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The biceps, or biceps brachii, is a large muscle that sits on the front of the upper arm between the shoulder and elbow. It is one of four muscles that make up the upper arm, alongside the brachialis, brachioradialis, and coracobrachialis. The biceps is a two-joint muscle, and its primary function is to flex the elbow and rotate the forearm. The brachialis muscle, which lies deep to the biceps, serves as a counter to the biceps, providing stability and aiding in forearm flexion. This relationship between the biceps and brachialis muscles is essential for various physical activities, such as lifting, throwing, and gesturing.
| Characteristics | Values |
|---|---|
| Muscle countering the bicep | Brachialis |
| Location of Brachialis | Underneath the biceps brachii |
| Function of Brachialis | Flexion at the elbow |
| Innervation | Musculocutaneous nerve, with contributions from the radial nerve |
| Other muscles that work with the biceps | Brachioradialis, supinator, and coracobrachialis |
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What You'll Learn
Brachialis muscle
The brachialis muscle is a prime flexor of the forearm at the elbow joint. It is located in the anterior 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 of its extremities. It is sometimes divided into two parts and may fuse with the fibres of the biceps brachii, coracobrachialis, or pronator teres muscles. The brachialis muscle originates from the anterior surface of the distal half of the humerus, just distal to the insertion of the deltoid muscle, which it embraces by two angular processes.
The brachialis is the prime mover of elbow flexion, generating about 50% more power than the biceps. It is the major flexor of the elbow, with a large cross-sectional area, providing it with more strength than the biceps brachii and the coracobrachialis. 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. The brachialis steadies the movement of the elbow joint by relaxing at an even pace during controlled extension. This motion is used for precision movements, such as carefully lowering a teacup onto a flat surface.
The brachialis is commonly injured by repetitive forceful contractions or muscular contractions with the arm in hyperextension. This is often seen in climbers due to the pronation of the hand and the extended starting position. Physical activity that involves a lot of pull-ups, curls, and rope climbing can also cause brachialis muscle pain. A strain to the brachialis tendon can cause a patient to present with a lacking elbow extension due to painful end-range stretching of the tendon.
The brachialis is innervated by the musculocutaneous nerve, which runs on its superficial surface, between it and the biceps brachii. In 70-80% of people, the muscle has double innervation with the radial nerve. The brachialis is supplied by muscular branches of the brachial artery and by the recurrent radial artery.
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Brachioradialis muscle
The brachioradialis is a muscle in the forearm that flexes the forearm at the elbow. It is also capable of both pronation and supination, depending on the position of the forearm. It is attached to the distal styloid process of the radius by way of the brachioradialis tendon and to the lateral supracondylar ridge of the humerus.
The brachioradialis is a superficial, fusiform muscle on the lateral side of the forearm. It originates proximally on the lateral supracondylar ridge of the humerus and inserts distally on the radius, at the base of its styloid process. It is also known as supinator longus.
The brachioradialis is considered a posterior or extensor-compartment muscle, though it functions as a flexor. It is one of two forearm extensor-compartment muscles that do not cross the wrist, the other being the supinator. The brachioradialis flexes, pronates, and supinates the forearm but is innervated by the radial nerve.
The brachioradialis is the most superficial muscle on the radial side of the forearm. It forms the lateral side of the cubital fossa. It has a thin belly that descends in the mid-forearm, where its long flat tendon begins, then the tendon continues to the radius.
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Triceps brachii muscle
The triceps brachii muscle is the only constituent of the posterior muscle group of the arm, spanning almost the entire length of the humerus. It is the only muscle in the posterior compartment of the arm. The triceps brachii has three heads: the long, medial, and lateral. These heads originate from their respective attachments on the humerus and scapula, and converge onto a common tendon, which inserts on the ulna.
The triceps brachii muscle is innervated by the radial nerve, which gives off a separate branch for each head. The C6 root value of the radial nerve innervates the lateral head, the C7 root value innervates the long head, and the C8 root value supplies the medial head. The triceps brachii muscle is mainly supplied by the deep brachial artery and the superior ulnar collateral artery, which arises from the brachial artery.
The medial head of the triceps brachii is innervated by the ulnar nerve. The long head of the triceps is innervated by the axillary nerve. The medial head is predominantly formed by small type I muscle fibres and motor units, while the lateral head is comprised of a great quantity of large type IIb fibres and motor units. The long head consists of a more balanced mixture of fibre types and motor units.
The triceps brachii plays a role in creating anatomical spaces which are traversed by neurovascular structures. This makes the triceps brachii an important surgical landmark. The space between the medial head of the triceps and the biceps brachii forms the medial bicipital groove, which provides a passageway for the brachial artery and median and ulnar nerves.
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Coracobrachialis muscle
The coracobrachialis muscle is a small muscle located in the upper medial part of the arm. It originates from the coracoid process of the scapula and inserts onto the middle of the medial aspect of the body of the humerus. It is innervated by the musculocutaneous nerve, which arises from the anterior division of the upper trunk (C5).
The coracobrachialis muscle plays a supporting role in several functions of the arm. These include flexion, which is bringing the upper arm forward, and adduction, which is returning the arm to the side from an "airplane" position. It also assists in internal rotation, bringing the arms inward toward the body, and helps to maintain shoulder stability by holding the upper arm bone within the shoulder socket during rotation.
The coracobrachialis muscle is not one of the most prominent arm muscles, and it does not play a major role in arm movement. However, it can become tight or strained from certain activities, leading to shoulder or arm pain. The muscle can be strained by activities such as push-ups, swinging on gymnast rings, or carrying heavy bags at the side.
The coracobrachialis muscle is classified into distinct superficial and deep layers. In most individuals, these layers are not completely separated, but in some cases, there may be a full or partial division. The muscle is considered functionally insignificant and is a weak flexor and adductor of the arm at the shoulder joint.
Rupture of the coracobrachialis muscle is extremely rare and is typically caused by direct trauma to the contracted muscle. Overuse of the muscle can lead to stiffening and may cause pain in the arm, shoulder, and hand. Common causes of overuse include chest workouts and gymnastics.
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Supinator muscle
The supinator muscle is a broad muscle located in the posterior compartment of the forearm. It curves around the upper third of the radius and consists of two layers of fibres, or planes, that originate from the ulna and humerus. The superficial layer arises from tendinous fibres, while the deep layer originates from muscular slips. The supinator muscle is innervated by the deep branch of the radial nerve, which becomes the posterior interosseous nerve upon exiting the muscle. Its nerve roots are primarily from C6, with some C5 involvement and possible additional C7 innervation.
The main function of the supinator muscle is to supinate the forearm, or turn the palm upwards. This movement can be done with the elbow in any position of flexion or extension. The supinator works with the biceps brachii to achieve powerful supination, except when the elbow joint is extended. The supinator is the most active muscle in forearm supination during unresisted supination, while the biceps become more active with heavy loading. Supination strength decreases by 64% if the supinator is disabled, for example, by injury.
The supinator muscle rotates the radius laterally at the proximal radioulnar joint, which puts the radius parallel to the ulna. This action brings the hand into the supine position, facing anteriorly with the palm up, like when holding a bowl of soup. When producing a slow and unopposed supination movement, the supinator muscle acts alone. However, for quick, strong, or forceful supination, or when the movement is resisted, the supinator is assisted by the biceps brachii. The most powerful supination occurs when the elbow joint is flexed to 90 degrees, such as when turning a screwdriver or opening a bottle of wine.
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Frequently asked questions
The triceps brachii is the muscle that counters the biceps brachii.
The biceps brachii is a large muscle situated on the front of the upper arm between the shoulder and the elbow.
The primary function of the biceps is to flex the elbow and rotate the forearm. It also aids in lifting, throwing, and gesturing.
Common conditions affecting the biceps include biceps tendinopathy, strains, and tendon tears. These issues often arise from physical trauma or repetitive activities.
The Popeye Sign or Popeye Deformity refers to an obvious deformity where a bulge forms in the muscle belly due to a rupture of the long head tendon of the biceps. This condition is common among baseball pitchers due to chronic wear and tear.
