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.

Fig. 1.

Thigh muscle activation during an extension-flexion-extension cycle of the knee extension (KE) exercise. Electromyography (EMG) signal was processed by root mean square (RMS) algorithm and expressed as a percentage of the value obtained during maximal voluntary contraction (MVC). During the extension phase, RMS values for biceps femoris (BF) muscle were markedly lower than those for vastus lateralis (VL) and rectus femoris (RF) muscles. Substantially no muscle activation was observed for any muscle during flexion. These data confirm that knee flexors (BF) were not significantly involved during KE, and that the flexion phase was completely passive (see text for further details).

Fig. 2.

A: mean (SD) O₂ uptake (V̇o₂) values (l/min) as a function of work rate (Watts) during KE incremental exercise. Knee extensor muscle-specific peak V̇o₂ (V̇o_2peak; indicated by the arrows) was determined by extrapolating the V̇o₂ vs. work rate linear regressions, obtained before the inflection point, to peak work rate. See text for further details. B: mean (SD) V̇o_2peak values normalized per unit of quadriceps femoris muscle mass, before and after bed rest (BR). *P < 0.05.

Fig. 3.

Near-infrared spectroscopy (NIRS)-obtained brain (frontal cortex) oxygenation data during KE before and after BR. Mean (SD) values of changes in concentration of oxyhemoglobin (Δ[oxyHb]; A), changes in concentration of deoxyhemoglobin (Δ[deoxyHb]; B), and changes in concentration of total (oxy + deoxy) hemoglobin (Δ[oxyHb+deoxyHb]; C), all expressed as micromolar changes relative to an initial value arbitrarily set equal to zero, are shown as a function of work rate. *P < 0.05, before vs. after BR at the same relative work rate. See text for further details.

Fig. 4.

Mean (SD) values of the NIRS-obtained muscle oxygenation index {concentration changes of deoxygenated Hb + myoglobin (Mb); Δ[deoxy(Hb+Mb)]}, which was utilized to estimate VL fractional O₂ extraction, are shown as a function of work rate during KE. The Δ[deoxy(Hb+Mb)] data are expressed as a percentage of those obtained during a transient limb ischemia induced at the end of the test. The sigmoid function (Eq. 1) utilized to fit the data before BR is also shown. *P < 0.05 vs. before BR data. See text for further details.

Fig. 5.

Mean (SD) integrated EMG (iEMG) values during the last 10 flexion-extension cycles of each work rate during KE, divided by the duration of the considered phase for VL (A), RF (B), and BF (C) muscles. *P < 0.05 vs. before BR. See text for further details.