Declines in the control of movement experienced by older adults are mediated by adaptations in motor unit activity. It has been proposed, for example, that the reduced ability of older adults to perform steady contractions at low forces is attributable to a greater variability in the discharge of action potentials by motor neurons. To evaluate this hypothesis, the discharge characteristics of motor units in a hand muscle of young and old adults were determined during brief isometric contractions over a range of contraction intensities. The experimental measurements were imported into a computational model of motor unit recruitment and rate coding to examine the association between discharge variability and force steadiness. The results indicated a strong association between population levels of motor unit discharge variability and steadiness, but there were no differences in discharge variability between young and old adults. Nonetheless, old adults are often less steady than young adults, especially when exerting low forces and after being exposed to a stressor that increases physiological arousal. Furthermore, there are moderate levels of association across the lifespan between measures of steadiness during weak contractions with hand muscles and performance on tests of hand function. Collectively, these findings underscore the significant influence of task specificity when comparing the performance capabilities of young and old adults. As the task requirements change, so does the synaptic input received by the motor neurons. A striking example of a difference between young and old adults is the discharge characteristics of motor units recruited during sustained, submaximal contractions. Discharge variability can be much greater for young adults when they sustain isometric contractions at different target forces compared with old adults, which suggests that some combination of the synaptic input and the intrinsic properties of the motor neurons differ for these groups of individuals. Furthermore, similar improvements in the accuracy of a rapid isometric contraction to a target force were accomplished by different adaptations in the agonist and antagonist muscles by young and old adults. These comparisons indicate that understanding the declines in the control of movement by older adults requires studies that focus on the relevant adaptations that influence the control of the involved muscles.