A new study published in Nature Communications found that the skeletal muscle changes seen in Long COVID and pre-pandemic ME/CFS differ from those caused by prolonged physical inactivity. Researchers compared 25 people with Long COVID, 26 people with pre-pandemic ME/CFS, 30 healthy controls who had fully recovered from SARS-CoV-2, and 24 healthy participants who underwent 60 days of strict bed rest. While prolonged bed rest caused expected signs of deconditioning, including muscle atrophy, people with Long COVID and ME/CFS showed a different pattern. Researchers found changes in muscle fiber composition, capillarization, oxygen delivery and utilization, and mitochondrial function that were not reproduced by bed rest alone. In ME/CFS, participants had reduced capillary-to-fiber ratios and changes involving type I muscle fibers. Long COVID and ME/CFS participants also showed evidence of altered oxidative phosphorylation and exercise responses suggestive of impaired oxygen extraction or reduced stroke volume. The findings suggest that reduced exercise capacity in Long COVID and ME/CFS cannot be explained by inactivity or deconditioning alone. Instead, underlying mitochondrial, vascular, and skeletal muscle abnormalities may contribute to exercise intolerance and post-exertional malaise. Importantly, the findings also support the need for rehabilitation approaches that recognize the underlying disease physiology rather than treating patients as simply deconditioned. Read the full study: DOI 10.1038/s41467-026-75725-y You can find a clickable link in our linktree: linktr.ee/rthm_health