Aging and skeletal muscle force control: Current perspectives and future directions

During voluntary muscle contractions, force output is characterized by constant inherent fluctuations, which can be quantified either according to their magnitude or temporal structure, that is, complexity. The presence of such fluctuations when targeting a set force indicates that control of force is not perfectly accurate, which can have significant implications for task performance. Compared to young adults, older adults demonstrate a greater magnitude and lower complexity in force fluctuations, indicative of decreased steadiness, and adaptability of force output, respectively. The nature of this loss-of-force control depends not only on the age of the individual but also on the muscle group performing the task, the intensity and type of contraction and whether the task is performed with additional cognitive load. Importantly, this age-associated loss-of-force control is correlated with decreased performance in a range of activities of daily living and is speculated to be of greater importance for functional capacity than age-associated decreases in maximal strength. Fortunately, there is evidence that acute physical activity interventions can reverse the loss-of-force control in older individuals, though whether this translates to improved functional performance and whether lifelong physical activity can protect against the changes have yet to be established. A number of mechanisms, related to both motor unit properties and the behavior of motor unit populations, have been proposed for the age-associated changes in force fluctuations. It is likely, though, that age-associated changes in force control are related to increased common fluctuations in the discharge times of motor units.