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The effects of aging on maximal voluntary strength and on the isometric twitch were determined in the ankle dorsiflexor and plantarflexor muscles of 111 healthy men and women aged 20-100 yr. Men were found to be stronger than women at all ages. In both sexes, the average values for maximum voluntary strength of the dorsiflexors and plantarflexors began to decline in the 6th decade. Although the absolute loss of strength was greater for the plantarflexor muscles, the relative losses were similar in the two muscle groups. During maximum voluntary effort, stimulation of motor nerves produced no additional torque in the majority of elderly men and women, indicating that these subjects remained able to utilize their descending motor pathways for optimal muscle activation. Comparisons of muscle compound action potentials, twitch torques, and muscle cross-sectional areas suggested that a decrease in excitable muscle mass was entirely responsible for the lower strength of the elderly. An additional effect of aging was the gradual prolongation of twitch contraction and half-relaxation times throughout the adult life-span.

SUMMARY Physiological studies have been made of extensor digitorum brevis muscles in 28 healthy subjects aged between 60 and 96. Within this elderly population there was evidence of muscle wasting and weakness. These changes were shown to result from a loss of functioning motor units. The surviving motor units were often enlarged and tended to have relatively slow twitches. In some subjects the maximum impulse conduction velocities were reduced in motor nerves; there was evidence that slowing of impulse conduction could be especially marked in distal regions of axons. The findings are considered to indicate the presence of motoneurone dysfunction in old age.

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