Motor Unit Activity and Force Steadiness Changes w Aging

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DESCRIPTION (provided by applicant): The neuromuscular system undergoes significant denervation and muscle atrophy during aging. Muscle force production becomes less steady, particularly during the controlled release of muscle force and during low-force contractions. The loss of control of hand dexterity is extremely detrimental to the quality of life in many older individuals and to those affected by disuse atrophy from musculoskeletal disease. This study will investigate possible mechanisms behind the decrease in hand steadiness during aging. We hypothesize that there is a higher incidence of self sustained firing in motoneurons of older adults and that this behavior contributes to the increased difficultly to release muscle force smoothly. Secondly, we hypothesize that there is an increase in motor unit synchronization and coherence across the muscles of the thumb and index finger in older adults and that this could account for the decrease in steadiness observed during functional hand tasks such as pinching. Finally, we will determine if changes in motor unit firing rate synchronization and coherence across the thumb and index finger are related to increases in force steadiness following resistance training in older adults. This study will encompass two sets of experiments performed before and after one-month training programs. The first will investigate single motor unit firing rates and the incidence of self sustained firing of muscles serving the thumb and index finger during independent thumb and finger contractions in younger and older adults. In a second set of experiments, all subjects will perform a pinching force-accuracy task and the degree of synchronization and coherence between the thumb and index finger in younger and older adults will be compared with cross correlation analysis. Twenty older adults will be divided into two groups which will undergo different training programs for four weeks. One group will perform a take-home resistance training program and a matched group will perform a precision motor learning program. The incidence of self-sustained firing in single motor units and motor unit firing rate synchronization and coherence during pinching will be measured again post training. This elucidation of the neuromuscular mechanisms that enable hand steadiness will provide the foundation for the design of advanced rehabilitation systems.