Association between changes in muscle activation and motor performance with advancing age
The human neuromuscular system undergoes substantial reorganization after the age of 60 years. Many of these processes have significant functional consequences on the ability of old adults to perform normal activities of daily living. This thesis focuses on the function of an intrinsic hand muscle, the first dorsal interosseous, which is located between the thumb and index finger and is solely responsible for abduction of the index finger about the first metacarpophalangeal joint. The first dorsal interosseous also contributes to flexion of the index finger, and is especially important to the precision pinch grip. When an individual is instructed to exert a submaximal, constant abduction force with the index finger, the force is not constant, but varies about an average value. The magnitude of the force fluctuations is greater for old subjects compared with young subjects. Previous studies have focused on age-related differences in the size of individual motor units as a potential mechanism underlying the reduced steadiness. The purpose of this thesis was to examine the effects of agonist antagonist coactivation, motor unit discharge variability, and light- and heavy-load strength training on the ability of young and old adults to exert steady muscle forces during slow finger movements. The results indicate that the recruitment and modulation of the discharge rate of motor units changes with age, which contributes to the decline in movement capabilities.