The temporalis and masseter muscles have a complex architectural design with large attachment areas. As a consequence, each of these muscles is capable of producing a large number of mechanical effects. In addition, within each muscle the muscle fibres and sarcomeres undergo different excursions during jaw movements. This leads to intramuscular differences in the possible excursion range and the capacity to produce force. Depending on the desired motor task, the nervous system is able to vary both the magnitude and direction of muscle force by selective activation of specific muscle portions. The anterior temporalis and the superficial and deep masseter are capable of generating large forces, e.g. during biting and chewing. The posterior temporalis and the anterior and posterior deep masseter are suited to establish a precise adjustment of forces and movements during laterotrusion and protrusion/retrusion.