Dominic Stone
The elbow is a complex joint that connects the upper arm and forearm, allowing for flexion and extension as well as pronation and supination. The triceps brachii and anconeus muscles act as the extensors of the forearm, working together to straighten the arm. Heavy pushing activities, such as push-ups, require strong activation of all three heads of the triceps and the anconeus, while daily activities typically require less force and only activate the one-joint extensor muscles. The elbow joint also enables the wrist to rotate through the action of the pronator teres and supinator muscles. Understanding the muscular coordination and biomechanics of the elbow joint is important for diagnosing and treating neuromuscular dysfunctions and orthopedic conditions.
| Characteristics | Values |
|---|---|
| Elbow extension muscles | Triceps Brachii, Anconeus |
| Elbow flexors | Biceps Brachii, Brachialis, Brachioradialis |
| Elbow joint | A synovial hinge joint |
| Elbow joint bones | Humerus, Ulna, Radius |
| Elbow joint movement | Flexion and extension of the forearm |
| Elbow joint functions | Rotation of the forearm, wrist, and fingers |
| Elbow extension force | Low force requires activation of one-joint extensor muscles; high force requires strong activation of all three heads of the triceps and the anconeus |
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What You'll Learn
Triceps Brachii
The triceps brachii is a three-headed muscle in the arm, consisting of the medial, lateral, and long head. It is the only muscle in the posterior compartment of the arm and is responsible for elbow extension. The triceps brachii muscle originates from the humerus and scapula and inserts via a common tendon on the ulna. Its main function is to extend the forearm at the elbow joint.
The long head of the triceps brachii arises from the infraglenoid tubercle of the scapula and contributes to the extension and adduction of the arm at the shoulder joint. The medial head arises from the humerus, just below the groove of the radial nerve, and its distal part arises from the lateral intermuscular septum. The lateral head originates from a narrow, linear ridge on the posterior surface of the humerus, above the radial groove.
The triceps brachii is innervated by the radial nerve, which provides a separate branch for each head. The C6 root value of the radial nerve innervates the lateral head, C7 innervates the long head, and C8 supplies the medial head. However, recent studies have found that in some individuals, the long head may be innervated by the axillary nerve or receive dual innervation from both the radial and axillary nerves.
The triceps brachii can be trained through isolation or compound elbow extension movements, such as push-ups, bench presses, and tricep dips. It can also be contracted statically to keep the arm straight against resistance. Ruptures of the triceps brachii muscle are uncommon and typically occur in anabolic steroid users or due to a fall on an outstretched hand or a direct blow to the triceps tendon.
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Anconeus
The anconeus is a small, triangular muscle located at the elbow, attaching the humerus and ulna. It is considered a continuation of the triceps brachii muscle, with their fibres often being partially or completely blended together. The anconeus is innervated by a branch of the radial nerve (cervical roots 7 and 8) from the posterior cord of the brachial plexus. It is supplied by the recurrent interosseous branch of the posterior interosseous artery.
Functionally, the anconeus performs the same tasks as the triceps muscle at the elbow. Its contraction leads to the extension of the forearm, allowing for elbow extension. It also provides support for the dorsal joint capsule, preventing damage during hyperextension. Additionally, the anconeus is believed to stabilise the ulna, especially during pronation movements of the forearm.
The anconeus muscle can be easily palpated at the lateral side of the forearm near the elbow, particularly during pronation and supination movements. It is important to note that the anconeus may not play a significant role in elbow extension, with the triceps brachii muscle being the major muscle responsible for this movement.
An anatomical variation of the anconeus, called the anconeus epitrochlearis muscle, is found in about 34% of humans. While most variations are harmless, the anconeus epitrochlearis can cause compression of the ulnar nerve, resulting in cubital tunnel syndrome. This syndrome is characterised by numbness in the ulnar side of the hand and pain in the elbow. Treatment of pain associated with the anconeus muscle can be achieved by releasing the trigger point causing the pain.
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Biceps Brachii
The biceps brachii, commonly known as the biceps, is a large, thick muscle on the front of the upper arm between the shoulder and the elbow. The muscle is composed of a short head and a long head. The long head is located on the lateral side of the biceps brachii, while the short head is located on the medial side. The short head is sometimes referred to as "caput breve", while the long head is also called "caput longum".
The biceps brachii works across three joints and is able to generate movements in glenohumeral, elbow, and radio-ulnar joints. The muscle is a forearm flexor when extended but becomes the forearm's most powerful supinator when flexed. It turns the palm upwards, with the help of the supinator muscle. This action requires the humeroulnar joint of the elbow to be at least partially flexed. The biceps are involved in tasks such as lifting, sports involving throwing and racket use, and gesturing.
The biceps brachii is an important landmark for locating the brachial artery during physical examination and ultrasound-guided arterial cannulation. The primary arterial blood supply for the biceps brachii muscle comes from the muscular branches of the brachial artery. The musculocutaneous nerve provides the biceps' sensory and motor innervation.
The biceps brachii is traditionally described as a two-headed muscle, but it has a third head arising from the humerus in 10% of cases. In rare cases, four, five, or even seven supernumerary heads have been reported. The short head of the biceps brachii assists with horizontal adduction (bringing the arm across the body) when the arm is internally (or medially) rotated. It also assists in stabilization of the shoulder joint when carrying a heavy weight.
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Brachialis
The brachialis (brachialis anticus) is a muscle in the upper arm that flexes the elbow. It is the prime mover of elbow flexion, generating about 50% more power than the biceps. 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.
The brachialis is located on the anterior surface of the shaft of the humerus, deep to the muscle belly of the 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.
The brachialis is innervated by the musculocutaneous nerve, which runs on its superficial surface, between it and the biceps brachii. However, in 70-80% of people, the muscle has double innervation with the radial nerve (C5-T1). The divide between the two innervations is at the insertion of the deltoid. The majority of the motor supply is supplied by the musculocutaneous nerve. The radial nerve descends in the groove between the brachialis and brachioradialis muscles, above the elbow.
The brachialis muscle can be commonly injured by repetitive forceful contractions or muscular contractions with the arm in hyperextension. This is commonly 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 initiate brachialis muscle pain. A strain to the brachialis tendon can also cause a patient to present with a lacking elbow extension due to painful end-range stretching of the tendon.
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Brachioradialis
The brachioradialis is a fusiform muscle located in the lateral part of the posterior forearm. It is the most superficial muscle on the radial side of the forearm. It forms the lateral side of the cubital fossa and is often fused proximally with the brachialis. It has a thin belly that descends in the mid-forearm, where its long, flat tendon begins. The muscle is also known as supinator longus.
The brachioradialis is considered a posterior or extensor-compartment muscle, although it functions as a flexor. It flexes, pronates, and supinates the forearm but is innervated by the radial nerve. Trauma and chronic overuse are the usual causes of brachioradialis impairment. In surgery, the brachioradialis is vital to hand defect reconstruction, forearm tendon transfers, and carpal tunnel release.
The brachioradialis tendon is used clinically to test the C6 spinal nerve root. The muscle is active during elbow flexion, whether the forearm is supinated, neutral, or pronated. A study by Tim Kleiber et al found increased activity of the brachioradialis muscle in elbow flexion from a forearm pronation position. This increased activity compensates for the mechanically disadvantaged biceps brachii muscle in forearm pronation as its tendon is wrapped around the radial tuberosity.
The brachioradialis is one of the first muscles to recover following radial nerve injury. It arises with the extensor carpi radialis longus muscle from the lateral humeral supracondylar ridge. The brachioradialis is one of seven muscles in the forearm's superficial layer that attach to the lateral epicondyle and supracondylar ridge of the humerus.
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Frequently asked questions
The triceps brachii and the anconeus are the two muscles that act as extensors of the forearm.
The triceps brachii is a long muscle that runs from the scapula to the ulna, and it is one of the extensors of the forearm.
The anconeus is a small muscle that begins at the distal end of the humerus and ends at the ulna, and it is also an extensor of the forearm.
Yes, the brachialis, biceps brachii, and brachioradialis are the flexors of the elbow, and they work together with the extensors to allow for a full range of motion.
If your elbow extensors are not functioning properly, it can be difficult to transition from a supine to a sitting position without assistance. It can also impact your ability to perform daily functions that require low elbow extension force.
