Mandible position and head posture: electromyography of sternocleidomastoids.

A study was performed to evaluate relationships between mandible position, dynamics, muscle activity and head posture while swallowing by use of surface EMG and mandible kinesiograph on two population groups (118 pathologics and 31 controls). The study produced the following: 1. specific mandible dynamics with a very fast rising phase (0.3 sec) and longer phase of stabilization (1.5 sec); 2. more than 60% of the subjects presented deglutition at occlusion level, the others swallowing at a distance of 0.1-4.6 mm; 3. the whole muscle activity (temporals, masseters, digastrics, sternocleidomastoids) lasted 1.5 sec with no correlation of duration to age; 4. sternocleidomastoids fired at swallowing with an effort of one-half of temporals or masseters; 6. firing order presented a particular pattern: digastrics more often first, sternocleidomastoids more often last. No differences were found between the pathologics and controls. Findings suggest that the oral phase or mandible dynamic, and the stabilization phase or oropharyngeal phase of swallowing, has an individual role that is important in head postural equilibrium.