Control of steering in the presence of unexpected head yaw movements - Influence on sequencing of subtasks

Control of the head during locomotion has been suggested as a means of faci litating overall postural control of the body. The control of online steeri ng is challenging, as it requires the central nervous system (CNS) to simul taneously control body reorientation in a new direction while modifying the ongoing step cycle. Stable body posture during steering is maintained via appropriately organized postural responses to error signals detected by the visual, vestibular, and/or proprioceptive systems. Modifications to the ga it cycle include step-width regulation and movement of body center of mass (COM) in the direction of travel, and may be preceded by independent contro l of head orientation to see where one is going. The purpose of this invest igation was to examine how the ability to successfully steer is influenced by unexpected head perturbations and how various body segments are coordina ted and controlled to successfully steer along different pathways. Body kin ematics were monitored as participants changed their direction of travel by varying amounts when visually cued one stride before the turn. Perturbatio ns to the head were applied to either assist or oppose the change in direct ion one step prior to initiation of the turn. Analyses focused on the timin g of the changes in head yaw, trunk yaw, and COM trajectories in the mediol ateral plane. Results indicate that the order of control over the body segm ents was head and trunk reorientation in the direction of travel and finall y movement of the COM in the intended direction. Thus gaze, inferred from h ead movement, preceded changes in COM trajectory. This suggests that lookin g where you are going is critical for steering. When steering is potentiall y compromised by unexpected head movements, the CNS delays committing movem ent of the COM until it has a chance to look at the new travel path.