This study investigated the functional roles of the inferior of the lateral pterygoid muscle (LPt) and the anterior belly of the digastric muscle (Da) during jaw-opening, in which EMG activity was simultaneously recorded at the condylar and incisor points. The LPt was found to slide the condyle along the articular tubercles and the Da was moved the incisor region downward-backward for condylar rotation. Therefore, rotation of the condyle and sliding of the mandible downward-forward apparently combined in a coordinated movement. Direct relationships were found between LPt activity and the distance of condylar movement, and between Da activity and the velocity of incisor movement during opening of the mandible. Therefore, the muscles primarily engaged in wide opening movement were mainly the LPt, and for rapid opening, mainly the Da.