During constant-load submaximal exercise above the ventilatory threshold (VT), the fundamental response of pulmonary oxygen uptake (V_O2) is supplemented by a slow component (SC) that causes it to rise above the anticipated steady-state value. It has been suggested that the V_O2 SC is related to the recruitment of type II fibres at high-exercise intensities (Barstow et al. 1996; Pringle et al. 2002). There is evidence that the recruitment of type II motor units is enhanced at high pedal rate (Sargeant, 1994). However, only few studies have examined the V_O2 kinetic responses to heavy exercise at different pedal rates and these have used a limited range of pedal rates (Barstow et al. 1996). In the present study, we manipulated pedal rate during heavy exercise in order to test the hypothesis that the V_O2 SC was related to the recruitment of type II muscle fibres.

Ten recreationally active subjects (8 male, 2 female, mean ± S.D., age 26 ± 4 years; mass 71.5 ± 7.9 kg) volunteered to participate in this study that was approved by the Manchester Metropolitan University ethics committee. The subjects completed three separate incremental exercise tests at 35, 75 and 115 rev min^-1, on an electrically braked cycle ergometer to determine the VT and peak V_O2 from breath-by-breath pulmonary gas exchange responses. Subsequently, the subjects performed two transitions of 6 min duration at each pedal rate at an intensity equivalent to half way between the pedal rate-specific VT and peak V_O2. The power output was adjusted during the baseline cycling period at 35 and 75 rev min^-1 in order that the V_O2 was equivalent to that during unloaded cycling at 115 rev min^-1. For each of the three conditions, breath-by-breath V_O2 data were interpolated, time-aligned and ensemble averaged, and then modelled using non-linear regression techniques to determine the amplitude of the V_O2 primary and slow components. ANOVA with Bonferroni adjusted paired t tests were used to test for differences across pedal rates. Results are reported as means ± S.E.M.

The temporal aspects of the V_O2 kinetic responses and the absolute V_O2 at the end of the primary component were not significantly different across the pedal rates. The gain (ΔV_O2/ΔWR) of the V_O2 primary component fell as pedal rate increased (10.6 ± 0.3 vs. 9.5 ± 0.2 vs. 9.0 ± 0.4 ml min^-1 W^-1 at 35, 75 and 115 rev min^-1, respectively; P < 0.05 for 75 and 115 vs. 35 rev min^-1). The amplitude of the V_O2 SC increased as pedal rate increased (109 ± 30 vs. 202 ± 38 vs. 328 ± 29 ml min^-1 at 35, 75 and 115 rev min^-1, respectively; P < 0.01 for 115 vs. 35 rev min^-1).

In conclusion, our results demonstrate that both the primary and slow components of V_O2 are affected by differences in pedal rate during heavy exercise. These effects are presumably mediated by altered motor unit recruitment patterns at the onset of exercise and the associated changes in these and in the rate of fatigue development and efficiency as exercise progresses.