The upper arm is between the shoulder and elbow joint and has four muscles. Three of them are in the anterior compartment - brachialis, coracobrachialis, and biceps brachii. The remaining muscle – triceps brachii, is in the posterior compartment.
The biceps brachii is anterior to the humerus and has two heads that join together – the short and the long heads. Note that the biceps brachii does not attach to the humerus. The long head originates from the scapula at the supraglenoid tubercle. The short head originates from the scapula’s coracoid process. At the distal end of the biceps brachii, the short head inserts distally on the radial tuberosity, while the long head inserts proximally. Here we see the bicipital aponeurosis, also called the lacertus fibrosus. This is a thick sheet of connective tissue which blends with deep fascia of the anterior forearm. The biceps brachii flexes the arm at the elbow and at the shoulder and causes supination of the forearm.
Coracobrachialis can be seen here posterior to the biceps brachii. Like the short head of the biceps brachii, it originates from the scapula’s coracoid process. After passing through the axilla, or armpit, the coracobrachialis attaches to the medial side of the humerus, at the level of the deltoid tuberosity, which is where the deltoid attaches. The coracobrachialis flexes the arm at the glenohumeral joint, or shoulder, causes medial rotation, and causes weak adduction, bringing the arm in towards the body.
Brachialis lies deep to biceps brachii. It originates from the humeral shaft and inserts into the ulna tuberosity and coronoid process. It makes up the floor of the cubital fossa, or elbow pit. Brachialis causes flexion at the elbow.
The triceps brachii muscle has three heads. The long head originates from the infraglenoid tubercle. The lateral head originates from the humerus, superior to the radial groove. The medial head is under the other two, which cover it. The medial head originates from the humerus, inferior to the radial groove. Distally, the heads converge onto one tendon and insert into the olecranon and fascia of the ulna. The triceps brachii serves to extend the arm at the elbow, providing enough power for you to complete a pushup!
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