Motor unit decomposition detects altered trunk neural output during seated postural maintenance after stroke

Trunk control after stroke is clinically important, yet the neural output supporting seated postural maintenance remains poorly characterized. This limits not only the physiological interpretation of trunk dysfunction but also the development of neuromodulation technologies. We therefore examined post-stroke trunk muscle control across three levels: motor unit firing behavior, the frequency distribution of surface electromyography (sEMG) envelope power, and low-frequency sEMG-kinematic coherence. Approach. Twenty individuals with subacute stroke and 20 elderly controls performed a seated postural maintenance task with bilateral arm elevation. High-density sEMG was recorded from the lumbar longissimus, and pelvic motion was measured using an inertial measurement unit. Motor unit discharge metrics, EMG envelope power, and sEMG-kinematic coherence were quantified. Main results. Twenty-two motor units from 12 participants with stroke and 14 motor units from 10 elderly participants were analyzed. Mixed-effects models showed higher 95th percentile discharge rate after stroke (beta = 4.70 Hz, p = 0.037; cluster-bootstrap p = 0.0098), whereas mean discharge rate and inter-spike interval variability were not different. EMG envelope power showed frequency-band and group-by-band effects, with the clearest difference at 0.5-5 Hz. sEMG-kinematic coherence showed no significant effects. Significance. Post-stroke trunk motor output was most clearly detected as an upper-tail motor unit discharge-rate difference rather than altered mean firing. EMG envelope power provided secondary evidence of low-frequency modulation, whereas muscle-motion coupling showed no stroke-specific modulation. Motor unit decomposition may therefore serve as a useful physiological reference for trunk motor control after stroke.