The accurate measurement of hip motion is critical when rating impairments of this joint, making an initial diagnosis, assessing progression over time, and evaluating treatment outcome. The hip permits all motions typical of a ball and socket joint: abduction and adduction, extension and flexion (hence, also circumduction), as well as external and internal rotation.

In contrast to its upper extremity counterpart, the glenohumeral joint, the hip sacrifices some motion in each of the aforementioned directions to achieve greater strength and stability. The femoral head consists of more than half a sphere, in contradistinction to the humeral head, which comprises less than half. The acetabulum also accommodates more than half a sphere, as opposed to the shallow socket formed by the glenoid. While both the acetabulum and the glenoid are deepened by a fibrocartilaginous labrum, the fibrocartilaginous labrum...

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