The aim of this study was to investigate whether the medial pterygoid muscle shows differential activation under experimental conditions simulating force generation during jaw clenching. To answer this question, the electromyographic activity of the right medial pterygoid was recorded with two intramuscular electrodes placed in an anterior and posterior muscle region, respectively. Intraoral force transfer and force measurement were achieved by a central bearing pin device equipped with strain gauges. The activity distribution in the muscle was recorded in a central mandibular position during generation of eight different force vectors at a constant amount of force (F=150 N). The investigated muscle regions showed different amounts of EMG activity. The relative intensity of the activation in the two regions changed depending on the task. In other words, the muscle regions demonstrated heterogeneous changes of the EMG pattern for various motor tasks. The results indicate a heterogeneous activation of the medial pterygoid muscle under test conditions simulating force generation during clenching. This muscle behaviour might offer an explanatory model for the therapeutic effects of oral splints.