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

Abstract

Oxygen uptake (V̇o2) kinetics are slowed in the supine (S) position purportedly due to impaired muscle O2 delivery ([Formula: see text]); 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 V̇o2 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 V̇o2 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 V̇o2 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 µM; P < 0.05) and total[heme] amplitude (S: 17-20 vs. U: 9-16 µM; 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 [Formula: see text] in S, even within deep muscle, necessitated a greater reliance on fractional O2 extraction and diffusive [Formula: see text]. The slower V̇o2 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 O2 delivery gradient toward deep versus superficial muscle is insufficient to mitigate impairments in oxidative function in response to reduced whole muscle O2 delivery. More heterogeneous muscle deoxygenation is associated with slower V̇o2 kinetics.