Arm Muscles in Action
Diagram of the muscles of the arm in action. Labels: 1, humerus; 2, ulna, 3, biceps muscle; 4, attachment to the humerus; 6, triceps muscle; 7, tendon; 8, a ball to be moved. 9, 10, 11, direction the ulna moves when the biceps muscle contracts. When the triceps muscle contracts, the forearm extends.