Masseter thickness, endurance and exercise-induced pain in subjects with different vertical craniofacial morphology.

The aim of the study was to compare neuromuscular features of the masseter muscle in subjects with different vertical craniofacial morphology. Fifteen short-faced (mandibular plane-Frankfurt plane angle < 15 degrees) and 15 normal- to long-faced (mandibular plane-Frankfurt plane angle > or = 23 degrees) male students participated. The thickness of the masseter was assessed by ultrasonography. Onset and endurance of exercise pain were recorded during sustained biting at a level of 15% of maximum voluntary contraction and 30 micro V electromyographic activity. Pain and fatigue was measured on visual analog scales before and after the biting, as well as before and after 10 min chewing. Statistical comparison showed that the masseter muscle was significantly thicker (+15%) in the short-faced than the normal- to long-faced subjects. The pain onset time and endurance time were also consistently shorter in short-faced subjects, whereas the intensity of pain and fatigue did not differ significantly between the two groups. Multiple stepwise regression showed positive influence from the mandibular plane inclination and the masseter thickness on the pain onset time and endurance time. The present findings support the concept that subjects with different craniofacial morphology show neuromuscular differences.