Inﬂuence of jaw gape on EMG of jaw muscles and jaw-stretch reﬂexes Kelun Wang a, *, Frank Lobbezoo b , Peter Svensson a,c , Lars Arendt-Nielsen a a Center for Sensory-Motor Interaction, Orofacial Pain Laboratory, Aalborg University, Denmark b Department of Oral Function, Academic Centre for Dentistry Amsterdam (ACTA), Louwesweg 1, 1066 EA Amsterdam, The Netherlands c Department of Clinical Oral Physiology, School of Dentistry, University of Aarhus, Denmark 1. Introduction A sudden stretch of the jaw-closing muscles can elicit a short- latency excitatory response in the muscles, the so-called stretch reﬂex. The modulation of the stretch reﬂex in jaw- closing muscle has been studied previously. 1–8 The human jaw-closing muscles are richly endowed with muscle spindles, and the spindle afferents make excitatory connections to the archives of oral biology 52 (2007) 562–570 article info Article history: Accepted 5 December 2006 Keywords: Human Jaw gape Stretch reﬂex Jaw muscle EMG abstract The inﬂuence of jaw gapes on jaw-stretch reﬂexes and jaw muscles activity was studied in order to test the sensitivity of human muscle spindle afferents in various jaw muscles. Twelve healthy men (mean age  S.E.M.: 25.0  1.2 yr) participated in the study. Short- latency excitatory reﬂex responses were evoked by a custom-made stretch device with the subjects biting on a jaw-bar with their front teeth. Surface electromyographic (sEMG) recordings from right masseter (MAR), and right temporalis (TAR), intramuscular EMG (imEMG) recordings from right lateral pterygoid (LPR) and right anterior digastric (ADR) muscles were made. The reﬂex at different jaw gapes of 6, 10, 14, 18, 22, 26, 30, 34, and 38 mm were examined in random order with standard stretch conditions of 1 mm displacement and 10 ms ramp time. Twenty sweeps of the reﬂex were recorded at each level with at least 5 s interval between each sweep with online monitoring of the visual feed back at 15% of maximum voluntary contraction (MVC) of each jaw gape from MAR. The results showed that the peak-to-peak amplitude of the jaw-stretch reﬂex in MAR was signiﬁcantly higher at 14 mm compared to 30, 34, and 38 mm (P < 0.038), whereas the reﬂex amplitude in TAR increased with jaw gape until a maximum at 34 mm. There was no signiﬁcant effect of jaw gape in LPR muscles (P = 0.825) and no obvious stretch reﬂex was observed in ADR. When the amplitude was normalised to the pre-stimulus EMG at each jaw gape, the highest normal- ised amplitude was observed at 14 mm jaw gape in MAR, however there was no signiﬁcant effect of jaw gape on the normalised amplitude in TAR and LPR. In addition, masseter EMG at MVC signiﬁcantly decreased with the increase of the gapes, i.e. biting at 6, 14, and 18 mm gapes had a signiﬁcantly higher MVC compared to 26, 30, 34, and 38 mm (ANOVA: P < 0.013). It is concluded that the jaw gapes inﬂuence the sensitivity of the human muscle spindle afferents in jaw-closing muscles with a distinct peak, which is within normal jaw gapes during function. # 2006 Elsevier Ltd. All rights reserved. * Corresponding author at: Center for Sensory-Motor Interaction, Orofacial Pain Laboratory, Aalborg University, Fredrik Bajers Vej 7 D-3, DK-9220 Aalborg S, Denmark. Tel.: +45 9635 8745; fax: +45 9815 4008. E-mail address: kelun@smi.auc.dk (K. Wang). available at www.sciencedirect.com journal homepage: www.intl.elsevierhealth.com/journals/arob 0003–9969/$ – see front matter # 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.archoralbio.2006.12.004