Modulation of Motor Unit Discharge Has Task-Dependent Influences on Fluctuations in Motor Output

Abstract

2194 The rate of increase in the fluctuations in motor output during a sustained isometric contraction with the elbow flexor muscles is less when the wrist is restrained in a force transducer (force task) compared with when the wrist supports an equivalent inertial load (position task). PURPOSE: To determine the contribution of modulation in motor unit discharge to the difference in the output fluctuations during the two submaximal contractions. METHODS: The discharge of 17 motor units was recorded in 10 men (23.9 ± 4.3 yr, mean ± SD) who performed the force and position tasks in a randomized order with the non-dominant arm. The load torque applied at the wrist and contraction time (range 45–245 s) for the two tasks were identical for each subject. Motor output was quantified as the fluctuations (standard deviation, SD) and power spectrum of force (force task) and acceleration (position task). Motor unit discharge was characterized in terms of its variability (coefficient of variation, CV) and power spectrum. RESULTS: The rate of increase in the acceleration SD during the position task was greater than the rate of increase in the force SD during the force task (p<0.01). Although the CV of motor unit discharge was higher for the position (23.7 ± 10.7%) compared with the force task (20.8 ± 7.8%), the change in the CV for discharge rate was significantly associated only with the change in acceleration SD (r2 = 0.3, p<0.01), but not the change in the force SD. The power spectra for motor unit discharge were similar for the two tasks, with peaks at ∼16–20 Hz. Nonetheless, the change in vertical force SD (r2 = 0.34, p<0.01) was associated with the change in power of motor unit discharge at 17–18 Hz (r = 0.53) and 31–32 Hz (r = −0.29), whereas the change in vertical acceleration SD (r2 = 0.21, p<0.01) was associated with the change in power of motor unit discharge at 7–8 Hz (r = 0.46). CONCLUSIONS: The rates of increase in the fluctuations of motor output for the two tasks were associated with modulation of different bandwidths in motor unit discharge. Supported by NIH award R01 AG09000 to RME and ACSM doctoral student research grant (FRG 26) to CJM.