Age-related change in motor unit activation strategy in force production: a mechanomyographic investigation

This study was designed to examine the effect of increasing age on the recruitment and activation of motor units (MU) in the biceps brachii muscle, using the mechanomyogram (MMG)/force relationship during isometric ramp contractions. The relationships between the root mean squared amplitude (RMS) and mean power frequency (MPF) of the MMG and relative force (% MVC) in the elderly (male, n = 10, age = 69.8 +/- 4.7 years, mean +/- SD) were markedly different from those in the young group (male, n = 15, age = 22.7 +/- 1.8 years). In elderly individuals, the RMS increased progressively with force up to 57.6 +/- 3.4% MVC, when a brief rapid increase was followed by a stable trend beyond 63.6 +/- 3.7% MVC. The MPF increased slowly up to 59.4 +/- 2.3% MVC; after a temporary reduction from 59.4% to 64.3 +/- 2.0% MVC, it then increased progressively again. In conjunction with absolute force (F(abs)), both the RMS and MPF in the elderly were smaller than those in the young group throughout the submaximal levels of force exerted. The results reflect an alteration in MU activation strategy, with a predominant role for MUs with slow-twitch fibers and an effective fused tetanus induced at lower firing rate of the MUs, resulting from age-related neuromuscular changes.