"Central" and "peripheral" limitations to oxidative metabolism were evaluated on 10 young untrained male volunteers following a 35-day horizontal bed rest (BR). Incremental and constant-load exercises were carried out on a cycloergometer before and after BR (Valdoltra Hospital, Ankaran, Slovenia). Pulmonary ventilation and gas exchange, heart rate (HR) and cardiac output (Q') (by impedance cardiography), skeletal muscle (vastus lateralis) and brain (frontal cortex) oxygenation (by near infrared spectroscopy [NIRS]) were determined. After BR "peak" (that is, determined at exhaustion) O₂ uptake (V'O₂peak), peak stroke volume, peak Q'and peak O₂ extraction by muscles were decreased (-18%, -22%, -19%, -33%, respectively), whereas peak HR was unaffected. The gas exchange threshold (% of V'O₂peak) was unaffected by BR. The efficiency of cycling (ratio between external mechanical power output and oxidative energy output) was unaffected by BR. At the highest workloads brain oxygenation data suggest an increased O₂ extraction, which was not affected by BR. Pulmonary V'O₂ kinetics during constant-load exercises were slowed and the amplitude of the "slow component" (for the same absolute workload) was higher after BR. Skeletal muscles contribute to the impairment of oxidative metabolism after BR. The reduced capacity of cardiovascular O₂ delivery does not seem to determine a "competition" for the available O₂ between skeletal muscles and brain.

Supported by ASI – OSMA, Workpackage 1-B-32-1.