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The Glenohumeral (GH) Joint or shoulder joint is comprised of incongruent surfaces: the large head of the humerus or the ball and the small glenoid fossa of the scapula which functions as the socket. This composition does not allow for the joint to rotate in a spin motion, but rather facilitates movement through a combination of rolling or a slide and glide motion of the humeral head in the opposite direction of the humerus. The structure of the GH joint allows for three degrees of freedom: flexion-extension, abduction-adduction, and medial-lateral rotation. The range of each of these motions at the GH joint varies. Among the factors that influence these variations are arm position and whether the motion is done actively or passively. (Joint Structure & Function, Norkin & Levangie)
Additionally, the shallow loose structure of the GH joint is why the shoulder is the most mobile or versatile joint in the body. There are other ball and socket joints in the human body, for example, the hip joint which offers more stability but less range of motion due to its deep & tight fit. In contrast to the flexibility allowed by ball and socket joints, the hinge joint, the simplest of joints, typically allows motion in one direction.
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