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]/VO(2) ratio was calculated as the average Δ[HHb]/VO(2) 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]/VO(2) 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 VO(2) functional gain (G; ΔVO(2)/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]/VO(2) 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 O(2) delivery limitation; and (3) that increasing Mod WR increments elicits an increased functional G, regardless of the τVO(2p) response.