Fig. 2.

Left frontal bone strain during mastication, pig 158. The B element of the rosette is parallel to the sagittal axis and records no strain, whereas the 45° A and C elements are reciprocally tensed (positive signal) or compressed (negative signal). The pig alternates between left (L) and right (R) side chewing. Chewing on the left features early activity in the right masseter (RM) and left zygomaticomandibularis (LZM) to move the mandible to the left, and late activity in the left masseter (LM) and right zygomaticomandibularis (RZM) to complete the power stroke. For chewing on the right, LM and RZM are early, RM and LZM are late. Strain in the left frontal bone is best predicted by masseteric activity—predominance of the LM is associated with tension along the A element and compression in the C element, predominance of the RM causes the opposite pattern. Note that strain is slightly delayed relative to muscle contraction; this is because the EMG signal of the pig masseter precedes the development of mechanical force by approximately 50 msec (Anapol and Herring, unpublished data).