The aim of this study was to estimate numerically the properties of masseter motor units (MUs) in relation to bite force magnitude and direction three-dimensionally and to confirm the hypothesis that the properties differ between different parts of the muscle by means of simultaneous recording of MU activity along with the MU location and three-dimensional (3D) bite force. The MU activity of the right masseter of four healthy men was recorded using a monopolar needle electrode in combination with a surface reference electrode. The location of the needle electrode was estimated stereotactically with the aid of magnetic resonance images and a reference plate. The magnitude and direction of the bite force was recorded with a custom-made 3D bite force transducer. The recorded bite force was displayed on a signal processor, which enabled the participant to adjust the direction and magnitude of the force. The activities of 65 masseter MUs were recorded. Each MU had specific ranges of bite force magnitude and direction (firing range: FR) and an optimum direction for recruitment (minimum firing threshold: MFT). There was a significant negative correlation between MFT and FR width. There were functional differences in MU properties between the superficial and deep masseter and between the superficial layer and deep layer in the superficial masseter. These results indicate that the contribution of human masseter motor units to bite force production is heterogeneous within the muscle.
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