While the anterior deltoids and pectoralis major muscles work to horizontally adduct the upper arms during the upward phase of a push-up, the triceps brachii muscles, or triceps for short, are also hard at work extending the elbow joints so the arms can be fully extended. The triceps also control the speed of elbow-joint flexion during the downward phase of the exercise. The closer together the hands are placed during a push-up, the harder the triceps work. The muscle is divided into three heads — the lateral head, long head and medial head. The lateral and medial heads attach to the back of the humerus bone, and the long head attaches just behind the shoulder socket on one end; all three heads combine and attach to the back of the elbow on the other. There is a special sub-set of the diamond push-up (so named for the diamond shaped space between the hands when the thumb and forefinger of the left hand is placed on the floor up against the thumb and forefinger of the right hand. ) The special version of this push-up is when the diamond is placed directly below the nose instead of the solar plexus. The nose must almost touch the floor in the center of the diamond. This special diamond push-up is done by the United States Marine Corps. The lips must come within 1 inch of the floor while keeping the neck in line with the straight spine to qualify as a valid push-up. This can be verified by placing a 1-inch foam disposable earplug on the floor in the center of the diamond and picking it up with the lips.