Unraveling neuro-motor control deficits in healthy aging: Implications for neurorehabilitation

Introduction: People frequently interact with another person, and stability of such actions is crucial (patient-caregiver interactions, etc.). When a person executes a task alone, the central nervous system (CNS) exploits the variability of sensory-motor elements. However, when two persons share a task, their CNS needs to account for the extra-personal source of movement variability. The neurological effects of healthy aging may impair modulation of motor control processes to stabilize two-person motor tasks. We used the Uncontrolled Manifold (UCM) to compute the index of stability (DV), which quantifies how CNS utilizes movement variability to stabilize the performance of a motor task. Recently, the UCM was proposed as a sensitive and individualized biomarker of movement quality – a significant component in precision neurorehabilitation. This study explored neural motor control deficits in old adults while performing interpersonal prehension tasks with younger and age-matched counterparts. Methods: Ten young (24.6±0.8yrs) and ten older adults (73.5±3.1yrs) were randomly assigned to 10 matched-age and mixed-age pairs for two-person conditions. Subjects sat on a chair holding the handle instrumented with force sensors vertically using one hand (in two-person conditions) or two hands (in one-person conditions). In one-person condition, each subject held the handle with both hands. Subjects held the handle with matched-aged or different age partners in two-person conditions. Subjects used only finger flexion and extension to cyclically transfer the handle from the left to right hand while keeping the handle’s orientation and position steady. We computed grip forces and UCM-parameters for the vertical and horizontal handle stability. Results: We found no differences in clinical hand dynamometer measures between age groups (p=0.4). Old subjects had higher grip forces than young in all conditions (p=0.01). Only older adults increased their grip forces when paired with another older adult (p=0.05). Grip forces of young participants did not vary between conditions (p=0.6). For horizontal stability, young subjects decreased DV values when matched with another young person (p=0.03), compared to a task performed alone. Old adults showed no difference in DV between one-person and two-person condition with another older adult; however, they showed a decrease in DV when matched with a young person (p=0.04). For vertical stability, young and old participants behaved similarly, with one-person conditions having the largest DV values, than two- person conditions. Conclusions: Results show changes in multi-digit coordination with age at the neural control level. Notably, young adults showed the ability to modulate the performance stability as an adaptation for tasks performed with another person of the same age. Old adults modulated stability of action only when sharing the task with younger counterparts. These findings suggest the potential role of motor control deficits in healthy aging. Future studies should clarify if targeted therapies could improve these deficits.