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

The biceps brachii is the only muscle of the answer choices that flexes the elbow but also is a strong supinator of the forearm. It inserts on the radius, which accounts for its supinating action. Brachialis inserts onto the ulna, and thus, has no supinating action.

Flexor digitorum profundus is a deep muscle of the forearm, all the other choices are superficial muscles of the forearm.

The lateral and medial heads of the triceps brachii originate on the humerus and attach to the ulna. The long and short heads of the biceps brachii attach to the scapula at the supraglenoid tubercle and the coracoid process, respectively. The long head of the triceps brachii attaches to the scapula at the infraglenoid tubercle. The supraspinatus attaches to the scapula at the supraspinous fossa.

The insertions of the trapezius muscle are the superior spine of the scapula, posterior lateral one-third of the clavicle, and the acromion (spinotrapezius, clavotrapezius, and acromiotrapezius). The acromion is one origin for the deltoid, however, it's insertion is only the deltoid tuberosity of the humerus. All of the other answers are incorrect since their insertions are on the posterior medial border of the scapula.

The brachialis only produces motion across one joint; it flexes the elbow. The biceps brachii flex the shoulder and elbow and supinate the forearm. The triceps brachii extends both the shoulder and elbow. The extensor carpi radialis longus produces motion across the elbow and the wrist.

The upper and lower trapezius function to upwardly rotate the scapula, as does the serratus anterior. Although the rhomboids (major and minor) can elevate the scapula, they are downward scapular rotators. Additionally, although the middle trapezius attaches to the scapula, it does not contribute to scapular upward rotation, due to the transverse orientation of its fibers.