Résumé
Prolonged bed rest and immobilization have deleterious effects on skeletal muscle mass and function, especially in older adults. These can lead to reduced physical capacity and quality of life. Previous experiments in younger individuals show that exercise can mitigate these effects, but evidence remains scarce and inconclusive in older adults. Therefore, the aim of this study was to evaluate the effects of a mixed‐exercise countermeasure on body composition, muscle volume, strength and quality in healthy, ageing adults during 2 weeks of head‐down tilt bed rest and in the subsequent recovery period, and to explore the underlying mechanisms. Twenty‐two participants (11 male and 11 female) aged 55–65 years completed the head‐down tilt bed rest protocol. Participants were randomly assigned to either an exercise group ( n = 11) undertaking three daily exercise sessions or a passive‐mobilization control group ( n = 11). Assessments of body composition, thigh muscle volume, muscle strength, blood markers and muscle biopsies were performed before, during and after bed rest, and after 4 weeks and 4 months of recovery. Bed rest induced significant declines in total and lower‐limb lean body mass, knee‐extensor strength and muscle quality. The mixed‐exercise intervention mitigated the loss of thigh muscle volume but did not prevent reductions in total or lower‐limb lean mass, strength or muscle quality. All participants had recovered fully after 4 weeks of recovery. In conclusion, daily participation in an exercise countermeasure preserves thigh muscle volume during 2 weeks of bed rest in healthy older adults but is insufficient to mitigate muscle strength and total lean mass.