The structural foundation of the arm begins with the humerus, radius, and ulna. In motion, the relationship between the radius and ulna is the primary driver of forearm transformation. During supination and pronation, the radius crosses over the stationary ulna, causing the muscular masses of the forearm to shift and twist. A sculptor must account for this rotation; the fleshy part of the forearm follows the radius, meaning the silhouette of the arm changes drastically depending on whether the palm is facing up or down. Without this understanding, a sculpted arm often looks rigid or "broken" at the wrist.
How the knuckles align on an arc rather than a straight line. arm and hand in motion by anatomy for sculptors pdf better