Fig. 1.

Experimental setup. Participants were seated in a dental chair, and their head was positioned in the Frankfurt horizontal plane and immobilized with a cephalostat mounted from the ceiling. A bite block was used to fix the jaw with a 1-cm incisor gap. The movement of the lips and jaw was tracked with a standard strain-gauge cantilever system. Surface electromyography (EMG) recorded the muscle activity of the upper and lower lips. Perturbations in the lower lip were delivered using a commercial lever system