Discharge rate during low-force isometric contractions influences motor unit coherence below 15 Hz but not motor unit synchronization

The purpose of the study was to determine whether pairs of motor units that discharge action potentials at different rates during isometric contractions exhibit different levels of motor unit synchronization or coherence. Twelve subjects (28.6 +/- 6.1 years) performed isometric contractions at target forces slightly above the recruitment threshold (1.02-20.9%) of an isolated motor unit. Based on audio feedback, subjects maintained a relatively constant discharge rate of the isolated unit for about 80 s. Intramuscular electrodes were used to record the discharge of 47 pairs of motor units at rates that ranged from 8.07 to 13.6 pps. Correlated discharge between pairs of motor units was quantified with the common input strength (CIS) index, k' index, and coherence spectrum. Greater discharge rates across pairs of motor units were predicted (R-2 = 0.36, P < 0.001) by higher coherence from 8 to 13 Hz (r = -0.52) and lower coherence from 0 to 4 Hz (r = 0.37). Indexes of motor unit synchronization (CIS and k') were strongly associated with motor unit coherence from 16 to 32 Hz (CIS: R(2)p = 0.63; k': R-2 = 0.4; P = 0.001). The CIS index of motor unit synchronization and the motor unit coherence from 16 to 32 Hz did not vary with discharge rate. In contrast, the k' index of motor unit synchronization declined with discharge rate (r(2) = 0.20, P = 0.001). Furthermore, greater discharge rates across pairs of motor units were accompanied by higher motor unit coherence in the 8-13 Hz band and lower motor unit coherence in the 0-4 Hz band. These results demonstrate that differences in discharge rate between pairs of motor units in first dorsal interosseus during low-force, isometric contractions were associated with modulation of the correlation in the discharge times of the two motor units at frequencies less than 15 Hz.