Online steering: coordination and control of body center of mass, head andbody reorientation

Steering is an integral component of adaptive locomotor behavior. Along wit h reorientation of gaze and body in the direction of intended travel, body center of mass must be controlled in the mediolateral plane. In this study we examine how these subtasks are sequenced when steering is planned early or initiated under time constraints. Whole body kinematics were monitored a s individuals were required to change their direction of travel by varying amounts when visually cued either at the beginning of the walk or one strid e before. The analyses focused on the transition stride from one travel dir ection to another. Timing of changes (with respect to first right foot cont act) in trunk roll angle, head and trunk yaw angle, and right foot displace ment in the mediolateral plane were analyzed. The magnitude of these measur es along with right and left foot placement at the beginning and right foot placement at the end of the transition stride were also analyzed. The resu lts show the CNS uses two mechanisms, foot placement and trunk roll motion (piking action about the hip joint in the frontal plane), to move the cente r of mass towards the new direction of travel in the transition stride, pre ferring to use the first option when planning can be done early. Control of body center of mass precedes all other changes and is followed by initiati on of head reorientation. Only then is the rest of the body reorientation i nitiated.