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

© 2020 the American Physiological Society.Oxygen uptake (V O2) kinetics are slowed in the supine (S) position purportedly due to impaired muscle O2 delivery (Q O2 ); however, these conclusions are predicated on singlesite measurements in superficial muscle using continuous-wave nearinfrared 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 O2 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 O2 was measured breath-bybreath 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 O2 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 perfusiveQ O2 in S, even within deep muscle, necessitated a greater reliance on fractional O2 extraction and diffusive Q O2 . The slowerV O2 kinetics in S versus U demonstrates that, ultimately, these adjustments were insufficient to prevent impairments in whole body oxidative metabolism.