Masseter Muscle Single Motor Units: Effect of Bite Angle on Recruitment

Objectives: Recruitment of motor units is well understood as force is increased but poorly understood as the intended angle of force is altered. We investigated the effect of bite force angle on recruitment of single motor units in the masseter muscle. Methods: In two subjects fine wires were inserted into the right masseter muscle to obtain EMG from single motor units. Specially designed beams were affixed to the posterior teeth with acrylic resin to provide a stable mounting for a calibrated force transducer. This transducer could be placed at any of seven frontal plane angles or seven sagittal plane angles. Computer programs were written to sample both the EMG and the bite force at 10 kHz for 100 ms when an action potential exceeded a pre-set threshold and to analyze the data off-line. Five trials were obtained at each angle. Results: We obtained data from four single motor units in both the frontal and sagittal planes. In the frontal plane the lowest force thresholds for single motor unit recruitment occurred near vertical bite angles. As the angle departed from vertical the force threshold increased. In the sagittal plane the force threshold of the motor units decreased as the bite angle rotated clockwise from vertical as viewed from the subject’s right. Conclusions: Our data suggest that recruitment of these motor units depends on the intended angle of bite force in both the frontal and sagittal planes. Supported in part by USPHS Grant DE-07106.