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

The relationship between the adjustment of muscle deoxygenation (a dagger[HHb]) and phase II VO2p was examined in subjects presenting with a range of slow to fast VO2p kinetics. Moderate intensity VO2p and a dagger[HHb] kinetics were examined in 37 young males (24 +/- A 4 years). VO2p was measured breath-by-breath. Changes in a dagger[HHb] of the vastus lateralis muscle were measured by near-infrared spectroscopy. VO2p and a dagger[HHb] response profiles were fit using a mono-exponential model, and scaled to a relative % of the response (0-100%). The a dagger[HHb]/a dagger VO2p ratio for each individual (reflecting the matching of O-2 distribution to O-2 utilization) was calculated as the average a dagger[HHb]/a dagger VO2p response from 20 to 120 s during the exercise on-transient. Subjects were grouped based on individual phase II VO2p time-constant (tau VO2p): < 21 s [very fast (VF)]; 21-30 s [fast (F)]; 31-40 s [moderate (M)]; > 41 s [slow (S)]. The corresponding a dagger[HHb]/a dagger VO2p were 0.98 (VF), 1.05 (F), 1.09 (M), and 1.22 (S). The larger a dagger[HHb]/a dagger VO2p in the groups with slower VO2p kinetics resulted in the a dagger[HHb]/a dagger VO2p displaying a transient "overshoot" relative to the subsequent steady state level, which was progressively reduced as tau VO2 became smaller (r = 0.91). When tau VO2p > similar to 20 s, the rate of adjustment of phase II VO2p appears to be mainly constrained by the matching of local O-2 distribution to muscle VO2. These data suggest that in subjects with "slower" VO2 kinetics, the rate of adjustment of VO2 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.