Electromyographic activity of the human lateral pterygoid muscle during contralateral and protrusive jaw movements

Understanding of the normal function of the lateral pterygoid muscle is lim ited. The principal aim here was to determine whether there is a progressiv e increase in lateral pterygoid activity as the mandibular condyle moves do wnwards and forwards as would be expected if the muscle is concerned with t he precise horizontal positioning of the mandible. In eight humans, recordi ngs were made of the activity of the superior (SHLP) and inferior (IHLP) he ads of the lateral pterygoid and the masseter, anterior temporal, posterior temporal and digastric muscles, together with the movement of the palpated lateral condylar pole (JAWS-3D tracking system) during trials of a contral ateral and a protrusive jaw movement. Recording sites in SHLP and, in one p articipant, IHLP were verified by computed tomography. In each participant there was a progressive increase in the rectified and smoothed SHLP and IHL P activity in association with condylar movement during the contralateral a nd protrusive jaw movement. Further, irregularities in condylar movement, w hich reflected variations in the rate at which the jaw was moved, were corr elated in time with prominent bursts of SHLP and IHLP activity. In all part icipants there was a consistently high correlation coefficient between the rectified and smoothed SHLP and IHLP activity and condylar displacement dur ing the contralateral or protrusive jaw movements. For example, the mean (/-SD) correlation between anterior condylar translation during contralatera l excursion and SHLP activity was 0.91 +/- 0.09, and for IHLP 0.96 +/- 0.02 . For the masseter, anterior temporal, posterior temporal and digastric mus cles, mean r-values were, respectively, 0.10 +/- 0.77; -0.14 +/- 0.72; 0.24 +/- 0.78; 0.54 +/- 0.47. When treated as a group the correlation coefficie nts for SHLP and IHLP were statistically significantly different from the c orrelation coefficients for the other muscles treated as a group (ANOVA; p < 0.002 for correlation with anterior translation). These observations supp ort the notion that the lateral pterygoid provides the principal driving fo rce for moving the jaw forwards or laterally in protrusive or lateral excur sive condylar movements. Further, the data suggest that the muscle plays a part in the fine control of jaw movements. (C) 1999 Elsevier Science Ltd. A ll rights reserved.