Motoneuron Loss Is Associated With Sarcopenia

Abstract Objectives Sarcopenia, age-related muscle wasting, is associated with increased morbidity and mortality in the affected individuals. The pathogenesis of sarcopenia is not yet fully understood. A multifactorial concept is currently favored. The reduced number of motor units as a potential mechanism of muscle mass loss is explored in the present study. Design This is a cross-sectional study. Setting The participants were community-dwelling older adults. Participants The participants were sarcopenic (75) and nonsarcopenic (74) according to the criteria of the European Working Group on Sarcopenia in Older People aged 65 to 94 years. Measurements The motor unit number index (MUNIX) of the hypothenar muscle was used to assess the number and size [motor unit size index (MUSIX)] of motor units. Results The participants with pathologic MUNIX and MUSIX (n = 23) are significantly more frequently sarcopenic (n = 17, P = .029) than nonsarcopenic (n = 6). The participants with pathologic MUNIX and MUSIX (n = 23) had significantly less muscle mass than the nonsarcopenic controls ( P < .001). After adjusting for age and sex, only gait speed has shown no difference between the 2 groups. Pearson's correlation coefficient between MUSIX and the reciprocal value of MUNIX is 0.87 ( P < .001). Conclusions Sarcopenia induced by a small number of motoneurons can be identified by applying the MUNIX method to the hypothenar muscle. An enlargement of motor units because of motoneuron loss seems to preserve physical performance.