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Co-activation of Sternocleidomastoid Muscles During Maximum Clenching

UCLA Dental Research Institute, School of Dentistry, 73-017 Center for Health Sciences, 10833 Le Conte Avenue, Los Angeles, California 90024-1668

P.A. Browne

Division of Physical Therapy, Chapman University, Orange, California 92666

M. Nakano

Tokushima University, School of Dentistry, Department of Fixed Prosthetics, Tokushima 770, Japan

Q. Yang

UCLA Dental Research Institute, School of Dentistry, 73-017 Center for Health Sciences, 10833 Le Conte Avenue, Los Angeles, California 90024-1668

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.

Journal of Dental Research, Vol. 72, No. 11, 1499-1502 (1993)
DOI: 10.1177/00220345930720110701


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