INTRODUCTION C lenching forces are generated by complex co-activation of masticatory muscles. Their individual contributions to the motor task are traditionally estimated by electromyography (EMG) (Gibbs et al., 1984; Wood, 1986). However, only biomechanical modeling permits masticatory muscle and temporomandibular joint forces to be calculated. In principle, two different methods are available: (1) optimization algorithms, based on physiologically reasonable neuromuscular objectives for coordinated muscle co-contraction, e.g., minimization of joint forces, minimization of overall muscle forces, or minimization of applied muscle energy (Pruim et al., 1980; Throckmorton et al., 1990; Koolstra and van Eijden, 1992; Osborn, 1995; Trainor et al., 1995; Raikova, 1999; Iwasaki et al., 2004); and (2) simultaneous in vivo measurement of EMGs of all relevant masticatory muscles and the resultant bite force, and subsequent calculation of all forces from the static equilibrium conditions (van Eijden, 1990; van Eijden et al., 1990; Iwasaki et al., 2004). Pre-conditions for both methods are geometric data from the skull, lines of action, physiological cross-sectional areas (A i ) of the muscles, their pennation angles, and intrinsic muscle strength (P). Commonly, P is used as a fixed conversion constant based on generic mammalian skeletal muscle properties (Koolstra and van Eijden, 1992; Langenbach and Hannam, 1999). With optimization strategies, joint force directions are usually pre-supposed. The second method allows them to be calculated. To date, however, no such computations are available for bilateral biting with all essential measurements acquired from one sample. Jaw clenching is assumed to be a risk factor for temporomandibular disorders (Velly et al. , 2003; Magnusson et al. , 2005). Biomechanical modeling based on all feasible in vivo measurements from a group of individuals may provide realistic insight into joint and muscle loads generated during clenching activities. Furthermore, it broadens the insight into control strategies of the motor system during clenching, and may help to reveal risk factors for the structures involved. We undertook this study to determine the realistic loading of masticatory muscles and temporomandibular joints during biting under various resultant force vectors. Additionally, we used optimization algorithms to investigate possible control strategies for muscle co- contractions. Muscle and joint forces were calculated based on feedback- controlled electromyograms of all jaw muscles, their lines of action and physiological cross-sectional areas, geometric data from the skull, and individually calculated P gathered from one defined sample. MATERIALS & METHODS Participants Ten healthy males (average age, 31 ± 2.3 yrs) gave written informed consent to participate in this study. They had Angle class I or mild class II dentition. Exclusion criteria were skeletal anomalies (e.g., short-faced or long-faced) or distinct malocclusions. The study was approved by the Ethics Committee of the University of Freiburg, Germany (No. 25/02, amendment 04). ABSTRACT Realistic masticatory muscle and temporo - mandibular joint forces generated during bilateral jaw clenching are largely unknown. To determine which clenching directions load masticatory muscles and temporomandibular joints most heavily, we investigated muscle and joint forces based on feedback-controlled electromyograms of all jaw muscles, lines of action, geometrical data from the skull, and physiological cross-sectional areas acquired from the same individuals. To identify possible motor control strategies, we applied objective functions. The medial pterygoid turned out to be the most heavily loaded muscle for all bite directions. Biting with accentuated horizontal force components provoked the highest loading within the medial and lateral pterygoids. The largest joint forces were also found for these bite directions. Conversely, the lowest joint forces were detected during vertical biting. Additionally, joint forces with a clear posterior orientation were found. Optimization strategies with the elastic energy as objective function revealed the best fit with the calculated results. KEY WORDS: jaw muscles, joint forces, muscle forces, EMG, optimization. Received June 16, 2006; Last revision March 23, 2007; Accepted April 16, 2007 A supplemental appendix to this article is published electronically only at http://www.dentalresearch.org. Jaw Clenching: Muscle and Joint Forces, Optimization Strategies H.J. Schindler 1 * ,3 , S. Rues 1 , J.C. Türp 2,4 , K. Schweizerhof 1 , and J. Lenz 1 1 Research Group Biomechanics, Faculty for Mathematics, University of Karlsruhe, D-76128 Karlsruhe, Germany; 2 Department of Reconstructive Dentistry and Temporomandibular Disorders, Dental School, University of Basel, Switzerland; 3 Department of Prosthodontics, University of Heidelberg, Germany; and 4 Department of Prosthodontics, Dental School, University Hospital Freiburg, Germany; *corresponding author, myo.schindler@t-online.de J Dent Res 86(9):843-847, 2007 RESEARCH REPORTS Clinical 843