Effect of age and gender on the number of motor units in healthy subjects estimated by the multipoint incremental MUNE method.

Abstract

Motor unit number estimation (MUNE) is a tool for estimating the number of motor units. The aim was to evaluate the multipoint incremental MUNE method in a healthy population, to analyze whether aging, gender, and the dominant hand side influence the motor unit number, and to assess reproducibility of MUNE with the Shefner modification. We studied 60 volunteers (mean age, 47 ± 17.7 years) in four groups aged 18 to 30, 31 to 45, 46 to 60, and above 60 years. Motor unit number estimation was calculated in the abductor pollicis brevis (APB) and the abductor digiti minimi (ADM) by dividing the single motor unit action potential amplitude into the maximal compound motor action potential amplitude. Test-retest variability was 7%. The mean value of MUNE for APB was 133.2 ± 43 and for ADM was 157.1 ± 39.4. Significant differences in MUNE results were found between groups aged 18 to 30 and 60 years or older and between groups aged 31 to 45 and 60 years or older. Motor unit number estimation results correlated negatively with the age of subjects for both APB and ADM. Single motor unit action potential, reflecting the size of motor unit, increased with the age of subjects only in APB. Compound motor action potential amplitude correlated negatively with the age of subjects in APB and ADM. Significant correlations were seen between MUNE in APB or ADM and compound motor action potential amplitude in these muscles and the age of female subjects. A similar relationship was not found in males. Multipoint incremental MUNE method with the Shefner modification is a noninvasive, easy to perform method with high reproducibility. The loss of motor neurons because of aging could be confirmed by our MUNE study and seems to be more pronounced in females.