High-Density Surface Electromyography Detects Fiber-Type-Related Profiles and Disease-Relevant Fatigue Trajectories in Amyotrophic Lateral Sclerosis

Fiber-type composition contributes to contractile behavior and fatigue resistance. Characterizing these physiological differences could help study and monitor disease-related changes. Direct fiber-type measurement requires biopsy, whereas animal findings may not fully translate to humans. High-density surface electromyography (HD-sEMG) may offer repeatable, noninvasive assessment. We recorded HD-sEMG from tibialis anterior (TA) and soleus, muscles with contrasting fast- and slow-twitch-like physiology, during sustained contractions at 30% and 50% maximum voluntary contraction in participants with amyotrophic lateral sclerosis (ALS; n = 7) and healthy controls ( n = 10). Given fast-fatigable motor unit vulnerability in ALS models, we compared TA-soleus profiles and fatigue-related changes. Amplitude, spectral, motor unit firing, and motor unit action potential (MUAP) features were quantified in the first 10 s of stable contraction, with firing and MUAP features derived from decomposition. Exploratory fatigue analyses assessed early-to-late changes and trajectories. Early-window root mean square (RMS), spectral power, firing rate, and MUAP amplitude were higher in TA than soleus in both groups. Group x Muscle interactions were nonsignificant. Despite higher early values, TA showed larger fatigue-related declines in high-frequency power, RMS, and firing rate than soleus, particularly in ALS-affected TA. These findings suggest HD-sEMG detects muscle-type-related electrophysiological differences, with ALS-related changes more evident during fatigue.