Motor unit synchronization is enhanced during slow shortening and lengthening contractions of the first dorsal interosseus muscle
Semmler, JG, Kutscher, DV, Zhou, S & Enoka, RM 2000, 'Motor unit synchronization is enhanced during slow shortening and lengthening contractions of the first dorsal interosseus muscle', Annual meeting of Society for Neuroscience: abstract volume 26, 170.10, New Orleans, 4-9 November, Society for Neuroscience, Washington, DC.
Motor unit synchronization is a measure of the near simultaneous discharge in pairs of motor units, and provides information on the shared synaptic input to motor neurons. Due to technical difficulties, most measurements of motor unit synchronization have been made during isometric contractions. The purpose of the study was to compare the strength of motor unit synchronization (CIS index) during isometric, position-holding, and slow (1.7 degrees/s) shortening and lengthening (anisometric) contractions of the first dorsal interosseus muscle (FDI). Motor unit activity was recorded from the same motor units during these tasks with two fine-wire intramuscular electrodes in FDI. For comparison between the isometric and the position-holding task, 9 motor unit pairs were sampled from 6 subjects. The strength of motor unit synchronization (CIS ± SD imp/s) was greater (P < 0.05) during position holding (0.86 ± 0.31 imp/s) than isometric contractions (0.69 ± 0.40 imp/s). For all task conditions, 5 motor unit pairs were sampled from 4 subjects. Motor unit synchronization was greatest for the anisometric contractions (1.24 ± 0.66 imp/s), intermediate for the position holding (0.81 ± 0.38 imp/s) and least for the isometric contractions (0.70 ± 0.48 imp/s). Preliminary data from 3 motor unit pairs suggested that motor unit synchronization was consistently greater during lengthening contractions (1.75 ± 0.60 imp/s) compared with shortening contractions (1.41 ± 0.30 imp/s). These findings indicate that the strength of common input to FDI motor neurons is increased during tasks that require movement of the index finger.