Speeding of VO₂ kinetics during moderate-intensity exercise subsequent to heavy-intensity exercise is associated with improved local O ₂ distribution

The relationship between the adjustment of muscle deoxygenation (Δ[HHb]) and phase II V_O2p during moderate-intensity exercise was examined before (Mod 1) and after (Mod 2) a bout of heavy-intensity "priming" exercise. Moderate intensity V_O2p and Δ [HHb] kinetics were determined in 18 young males (26 ± 3 yr). V _O2p was measured breath-by-breath. Changes in Δ [HHb] of the vastus lateralis muscle were measured by near-infrared spectroscopy. V _O2p and Δ [HHb] response profiles were fit using a monoexponential model, and scaled to a relative % of the response (0 -100%). The Δ [HHb]/V? O₂ ratio for each individual (reflecting the local matching of O₂ delivery to O₂ utilization) was calculated as the average Δ [HHb]/ V ? O₂ response from 20 s to 120 s during the exercise on-transient. Phase II τV_O2p was reduced in Mod 2 compared with Mod 1 (P < 0.05). The effective τ' Δ [HHb] remained the same in Mod 1 and Mod 2 (P > 0.05). During Mod 1, there was an "overshoot" in the Δ [HHb]/V? O₂ ratio (1.08; P > 0.05) that was not present during Mod 2 (1.01; P < 0.05). There was a positive correlation between the reduction in the Δ [HHb]/V? O₂ ratio and the smaller τV_O2p from Mod 1 to Mod 2 (r = 0.78; P < 0.05). This study showed that a smaller τV_O2p during a moderate bout of exercise subsequent to a heavy-intensity priming exercise was associated with improved microvascular O₂ delivery during the on-transient of exercise, as suggested by a smaller Δ [HHb]/VO₂ ratio.