blood samples for plasma lactate [La](pl) and blood acid-base balance variables were taken at 1 min intervals. Oxygen uptake was determined breath-by-breath, The total net oxygen consumed throughout the 6 min cycling period at pedalling rates of 40, 60, 80, 100 and 120 rev.min-(1) amounted to 7.727 +/- 1.197, 7.705 +/- 1.548, 8.679 +/- 1.262, 9.945 +/- 1.435 and 13.720 +/- 1.862 l, respectively for each pedalling rate. The Vo(2) during the 6 min of cycling only rose slowly by increasing the pedalling rate in the range of 40-100 rev.min(-1). This increase, was 0.142 l per 20 rev.min(-1) on the average. Plasma lactate concentration during the sixth minute of cycling changed little within this range of pedalling rates: the values were 1.83 +/- 0.70, 1.80 +/- 0.48, 2.33 +/- 0.88 and 2.52 +/- 0.33 mmol.l(-1). The values of [La](pl) reached in the 6th minute of cycling were not significantly different from the pre-exercise levels. Blood pH was also not affected by the increase of pedalling rate in the range of 40-100 rev.min(-1). However, an increase of pedalling rate from 100 to 120 rev.min(-1) caused a sudden increase in the Vo(2) amounting to 0.747 l per 20 rev.min(-1), accompanied by a significant increase in [La](pl) from 1.21 +/- 0.26 mmol.l(-1) in pre-exercise conditions to 5.92 +/- 2.46 mmol.l(-1) reached in the 6th minute of cycling (P < 0.01). This was also accompanied by a significant drop of blood pH, from 7.355 +/- 0.039 in the pre-exercise period to 7.296 +/- 0.060 in the 6th minute of cycling (P < 0.01). The mechanical efficiency calculated on the basis of the net Vo(2) reached between the 4th and the 6th minute of cycling amounted to 26.6 +/- 2.7, 26.4 +/- 2.0, 23.4 +/- 3.4, 20.3 +/- 2.6 and 14.7 +/- 2.2%, respectively for pedalling rates of 40, 60, 80, 100 and 120 rev.min(-1). No significant increase in the Vo(2) from the 3rd to the 6th min (representing the magnitude of the slow component of Vo(2) kinetics) was observed at any of the pedalling rates (-0.022 +/- 0.056, -0.009 +/- 0.029, 0.012 +/- 0.073, 0.030 +/- 0.081 and 0.122 +/- 0.176 Lmin(-1) for pedalling rates of 40, 60, 80, 100 and 120 rev.min(-1), respectively). Thus a significant increase in [La](pl) and a decrease in blood pH do not play a major role in the mechanism(s) responsible for the slow component of Vo(2) kinetics in humans.
