INTER-INDIVIDUAL DIFFERENCE AND RELATIONSHIP WITH MOTOR FUNCTION AND MUSCLE MORPHOLOGY IN MOTOR UNIT NUMBER OF ABDUCTOR POLLICIS BREVIS MUSCLE IN HEALTHY MIDDLE-AGED MALES AND FEMALES

Author(s): WATANABE, K., IGAWA, K., TAKEDA, R., NOTO, Y., Institution: CHUKYO UNIVERSITY, Country: JAPAN, Abstract-ID: 824

INTRODUCTION:
The number of motor units (MUs) decreases with aging and this accelerate from 40-50s. This anatomical characteristic would be a crucial biomarker for age-related muscle dysfunctions because MU numbers should contribute to both motor functions and muscle morphology. However, MU number has been mainly measured in patients with neurological diseases, and its inter-individual variability and the associations between MU number and muscle functions and/or muscle morphology have not been investigated in healthy subjects. We aimed to investigate the inter-individual variability of MU number and its associations with motor function and muscle morphology in middle-aged healthy males and females.
METHODS:
The participants were 93 healthy middle-aged males and females (40s: n=59, 50s: n=29, 60s: n=5). The maximal voluntary contraction pinch force (MVC) and muscle thickness of abductor pollicis brevis muscle measured by ultrasonography (MT). MU number was estimated from a detailed electrical stimulation-electromyographic response curve by MScanFit-2 for abductor pollicis brevis muscle (MUNE). Inter-individual variability of MVC, MT, and MUNE were quantified by coefficient variation (CoV). Relationships between MVC, MT, and MUNE were quantified by the Spearmans rank correlation coefficient.
RESULTS:
MVC, MT, and MUNE were 9.7 ± 2.5kg, 0.4 ± 0.1cm, and 107.6 ± 28.7 and their CoV were 25.3%, 28.4%, and 26.7%, respectively. A significant correlation between MVC and MT was detected (r = 0.334, p = 0.001). There were no significant correlations with MUNE in MVC and MT (p > 0.05).
CONCLUSION:
In healthy middle-aged populations, MU number has similar amount of inter-individual variability with those of motor function and muscle morphology and is a biomarker independent of motor function or muscle morphology.