Suprahyoid biomechanics and head posture. An electromyographic, videofluorographic and dynamographic study of hyo-mandibular function in man.

Suprahyoid and masseter muscle function was studied in 49 adult males with normal dentofacial morphology and normal soft tissue profiles, with special reference to head posture. The different parameters were investigated by EMG during chewing, by EMG synchronized to an opening force dynamograph during static and dynamic conditions, and by EMG synchronized to videofluorography during the open-close-clench cycle. Cephalometry was applied to evaluate the videofluorographic images of the hyo-mandibular complex. The EMG amplitude for the entire group did not vary significantly between different test occasions with an interval of 5-24 hours. The individual variation was large, however. Repeated EMG registrations within such time intervals should therefore be reliable for groups of individuals. Hook electrodes gave about 10% higher maximal EMG compared to surface electrodes. Registrations with the two types of electrodes could therefore not be directly compared. An increased interelectrode distance resulted in larger EMG amplitudes. The interelectrode distance should therefore be standardized. Head posture influenced the EMG amplitude and should therefore also be standardized. The electrical activity in suprahyoid muscle contraction preceded the mechanical activity. During opening, a time- lag of approximately 200 msec was found between the peak EMG activity of the anterior belly of the digastric muscle and the maximal opening force. A direct correlation existed between the IEMG activity of the digastric muscle and force during static (r = 0.89) and dynamic (r = 0.72) conditions. The maximal recorded force of the opening muscles of the mandible was found to be 70 N. No absolute reciprocity existed between opening and closing EMG activity. During chewing, suprahyoid maximal IEMG activity was less than one-third of that from the masseter. Maximal IEMG activity was reduced for the masseter during forward flexion, and for the suprahyoid muscle group during backward extension. Head posture is an important factor in EMG registrations of opening and closing muscle function during chewing. During upright head posture, the hyoid bone exhibited on average an elliptical movement pattern and reached its most elevated position at the start of opening and its most depressed position at the start of closing. A small movement before opening an upward and forward direction and before closing in a further downward and backward direction was found. Suprahyoid EMG activity was registered before start of opening, during opening and during the first half of the open phase.(ABSTRACT TRUNCATED AT 400 WORDS)