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14 days of bed rest induces a decline in satellite cell content and robust atrophy of skeletal muscle fibers in middle‐aged adults

We examined the effect of 14 days of bed rest on skeletal muscle satellite cell content and fiber type atrophy and in middle‐aged adults. Bed rest, a ground‐based space flight analog, induces robust atrophy of skeletal muscle, an effect that is exacerbated with increasing age. With the average age of astronauts increasing, we wanted to determine the mechanistic changes during bed rest on an understudied age‐cohort. Muscle biopsies were obtained from the vastus lateralis of healthy middle‐aged adults (n=7 [4M;3F]; age: 51.0±0.6 y) before (Pre‐BR) and after (Post‐BR) 14 days of bed rest. Immunohistochemical analyses were used to quantify myosin heavy chain (MyHC) isoform expression, cross‐sectional area (CSA), satellite cell and myonuclear content and capillary density. Peak oxygen consumption and body composition were measured Pre‐ and Post‐BR. Post‐BR MyHC Type 2a fiber percentage was reduced and mean CSA decreased in all fiber types (25% decrease; P<0.05). Satellite cell content was also reduced Post‐BR (41% decrease; P<0.05), while Pre‐BR Type 2 satellite cell frequency was correlated with the decline in leg lean mass as measured by DXA scan. A decline in capillary density was observed Post‐BR (25% decrease; P<0.05), and Post‐BR capillary density was associated with Post‐BR peak oxygen consumption. A subtle decline in myonuclear content occurred during bed rest. The rapid maladaptation of skeletal muscle to 14 days of mechanical unloading in middle‐aged adults emphasizes the need for appropriate counter measures to preserve muscle function in astronauts.