Long COVID and ME/CFS drive distinct skeletal muscle changes beyond bed-rest effects

60-day bed rest in healthy adults causes muscle atrophy and altered maximal exercise responses without changing fiber type, whereas long COVID and ME/CFS patients show more glycolytic fibers (with ME/CFS showing type I–specific atrophy); oxidative phosphorylation capacity only links to maximal oxygen uptake after bed rest, indicating that inactivity alone cannot explain the reduced exercise capacity in these patients, suggesting disease-specific skeletal muscle remodeling.