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Key ideas. Origin is where a muscle starts, usually proximal, closer to the body. Insertion is where it ends, usually distal, the part that moves. Action is the movement the muscle creates. Joint is what joint the muscle crosses. Supination means palm up. Pronation means palm down. Joint basics. The shoulder is a ball and socket joint. The elbow is a hinge joint. The radioulnar joint is a pivot joint. The wrist is a condyloid joint. Hinge joints bend and straighten. Pivot joints twist. Ball