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The changes in lip activity associated with retraction of the upper labial segment during treatment of Class II division 1 malocclusion were investigated electromyographically. In the lip seal position muscle activity of the circumoral muscles was much reduced after treatment, but the amount of reduction appeared to be unrelated to the amount of incisor retraction. In some cases lips which were judged to be incompetent before treatment were competent after treatment. Activity in the suprahyoid muscle group associated with upper incisor inclination and overjet when the lips were sealed was eliminated during swallowing.
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Mary McF. Simpson (1977) studied this question.
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