The role of the inferior head of the human lateral pterygoid muscle in thegeneration and control of horizontal mandibular force

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 contr ol of horizontal isometric mandibular force. Although previous studies prov ided some evidence for this, they had limitations that necessitate a re-exa mination. In eight participants, electromyographic (EMG) activity was recor ded from the IHLP unilaterally, as well as bilateral surface recordings fro m the masseter (M) and anterior temporalis (AT), and the submandibular grou p of muscles (SUBM), during the generation of horizontal isometric mandibul ar force in a direction contralateral to the side of the IHLP recording. Is ometric force at 5-8 mm open from the intercuspal position was exerted on a transducer (attached by a bar to the upper teeth) by a rod attached to the lower teeth. 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). The mean of multi-unit EMG activity from all muscles duri ng the most stable 2-s force-holding periods increased significantly with e ach force increment (GLM repeated measures: P < 0.0001). When normalized, t he multi-unit data from the IHLP exhibited the steepest rate of increase. T he mean firing rates of 21 IHLP single motor units (SMUs) significantly inc reased with force (GLM repeated measures: P < 0.0001). Two SMUs fired in ad vance of force onset, which suggests a role in force initiation. There were close associations between fluctuations in force and in IHLP SMU firing ra tes and multi-unit activity, but a similar correspondence was not as clear for the other recorded jaw muscles. These findings suggest that the IHLP is important in the generation and fine control of contralaterally directed, horizontal jaw forces. (C) 2001 Elsevier Science Ltd. All rights reserved.