Bed Rest Decreases Skeletal Muscle O2 Uptake At Rest In Young And Elderly Subjects

We recently demonstrated that, in young subjects, 10 days of horizontal bed rest (BR) determine a 25% decrease in vastus lateralis muscle O2 uptake (V̇O2m) at rest, as estimated by near-infrared spectroscopy (NIRS) on the basis of the slope of the linear increase in Δ[deoxy(Mb + Mb)] during the first seconds of a transient limb ischemia. PURPOSE: To assess the effects on resting V̇O2m of 10-d BR in elderly subjects (E, n = 10, 69 ± 3 years), and in young subjects (Y, n = 9, 23 ± 4 years) after a longer BR (21-d) compared to that of the previous study. METHODS: Measurements were performed before (PRE) and after (POST) BR. V̇O2m at rest was determined by NIRS (see above), and whole-body resting energy expenditure (REE) was determined by indirect calorimetry (pulmonary gas exchange). Body composition was evaluated by bioelectrical impedance analysis. Skeletal muscle citrate synthase (CS) activity, an estimate of mitochondrial volume density, was measured on vastus lateralis fibers obtained by biopsy. RESULTS: Body mass (BM) decreased in both groups (-5% in Y and -3% in E; p < 0.001), in association with a decreased fat free mass (FFM) (-5% and -4%, in Y and E, respectively; p < 0.01). CS activity decreased both in E (-31%) and in Y (-20%). Resting V̇O2m decreased both in Y (-39% [-1.9%/day]; p = 0.04) and in E (-30% [-3.0%/day]; p = 0.049). No differences in POST vs PRE were observed when resting V̇O2m data were normalized for CS activity (p > 0.05). REE was lower in POST vs PRE both in Y (1522 ± 307 vs 1827 ± 207 kcal/d; p < 0.01) and in E (1568 ± 214 vs 1788 ± 202 kcal/d; p = 0.01), even when adjusted for FFM (-12% in Y and -9% in E; p ≤ 0.05). CONCLUSIONS: Both Y and E exhibited a decreased resting V̇O2m following BR, associated with a decreased mitochondrial volume density. The decreased resting V̇O2m was accompanied, at the whole-body level, by a decreased resting energy expenditure. These phenomena may represent adaptive responses to prolonged periods of profound inactivity and disuse. Funding: MUR; PRIN Project 2020EM9A. Supported by: Italian Ministry of University and Research, PRIN Project 2020EM9A