Alteration of motor-unit discharge characteristics in aged humans.

The purpose of this study was to determine and compare the discharge characteristics of single motor-unit action potentials (SMUAPs) in young and aged subjects during 5, 10, and 20 percent of maximum voluntary contraction (MVC) of the abductor digiti minimi manus muscle. Three decades of aged subjects ranging from 60 to 90 years of age were compared with a group of 20 to 35 year olds. Specially constructed fine-wire bifilar recording electrodes were used, and SMUAPs were identified by interactive computer processing. Although the results did not reach a statistically significant level, they showed the 70- to 79-year-old age group had the longest mean interspike intervals (ISI) in each of the three percentages of MVC. The prolonged ISI indicate a slow motor-unit discharge rate. In addition, the 70- to 79-year-old group had a statistically significant floating standard deviation, indicating increased variability of discharge, and a significantly different negative floating serial correlation coefficient. All three factors describing motor-unit discharge behavior in the 70- to 79-year-old age group suggest possible substitution of larger motor units for smaller motor units normally active at low tensions.