Muscles of the Upper Arm
The upper arm is located between the shoulder joint and elbow joint. It contains four muscles – three in the anterior compartment (biceps brachii, brachialis, coracobrachialis), and one in the posterior compartment (triceps brachii).
In this article, we shall look at the anatomy of the muscles of the upper arm – their attachments, innervation and actions.
There are three muscles located in the anterior compartment of the upper arm – biceps brachii, coracobrachialis and brachialis. They are all innervated by the musculocutaneous nerve. A good memory aid for this is BBC – biceps, brachialis, coracobrachialis.
Arterial supply to the anterior compartment of the upper arm is via muscular branches of the brachial artery.
The biceps brachii is a two-headed muscle. Although the majority of the muscle mass is located anteriorly to the humerus, it has no attachment to the bone itself.
As the tendon of biceps brachii enters the forearm, a connective tissue sheet is given off – the bicipital aponeurosis. This forms the roof of the cubital fossa and blends with the deep fascia of the anterior forearm.
- Attachments: Long head originates from the supraglenoid tubercle of the scapula, and the short head originates from the coracoid process of the scapula. Both heads insert distally into the radial tuberosity and the fascia of the forearm via the bicipital aponeurosis.
- Function: Supination of the forearm. It also flexes the arm at the elbow and at the shoulder.
- Innervation: Musculocutaneous nerve. The bicep tendon reflex tests spinal cord segment C6.
The coracobrachialis muscle lies deep to the biceps brachii in the arm.
- Attachments: Originates from the coracoid process of the scapula. The muscle passes through the axilla, and attaches the medial side of the humeral shaft, at the level of the deltoid tubercle.
- Function: Flexion of the arm at the shoulder, and weak adduction.
- Innervation: Musculocutaneous nerve.
The brachialis muscle lies deep to the biceps brachii, and is found more distally than the other muscles of the arm. It forms the floor of the cubital fossa.
- Attachments: Originates from the medial and lateral surfaces of the humeral shaft and inserts into the ulna tuberosity, just distal to the elbow joint.
- Function: Flexion at the elbow.
- Innervation: Musculocutaneous nerve, with contributions from the radial nerve.
Clinical Relevance: Rupture of the Biceps Tendon
A complete rupture of any tendon in the body is rare. However, the long head of the biceps brachii is one of the more common tendons to rupture.
This produces a characteristic sign on flexing the elbow – a bulge where the muscle belly is, called the ‘Popeye Sign’. The patient would not notice much weakness in the upper limb due to the action of the brachialis and supinator muscles.
The posterior compartment of the upper arm contains the triceps brachii muscle, which has three heads. The medial head lies deeper than the other two, which cover it.
Arterial supply to the posterior compartment of the upper arm is via the profunda brachii artery.
- Attachments: Long head – originates from the infraglenoid tubercle. Lateral head – originates from the humerus, superior to the radial groove. Medial head – originates from the humerus, inferior to the radial groove. Distally, the heads converge onto one tendon and insert into the olecranon of the ulna.
- Function: Extension of the arm at the elbow.
- Innervation: Radial nerve. A tap on the triceps tendon tests spinal segment C7.
- Note: In some individuals, the long head of the triceps brachii is innervated by the axillary nerve.