Characteristics of the Electrophysiological Properties of Neuromuscular Motor Units and Its Adaptive Strategy Response in Lower Extremity Muscles for Seniors with Pre-Sarcopenia: A Preliminary Study

Abstract

Older adults with sarcopenia, which is an aging-related phenomenon of muscle mass loss, usually suffer from decreases in both strength and functional performance. However, the causality between function loss and physiological changes is unclear. This study aimed to explore the motor unit characteristics of the neurological factors between normal subjects and those with sarcopenia. Five risk-sarcopenia (age: 66.20 ± 4.44), five healthy (age: 69.00 ± 2.35), and twelve young (age: 21.33 ± 1.15) participants were selected. Each participant performed knee extension exercises at a 50% level of maximal voluntary isometric contraction. Next, electromyogram (EMG) signals were collected, and information on each parameter-e.g., motor unit number, recruitment threshold, the slope of the mean firing rate to recruitment threshold, y-intercept, firing rate per unit force, and mean motor unit firing rate (MFR)-was extracted to analyze muscle fiber discrimination (MFD). Meanwhile, force variance was used to observe the stability between two muscle groups. The results suggested that there was no difference between the three groups for motor unit number, recruitment threshold, y-intercept, mean firing rate, and motor unit discrimination (p > 0.05). However, the slope of MFR and firing rate per unit force in the risk-sarcopenia group were significantly higher than in the young group (p < 0.05). Regarding muscle performance, the force variance in the non-sarcopenia group was significantly higher than the young group (p < 0.05), while the risk-sarcopenia group showed a higher trend than the young group. This study demonstrated some neuromuscular characters between sarcopenia and healthy elderly and young people when performing the same level of leg exercise tasks. This difference may provide some hints for discovering aging-related strength and function loss. Future studies should consider combining the in vivo measurement of muscle fiber type to clarify whether this EMG difference is related to the loss of muscle strength or mass before recruiting symptomatic elderly participants for further investigation.

Keywords: Decomposed Electromyography; aging; motor unit; sarcopenia.

Figure 1

Characteristics of motor units. (a) Motor units are recruited from the smallest to largest with increasing force production. Each circle represents a motor unit, and the area of each circle shows the different sizes of the motor unit pool. (b) The mean value of the motor unit firing rates plotted as functions of recruitment threshold. Regression lines are drawn through the data from individual contractions, with each data point representing an individual motor unit.

Figure 2

Steady contraction during 50% MVC. The black line represents the force output target; the red line represents actual measurement of subject’s muscle contraction force form the real-time monitor.

Figure 3

The relationship between muscle force output and motor unit MFR in the time-domain. (a) The line shows the muscle force output in the maximum voluntary contraction force percentage during the task. (b) Each grey curve represents the time registered firing rate of a motor unit during the contraction task; the short dash line indicated the range of one standard deviation, and the solid line represented the mean firing rate of all detected motor units between 10–20 s.