Recruitment thresholds of lower-lip motor units with changes in movement direction.

Recruitment threshold may provide a useful measure for understanding the functional organization of speech muscles. The objective of the present study was to describe the recruitment thresholds of orbicularis oris inferior ( OOI ) and mentalis ( MENT ) motor units with changes in the direction of lower lip movement. Three subjects produced slow ramp displacements of the lower lip in three directions while simultaneous single motor unit recordings were obtained. Analysis was performed on 28 OOI units and 20 MENT units. The OOI motor units had their lowest recruitment thresholds for anterior lip movement, and the MENT units had their lowest thresholds for superior movements. In general, motor unit recruitment order was fixed and recruitment thresholds remained proportional with changes in the direction of movement. There was one instance of a recruitment order reversal in a pair of MENT motor units.