Single motor units' study on the role of the human lateral pterygoid muscle in the generation of horizontal mandibular force

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The aim was to test the hypothesis that the inferior head (IH) of the human lateral pterygoid muscle (LP) is involved in the generation and fine control of horizontal isometric mandibular force. In eight participants, single motor units' activity (SMUs) was recorded from IHLP unilaterally during the generation such forces to a target in contralateral (CL), ipsilateral (IL), protrusive (P) directions and in two intermediate directions (contralateral-protrusive (CL-P), ipsilateral-protrusive (IL-P)). Isometric force at 5-8 mm open from the intercuspul position was exerted on a transducer (attached by a bar to the upper teeth) by a rod attached to the lower teeth of horizontal isometric mandibular force in a direction. Participants tracked a target on a video screen that required 5-s holding periods at each 100 gwt (0.98 N) between 400 gwt (3.92 N) and 800 gwt (7.84 N).We could reliably discriminate 21 SMUs during isometric CL contraction. The mean firing rates of 21 IHLP SMUs significantly increased with force (p<0.0001). Three patterns of SMUs were identified at CL: (a) tonic (t-unit; 10/21, 48%), (b) intermediate (I-unit; 4/21, 19%) and (c) phasic or predominantly phasic (P-unit; 7/21, 83%). The mean firing rates of 21 IHLP SMUs were also affected by direction, with the highest activity being CL, and graded decreases occurring as the direction shifted to IL.The data suggest that the IHLP is involved in the creation and fine control of mandibular forces in different horizontal directions.