Human lateral pterygoid muscle activity on the return phase of contralateral and protrusive jaw movements

Normal function of the lateral pterygoid muscle is not well understood. The principal aim of this study was to determine whether there is a progressiv e decrease in lateral pterygoid activity as the condyle moves posteriorly a nd superiorly during the return phase of a contralateral or protrusive jaw movement; as would be expected if the muscle is involved in controlling or stabilizing the condyle during the return phase of these movements. In seve n humans, electromyographic activity was recorded in the superior (SHLP) an d inferior (IHLP) heads of the lateral pterygoid, the masseter, anterior te mporal, posterior temporal and submandibular group of muscles, together wit h condylar movement, during contralateral and protrusive jaw movement. In m ost individuals, there was a progressive decrease in rectified and smoothed IHLP activity in relation to condylar movement during the return phase of contralateral and protrusive jaw movement. However, this pattern usually wa s not seen when SHLP activity was studied in relation to condylar movement. Further, there was a high correlation coefficient between condylar displac ement and the rectified and smoothed IHLP and anterior temporal muscle acti vities during the return phase of contralateral or protrusive jaw movement, while SHLP presented a much lower correlation. For example, the mean (SD) correlation coefficient between posterior condylar movement (along anteropo sterior axis) and IHLP activity during the return phase of a protrusive jaw movement was -0.73 +/- 0.36 (for contralateral movement: -0.71 +/- 0.56), for the anterior temporal 0.69 +/- 0.21 (contralateral: 0.81 +/- 0.09), and for the submandibular muscles, -0.77 +/- 0.15 (contralateral: -0.34 +/- 0. 71). For the SHLP, masseter and posterior temporal, values were -0.34 +/- 0 .61 (contralateral: -0.48 +/- 0.37), -0.24 +/- 0.57 (contralateral: 0.16 +/ - 0.80), and 0.16 +/- 0.77 (contralateral: 0.64 +/- 0.14), respectively. Th ese findings suggest an important role for the IHLP and anterior temporal i n controlling the movement of the condyle to the glenoid fossa on the retur n phase of contralateral and protrusive jaw movements. Further studies are needed to clarify the function of the lateral pterygoid muscle during these and other jaw movements. (C) 2000 Elsevier Science Ltd. All rights reserve d.