title: Co-activation of sternocleidomastoid muscles during maximum clenching.
abstract: In an attempt to determine the degree of co-activation present in selected cervical muscles during clenching, we instructed 12 male subjects to produce four brief maximum voluntary contraction (MVC) efforts (clenching) in a position of maximum intercuspation. Surface EMG activity was recorded bilaterally from the masseter and sternocleidomastoid (SCM) muscles. The contraction level for the SCM during clenching was reported as a percentage of the SCM's maximum activity achieved during maximum neck flexion against resistance. All EMG signals for the masseter and SCM were converted to a true RMS voltage signal and digitized at a 100-Hz sampling rate. Mean peak EMG voltage levels were determined for the activity recorded during each brief MVC task. All subjects demonstrated co-activation of the SCM during strong abrupt clenching efforts. The mean levels (+/- S.D.) of SCM activity were 11.8 +/- 9.6% (right) and 14.2 +/- 9.4% (left) of the MVC capacity. Fifty percent of masseter activity was required to achieve 5% activity of the SCM bilaterally, and there was a progressive development of the SCM co-activation which paralleled the masseter activation.
authors.affiliation: UCLA Dental Research Institute, School of Dentistry 90024-1668.
authors.forename: G T
authors.initials: GT
authors.lastname: Clark
authors.forename: P A
authors.initials: PA
authors.lastname: Browne
authors.forename: M
authors.initials: M
authors.lastname: Nakano
authors.forename: Q
authors.initials: Q
authors.lastname: Yang
mesh_terms: D000328:Adult; D003766:Dental Occlusion; D004576:Electromyography / Q000379:methods; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008297:Male; D008334:Mandible / Q000502:physiology*; D008406:Masseter Muscle / Q000502:physiology; D009119:Muscle Contraction / Q000502:physiology*; D009334:Neck Muscles / Q000502:physiology*; D011187:Posture; D013703:Temporal Muscle / Q000502:physiology