The coordination of head and trunk movement during postural sway in the anterior/posterior plane was examined in three normal adults. Postural sway about the ankles or hips was elicited in two ways: (1) In free-fall sway trials, the subject passively fell forward while the feet remained in place on the support surface (ankle sway). (2) In perturbed sway trials, subjects stood on either a flat surface (ankle sway) or a narrow beam (hip sway) which was displaced backwards at the onset of each trial. In all cases, postural responses were initiated before significant horizontal head motion was recorded. For subjects swaying about the ankles, changes in neck angle followed changes in ankle angle as the effect of the postural movement was propagated up the body. Neck muscle activation for ankle sway thus appeared to be elicited by neck stretch resulting from the postural correction. These results suggest that head and body motions may be controlled independently during active postural movements for ankle sway. For subjects swaying about the hip, however, changes in neck and hip angles were coordinated to approximately stabilize the rotational position of the head, and neck and hip muscles were activated simultaneously. These results, in contrast to those for ankle sway, suggest that control of head and body motion is coordinated on a feedforward basis during hip sway.
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