Juan M. Murias, Matthew D. Spencer, John M. Kowalchuk, Donald H. Paterson.
European Journal of Applied Physiology

The relationship between the adjustment of muscle deoxygenation (∆[HHb]) and phase II VO2p was examined in subjects presenting with a range of slow to fast VO2p kinetics. Moderate intensity VO2pand ∆[HHb] kinetics were examined in 37 young males (24 ± 4 years). VO2p was measured breath-by-breath. Changes in ∆[HHb] of the vastus lateralis muscle were measured by near-infrared spectroscopy. VO2p and ∆[HHb] response profiles were fit using a mono-exponential model, and scaled to a relative % of the response (0–100%). The ∆[HHb]/∆VO2p ratio for each individual (reflecting the matching of O2 distribution to O2 utilization) was calculated as the average ∆[HHb]/∆VO2p response from 20 to 120 s during the exercise on-transient. Subjects were grouped based on individual phase II VO2p time-constant (τVO2p): <21 s [very fast (VF)]; 21–30 s [fast (F)]; 31–40 s [moderate (M)]; >41 s [slow (S)]. The corresponding ∆[HHb]/∆VO2p were 0.98 (VF), 1.05 (F), 1.09 (M), and 1.22 (S). The larger ∆[HHb]/∆VO2p in the groups with slower VO2p kinetics resulted in the ∆[HHb]/∆VO2p displaying a transient “overshoot” relative to the subsequent steady state level, which was progressively reduced as τVO2 became smaller (r = 0.91). When τVO2p > ~20 s, the rate of adjustment of phase II VO2p appears to be mainly constrained by the matching of local O2distribution to muscle VO2. These data suggest that in subjects with “slower” VO2 kinetics, the rate of adjustment of VO2 may be constrained by O2 availability within the active tissues related to the matching of microvascular O2 distribution to muscle O2 utilization.

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