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

Oxygen uptake (VO2) kinetics are slowed in the supine (S) position due to impaired muscle O2 delivery (QO2), 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 vs. S) on deep and superficial muscle deoxygenation (deoxy[heme]) using time-resolved (TR-) NIRS, and how these relate to slowed pulmonary VO2 kinetics. 17 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 VO2 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 VO2 time constant was increased during S compared to 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: 9-16µM, P<0.05) were greater in S compared to 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 QO2 in S, even within deep muscle, necessitated a greater reliance on fractional O2 extraction and diffusive QO2. The slower VO2 kinetics in S vs. U demonstrates that, ultimately, these adaptations were insufficient to prevent impairments in whole-body oxidative metabolism.