Neural Substrates Underlying Adaptations in Manual Dexterity of Older Adults

Managing fine motor function is essential for independence and quality of life in older adults. However, the neural mechanisms underlying age-related declines in manual dexterity remain poorly understood. Traditional models of motor control suggest that the nervous system coordinates movement through shared motor commands across muscles-so-called "motor modules" or "muscle synergies". Yet, emerging evidence reveals that synaptic inputs to motor neurons can vary even within a single muscle, challenging this muscle-level concept and prompting a shift toward more a granular, motor-unit level framework. These "motor unit modes" offer a more accurate representation of the neural architecture of motor control.

This project will be the first to investigate whether improvements in manual dexterity-a core marker of neurological health in aging-are associated with neuroplastic changes in the strength of functionally relevant motor unit modes. Older adults (54-89 yrs) will practice a test of manual dexterity (Grooved Pegboard) with or without performance-enhancing transcutaneous electrical nerve stimulation (TENS). Outcomes will include force steadiness and motor unit activity derived from high-density electromyography during low-intensity contractions.

Our central hypothesis is that improvements in manual dexterity will be mediated by neuroplastic strengthening of functionally relevant motor unit modes. The project has three specific aims:

• Characterize short-term neuroplastic adaptations following task familiarization.
• Determine the effects of steady-contraction training on neuromuscular control.
• Evaluate the added benefit of somatosensory augmentation with TENS.

Innovation. This study introduces two key innovations: (1) It quantifies, for the first time, the extent to which improvements in a dynamic behavior are mediated by changes in shared synaptic inputs across motor units during low-intensity contractions; (2) it evaluates the capacity of TENS-induced somatosensory feedback to boost neuroplasticity in the aging motor system.

Expected Outcomes. We expect that gains in force steadiness and pegboard performance will strongly correlate with increased strength and consistency of motor unit modes. These findings will clarify the neural mechanisms underlying motor adaptation in older adults and define new markers for assessing motor function.

Impact. Aligned with the goals of PA-25-303 and the missions of NINDS and NIA, this research will generate foundational knowledge of spinal motor control and establish motor unit modes as a new biomarker for evaluating motor function and therapeutic efficacy. This work has the potential to inform targeted interventions aimed at preserving dexterity and independence in older individuals and those with neurological dysfunction.

Inclusion criteria

Community-dwelling men and women who are 54-89 years of age and able to give fully informed consent; able to read, write, and speak English to ensure safe participation in the project; and able to arrange own transportation to Boulder campus.

Exclusion criteria

Cognitive impairment, major psychiatric condition, or unstable depressive disorder that would influence the ability to understand the study and cooperate fully in the proposed protocols; any progressive neurological, muscular, cardiovascular, or skeletal disorder that limits participation, such as (but not limited to) (1) amyotrophic lateral sclerosis, multiple sclerosis, multiple system atrophy, muscular dystrophy, myasthenia gravis, Parkinson's disease, spinal muscular atrophy, spinocerebellar ataxia, or spasticity; (2) congenital, mitochondrial, or thyrotoxic myopathies, fibromyalgia, or myositis; (3) peripheral neuropathy, diabetes, or hypertension; or (4) cancer, gout, osteoarthritis with severe pain, or rheumatoid arthritis; chronic pain condition that would impair the ability to participate in the study; currently taking prescribed medication known to influence neuromuscular function, such as carisoprodol, cyclobenzaprine, metaxalone, and methocarbamol; function-limiting injury to the hands, arms, shoulders, neck, or legs; recent hospitalization (within the last 3 months) or enforced bedrest/sedentary state; inability to attend the evaluation and practice sessions in 2 weeks.