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How does the radius rotate during pronation and supination of the forearm?

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Front and back views of the human skeleton are shown side by side, with labeled bones including the skull, spine, ribs, pelvis, arms, legs, hands, and feet. Insets detail the bones of the foot and hand, highlighting major bone groups and anatomical terms. — human skeleton Front and back views of the human skeleton.

The rotation of the radius is central to the movements of the forearm. The radius features a disk-shaped head that articulates with the humerus above and the ulna at its side. Although the head is secured against the ulna by a strong, ring-shaped annular ligament, it remains free to rotate.12

During these movements, the rotation of the radial head causes the shaft and the outer end of the radius to swing in an arc:2

Supination: The radius and ulna remain parallel, positioning the palm to face forward with the thumb away from the body.23
Pronation: The radius moves around and crosses over the ulna, causing the palm to face backward (or downward) with the thumb positioned toward the body.123

This complex mechanical interaction between the radius and the ulna allows for the precise twisting motions essential for many everyday human activities.3