Age-related differences in force steadiness and motor unit behavior during dynamic ankle dorsiflexions

The aim of this study was to describe the age-related differences in motor unit behavior during concentric, isometric, and eccentric ankle dorsiflexions. Fourteen young adults (age: 23 ± 3 yr) and 12 older adults (age: 68 ± 5 yr) performed cycles of concentric/isometric/eccentric ankle dorsiflexions at low velocity (5°/s) and low force level (10% maximum isometric voluntary contraction). Muscle activity was recorded using high-density surface electromyography (HD-sEMG) and decomposed using blind source separation. Motor units were divided into continuous motor units (CNTMU, e.g., units recruited ≥90% of the task duration) and intermittent motor units (INTMU, e.g., units recruited <90% of the task duration). The average discharge rate (AVRDR) and discharge rate slopes (SLOPEDR) were estimated from each extracted motor unit. Joint torque, position, and motor unit discharge rate variability were assessed using coefficient of variation (COV). The results revealed that older adults present significantly greater variability in torque, position, and discharge rates, especially in dynamic contractions. Regarding motor unit discharge properties statistics, older adults presented reduced AVRDR for CNTMU during concentric contractions, whereas their AVRDR was increased for INTMU during eccentric contractions compared with young adults, with no differences during isometric contractions. Moreover, older adults presented reduced concentric SLOPEDR for INTMU when compared with young adults. Our results demonstrate that older adults present altered neural drive to the muscles, reducing their ability to modulate rate coding and subsequently maintain force steadiness at low force levels in concentric and eccentric contractions.NEW & NOTEWORTHY Our study reveals how aging affects tibialis anterior's motor unit behavior during slow dynamic contractions at low force levels. Using high-density surface EMG decomposition, we uncovered age-specific changes in neural drive during concentric and eccentric contractions. Older adults showed greater force and discharge rate variability, with distinct differences in discharge properties across motor unit types and contraction modes. These findings advance our understanding of age-related declines in muscle function.