Abstract: Objective: To investigate whether force level, visual feedback and maintenance effort had synergistic effects on the electromyographic (EMG) activities of jaw-closing muscle during simulated clenching. Methods: T-Scan III and BioEMG III systems were used to evaluate the interrelationships between occlusal force and jaw-closing muscles activity of bilateral anterior temporalis (TA) and masseter (MM) muscles in 25 asymptomatic volunteers, who were enrolled to clench with low, medium, and maximum force levels, with or without visual feedback and maintenance effort. Corresponding statistics were calculated using multivariate factorial variance analysis. Results: Statistics indicated that force level and maintenance effort contributed to the increase of SEMG values in all tested jaw muscles during clenching (P<0.001) and there was an association between two factors (P<0.01). The contribution of visual feedback to the SEMG values was only noticed in TAR and MML (P>0.05). Conclusion: The SEMG activity of jaw muscles was increased when maintaining appropriate occlusal force. There was an interaction between maintaining and occlusal force. The visual feedback of occlusal force might decrease the level of SEMG activity.