Influence of body position on muscle deoxy[Hb + Mb] during ramp cycle exercise

We used near-infrared spectroscopy (NIRS) to test the hypothesis that body position alters the sigmoidal response profile of muscle fractional O2 extraction (estimated using deoxy[Hb + Mb]) during incremental cycle exercise. Seven male subjects (mean ± SD age 32 ± 13 years) completed a ramp incremental cycling test to exhaustion (30 W/min) in both the supine and upright body positions. The sigmoidal (as opposed to hyperbolic) model that provided the better fit to deoxy[Hb + Mb] data during upright cycling was also present for the supine response; however, the slope of the sigmoid was increased (upright: 0.052 ± 0.012 vs  . supine: 0.090±0.036%⋅%Ppeak−1; P < 0.05) and a plateau occurred at a lower work rate (upright: 83 ± 8 vs  . supine: 68±19%Ppeak−1; P < 0.05) during supine exercise. These changes occurred in the absence of a leftward shift of the sigmoid. We also found a significantly greater ΔV˙O2/ΔW slope above compared to below gas exchange threshold (GET) for both conditions (upright: 9.8 ± 0.5 vs. 8.2 ± 0.9; supine: 10.7 ± 0.9 vs. 8.0 ± 0.8) and for supine compared to upright cycling above GET. These findings suggest that the supine posture affects O2 extraction and V˙O2 kinetics to a greater extent as work rate progresses during ramp incremental exercise.