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Motor unit (MU) potentials were recorded from brachial biceps of healthy subjects aged 5.5-79 years. The subjects were subdivided into young (5.5-19 year) and adult (37.5-79 year) groups, between which single MU discharge characteristics were compared. Firing rates were in the ranges of 8.3-21.7 s −1 (mean 12.87 s −1 ) and 5.9-18.7 s −1 (mean 11.08 s −1 ) for young and adult groups, respectively. Standard deviations (S.D.) of interspike intervals (ISIs) were in the range 4.84-11.57 ms (mean 8.39 ms) for the young group and 4.26-12.23 ms (mean 7.76 ms) for the adult group. Both differences were statistically significant (P < 0.001). Special attention was paid to the previously developed method of ISI variability analysis, which enabled the comparison of MUs with respect to afterhyperpolarization (AHP) duration of their motoneurones (MNs). The results show that AHP duration increases gradually with increasing age, which is in line with the transformation of muscle properties towards a slower phenotype. This transformation seems to be a continuous process, covering the entire lifespan of a human being, from childhood to senescence. The results presented here are significant for their insight into the ageing process of the neuromuscular system. The age-related change in AHP duration has not been investigated previously in human studies and has been met with ambiguous results in animal studies.

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