3-DIMENSIONAL HEAD AXIS OF ROTATION DURING TRACKING MOVEMENTS - A TOOL FOR ASSESSING NECK NEUROMECHANICAL FUNCTION

The purpose of this article is to report normal variation in the screw (helical) axis of rotation of the head during various types of natura l tracking movements. Nine normal subjects and eighteen subjects with neck injury faced a grid of targets separated by 10-degree intervals, and were instructed to use a head pointer (laser) to track whatever ta rget was lit. Various horizontal, vertical, and oblique target sequenc es were employed. The normal subjects exhibited several consistent tre nds in finite screw axis parameter variation: vertical movements have a laterally-directed axis whose midsagittal plane crossing position is a function of the head orientation (typical range C3-T1); oblique mov ements have a diagonally-directed axis and an even greater orientation -specific range (Cl-T1); and horizontal movements have a vertical axis that is modified near horizontal orientation extremes and is asymmetr ically influenced by upward and downward bias orientations. Subjects w ith neck injury were seen to exhibit a variety of abnormal screw axis patterns.