Motor unit activation and fluctuations in motor output in old adult

Aging is associated with a deterioration of movement performance. When compared with young adults, older individuals typically exhibit a decreased ability to execute steady movements. Because the force exerted by a muscle depends upon the activation pattern of motor units, it is likely that differences in motor unit activation contribute to the greater fluctuations in motor output (reduced steadiness) exhibited by old adults. The purpose of this dissertation was to quantify the contribution of individual and correlated motor unit discharge to the fluctuations in index forger acceleration during isometric and anisometric (shortening and lengthening) contractions of the first dorsal interosseus muscle. Four studies were performed to achieve this purpose. The first two studies involved validating a training program to improve steadiness, then utilizing the intervention to measure the contribution of single motor unit discharge characteristics to the acceleration fluctuations. It was determined that variability in the discharge rate of motor units significantly contributed to the acceleration fluctuations and that the discharge rate variability was reduced in parallel with improvements in steadiness. Furthermore, the changes in discharge rate variability and acceleration fluctuations were associated with improvements in performance on a test of manual dexterity. The next two studies measured the effect of correlated motor unit activity on the acceleration fluctuations. Correlated motor unit discharge was found to be similar in young and old adults and was weakly associated with fluctuations in acceleration only during anisometric contractions. The results of these experiments indicated that the fluctuations in index finger acceleration are functionally relevant, and related to the degree of variability in motor unit discharge rate and motor unit synchronization but not motor unit coherence.