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

Older adults exhibited significantly greater variability in torque, position, and discharge rates compared to young adults, especially during dynamic contractions.

• Fourteen young adults (age: 23 ± 3 yr) and 12 older adults (age: 68 ± 5 yr) participated in the study.
• Variability was assessed using coefficient of variation (COV) for joint torque, position, and motor unit discharge rates.
• Greater variability was observed in older adults particularly during concentric and eccentric contractions rather than isometric contractions.
• Tasks were performed at low velocity (5°/s) and low force level (10% maximum isometric voluntary contraction).

Older adults showed reduced average discharge rate (AVRDR) for continuous motor units (CNTMU) during concentric contractions compared to young adults.

• Continuous motor units (CNTMU) were defined as units recruited ≥90% of the task duration.
• The reduction in AVRDR for CNTMU was specific to concentric contractions, with no significant differences observed during isometric contractions.
• Muscle activity was recorded using high-density surface electromyography (HD-sEMG) and decomposed using blind source separation.
• The muscle examined was the tibialis anterior during ankle dorsiflexion tasks.

Older adults showed increased average discharge rate (AVRDR) for intermittent motor units (INTMU) during eccentric contractions compared to young adults.

• Intermittent motor units (INTMU) were defined as units recruited <90% of the task duration.
• The increase in AVRDR for INTMU was specific to eccentric contractions.
• No differences in AVRDR were found between age groups during isometric contractions.
• This finding suggests age-related differences in neural drive are contraction-mode specific and motor unit type specific.

Older adults presented reduced concentric discharge rate slopes (SLOPEDR) for intermittent motor units (INTMU) compared to young adults.

• SLOPEDR reflects the modulation of discharge rate across the contraction phase.
• This reduction was observed specifically for INTMU during concentric contractions.
• Reduced SLOPEDR indicates impaired ability to modulate rate coding during dynamic contractions in older adults.
• No equivalent difference in SLOPEDR for CNTMU during concentric contractions was reported.

Age-related changes in motor unit behavior were specific to contraction mode (concentric vs. eccentric vs. isometric), with isometric contractions showing no significant age-related differences in discharge properties.

• The study design included cycles of concentric, isometric, and eccentric ankle dorsiflexions performed at 5°/s and 10% MVIC.
• No differences in AVRDR were found between young and older adults during isometric contractions for either CNTMU or INTMU.
• Age-related differences in AVRDR emerged differentially: reduced for CNTMU in concentric and increased for INTMU in eccentric contractions.
• These findings highlight the importance of examining dynamic rather than solely isometric contractions when studying age-related neuromuscular changes.

The study used high-density surface EMG decomposition via blind source separation to identify and classify individual motor units in the tibialis anterior during dynamic contractions.

• HD-sEMG decomposition allowed separation of motor units into CNTMU (recruited ≥90% of task duration) and INTMU (recruited <90% of task duration).
• Average discharge rate (AVRDR) and discharge rate slopes (SLOPEDR) were estimated from each extracted motor unit.
• The use of blind source separation on HD-sEMG signals enabled analysis of motor unit behavior across concentric, isometric, and eccentric phases within the same contraction cycle.
• The approach is described as noteworthy for uncovering age-specific changes in neural drive during dynamic contractions.