Final answer:
During pronation at the distal radioulnar joint, the radius crosses over the ulna, transitioning from a parallel arrangement to an 'X' shape, facilitated by the pivot motion at the proximal radioulnar joint.
Step-by-step explanation:
The motion of the distal radioulnar joint during pronation involves the radius moving around the ulna. Pronation moves the forearm from a supinated position, where the palm faces forward and the bones are parallel, to a pronated position, where the palm faces backward, forming an 'X' shape with the ulna. At the distal radioulnar joint, this rotation is accompanied by the crossing-over of the radius over the ulna due to the joint's pivot nature and the radius's slight curvature.
The proximal radioulnar joint, acting as a pivot, allows for the rotation of the head of the radius. This motion is stabilized by a ligament that encircles the head of the radius, aligning it with the radial notch of the ulna. These actions facilitate the crucial arm movements of pronation and supination, which are used in everyday activities such as turning the palm up or down.