A method for estimating the number of motor units in median innervated thenar muscles is described. In normal human subjects under 40 years of age, the mean and 1 SD was 253+34. The number of motor units falls with increasing age, especially after mid life, so that elderly subjects often have less than one-half the number in youth despite good strength. The cause is not known but probably includes peripheral nerve trauma and `primary neuronal cell death'.
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