Journal of Dental Research, Vol 72, 1499-1502, Copyright © 1993 by International & American Associations for Dental Research Online Journals
Co-activation of sternocleidomastoid muscles during maximum clenching
G. T. Clark, P. A. Browne, M. Nakano and Q. Yang
UCLA Dental Research Institute, School of Dentistry 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.
