In the elderly group, muscle thickness, RMS, and normalized RMS were significantly correlated with MVC (
p < 0.05) (Table
1) and in stepwise regression analysis muscle thickness, RMS, and normalized RMS were selected as determinants of MVC (Table
3). Also, stepwise regression analysis shows high standardized regression coefficient in RMS (0.288,
p = 0.001) in addition to MT (0.519,
p < 0.001) (Table
3). Although significant correlation was not observed, tendency to correlate between MVC and RMS was also shown in young (
p = 0.054) (Table
2). These results suggest that the variables related to neuromuscular function are also major determinants of muscle strength in addition to indicators of muscle volume such as muscle thickness [
16,
31,
42,
43].
Since voluntary muscle contraction is regulated by central nervous system, it would be reasonable to conclude that neuromuscular function contributes to individuals’ muscle strength. EMG amplitude variables, i.e., RMS and normalized RMS, mainly reflect number of recruited motor units and its firing rate during voluntary contraction. Decreases in number of motor unit [
24,
33] and in motor unit firing rate [
34] with ageing are well known. These changes would induce decrease in EMG amplitude in the elderly. Merletti et al. [
26] reported a significantly lower averaged rectified value of surface EMG of biceps brachii muscle during submaximal isometric contractions in elderly compared with young [
26]. On the other hand, age-related decrease in EMG amplitude may not be uniformly manifested among elderly individuals. Power et al. [
33] demonstrated that the estimated number of motor units in tibialis anterior muscle were significantly lower in elderly than young, but not in master runners [
33]. In our recent study that used a decomposition of motor units action potential from multi-channel surface EMG, positive correlation between MVC and motor unit firing rate was demonstrated in the VL muscle for elderly [
38]. Variation in EMG amplitude among individuals and relationship between EMG amplitude and muscle strength in the elderly may be brought about by these inter-individual differences in age-related alterations for anatomy and/or regulation of motor units [
24,
33,
34,
38]. However, we should note that surface EMG is strongly influenced by geometric or non-physiological parameters [
12,
27]. When amplitude variables are compared among the individuals, differences in the thickness of the subcutaneous tissue should be considered [
28]. To quantify the effect of inter-individual differences in the subcutaneous tissue on relationship between EMG amplitude and muscle strength, we additionally calculated partial correlation coefficient between RMS and MVC adjusted for the thickness of the subcutaneous tissue in the elderly. Adjusted correlation coefficient (
r = 0.291,
p = 0.009) was slightly lower than the value before adjustment (
r = 0.361,
p = 0.001). While this means that relationship between MVC and RMS was influenced by the thickness of the subcutaneous tissue, significance in correlation coefficient value is unchanged. We thus assumed that the thickness of the subcutaneous tissue was not critical effect on our results.