Lower lip displacement during voluntary activation of individual labial muscles in man

Displacement was measured at the lower lip midline and the corner of the mouth during isolated voluntary activation of individual labial muscles. Three normal adult subjects were studied and the procedure was repeated three times for one subject Each subject. was trained with auditory and visual feedback to develop electromyographic activity in each muscle separately without activity in other labial muscles. It was possible to isolate mentalis in four of the five sessions and depressor labii inferior in three sessions; however, in no instance, were subjects able to isolate orbicularis oris inferior or depressor anguli oris. Activation of mentalis moved the midline of the lower lip superiorly and anteriorly, but depressor labii inferior moved it inferiorly and posteriorly.