Effect of intensive care unit hospitalization for acute respiratory distress syndrome on strength, fatigue and motor unit behaviour.

Prolonged intensive care unit (ICU) hospitalization due to acute respiratory distress syndrome results in reduced muscle strength and increased fatigability. This study investigated the central and peripheral neuromuscular mechanisms underlying these impairments in sixty COVID-19 patients up to 12 months post-ICU discharge. Muscle strength (maximal voluntary contraction; MVC), functional exercise capacity (six-minute walk test; 6MWT), and perceived fatigue (Fatigue Severity Scale; FSS) were assessed at each follow-up. High-Density surface electromyography was collected from the Tibialis anterior muscle at 30%, 50%, and 70% MVC. 6MWT distance increased from 3 to 6 months, but not 6-12. FSS declined from 6 to 12 months, while MVC increased across both intervals. Motor unit (MU) discharge rate decreased from 6 to 12 months at all contraction intensities and from 3 to 6 months at 50% and 70% MVC. MU discharge variability decreased at all levels except 70% MVC from 3 to 6 months. MU action potential amplitude increased from 3 to 6 months but not 6-12. Central MU properties correlated with voluntary force at 3-6 months and with fatigue scores at 6-12 months. These interval-based analyses indicate that associations between neural adaptations and muscle strength were more prominent between 3 and 6 months following ICU discharge, whereas associations with perceived fatigue were more evident between 6 and 12 months.