Muscles - Posterior Forearm Flashcards
What is the general function of the muscles of the posterior forearm?
Extension of the wrist and the fingers.
They are all innervated by the radial nerve.
What are the superficial muscles of the posterior forearm?
There are seven superficial posterior forearm muscles. The ‘extensor carpi/digiti’ muscles share a common tendinous origin at the lateral epicondyle.
1) Brachioradialis
2) Extensor carpi radialis longus and brevis
3) Extensor digitorum communis
4) Extensor digiti minimi
5) Extensor carpi ulnaris
6) Anconeus
Brachioradialis
It is a paradoxical muscle - its origin and insertion displays characteristics of an extensor muscle, but it is actually a flexor at the elbow.
The muscle is most visible when the arm is half pronated, an flexing at the elbow against resistance - this is relevant for ‘reflex test’ in a neurological examination.
In this distal forearm, the radial artery and nerve are sandwiched between the radialis and the deep flexor muscles.
Origin - proximal aspect of the lateral supracondylar ridge of the humerus.
Insertion - distal end of the radius, just before the radial styloid process.
Actions - flexes at the elbow.
Innervation - radial nerve.
Extensor carpi radialis brevis and longus
These muscles are situated on the lateral aspect of the posterior forearm. Due to their position they able to produce abduction as well as extension of the forearm.
Origin (ECRL) - supracondylar ridge
Origin (ECRB) - lateral epicondyle
Insertion - metacarpal bones II and III
Extensor digitorum communis
The main extensor of the fingers. To test the function of the muscle, the forearm is pronated, and the fingers extended against resistance.
Origin - lateral epicondyle
Insertion - continues into to the distal forearm, where it splits into four, and inserts into the extensor hood of each finger.
Action - extends the medial four fingers at the MCP and IP joints.
Extensor digiti minimi
In some this muscle is fused to the extensor digiti communis. This muscle lies medially to the extensor digitorum.
Origin - lateral epicondyle.
Insertion - attaches to the extensor digitorum tendon, into the extensor hood of the little finger.
Actions - extends the little finger, and contributes to extension at the wrist.
Innervation - radial nerve (deep branch).
Extensor carpi ulnaris
Located on the medial aspect of the posterior forearm. Due to its position, it produces adduction as well as extension at the wrist.
Origin - lateral epicondyle of the humerus.
Insertion - base of metacarpal V.
Action - extension and adduction of the wrist.
Innervation - radial nerve (deep branch).
Anconeus
Situated medially and superiorly in the extensor compartment of the forearm. It is blended with the fibres of the triceps brachii, and the two muscles can be indistinguishable.
Origin - lateral epicondyle
Insertion - olecranon
Action - extends and stabilises the elbow joint. Abducts the ulna during pronation of the forearm.
Innervation - radial nerve.
Clinical relevance: lateral epicondylitis
Lateral epicondylitis (or tennis elbow) is inflammation of the periosteum lining the lateral epicondyle.
It is caused by repeated use of the superficial extensor muscles, which strain the common tendinous attachment to the lateral epicondyle.
What are the five muscles of the deep compartment?
1) Supinator
2) Abductor pollicis longus
3) Extensor pollicis brevis
4) Extensor pollicis longus
5) Extensor indicis proprius
With the exception of the supinator, these muscles act of the thumb and the index finger.
Supinator
It lies in the floor of the cubital fossa. It has two heads, which the deep branch of the of the radial nerve passes between.
Origin - lateral epicondyle and posterior surface of the ulna.
Insertion - posterior surface of the radius.
Actions - supinates the forearm.
Innervation - radial nerve (deep branch)
Abductor pollicis longus
Located immediately distal to the supinator. Its tendon contributes to the lateral border of the anatomical snuffbox.
Origin - interroseous membrane and the adjacent posterior surface of the radius and ulna.
Insertion - attaches to the lateral side of the base of metacarpal I.
Action - abducts the thumb.
Innervation - radial nerve (posterior interroseous branch)
Extensor pollicis brevis
Can be found medially and deep to the abductor pollicis longus. Its tendon contributes to the lateral border of the anatomical snuffbox.
Origin - posterior surface of the radius and interroseous membrane.
Insertion - base of the proximal phalanx of the thumb.
Actions - extends the metacarpophalangeal and carpometacarpal joints of the thumb.
Innervation - radial nerve (posterior interroseous branch).
Extensor pollicis longus
Large muscle belly than the EPB. The tendon runs medially to the dorsal tubercle at the wrist, using the tubercle as a ‘pulley’ to increase the force exerted.
The tendon of the EPL forms the medial border of the anatomical snuffbox.
Origin - posterior surface of the ulna and the interosseous membrane.
Insertion - distal phalanx of the thumb.
Actions - extends all joints of the thumb: carpametacarpal, metacarpophalangeal, and the interphalangeal
Innervation - radial nerve (posterior interroseous branch)
Extensor indicis proprius
This muscle allows the index finger to be independent to the other fingers during extension.
Origin - posterior surface of the ulna and the interroseous membrane, distal to the extensor pollicis longus.
Insertion - extensor head of the index finger.
Actions - extends the index finger.
Innervation - radial nerve (posterior interroseous branch).
