Anatomy of the Arm:

The arm is divided by a fascial layer known as the lateral and medial intermuscular septa which separates the muscles from two osteofascial compartments called the anterior and posterior compartments of the arm. The fascia merges with the periosteum (outer bone layer) of the humerus. The compartments contain muscles which which are innervated by the same nerve and perform the same action.

Other Muscles Partially of the Arm:

• The Deltoid muscle has part of its’ body in the anterior compartment. This muscle is the main abductor muscle of the upper limb and extends over the shoulder.

• The Brachioradialis muscle originates in the arm, but its’ insertion is into the forearm. This muscle functions to rotate the hand so it faces forward (an upward facing palm or supination).

The muscles of the upper arm are used for the flexion and extension of the forearm at the elbow joint. Flexion is accomplished through the brachialis, biceps brachii, and the brachioradialis. Most of the muscles that move the hand, wrist and fingers are located in the forearm. These are thin, strap-like muscles, and extend from the humerus, ulna and radius and insert into the carpals, metacarpals, and phalanges by the long tendons attached to them.

On the anterior side of the forearm, the flexor muscles such as the carpi radialis and the flexor digitorum superficialis, form the flexor group that flexes the hand at the wrist and each of the phalanges. The tendons of these muscles pass through the carpal tunnel.

On the posterior side of the arm are the extensor muscles, such as the extensor carpi ulnaris, and extensor digitorum which act as antagonist to the flexor muscles by extending the hand and fingers. The extensors usually do the work of opening the hand rather than gripping onto something.  They are usually weaker than the flexor muscles and also run as long, thin straps from the humerus to the metacarpals and phalanges.