[Effect of head posture on muscle tonus of the masticatory muscles]

The integrated electromyographic study allows to determine action of the variation amplitudes of tonic activity of the masseter muscles, the anterior temporalis muscles and the trapezius muscles induced by angular cephalic displacements of 10 degrees in the three space planes, in sitting position (P.A) and upright position (P.D.). The direction of the variations can be simultaneously positive or negative according to the vertical reference position (P.R.) in the different muscles studied. The masseter muscles showed the largest variations of the P.A. to the P.D. Twenty per cent of the subjects can be classified in a hypertonic category.