The brachialis muscle is a prime flexor of the forearm at the elbow joint. It is fusiform in shape and located in the anterior (flexor) compartment of the arm deep to the biceps brachii. The brachialis is a broad muscle, with its broadest part located in the middle rather than at either extremity.
It is sometimes divided into two parts, and may fuse with the fibers of the biceps brachii, coracobrachialis, or pronator teres muscles. It also functions to form part of the floor of the cubital fossa.
Distal half of anterior surface of humerus
|Insertions||Coronoid process of the ulna; Tuberosity of ulna|
|Innervation||Musculocutaneous nerve (C5,C6); Radial nerve (C7)|
|Functions||Strong flexion of forearm at the elbow joint|
The brachialis is located on the anterior surface of the shaft of the humerus, deep to the muscle belly of biceps brachii and distally to its tendon. The biceps brachii muscle is located immediately anterior to the brachialis, as are the brachial vessels, the musculocutaneous, and median nerves. The humerus and the capsule of the elbow joint lie posterior to the muscle.
Medially, the brachialis is separated from the triceps brachii and the ulnar nerve by the medial intermuscular septum and pronator teres. Laterally it is related to the radial nerve in the radial groove, the brachioradialis and the extensor carpi radialis longus muscles. The tendon of brachialis forms part of the floor of the cubital fossa.
Origin and Insertion
The brachialis muscle originates from the anterior surface of the distal half of the humerus, just distal to the insertion of the deltoid muscle. It is also attached to the intermuscular septa of the arm on either side, with a more extensive attachment to the medial intermuscular septum.
The fibers of brachialis extend distally to converge on a strong tendon. The tendon inserts onto the tuberosity of ulna and onto a rough depression on the anterior surface of the coronoid process of the ulna–passing between two slips of the flexor digitorum profundus muscle. It sometimes has an accessory attachment to the radius or the bicipital aponeurosis.
The brachialis is known as the workhorse of the elbow. It is a major flexor of the forearm at the elbow joint, flexing the elbow while it is in all positions. The brachialis is the only pure flexor of the elbow joint–producing the majority of force during elbow flexion. It is not affected by pronation or supination of the forearm, and does not participate in pronation and supination due to its lack of attachment to the radius.
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During controlled extension of the elbow joint, the brachialis steadies the movement by relaxing at an even pace. This is an eccentric contraction of the muscle. This motion is used for precision movements such as lowering a teacup onto a flat surface carefully. The brachialis is also responsible for holding the elbow in the flexed position, thus, when the elbow joint is flexed, the brachialis is always contracting.
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Innervation and Blood Supply
The brachialis is primarily supplied by the musculocutaneous nerve (C5, C6). In addition, a small lateral portion of the muscle is innervated by the radial nerve (C7).
Branches of the brachial artery and the radial recurrent artery supply the brachialis with contribution from accessory arteries. Occasionally, branches from the superior and inferior ulnar collateral arteries also contribute to the arterial supply of the brachialis muscle. The accessory arteries are small and highly variable. They can arise as branches from the brachial artery directly, the profunda brachii, or the superior and inferior ulnar collateral arteries. Venous drainage of the brachialis is by venae comitantes, mirroring the arterial supply and ultimately drain back into the brachial veins.
The brachialis can be clinically assessed by palpating the contracting muscle fibers during flexion of the elbow joint against resistance while the forearm is in the semi-prone position. If acting normally, the brachialis can be seen and palpated during this movement. If the muscle cannot be palpated, testing of the function of the muscle can be used to assess it. This can present as a weakness when flexing the arm against resistance, but also as an inability to fully extend the elbow joint due to painful stretching of the brachialis tendon.
The brachialis is the main muscle acting in common upper body exercises such as pull ups and elbow curls and overuse of it during exercises such as these can cause inflammation in the tendon of the muscle. This is called brachialis tendonitis. Climbers elbow is a form of brachialis tendonitis that is extremely common in climbers. It is caused by forceful contractions of the brachialis muscle, especially when the elbow is hyperextended. A common cause of this injury in climbers is reaching (hyperextending the elbow) and then pulling their body weight upwards by flexing the elbow joint, such as in rock climbing.
Symptoms of brachialis tendonitis are mainly a gradual onset of pain in the anterior elbow and swelling around the elbow joint. Patients often present with an inability to extend the elbow due to stiffness and soreness of the brachialis muscle. Treatment is by implementing the POLICE (Prevention, Optimal Loading, Ice, Compression, Elevation) method for acute sprains and strains, which has replaced the traditional PRICE (Prevention, Rest, Ice, Compression, Elevation) method. The POLICE method introduces an incremental rehabilitation procedure by slowly introducing stress to the injured muscle to restore its strength and morphology. Prevention of injuries to muscles can be achieved by correctly warming up before exercise, but may also include the use of external accessories such as bandages and tapes. Recovery time for brachialis tendonitis is dependent on the extent of damage to the tendon, but the elbow usually retains good function throughout the course of recovery. This gradually increases with the regaining of strength in the muscle. Most strains will heal with proper physiotherapy by the six week mark.