Reduced exercise capacity and muscle symptoms in people with post-COVID and myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) cannot simply be explained by a lack of physical activity or long-term deconditioning. That is the conclusion of a new study by researchers from Vrije Universiteit Amsterdam and Amsterdam UMC, including several Amsterdam Movement Sciences researchers, published in Nature Communications.
To investigate whether prolonged inactivity alone could explain the muscle abnormalities seen in patients, the researchers compared people with post-COVID and ME/CFS to healthy volunteers who had undergone 60 days of strict bed rest. While both groups showed reduced exercise capacity compared with healthy active controls, the underlying muscle characteristics were markedly different.
The study found that bed rest caused muscle loss, but did not reproduce the muscle changes observed in patients with post-COVID and ME/CFS. Instead, the patient groups showed distinct alterations in muscle fibre composition. Both patient groups had a higher proportion of glycolytic muscle fibres, while people with ME/CFS also showed atrophy of type I muscle fibres. These findings indicate that the muscle abnormalities in post-COVID and ME/CFS cannot be attributed to inactivity alone.
According to principal investigator Rob Wüst, the findings suggest that patients should not be treated as though they have simply lost their fitness. The researchers report indications of abnormalities in muscle fibres, energy production and oxygen supply, highlighting the need for future diagnostics and rehabilitation strategies to take these biological differences into account.
This work builds on earlier research from Amsterdam UMC and Vrije Universiteit Amsterdam that demonstrated a physical basis for the persistent fatigue experienced by people with post-COVID. Together, these studies strengthen the evidence that the reduced exercise capacity and post-exertional symptoms experienced by patients are linked to underlying biological changes rather than being solely the result of inactivity.