Association between motor unit remodeling and interindividual variability in muscle strength of older adults.

Marked interindividual variability in muscle strength exists among older adults, which cannot be fully explained by differences in muscle mass alone. Although motor unit remodeling is a characteristic feature of neuromuscular aging, whether variability in motor unit remodeling contributes to interindividual differences in muscle strength remains unclear. The purpose of this study was to examine the association between motor unit remodeling and muscle strength in older adults. Forty-nine older adults (25 men and 24 women) participated in this study. Motor unit parameters of the abductor pollicis brevis muscles were estimated using the MScanFit2 program. From the stimulus-response curve, motor unit number estimation (MUNE), size of motor units at the 50% mark of the cumulative amplitude expressed as a percentage (A50), the largest surface motor unit potential (L-sMUP) (motor unit size), estimated number of larger units making up 50% of the amplitude of compound muscle action potential (N50), and N50/MUNE (motor unit size composition) were derived. Associations between motor unit parameters and pinch strength were examined using partial correlation analyses adjusted for age, sex, and body weight. Sex-specific analyses were also performed. Pinch strength was significantly correlated with MUNE (r = 0.360; P = 0.014), A50 (rs = -0.394; P = 0.007), L-sMUP (rs = -0.389; P = 0.007), N50 (r = 0.416; P = 0.004), and N50/MUNE (r = 0.439; P = 0.002). These correlations remained significant only in men in sex-specific analyses. A lower motor unit number and shift toward larger motor unit sizes, reflecting greater motor unit remodeling, were associated with lower pinch strength. These findings suggest that motor unit remodeling may contribute to interindividual variability in muscle strength among older adults, particularly men.NEW & NOTEWORTHY This study demonstrated that, after adjustment for age, interindividual variation in motor unit remodeling is associated with muscle strength in older adults. Furthermore, by evaluating motor unit size composition in addition to conventional measures of motor unit number and size, this study provides a more comprehensive characterization of motor unit remodeling. These findings provide new insight into the neuromuscular mechanisms underlying marked interindividual variability in muscle strength during aging.