Speeding of VO2 kinetics during moderate-intensity exercise subsequent to heavy-intensity exercise is associated with improved local O2 distribution.

The relationship between the adjustment of muscle deoxygenation (Δ[HHb]) and phase II V(O(2p)) during moderate-intensity exercise was examined before (Mod 1) and after (Mod 2) a bout of heavy-intensity "priming" exercise. Moderate intensity V(O(2p)) and Δ[HHb] kinetics were determined in 18 young males (26 ± 3 yr). V(O(2p)) was measured breath-by-breath. Changes in Δ[HHb] of the vastus lateralis muscle were measured by near-infrared spectroscopy. V(O(2p)) and Δ[HHb] response profiles were fit using a monoexponential model, and scaled to a relative % of the response (0-100%). The Δ[HHb]/Vo(2) ratio for each individual (reflecting the local matching of O(2) delivery to O(2) utilization) was calculated as the average Δ[HHb]/Vo(2) response from 20 s to 120 s during the exercise on-transient. Phase II τV(O(2p)) 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]/Vo(2) 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]/Vo(2) ratio and the smaller τV(O(2p)) from Mod 1 to Mod 2 (r = 0.78; P < 0.05). This study showed that a smaller τV(O(2p)) during a moderate bout of exercise subsequent to a heavy-intensity priming exercise was associated with improved microvascular O(2) delivery during the on-transient of exercise, as suggested by a smaller Δ[HHb]/Vo(2) ratio.