During chewing, a small part of the observed muscle activity is needed for the basic open-close movements of the mandible, and additional muscle activity (AMA) is needed to overcome the resistance of the food. The AMA consists of two contributions: a large peripherally induced contribution, starting after food contact and a small anticipating contribution, starting before food contact. We investigated whether the latencies of these contributions depend on the expected or actual bolus size. Subjects made rhythmic open-close movements near their natural chewing frequency controlled by a metronome. This frequency was determined while the subjects were chewing gum. Food resistance was simulated by an external force, acting on the jaw in a downward direction during part of the closing movement. Bolus size was simulated by the jaw gape at which the force started. Jaw movement and surface EMG of the masseter and anterior temporal muscles on both sides and the suprahyoid muscles were recorded during experiments in which the jaw gape at which the force started was varied. The peripherally induced contribution to the AMA started about 20 ms after the onset of the force, irrespective of the jaw gape at which the force started. It is concluded that the onset of this contribution depends solely on food contact in the actual cycle. The function of the observed mechanism for jaw elevator muscle control may be to enable a highly automatic control of the muscle activity required to overcome the resistance of food of different hardness and different size. The onset of the anticipating contribution to the AMA showed neither a relationship with the actual jaw gape at which force onset occurred nor with the expected jaw gape of force onset. It is suggested that the onset of the anticipating AMA is related to the jaw gape at the onset of closing. The function of this contribution may be the regulation of the mechanical response of the jaw after an expected disturbance of the closing movement by food contact, by tuning the muscle stiffness to the expected hardness of the food.