Effects of Aging on Motor-Unit Control Properties

TLDR

Age‑related changes in motor unit morphology are thought to be accompanied by altered control mechanisms that help maintain smooth force production. The study aimed to examine how aging affects the simultaneous firing patterns of multiple motor units in the FDI muscle during submaximal isometric contractions. Data from 10 young and 10 elderly participants were analyzed to assess motor unit firing behavior. Elderly subjects exhibited reduced common firing fluctuations, disrupted firing‑rate versus recruitment‑threshold relationships, and a loss of the progressive firing‑rate decline seen in young, confirming earlier reports of lower average rates, lower recruitment thresholds, and multiphasic action potentials, and indicating major age‑related changes in motor‑unit control.

Abstract

It was hypothesized that the age-related alterations in the morphological properties of a motor unit would be accompanied by modifications in the control aspects of the motor unit, as either an adaptive or compensatory mechanism to preserve smooth force production. In specific, the objective of the study was to investigate the age-related alterations in the concurrent firing behavior of multiple motor units in the first dorsal interosseous (FDI) muscle in isometric contractions at 20 and 50% of the subject's voluntary contraction level. Analysis of the data collected from 10 young (24–37 yr of age) and 10 elderly (65–88 yr of age) subjects led to three novel observations regarding the firing behavior of aged motor units. 1) Among elderly subjects, there is a decrease in the common fluctuations that are observed among the firing rates of motor units in the young. 2) The relationship observed between the firing rate and recruitment threshold of young subjects is disturbed in the elderly. Although in young subjects, at any point in a given submaximal contraction, earlier recruited motor units have higher firing rates than later-recruited units; in aged subjects this dependency of firing rate on recruitment rank is compromised. 3) The progressive decrease observed in the firing rates of concurrently active motor units in constant-force contractions in the young is not seen in the aged. In addition to these original findings, this study provided support for earlier reports of 1) decreased average firing rates probably reflecting the slowing of the muscle, 2) a shift in recruitment thresholds toward lower force levels in line with the shift toward type I fibers, and 3) multiphasic action potential shapes indicative of the reinnervation process that takes place during aging. Taken as a whole, these findings indicate significant age-related modifications in the control properties of human motor units.