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Arthur W. English, Dario I. Carrasco, and Charles G. Widmer

Intramuscular microstimulation was used to evaluate the mechanical properties of 6 different neuromuscular compartments of four adult rabbit masseter muscles. Single compartment stimulation was verified by simultaneous multichannel electromyo-graphic recordings. A multi-axis force-moment sensor was used to measure evoked reaction forces at the mandibular symphysis, and these records were used to determine the magnitude and directions of the torques produced about the ipsilateral temoromandibular joint. The magnitude of the resultant torque vector differed between compartments. The most superficial compartments of the superficial masseter (MSSla, MSSlp) produced the largest torques, and compartments of the superficial masseter lying more caudal, such as MSS3, produced the smallest torques. The rise time of this vector was shortest for the posterior deep masseter (MPPo) compartment and longest for compartment MSSla. The directions of the torque vectors of different compartments varied considerably. Torque vectors of deep compartments such as MPPo, and the pars reflexa of the superficial masseter (MSPR) produced significant yaw torques toward the working masseter but very small lingual roll torques. Torque vectors from superficial compartments such as MSSI contain larger lingual roll components and smaller working yaw components. These observations are consistent with the hypothesis that neuromuscular compartments of the rabbit masseter muscle form output elements in the neural control of masticatory movements.