Fig. 5: Pathological features in skeletal muscle in patients with long COVID.

A Very small and angulated atrophic fibers were more abundant in patients with long COVID and in the post-exercise biopsy in both groups (Group: p < 0.001). B Large areas of necrotic fibers were observed in 36% of patients with long COVID after exhaustive exercise (Group p-value: 0.09, as compared to healthy controls. C Internal nuclei, indicative of fiber repair, were significantly more abundant (Group: p = 0.002) in the skeletal muscle of patients with long COVID, but did not increase during post-exertional malaise (PEM). D Regenerative fibers were not different between groups, but were more abundant in the post-exercise biopsy (Time: p < 0.001). E: More patients with long COVID had CD3+ T-cell infiltration (Group: p = 0.046). F The presence of CD68+ macrophages was higher in long COVID patients (Group: p = 0.03). G CD20+ B-cells were not abundantly present in skeletal muscle. All panels: n = 25 long COVID, n = 21 healthy controls. Categorical longitudinal data were analyzed using logistic regression with “time” as a covariate. Post-hoc comparisons (E) were using the Empirical Mean Differences using R package emmeans with BH adjustment. Bar: 100 μm. Abbreviation: PEM; post-exertional malaise. Source data are provided in the Source Data File.