title: Functional impairment of skeletal muscle oxidative metabolism during knee extension exercise after bed rest.
abstract: 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 O(2) 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 O(2) 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(-1)·100 g(-1)). Skeletal muscle peak fractional O(2) 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 O(2) delivery, a decrease in peak O(2) uptake and muscle peak capacity of fractional O(2) 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/O(2) utilization inside the muscle, peripheral O(2) diffusion, and intracellular oxidative metabolism.
authors.affiliation: Department of Medical and Biological Sciences, University of Udine, Udine, Italy.
authors.forename: Desy
authors.initials: D
authors.lastname: Salvadego
authors.forename: Stefano
authors.initials: S
authors.lastname: Lazzer
authors.forename: Mauro
authors.initials: M
authors.lastname: Marzorati
authors.forename: Simone
authors.initials: S
authors.lastname: Porcelli
authors.forename: Enrico
authors.initials: E
authors.lastname: Rejc
authors.forename: Bostjan
authors.initials: B
authors.lastname: Simunic
authors.forename: Rado
authors.initials: R
authors.lastname: Pisot
authors.forename: Pietro Enrico
authors.initials: PE
authors.lastname: di Prampero
authors.forename: Bruno
authors.initials: B
authors.lastname: Grassi
mesh_terms: D000328:Adult; D001510:Bed Rest / Q000009:adverse effects*; D002302:Cardiac Output; D015444:Exercise / Q000502:physiology*; D005080:Exercise Test; D006339:Heart Rate; D006801:Humans; D007717:Knee; D008297:Male; D018482:Muscle, Skeletal / Q000098:blood supply / Q000378:metabolism*; D010101:Oxygen Consumption / Q000502:physiology; D018474:Weightlessness Simulation; D055815:Young Adult