The arm consists of three bones:
Your best reference tool is attached to your torso. Hold a heavy object, twist your forearm, squeeze your fist, and physically feel how the muscles harden, shift, and stretch.
Supination (Palm Up) ---> Muscles run parallel, forearm looks wider laterally. Pronation (Palm Down) ---> Muscles twist diagonally, forearm forms a sleek taper. The Brachioradialis Twist