The upper arm is located between the shoulder joint and elbow joint and 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 these muscles - their attachments, innervation and actions.
The anterior compartment of the upper arm houses three muscles - biceps brachii, coracobrachialis and brachialis. All three muscles are innervated by the musculocutaneous nerve - an easy way to remember this is BBC - biceps, brachialis, coracobrachialis. The muscular branches of the brachial artery provide arterial supply to this compartment.
The biceps brachii is a two-headed muscle, but it does not have an attachment to the humerus itself. The long head of the biceps brachii originates from the supraglenoid tubercle of the scapula, while 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, which is a connective tissue sheet that is given off as the tendon of biceps brachii enters the forearm. The biceps brachii takes part in several actions, including supination of the forearm, flexion of the arm at the elbow and flexion of the arm at the shoulder. It is innervated by the musculocutaneous nerve and the bicep tendon reflex tests spinal cord segment C6.
The coracobrachialis muscle lies deep to the biceps brachii in the arm. It originates from the coracoid process of the scapula and passes through the axilla, attaching to the medial side of the humeral shaft at the level of the deltoid tubercle. The coracobrachialis aids in the flexion of the arm at the shoulder and weak adduction. It is innervated by the musculocutaneous nerve.
The brachialis muscle lies deep to the biceps brachii and is found more distally than the other muscles of the arm. It originates from the medial and lateral surfaces of the humeral shaft and inserts into the ulnar tuberosity, just distal to the elbow joint. The brachialis muscle is responsible for flexion at the elbow and is innervated by the musculocutaneous nerve, with contributions from the radial nerve.
Complete rupture of any tendon of the body is a rare occurrence. However, the long head of the biceps brachii is one of the more common tendons to rupture. This produces a characteristic sign when the elbow is flexed, in the form of a bulge where the muscle belly is - the 'Popeye Sign'. The patient may not experience much weakness in the upper limb, as the brachialis and supinator muscles take up some of the slack.
The posterior compartment of the upper arm houses the triceps brachii muscle, which has three heads. The medial head lies deeper than the other two, which cover it. The profunda brachii artery provides arterial supply to the posterior compartment of the upper arm.
The long head of the triceps brachii originates from the infraglenoid tubercle while the lateral head originates from the humerus, superior to the radial groove. The medial head of the triceps brachii originates from the humerus, inferior to the radial groove. Distally, the heads converge onto one tendon and insert into the olecranon of the ulna. The triceps brachii is involved in the extension of the arm at the elbow and is innervated by the radial nerve. A tap on the triceps tendon tests spinal segment C7. There may be some individuals in whom the long head of the triceps brachii is innervated by the axillary nerve.
The muscles of the upper arm can be divided into three main compartments – the anterior, lateral and posterior. The most prominent muscle of the anterior compartment is the biceps brachii, which has a two-headed origin from the scapula and attaches to the radius at the level of the elbow. It functions to flex the arm at the elbow and shoulder, and to supinate the forearm. It is innervated by the musculocutaneous nerve and the tendon reflex tests spinal segment C6.
The coracobrachialis muscle lies in the lateral compartment of the upper arm, originating from the coracoid process of the scapula and attaching to the medial side of the humerus. It is responsible for flexing the arm at the shoulder as well as weak adduction. It’s innervated by the musculocutaneous nerve.
The brachialis muscle is found in the posterior compartment. Its origin is from the medial and lateral surfaces of the humerus and it attaches to the ulnar tuberosity, distal to the elbow joint. Its primary function is to flex the elbow joint and it is innervated by the musculocutaneous nerve, with contributions from the radial nerve.
The triceps brachii is the largest muscle of the posterior compartment of the upper arm. It has three heads – a long, lateral and medial. The long head originates from the infraglenoid tubercle, the lateral originates from the humerus superior to the radial groove, and the medial originates from the humerus inferior to the radial groove. All three heads converge onto one tendon which attaches to the olecranon of the ulna. This is responsible for extension of the arm at the elbow joint and is innervated by the radial nerve. Clinically, rupture of the long head of the triceps brachii can manifest in a ‘Popeye sign’, where the lower part of the arm is much more prominent due to the lack of a triceps counteraction to the contraction of the biceps. In addition, the triceps brachii is supplied by the radial nerve and the profunda brachii artery.
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.
The coracobrachialis muscle lies deep to the biceps brachii in the arm and originates from the coracoid process of the scapula. It passes through the axilla and attaches to the medial side of the humeral shaft, at the level of the deltoid tubercle. Its primary function is to flex the arm at the shoulder and weak adduction and it is innervated by the musculocutaneous nerve.
The brachialis muscle lies deep to the biceps brachii, found more distally than the other muscles of the arm. It forms the floor of the cubital fossa and originates from the medial and lateral surfaces of the humeral shaft and inserts into the ulnar tuberosity, just distal to the elbow joint. Its primary function is to flex the elbow joint and it is innervated by the musculocutaneous nerve, with contributions from the radial nerve.
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.
The long head of the triceps brachii arises from the infraglenoid tubercle of the scapula and is located on the posterior aspect of the upper arm. It then attaches to the proximal part of the ulna and acts to extend the elbow joint. Clinically, rupture of the long head of the triceps brachii can manifest in a ‘Popeye sign’ - where the lower part of the arm is much more prominent due to the lack of a triceps counteraction to the contraction of the biceps. The triceps brachii is supplied by the radial nerve and the profunda brachii and the brachial arteries are responsible for its arterial supply.
The muscles of the upper arm are complex and vary in shape and size depending on their particular function. The biceps brachii’s primary function is to flex the elbow and shoulder, and also to supinate the forearm. The coracobrachialis’ primary function is flexion of the arm at the shoulder and weak adduction, while the brachialis’ main function is flexion of the elbow. Last but not least, the long head of the triceps brachii’s primary function is extension of the elbow joint.
The long head of the triceps brachii is important for normal upper arm function and its muscle fibers extend posteriorly from the scapula to the proximal part of the ulna. Its innervation is the radial nerve and its arterial blood supply is provided by the profunda brachii artery. Clinically, the long head of the triceps brachii can demonstrate rupture in a ‘Popeye sign’ - where the lower part of the arm is much more prominent due to the lack of a triceps counteraction to the contraction of the biceps.
The anatomy of the upper arm muscles is complex and requires a good understanding in order to assess and treat the upper limb. The biceps brachii, coracobrachialis, brachialis and triceps brachii are the primary muscles of theupper arm and all have different origins and functions which are important for normal arm function. Clinicians need to be aware of the anatomy of these muscles and understand their clinical implications.
