The arm is the part between the shoulder joint and the wrist joint and has the following main groups of muscles: First, located on the inside of the upper arm, called the biceps brachii, which mainly functions to flex the elbow. Second, called triceps brachii, mainly to extend the elbow joint. Third, the deltoid muscle, mainly to abduct the shoulder joint. Fourth, including the rotator anterioris and rotator posterioris, the main function is to rotate the forearm back and forth. Fifth, the dorsal wrist extensors, the main function is to cock the wrist. Sixth, wrist flexors, the main function is to flex the wrist. Seventh, the finger flexors, the main function is the grip action of the fingers.