Motoneuron loss is associated with sarcopenia.

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.