Properties of single motor units of the extensor digitorum brevis in elderly humans.

The axonal conduction velocity and voluntary discharge properties of single motor units of the extensor digitorum brevis were studied in elderly subjects. The results were compared with the findings of a previous study of younger subjects. The range of the axonal conduction velocities differed only slightly between the age groups, the elderly subjects having velocities of 25-52 m/sec and the younger subjects 30-54 m/sec. In both age groups, motor units firing tonically within the range of 10-30 Hz had axonal conduction velocities below 45 m/sec, and motor units firing mainly phasically within the range of 20-60 Hz had axonal conduction velocities above 40 m/sec. Motor units with an intermediate type of discharge pattern were more frequently seen in the elderly. In the elderly subjects, these motor units had axonal conduction velocities distributed within the whole range, while in the younger subjects, they had axonal conduction velocities in the middle of the range. An autopsy study showed an increased proportion of muscle fibers with intermediate characteristics of ATPase staining in the elderly subjects. It is suggested that the decreased differentiation of the muscle fiber histochemistry might be secondary to the decreased differentiation of the motoneuron firing properties.