Synergist coactivation and substitution pattern of the human masseter and temporalis muscles during sustained static contractions.

OBJECTIVE Previous reports indicated that between-muscle substitution of active motor unit pools can be found in a variety of synergist muscles, including shoulder and leg muscles, but little information is available for the masticatory muscles. We hypothesized that, during a prolonged clenching effort performed at low- to moderate-bite force levels, a substitution pattern of activity can be found also in the masseter and anterior temporal muscles. METHODS Ten healthy volunteers were recruited and were asked to clench unilaterally on a force transducer for 10min at 10%, 15%, and 20% of the maximum bite force. During each session, bite force, perceived muscle pain and electromyographic activity were continuously assessed. Data analyses were performed by means of cross-correlation and periodogram analyses. RESULTS During sustained static contractions, different contraction patterns of jaw elevator muscles could be identified. These included a coactivation pattern, a substitution pattern, and several intermediate situations between coactivation and substitution. CONCLUSIONS The findings support the concept that the masticatory muscles are functionally heterogeneous and provide evidence that the neuromuscular strategies used by the masticatory system to perform sustained static contractions differ between individuals. SIGNIFICANCE Individual neuromuscular strategies might play a role in the development of masticatory muscle pain conditions.