Muscle deoxygenation to VO₂ relationship differs in young subjects with varying τVO₂.

The relationship between the adjustment of muscle deoxygenation (∆[HHb]) and phase II VO(2p) was examined in subjects presenting with a range of slow to fast VO(2p) kinetics. Moderate intensity VO(2p) and ∆[HHb] kinetics were examined in 37 young males (24 ± 4 years). VO(2p) was measured breath-by-breath. Changes in ∆[HHb] of the vastus lateralis muscle were measured by near-infrared spectroscopy. VO(2p) and ∆[HHb] response profiles were fit using a mono-exponential model, and scaled to a relative % of the response (0-100%). The ∆[HHb]/∆VO(2p) ratio for each individual (reflecting the matching of O(2) distribution to O(2) utilization) was calculated as the average ∆[HHb]/∆VO(2p) response from 20 to 120 s during the exercise on-transient. Subjects were grouped based on individual phase II VO(2p) time-constant (τVO(2p)): <21 s [very fast (VF)]; 21-30 s [fast (F)]; 31-40 s [moderate (M)]; >41 s [slow (S)]. The corresponding ∆[HHb]/∆VO(2p) were 0.98 (VF), 1.05 (F), 1.09 (M), and 1.22 (S). The larger ∆[HHb]/∆VO(2p) in the groups with slower VO(2p) kinetics resulted in the ∆[HHb]/∆VO(2p) displaying a transient "overshoot" relative to the subsequent steady state level, which was progressively reduced as τVO(2) became smaller (r = 0.91). When τVO(2p) > ~20 s, the rate of adjustment of phase II VO(2p) appears to be mainly constrained by the matching of local O(2) distribution to muscle VO(2). These data suggest that in subjects with "slower" VO(2) kinetics, the rate of adjustment of VO(2) may be constrained by O(2) availability within the active tissues related to the matching of microvascular O(2) distribution to muscle O(2) utilization.