O-2 uptake kinetics during exercise at peak O-2 uptake

Compared with moderate- and heavy-intensity exercise, the adjustment of O-2 uptake ((V) over dot O-2) to exercise intensities that elicit peak (V) over dot O-2 has received relatively little attention. This study examined the (V) over dot O-2 response of 21 young, healthy subjects (25 +/- 6 yr; mean +/- SD) during cycle ergometer exercise to step transitions in work rate (WR) corresponding to 90, 100, and 110% of the peak WR achieved during a preliminary ramp protocol (15 - 30 W/min). Gas exchange was measured breath by breath and interpolated to 1-s values. (V) over dot O-2 kinetics were determined by use of a two- or three-component exponential model to isolate the time constant (tau2) as representative of (V) over dot O-2 kinetics and the amplitude (Amp) of the primary fast component independent of the appearance of any (V) over dot O-2 slow component. No difference in (V) over dot O-2 kinetics was observed between WRs (tau(90) = 24.7 +/- 9.0; tau(100) = 22.8 +/- 6.7; tau(110) = 21.5 +/- 9.2 s, where subscripts denote percent of peak WR; P > 0.05); nor in a subgroup of eight subjects was tau(2) different from the value for moderate-intensity (< lactate threshold) exercise (tau(2) = 25 +/- 12 s, P > 0.05). As expected, the Amp increased with increasing WRs (Amp(90) = 2,089 +/- 548; Amp(100) = 2,165 +/- 517; Amp(110) = 2,225 +/- 559 ml/min; Amp(90) vs. Amp(110), P < 0.05). However, the gain (G) of the (V) over dot O-2 response (Delta(V) over dot O-2/Delta WR) decreased with increasing WRs (G(90) = 8.5 +/- 0.6; G(100) = 7.9 +/- 0.6; G(110) = 7.3 +/- 0.6 ml center dot min(-1) center dot W-1; P < 0.05). The Amp of the primary component approximated 85, 88, and 89% of peak (V) over dot O-2 during 90, 100, and 110% WR transitions, respectively. The results of the present study demonstrate that, compared with moderate- and heavy-intensity exercise, the gain of the (V) over dot O-2 response (as Delta(V) over dot O-2/DeltaWR) is reduced for exercise transitions in the severe-intensity domain, but the approach to this gain is well described by a common time constant that is invariant across work intensities. The lower Delta(V) over dot O-2/DeltaWR may be due to an insufficient adjustment of the cardiovascular and/or pulmonary systems that determine O-2 delivery to the exercising muscles or due to recruitment of motor units with lower oxidative capacity, after the onset of exercise in the severe-intensity domain.