Impact of supine exercise on muscle deoxygenation kinetics heterogeneity: mechanistic insights into slow pulmonary oxygen uptake dynamics

Oxygen uptake ((V) over doto(2)) kinetics are slowed in the supine (S) position purportedly due to impaired muscle O-2 delivery ((Q) over dot(O2)); however, these conclusions are predicated on single-site measurements in superficial muscle using continuous-wave near-infrared spectroscopy (NIRS). This study aimed to determine the impact of body position [i.e., upright (U) versus S] on deep and superficial muscle deoxygenation (deoxy[heme]) using time-resolved (TR-) NIRS, and how these relate to slowed pulmonary (V) over doto(2) kinetics. Seventeen healthy men completed constant power tests during 1) S heavy-intensity exercise and 2) U exercise at the same absolute work rate, with a subset of 10 completing additional tests at the same relative work rate as S. Pulmonary (V) over doto(2) was measured breath-by-breath and, deoxy- and total[heme] were resolved via TR-NIRS in the superficial and deep vastus lateralis and superficial rectus femoris. The fundamental phase (V) over doto(2) time constant was increased during S compared with U (S: 36 +/- 10 vs. U: 27 +/- 8 s; P < 0.001). The deoxy[heme] amplitude (S: 25-28 vs. U: 13-18 mu M; P < 0.05) and total[heme] amplitude (S: 17-20 vs. U: 9-16 mu M; P < 0.05) were greater in S compared with U and were consistent for the same absolute (above data) and relative work rates (n = 10, all P < 0.05). The greater deoxy- and total[heme] amplitudes in S vs. U supports that reduced perfusive (Q) over dot(O2) in S, even within deep muscle, necessitated a greater reliance on fractional O-2 extraction and diffusive (Q) over dot(O2). The slower (V) over doto(2) kinetics in S versus U demonstrates that, ultimately, these adjustments were insufficient to prevent impairments in whole body oxidative metabolism. NEW & NOTEWORTHY We show that supine exercise causes a greater degree of muscle deoxygenation in both deep and superficial muscle and increases the spatial heterogeneity of muscle deoxygenation. Therefore, this study suggests that any O-2 delivery gradient toward deep versus superficial muscle is insufficient to mitigate impairments in oxidative function in response to reduced whole muscle O-2 delivery. More heterogeneous muscle deoxygenation is associated with slower (V) over doto(2) kinetics.