Effects of aging on motor-unit control properties

It was hypothesized that the age-related alterations in the morphological p roperties of a motor unit would be accompanied by modifications in the cont rol aspects of the motor unit, as either an adaptive or compensatory mechan ism to preserve smooth force production. In specific, the objective of the study was to investigate the age-related alterations in the concurrent firi ng behavior of multiple motor units in the first dorsal interosseous (FDI) muscle in isometric contractions at 20 and 50% of the subject's voluntary c ontraction 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 observat ions regarding the firing behavior of aged motor units. 1) Among elderly su bjects, there is a decrease in the common fluctuations that are observed am ong the firing rates of motor units in the young. 2) The relationship obser ved between the firing rate and recruitment threshold of young subjects is disturbed in the elderly. Although in young subjects, at any point in a giv en submaximal contraction, earlier recruited motor units have higher firing rates than later-recruited units; in aged subjects this dependency of firi ng rate on recruitment rank is compromised. 3) The progressive decrease obs erved in the firing rates of concurrently active motor units in constant-fo rce contractions in the young is not seen in the aged. In addition to these original findings, this study provided support for earlier reports of I) d ecreased average firing rates probably reflecting the slowing of the muscle , 2) a shift in recruitment thresholds toward lower force levels in line wi th the shift toward type I fibers, and 3) multiphasic action potential, sha pes indicative of the reinnervation process that takes place during aging. Taken as a whole, these findings indicate significant age-related modificat ions in the control properties of human motor units.