Association of Motor Unit Number and Size Estimation With Muscle Thickness and Strength in Healthy Young, Middle-Aged, and Older Adults

PURPOSE: Although age-related declines in muscle mass and strength are believed to result from changes in motor unit number and size, the association of motor unit number and size with muscle mass and strength in healthy individuals remains unclear. We aim to examine the association of motor unit number estimation (MUNE) and motor unit size estimation (MUSE) of abductor pollicis brevis (APB) with muscle thickness of APB and maximal pinch-grip strength among young, middle-aged, and older adults. METHODS: Forty-eight young adults (20-39 years; young), 89 middle-aged adults (40-59 years; middle-aged), and 47 older adults (60 years and older; older) participated in this study. MUNE and the mean single motor unit potential as MUSE were calculated using the MScanFit2. Muscle thickness of the APB was measured from ultrasound images. Maximal pinch-grip strength was assessed during maximal voluntary isometric contraction of lateral pinch. RESULTS: MUNE and MUSE were not correlated with muscle thickness in any of the groups. In addition, MUNE was not correlated with maximal pinch-grip strength in either the young or middle-aged group. A significant positive correlation between MUNE and maximal pinch-grip strength was observed in the older group. MUSE was not significantly correlated with maximal pinch-grip strength in any group. CONCLUSIONS: MUNE, not MUSE, is associated with maximal pinch-grip strength in older adults, but not in young or middle-aged adults. These findings suggest that MUNE may explain interindividual differences in neuromuscular function among healthy older adults. MUNE could be a useful biomarker for neuromuscular function in older individuals.