1. The effect of age on the motor output of the first dorsal interosseous muscle of 22 (6 female, 16 male) human subjects was investigated. The purpose of the study was to determine the effect of age on the control of muscle force and the associated changes in the discharge behavior and mechanical properties of single motor units. 2. Each subject performed three tasks requiring isometric abduction of the left index finger: a maximum voluntary contraction (MVC), a constant-force task, and a threshold task. The ability to control force was assessed during the constant-force task by quantifying the variation in isometric force about four submaximal target forces (5, 20, 35, and 50% MVC). The threshold task involved sustaining the discharge of the isolated motor unit at a low, steady rate for ~3 min. 3. The discharge behavior and the mechanical properties of single motor units were determined during the threshold task by measuring the interimpulse intervals and the peak amplitude and time to peak of the spike-triggered average force. 4. The data indicated that age had an effect on the variation of force about submaximal target forces (range: 5-50% MVC), and that these force variations, when calculated relative to the target force, were greater at lower force levels in the elderly subjects. 5. On the basis of the mean interimpulse interval and the variability in the interimpulse interval, no differences were identified between the young and elderly subjects in the discharge behavior of 159 (young subjects: n = 83; elderly subjects: n = 76) identified single motor units when discharging at low, steady rates. 6. The mean spike- triggered average force (percentage of MVC) exerted by individual motor units discharging at low, steady rates was larger in elderly subjects. However, the mean time to peak of the spike-triggered average force was similar for the two age groups. 7. The force exerted during the threshold task was subjected to a spectral analysis. No difference was found in the frequency of the peak in the power spectrum between the young and elderly groups. The amplitude of the peak, however, was significantly greater for the elderly subjects. These results suggest that the fluctuation of force at frequencies associated with unfused discharge rates in motor units was larger in elderly individuals during the threshold task. 8. We conclude that the reduced ability of elderly subjects to maintain a constant submaximal force is at least partially due to the larger forces exerted by low-threshold motor units. This association is probably due to the peripheral reorganization of motor units that accompanies aging, although a role for an increase in the synchronization of motor unit discharge cannot be excluded.
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