We have previously demonstrated that, during jaw open-close movements, there is a functionalrhythmical coupling between the head and the mandible, and that the head undergoes a rotation-like movement whose center of rotation (CR) is located inside the head. We believe that such concomitant motion occurs to balance the head during jaw movement. This study was made to confirm this theory, and to verify the CR's location and functional purpose. The same 6-degree-of-freedom motion measuring device and analyzing method from our prior studies was used in 11 healthy subjects, but in some of them, a controlled external force was applied to the head during jaw movements, to induce resistance and changes in head motion. In the subjects without loads, there was a high incidence of concomitant head and jaw movements. This was in contrast to the subjects with loads applied, i.e., the incidence decreased, but the CR kept the location inside the head. Yet, the vertical location of the CR relative to the cervical spine was similar in all subjects. We concluded that concomitant head and jaw movement is influenced by the vertical shift of the CR, and that it serves to balance the head in relation to the body.