Functional impairment of skeletal muscle oxidative metabolism during knee-extension exercise after bed rest

A functional evaluation of skeletal muscle oxidative metabolism during dynamic knee extension (KE) incremental exercises was carried out following a 35-day bed rest (BR) (Valdoltra 2008 BR campaign). Nine young male volunteers (age: 23.5 +/- 2.2 yr; mean +/- SD) were evaluated. Pulmonary gas exchange, heart rate and cardiac output (by impedance cardiography), skeletal muscle (vastus lateralis) fractional O2 extraction, and brain (frontal cortex) oxygenation (by near-infrared spectroscopy) were determined during incremental KE. Values at exhaustion were considered “peak”. Peak heart rate (147 +/- 18 beats/min before vs. 146 +/- 17 beats/min after BR) and peak cardiac output (17.8 +/- 3.3 l/min before vs. 16.1 +/- 1.8 l/min after BR) were unaffected by BR. As expected, brain oxygenation did not decrease during KE. Peak O2 uptake was lower after vs. before BR, both when expressed as liters per minute (0.99 +/- 0.17 vs. 1.26 +/- 0.27) and when normalized per unit of quadriceps muscle mass (46.5 +/- 6.4 vs. 56.9 +/- 11.0 ml/min/100 g). Skeletal muscle peak fractional O2 extraction, expressed as a percentage of the maximal values obtained during a transient limb ischemia, was lower after (46.3 +/- 12.1%) vs. before BR (66.5 +/- 11.2%). After elimination, by the adopted exercise protocol, of constraints related to cardiovascular O2 delivery, a decrease in peak O2 uptake and muscle peak capacity of fractional O2 extraction was found after 35 days of BR. These findings suggest a substantial impairment of oxidative function at the muscle level, “downstream” with respect to bulk blood flow to the exercising muscles, that is possibly at the level of blood flow distribution/O2 utilization inside the muscle, peripheral O2 diffusion, and intracellular oxidative metabolism.