Final answer:
In Proximal Radioulnar Pronation, the rim of the radial head rotates at the pivot joint where it articulates with the ulna, supported by the annular ligament, thus moving the forearm from a supinated to a pronated position.
Step-by-step explanation:
The motion of the rim of the radial head in Proximal Radioulnar Pronation involves the rotational movement of the radius at the proximal radioulnar joint. This joint is a uniaxial pivot joint where the head of the radius articulates with the radial notch of the ulna, which is supported by the annular ligament. As the radius rotates during pronation, the forearm goes from the supinated position (palm facing up) to the pronated position (palm facing back). The slight curvature of the radius shaft allows its distal end to cross over the ulna at the distal radioulnar joint, forming an X-shape with the ulna. Conversely, supination involves the rotation of the radius back to parallel alignment with the ulna, turning the palm anteriorly.