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 (VO2p) and near-infrared spectroscopy (NIRS)-derived muscle deoxygenation (Delta[HHb]) during moderate-intensity (Mod) cycling. Fourteen males (24 +/- A 5 years) each completed four to eight repetitions of Mod transitions from 20 to 50, 70, 90, 110 and 130 W. VO2p and Delta[HHb] responses were modelled as a mono-exponential; responses were then scaled to a relative % of the respective response (0-100 %). The Delta[HHb]/VO2 ratio was calculated as the average Delta[HHb]/VO2 during the 20-120 s period of the on-transient. When considered as a single group, neither the phase II VO2p time constant (tau VO2p; 27 +/- A 9, 26 +/- A 11, 25 +/- A 10, 27 +/- A 14, 29 +/- A 13 s for 50-130 W transitions, respectively) nor the Delta[HHb]/VO2 ratio (1.04 +/- A 0.13, 1.10 +/- A 0.13, 1.08 +/- A 0.07, 1.09 +/- A 0.11, 1.09 +/- A 0.09, respectively) was affected by WR (p > 0.05); yet, the VO2 functional gain (G; Delta VO2/Delta WR) increased with increasing WR transitions (8.6 +/- A 1.3, 9.1 +/- A 1.2, 9.5 +/- A 1.0, 9.5 +/- A 1.0, 9.9 +/- A 1.0 mL min(-1) W-1; p < 0.05). When subjects were stratified into two groups [Fast (n = 6), tau VO2p130W < 25 s < tau VO2p130W, Slower (n = 8)], a group by WR interaction was observed for tau VO2p. The increasing functional G persisted (p < 0.05) and did not differ between groups (p > 0.05). The Delta[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 tau VO2p; (2) that tau VO2p 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 tau VO2p response.