A scanning electromyographic study of the topography of human masseter single motor units.

The length of scanning EMG recordings over which motor-unit activity could be recorded varied between 0.6 and 12.5 mm. The mean length of the motor unit cross-section was less than that reported for large arm and leg muscles, confirming that masseter motor units contain fewer muscle fibres than those in large limb muscles. However, three units in three muscles had remarkably large motor-unit territories. Small motor-unit territories may indicate local specialization and a potential for selective recruitment of separate motor regions, which would favour fine adjustment of jaw movements. In contrast, large territories may reflect widespread motor-unit actions, advantageous in force development where fine movement control is less important, as in biting in the intercuspal position or opposing gravity. These findings emphasize the unique structural and functional features of the human mandibular motor system.