Torques produced by different compartments of the rabbit masseter muscle

Intramuscular microstimulation was used to evaluate the mechanical properti es of 6 different neuromuscular compartments of four adult rabbit masseter muscles. Single compartment stimulation was verified by simultaneous multic hannel electromyographic recordings. A multi-axis force-moment sensor was u sed to measure evoked reaction forces at the mandibular symphysis, and thes e records were used to determine the magnitude and directions of the torque s produced about the ipsilateral temporomandibular joint. The magnitude of the resultant torque vector differed between compartments. The most superfi cial compartments of the superficial masseter (MSS1a, MSS1p) produced the l argest torques, and compartments of the superficial masseter lying more cau dal, such as MSS3, produced the smallest torques. The rise time of this vec tor was shortest for the posterior deep masseter (MPPo) compartment and lon gest for compartment MSS1a. The directions of the torque vectors of differe nt compartments varied considerably. Torque vectors of deep compartments su ch as MPPo, and the pars reflexa of the superficial masseter (MSPR) produce d significant yaw torques toward the working masseter but very small lingua l roll torques. Torque vectors from superficial compartments such as MSS1 c ontain larger lingual roll components and smaller working yaw components. T hese observations are consistent with the hypothesis that neuromuscular com partments of the rabbit masseter muscle form output elements in the neural control of masticatory movements.