Effect of moderate-intensity work rate increment on phase II τVO₂, functional gain and Δ[HHb]

This study systematically examined the role of work rate (WR) increment on the kinetics of pulmonary oxygen uptake (VO_2p) and near-infrared spectroscopy (NIRS)-derived muscle deoxygenation (Δ[HHb]) during moderate-intensity (Mod) cycling. Fourteen males (24 ± 5 years) each completed four to eight repetitions of Mod transitions from 20 to 50, 70, 90, 110 and 130 W. VO_2p and Δ[HHb] responses were modelled as a mono-exponential; responses were then scaled to a relative % of the respective response (0–100 %). The Δ[HHb]/VO₂ ratio was calculated as the average Δ[HHb]/VO₂ during the 20–120 s period of the on-transient. When considered as a single group, neither the phase II VO_2p time constant (τVO_2p; 27 ± 9, 26 ± 11, 25 ± 10, 27 ± 14, 29 ± 13 s for 50–130 W transitions, respectively) nor the Δ[HHb]/VO₂ ratio (1.04 ± 0.13, 1.10 ± 0.13, 1.08 ± 0.07, 1.09 ± 0.11, 1.09 ± 0.09, respectively) was affected by WR (p > 0.05); yet, the VO₂ functional gain (G; ΔVO₂/ΔWR) increased with increasing WR transitions (8.6 ± 1.3, 9.1 ± 1.2, 9.5 ± 1.0, 9.5 ± 1.0, 9.9 ± 1.0 mL min⁻¹ W⁻¹; p < 0.05). When subjects were stratified into two groups [Fast (n = 6), τVO_2p130W < 25 s < τVO_2p130W, Slower (n = 8)], a group by WR interaction was observed for τVO_2p. The increasing functional G persisted (p < 0.05) and did not differ between groups (p > 0.05). The Δ[HHb]/VO₂ ratio was smaller (p < 0.05) in the Fast than Slower group, but was unaffected by WR. In conclusion, the present study demonstrated (1) a non-uniform effect of Mod WR increment on τVO_2p; (2) that τVO_2p in the Slower group is likely determined by an O₂ delivery limitation; and (3) that increasing Mod WR increments elicits an increased functional G, regardless of the τVO_2p response.