Age-associated differences in discharge rate modulation of human motor units
The nervous system controls muscle force by modulating motor unit activity, which depends on the integration of synaptic input by the motor neurons. Due to significant remodeling of synaptic inputs and intrinsic properties of motor neurons with advancing age, synaptic integration presumably differs between young and old adults and thereby influences the modulation of discharge rate. The purpose of this dissertation research was to examine the consequences of aging on motor unit discharge characteristics. The first project examined age-related differences in the amount of discharge variability in a hand muscle during index finger abduction tasks. The purpose was to examine the influence of motor unit discharge variability on the force fluctuations in a hand muscle of old adults. The results provided evidence that discharge variability, which was similar for young and old adults, had a significant influence on force fluctuations and that the extent of discharge rate modulation was reduced in old adults. The purpose of the second study was to determine how long motor unit activity could be sustained during a voluntary contraction in humans. The study examined changes in mean motor unit discharge rate and discharge variability for the duration that young and old adults were able to keep a motor unit discharging action potentials. Despite the absence of change in discharge rate for young adults, discharge variability exhibited a marked increase. The duration of motor unit activity in old adults was briefer, yet discharge variability increased to a similar extent as for young adults. The purpose of the third project was to compare the discharge characteristics of motor units recruited during an isometric contraction that was sustained with the elbow flexor muscles by old adults at target forces that were less than the recruitment threshold force of each isolated motor unit. In contrast to motor units in young adults that discharged action potentials either repetitively or intermittently depending on the magnitude of the target-force difference, the motor units of old adults discharged action potentials repetitively regardless of the target-force difference. These results indicate that the rate modulation capabilities of the motor unit pool for biceps brachii are reduced in old adults. The purpose of the fourth study was to compare the discharge characteristics of motor units when recruited during contractions that required young and old adults to maintain a constant elbow angle while supporting a compliant load that was less than the recruitment threshold force of each isolated motor unit. The results indicated that the discharge of motor units in old adults was not modulated by an increase in load compliance, whereas it was in young adults. These findings demonstrate that old adults have a reduced ability to modulate motor unit discharge rate, which must contribute to the decline in motor system performance with advancing age.