PlanHab: hypoxia exaggerates the bed-rest-induced reduction in peak oxygen uptake during upright cycle ergometry

The study examined the effects of hypoxia and horizontal bed rest, separately and in combination, on peak oxygen uptake ((V) over doto(2) (peak)) during upright cycle ergometry. Ten male lowlanders underwent three 21-day confinement periods in a counterbalanced order: 1) normoxic bed rest [NBR; partial pressure of inspired O-2 (PIO2) = 133.1 +/- 0.3 mmHg]; 2) hypoxic bed rest (HBR; PIO2 = 90.0 +/- 0.4 mmHg), and 3) hypoxic ambulation (HAMB; PIO2 = 90.0 +/- 0.4 mmHg). Before and after each confinement, subjects performed two incremental-load trials to exhaustion, while inspiring either room air (AIR), or a hypoxic gas (HYPO; PIO2 = 90.0 +/- 0.4 mmHg). Changes in regional oxygenation of the vastus lateralis muscle and the frontal cerebral cortex were monitored with near-infrared spectroscopy. Cardiac output (CO) was recorded using a bioimpedance method. The AIR (V) over doto(2) (peak) was decreased by both HBR (similar to 13.5%; P <= 0.001) and NBR (similar to 8.6%; P <= 0.001), with greater drop after HBR (P = 0.01). The HYPO (V) over doto(2) (peak) was also reduced by HBR (-9.7%; P <= 0.001) and NBR (-6.1%; P <= 0.001). Peak CO was lower after both bed-rest interventions, and especially after HBR (HBR: similar to 13%, NBR: similar to 7%; P <= 0.05). Exercise-induced alterations in muscle and cerebral oxygenation were blunted in a similar manner after both bed-rest confinements. No changes were observed in HAMB. Hence, the bed-rest-induced decrease in (V) over doto(2) peak was exaggerated by hypoxia, most likely due to a reduction in convective O-2 transport, as indicated by the lower peak values of CO.