Contents I. INTRODUCTION
The human masticatory system consists of upper and lower jaw, connected by two temporomandibular joints (TMJs)and by four pairs of masticatory muscles: the masseter, the temporalis, the lateral pterygoid, and the medial pterygoid. Craniomandibular function is the result of a complex interaction between the teeth, the TMJs, the masticatory muscles and the nervous system [1]. A better understanding of this interaction, however, may also provide valuable information about craniomandibular malfunctioning or dysfunction. For instance, it is well known that in several circumstances the masticatory muscles may become tender to palpation yielding sensations of fatigue, pain, and discomfort whose underlying mechanisms are still largely unknown. Quantitative assessments of the masticatory muscle or muscle groups are frequently obtained by means of measurements of the electromyographic activity (EMG), the bite force, the amount of work performed, the endurance time to specific dynamic or static masticatory tasks [2], [3], [4]. Pain and fatigue occurring in the jaw muscles are most often assessed by means of visual analogue scales, which consist of 100 mm horizontal lines where the subjects marks the perceived amount of sensation [5].
