A method for standardizing jaw displacements in the horizontal plane whilerecording single motor unit activity in the human lateral pterygoid muscle

The normal function of the lateral pterygoid muscle is not well understood although this muscle is thought to play an important role in the control of jaw and jaw-joint function and is implicated in temporomandibular disorder s (TMD). The lack of a validated method for standardization of jaw movement in studies of lateral pterygoid function has contributed to the lack of un derstanding of the normal function of this muscle. An improved understandin g of normal function will allow valid comparisons to be made with TMD patie nts in order to identify whether purported differences in activity actually exist. This paper describes a methodology for standardizing command jaw mo vements in the horizontal plane, together with reliable recordings of singl e-motor-unit (SMU) activity. In six human participants, jaw movements were standardized by having participants track a linear bank of light-emitting d iodes (LEDs) aligned on a monitor displaying the mid-incisor point (MIPT). In all participants, the MIPT target (i.e. an illuminated LED) could be tra cked, according to a pre-determined criterion, during single- and multiple- step displacements at different rates (1.3-6.5 mm/s at MIPT) and magnitudes (0.65-12 mm) of movement. SMU activity from the superior (SHLP) or inferio r (IHLP) head of the lateral pterygoid muscle could be reliably discriminat ed during repeated trials of these defined tasks. This methodology establis hes a reliable technique for characterizing the firing properties of SMUs w ithin the lateral pterygoid, and has implications for analogous studies in other jaw muscles. (C) 2001 Elsevier Science B.V. All rights reserved.