Influence Of Body Position On Pulmonary Oxygen Uptake And Muscle Deoxygenation Kinetics During Cycle Exercise

Abstract

Whether pulmonary oxygen uptake (VO2) kinetics are limited by O2 delivery remains contentious. Previous studies have demonstrated that the time constant of pulmonary VO2 kinetics (τVO2) is greater during supine compared to upright cycle exercise, presumably reflecting the superimposition of an O2 availability limitation. However, interpretation of these studies is compromised by their use of superficial single-site measurements using continuous-wave near-infrared spectroscopy (NIRS), which is unable to determine absolute [heme]. PURPOSE: To determine the impact of body position (i.e. upright [U] vs. supine [S]) on the kinetics of pulmonary VO2, as well as muscle deoxygenation (deoxy[heme]) kinetics and total[heme] using time-resolved (TR-)NIRS. METHODS: 7 healthy men completed an incremental ramp test to determine VO2 max and the gas exchange threshold in the supine position. 4 visits followed whereby pulmonary VO2 and deoxy[heme] kinetics and total[heme] were determined via TR-NIRS at three muscle sites (deep [VLd] and superifical [VLs] vastus lateralis and superficial rectus femoris [RFs]) in two conditions: 1) during S heavy intensity constant work rate exercise at 40%Δ (between ventilatory threshold and VO2 max); and 2) during U exercise at the same absolute work rate. RESULTS: τVO2 was increased during S compared to U (S: 42 ± 12 vs. U: 32 ± 9 s, P = 0.03). The fundamental phase τdeoxy[heme] was greater (i.e. slower) in S compared to U for each muscle site (VLd S: 19 ± 10 vs. U: 8 ± 5 s; VLs S: 16 ± 7 vs. U: 10 ± 3 s; RFs S: 20 ± 7 vs. U: 11 ± 3 s, P = 0.002) and its amplitude was greater in S compared to U for RFs only (S: 27.4 ± 12.1 vs. U: 9.1 ± 2.5 μM, P = 0.008). Total[heme] did not differ between U and S for any muscle site (all P > 0.05). CONCLUSION: The slowing of pulmonary VO2 kinetics for S versus U occurs concomitant with a depressed rate of muscle(s) deoxygenation. This finding suggests that supine exercise results in a relatively greater fall in muscle VO2 when compared to O2 delivery kinetics at least for VLd and VLs. The increased amplitude of deoxy[heme] in S for RFs suggests an increase in O2 extraction to compensate for impaired muscle perfusion in S compared to U.