Muscle deoxygenation to VO ₂ relationship differs in young subjects with varying τvO ₂

The relationship between the adjustment of muscle deoxygenation (D[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 D[HHb] kinetics were examined in 37 young males (24 ± 4 years). VO _2p was measured breath-by-breath. Changes in D[HHb] of the vastus lateralis muscle were measured by nearinfrared spectroscopy. VO _2p and D[HHb] response profiles were fit using a mono-exponential model, and scaled to a relative % of the response (0-100%). The D[HHb]/DVO _2p ratio for each individual (reflecting the matching of O ₂ distribution to O ₂ utilization) was calculated as the average D[HHb]/DVO _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 δ[HHb]/δVO _2p were 0.98 (VF), 1.05 (F), 1.09 (M), and 1.22 (S). The larger δ[HHb]/δVO _2p in the groups with slower VO _2p kinetics resulted in the δ[HHb]/δVO _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.