Effect of exercise modality on oxygen uptake kinetics during heavy exercise.

The mechanisms responsible for the oxygen uptake (VO2) slow component during high-intensity exercise have yet to be established. In order to explore the possibility that the VO2 slow component is related to the muscle contraction regimen used, we examined the pulmonary VO2 kinetics during constant-load treadmill and cycle exercise at an exercise intensity that produced the same level of lactacidaemia for both exercise modes. Eight healthy subjects, aged 22-37 years, completed incremental exercise tests to exhaustion on both a cycle ergometer and a treadmill for the determination of the ventilatory threshold (defined as the lactate threshold, Th1a) and maximum VO2 (VO2max). Subsequently, the subjects completed two "square-wave" transitions from rest to a running speed or power output that required a VO2 that was halfway between the mode-specific Th1a and VO2max. Arterialised blood lactate concentration was determined immediately before and after each transition. The VO2 responses to the two transitions for each exercise mode were time-aligned and averaged. The increase in blood lactate concentration produced by the transitions was not significantly different between cycling [mean (SD) 5.9 (1.5) mM] and running [5.5 (1.6) mM]. The increase in VO2 between 3 and 6 min of exercise; (i.e. the slow component) was significantly greater in cycling than in running, both in absolute terms [290 (102) vs 200 (45) ml x min(-1); P<0.05] and as a proportion of the total VO2 response above baseline [10 (3)% vs 6 (1)%; P < 0.05]. These data indicate that: (a) a VO2 slow component does exist for high-intensity treadmill running, and (b) the magnitude of the slow component is less for running than for cycling at equivalent levels of lactacidaemia. The greater slow component observed in cycling compared to running may be related to differences in the muscle contraction regimen that is required for the two exercise modes.