In order to cause flexion or extension of a joint, a muscle must originate on one side of the joint and insert on the other. In other words, the muscle body must cross the joint in order to affect it.

The biceps brachii, brachialis, corocobrachialis, and brachioradialis originate from the scapula or humerus and insert on the radius or ulna. Each of these muscles thus crosses the elbow. In contrast, the flexor carpi radialis originates on the radius and inserts on the second metacarpal. When contracted, the flexor carpi radialis causes flexion of the hand at the wrist, but does not affect the position of the forearm.

A synergist is, by definition, a muscle that aids an agonist in its movement. The biceps brachii acts to flex the forearm. The brachialis also flexes the forearm and helps stabilize the motion created by the biceps brachii.

The triceps brachii is an antagonist to the biceps brachii, working to extend the forearm rather than flex. The pectoralis major helps to adduct the arm at the shoulder and the rectus femoris extends the leg at the knee.

The ulnar nerve is responsible for all motor innervation of the hand except for the thenar muscles and the lateral two lumbricals, which are innervated by the recurrent branch of the median and the palmar digital nerves of the median nerve, respectively.

The two most common sites for ulnar nerve injury are where the ulnar nerve passes posterior to the medial epicondyle of the humerus and on the medial side of the wrist near the pisiform bone. If the site of injury is where the ulnar nerve passes lateral to the pisiform bone in the wrist, the patient will still be able to flex her medial two distal phalanges. Remember, in the forearm, the ulnar nerve innervates the medial two tendons of the extensor digitorum profundi and the median nerve innervates the lateral two extensor digitorum profundi.

The flexor carpi radialis passes through a small gap in the flexor retinaculum, not through the carpal tunnel.

The carpal tunnel houses the median nerve and tendons for three muscles: the flexor digitorum profundus, the flexor digitorum superficialis, and the flexor pollicis longus. Compression of the carpal tunnel such that it affects the median nerve results in carpal tunnel syndrome.

The deltoid muscle is a triangle-shaped muscle responsible for the abduction of the arm beyond the initial 15 degrees accomplished by the supraspinatus muscle (rotator cuff muscle). The deltoid is innervated by the axillary nerve.

The infraspinatus is another muscle of the rotator cuff, and serves to externally rotate the arm at the shoulder. The trapezius serves some abduction functions, but cannot generate the same range of motion as the deltoid. The trapezius is more closely linked to establishing posture and maintaining shoulder position than active abduction.

The rhomboid major and rhomboid minor muscles are responsible for retraction and elevation of the scapula. Both the rhomboid major and minor are innervated by the dorsal scapular nerve. The levator scapulae also helps elevate the scapula. The important distinction is that the rhomboids both originate from the vertebrae and insert on the medial border of the scapula. It is this pathway that allows for both elevation and retraction.

The trapezius helps elevate the scapula and raise the upper limb above the head, but is not associated with retraction of the scapula. The deltoid muscle is responsible for abduction of the arm beyond the initial fifteen degrees accomplished by the supraspinatus muscle. The latissimus dorsi extends, adducts, and medially rotates the arm.

The coracobrachialis muscle originates at the apex of the coracoid process and inserts on the medial side of the midshaft of the humerus. It is responsible for flexion and adduction of the arm at the glenohumeral joint (shoulder). It is innervated by the musculocutaneous nerve, which pierces the muscle and passes through it.

The median nerve innervates muscles of the anterior compartment of the forearm, the muscles of the thenar eminence in the hand, and the first and second lumbricals of the hand. The ulnar nerve innervates intrinsic muscles of the hand, including the flexor carpi ulnaris, flexor digitorum profundis, the medial two lumbrical mucles, the oppenens digiti minimi, the flexor digiti minimi, the abductor digiti minimi, the dorsal and palmar interossei, and the adductor pollicis. The radial nerve innervates the muscles of the posterior compartment of the arm and forearm.

The quadrangular space is a small opening between four structures through which the axillary nerve and posterior circumflex artery pass. The superior border is defined by the teres minor and the inferior border by the teres major. The long head of the triceps forms the medial border and the surgical head of the humerus forms the lateral border. The result is essentially a small, square space through which the nerve and artery can travel around the proximal humerus.

There are four muscles that form the rotator cuff: the supraspinatus, the infraspinatus, the teres minor, and the subscapularis. These muscles work together to stabilize the shoulder joint. The supraspinatus abducts the arm, the infraspinatus and teres minor externally rotate the arm, and the subscapularis internally rotates the arm. The most common rotator cuff injuries occur to the supraspinatus, as it is the most exposed and superior of the four muscles.

The teres major adducts and internally rotates the arm at the shoulder, but does not insert on the head of the humerus like the rotator cuff muscles. Instead, it inserts lower on the shaft and has less involvement in shoulder stability.

The supraspinatus muscle initiates the first 15 degrees of shoulder abduction. The deltoid is responsible for abduction from 15 to 90 degrees