CHANGES IN MOTOR UNIT ESTIMATES WITH AGING

Elderly persons usually exhibit some degree of muscle atrophy, togethe r with a reduction in voluntary strength, but there is still argument concerning the nature of the cellular events involved. This issue was reexamined by estimating the numbers and relative sizes of motor units in three limb muscles, using a fully automated system (Galea et al., 1993). In 79 healthy volunteers aged 20-98 years, estimations of motor unit numbers were performed on the thenar, biceps brachii, and extens or digitorum brevis muscles. Motor unit populations were noted to decr ease significantly with age in the distal muscles but appeared to rema in constant in the biceps. The excitable muscle fiber mass, as reflect ed in the peak-to-peak amplitude and area of the maximum M-wave, was d iminished in all three muscles. Although the area of the average motor unit action potential was not significantly different between groups, the ratio of this potential to the M-wave area increased with increas ing age. The results suggest that muscle deterioration in the elderly is due to a combination of changes in the muscle fibers and in their n erve supply and that the extent may differ between proximal and distal muscles.