Motor unit firing behavior during prolonged 50% MVC dorsiflexion contractions in young and older adults.

The purpose of this study was to investigate changes in motor unit firing behavior during prolonged contractions in young and older adults. Motor unit activity was recorded from the tibialis anterior of 16 subjects (8 young and 8 older), while they performed isometric dorsiflexion at 50% MVC until task failure. Mean motor unit firing rate, the standard deviation (SD), and coefficient of variation (CV) of the interspike intervals, and number of doublet discharges were calculated for a total of 52 motor units, tracked for an average of 92.9+/-68.6s. There was no age-related difference in the time to task failure. A modest decline in firing rate was observed in 71% of the motor units, with no significant age-related difference. The SD and CV of the interspike interval had a positive slope in 65% and 69% of the motor units, respectively, with no significant age-related differences. The number of doublet discharges remained stable throughout the contraction. Both groups exhibited motor unit dropout (discharge cessation) during the contraction. Thus, a fatiguing task producing modest changes in firing rate in young and older adults is accompanied by an appreciable increase in firing rate variability. The incidence of doublet discharges is not increased during fatiguing contractions.