Muscles of the forearm
Introduction0:00–0:28
Anatomically speaking, the forearm is the part of the upper limb between the elbow and the wrist joints. It contains two bones, the ulna and the radius, which provide support to local muscles supplied by an intricate network of nerves and vessels.
On the anterior part of the elbow, there's a triangular fat filled pit called the cubital fossa, which is where the majority of important nerves and vessels to the forearm and hand can be found.
Forearm compartments0:28–1:17
Ok. Before we start, it's important to know that even though some of the muscles of the forearm attach proximately to the humerus, they still belong to the forearm.
The forearm is divided into two compartments which are separated by the radius and the ulna and the interosseous membrane running between them.
We have the anterior compartment which contains flexors and pronators. Next is the posterior compartment housing the extensors and supinators of the forearm.
Generally, muscles in the same compartment are innervated by the same nerve. So the muscles of the anterior compartment are generally innervated by the median nerve with a few muscles being innervated by the ulnar nerve.
Muscles of the posterior compartment. On the other hand, are innervated by the radial nerve.
Anterior Compartment1:17–1:50
Now, the muscles of the anterior compartment are divided into three groups or layers, superficial intermediate and deep in the superficial layer.
There are four muscles which all arise from a common tendon attached to the medial epicondyle of the humerus. So, this attachment site is called the common flexor origin.
Muscles attaching to the common flexor origin are the flexor carpi ulnaris, the palmaris, longus, the flexor carpi radialis and the pronator teres muscles.
Superficial layer1:50–4:17
Now let's look at these muscles one by one. On the most medial side lies the flexor carpi ulnaris.
As these two move distally to the wrist, they fuse into one tendon that crosses the wrist to distally attach to three bones.
The pisciform, the hook of hamate and fifth metacarpal bone. Although muscles of the superficial layer are primarily innervated by the median nerve.
The flexor carpi ulnaris muscle is an exception because it is fully innervated by the ulnar nerve. This muscle mainly flexes and adducts the hand at the wrist joint lateral to the flexor carpi ulnaris.
There is the palmaris longus muscle. This muscle proximately attaches to the medial epicondyle of the humerus and extends distally to form a tendon that attaches to the distal half of the flexor retinaculum and the apex of the palmar aponeurosis, the palmaris longus muscle is innervated by the median nerve.
When it contracts it flexes the hand at the wrist. A little fun fact about the palmaris longus, it is absent in about 14% of people.
But luckily, its absence doesn't come with any functional deficits lateral to the palmaris longus. There's the flexor carpi radialis muscle which also approximately attaches to the medial epicondyle of the humerus.
It extends distally and laterally to the middle of the forearm, forming a tendon that attaches at the base of the second metacarpal bone.
The flexor carpi radialis is innervated by the median nerve when it contracts it flexes and abducts the hand at the wrist joint.
Finally, there's the pronator teres, which is the most lateral muscle of the superficial muscles in the anterior compartment.
It also has a humeral and an ulnar head proximately. The humeral head attaches to the medial humeral epicondyle.
While the ulnar head attaches to the coronoid process of the ulna, as the muscle extends distally, it's two heads fuse into one tendon that attaches to the midshaft of the radius.
On its lateral aspect. The pronator teres is innervated by the median nerve and its contraction pronate and flexes the forearm at the elbow.
Quiz4:17–4:32
Ok. Now, what if we take a one minute pause to just flex your wrists at the same time, try to remember the muscles that attach to the common flexor origin.
Now, let's have a look at the intermediate layer of the anterior compartment of the forearm. It's a one muscle compartment containing only the flexor digitorum superficialis.
Intermediate layer4:32–5:45
This muscle has two heads, a humeroulnar head that attaches to the common flexor origin and the coronoid process and a radial head that attaches to the superior half of the anterior border of the radius.
Also called the anterior oblique line moving distally. The two heads of the flexor digitorum superficialis fuse together in the middle of the forearm.
And that eventually splits into four separate tendons at the level of the wrist. All of these tendons pass deep to the flexor retinaculum in the carpal tunnel, eventually attaching to the shafts of the middle phalanges of the medial four digits.
The flexor digitorum superficialis is innervated by the median nerve. And its actions depend on how hard it's contracting when it contracts moderately.
It flexes the middle phalanges at proximal interphalangeal joints of the medial four digits with a stronger contraction.
This muscle flexes the proximal phalanges at the metacarpal phalangeal joints. Finally let's look at the three muscles of the deep layer of the anterior compartment of the forearm.
Deep layer5:45–9:03
The flexor digitorum profundus, the flexor pollicis, longus and the pronator quadratus. The flexor digitorum profundus has two proximal parts, a medial and a lateral part.
And both of them attached to the proximal three quarters of the anterior aspect of the ulna. And the interosseous membrane distally, the medial part splits into two tendons to attach to the base of the 4th and 5th distal phalanges.
And the lateral part splits into two tendons to attach to the bases of the 2nd and 3rd distal phalanges. All the tendons pass posterior to the flexor digitorum superficialis and flexor retinaculum.
The medial part functions to flex the distal phalanges of digit four and five and the distal interphalangeal joints. While the lateral part does the same thing for the 2nd and 3rd digits.
An important distinction here is that the lateral part is innervated by the anterior interosseous nerve, a branch of the median nerve.
While the medial part is actually innervated by the ulnar nerve along with the flexor carpi ulnaris. To easily remember this, think of how the flexor carpi ulnaris and the medial part of the flexor digitorum profundus are both medial and are closer to the ulnar side of the forearm.
This will remind you that they are innervated by the ulnar nerve as it travels more medially in the forearm as well. Next up, there's the flexor pollicis Longus which attaches proximally to the anterior surface of the radius and interosseous membrane.
As the flexor pollicis longus extends distally, it transforms into a flat tendon that passes under the flexor retinaculum of the hand through the carpal tunnel to insert to the base of the distal phalanx of the thumb.
The flexor pollicis longus is innervated by the anterior interosseous nerve which is a branch of the median nerve and it causes flexion of the thumb.
Finally, there's the pronator quadratus muscle, which is a quadrangular muscle found deep to the other muscles in the anterior compartment proximately.
It attaches to the distal quarter of the anterior surface of the ulna and it extends laterally and distally to attach to the distal quarter of the anterior surface of the radius.
The pronator quadratus is innervated by the anterior interosseous branch of the median nerve when it contracts, it is the primary pronator of the forearm while also helping to hold the radius and ulna together.
Now, it's also important to know that the majority of these flexor tendons from the anterior compartment are held in place to prevent bowing by both the palmar carpal ligament and the distal and deeper flexor retinaculum, which is also known as the flexor aretina culum.
Both of these can be found on the anterior surface of the hand and wrist. Furthermore, a lot of these tendons are enclosed in either common synovial sheaths or individual synovial sheaths which allow the tendons to slide freely over each other during movements.
Quiz9:03–9:20
Ok. Let's take another pause here and see if you can remember the only muscle of the intermediate layer and the three muscles in the deep layer of the anterior compartment.
So let's now move to the posterior compartment of the forearm. The muscles of the posterior compartment are all innervated by the radial nerve and they're mainly extensors of the wrist and digits in a similar fashion to the flexor muscle tendons.
Posterior Compartment9:20–9:58
The extensor muscle tendons are held in place posteriorly by the extensor retinaculum, which is this thickened part of fascia on the posterior aspect of the wrist to prevent bowing of the tendons.
The extensor tendons also have synovial tendon sheaths to reduce friction during movement. The posterior compartment muscles are divided into a superficial and deep layer.
The superficial layer has six muscles. The brachial radialis, the extensor carpi, radialis longus, the extensor carpi radialis brevis, the extensor digitorum, the extensor digiti minimi and the extensor carpi ulnaris muscle.
Superficial layer9:58–14:01
The brachial radialis is the most lateral muscle in the superficial layer. Although it is anatomically grouped in as an extensor muscle of the posterior compartment.
Due to its location, the brachial radialis is functionally counted as a flexor because it is a weak flexor of the forearm.
Its proximal broad part attaches to the proximal two thirds of the lateral supra epicondylar ridge of the humerus and then narrows down as it extends distally to attach to the lateral surface of the distal radius, proximal to the styloid process when it contracts it flexes the forearm, especially if it was in a mid pronated position.
This muscle is also known as the beer drinking muscle because it gets a real workout when you drink a can of beer. The second muscle is the extensor carpi radialis longus.
Its name itself, the extensor carpi radialis longus implies that it's a long muscle that runs alongside the radial bone and that it extends the wrist.
So it attaches proximately from the lateral supra epicondylar ridge of the humerus and crosses the wrist to attach to the base of the second metacarpal bone.
When it contracts it extends and abducts the hand at the wrist joint. Next up, there are four muscles which have a common proximal attachment.
The extensor carpi, radialis, brevis, the extensor digitorum, the extensor digiti minimi and the extensor carpi ulnaris, they all attach to the lateral epicondyle of the humerus.
So this is the common extensor origin in the distal forearm. The extensor carpi radialis brevis crosses the wrist and attaches to the base of the third metacarpal bone.
It functions in the same way as the extensor carpi radialis longus to extend and abduct the wrist. The extensor digitorum occupies most of the posterior surface of the forearm and from its attachment to the lateral epicondyle, it eventually splits into four tendons and travels deep to the extensor retinaculum.
These tendons pass deep to the retinaculum and then spread as they move along the dorsum of the hand where they attach to the extensor expansions of the medial four digits.
The extensor expansions wrap around the dorsum and sides of the metacarpal head and proximal phalanx forming a hood and keeping the extensor tendon in the middle of each digit.
This muscle acts to extend the medial four digits at both the metacarpophalangeal and interphalangeal joints. Now as the extensor digiti minimi extends distally.
It forms a tendon that also passes deep to the extensor retinaculum. But in a separate compartment in the hand, it ends up attaching in the extensor expansion of the fifth digit and extends the fifth digit, both the metacarpophalangeal and interphalangeal joints.
The sixth and final muscle in the superficial layer of the posterior compartment is the extensor carpi ulnaris muscle. It also attaches to the lateral epicondyle of the humerus as well as the posterior border of the ulna and then extends distally in the forearm forming a tendon that crosses the wrist to attach to the base of the fifth metacarpal bone.
Its function is to extend and abduct the hand at the wrist joint. The extensor carpi, radialis, brevis, extensor digitorum, the extensor digiti minimi and the extensor carpi ulnaris are all innervated by the deep branch of the radial nerve, which is a distal branch of the radial nerve.
Deep layer14:01–17:21
Now let's switch gears and look at the deep layer of the posterior compartment. It contains five muscles, the supinator, the abductor pollicis Longus, the extensor pollicis, brevis, the extensor pollicis, longus and the extensor indices.
The abductor pollicis longus and the extensor pollicis, brevis and Longus are referred to as the outcropping muscles of the deep layer because they emerge or crop out in a small trench that separates the extensors in the lateral forearm.
First. The supinator lies deep in the cubital fossa forming part of its floor.
It then spirals medially and distally to broadly attach to the lateral posterior and anterior surfaces of the proximal third of the radius.
It is innervated by the deep branch of the radial nerve and its contraction supinate the forearm by externally rotating the radius.
All right. Now, let's move on to a group of three muscles referred to as the outcropping muscles of the deep layer.
These three muscles are all innervated by the posterior interosseous nerve. The continuation of the deep branch of the radial nerve.
The first outcropping muscle is the abductor pollicis longus, the most lateral of the three muscles, it proximally attaches to the upper halves of the ulna and radius plus the interosseous membrane.
It then attaches to the base of the first metacarpal bone. When the abductor pollicis longus contracts it abducts the thumb and extends it at its carpal metacarpal joint.
Next, there is the extensor pollicis brevis muscle which runs closely beside the abductor pollicis longus. It proximately attaches to the distal third of the radius and the interosseous membrane.
It then attaches at the base of the proximal phalanx of the thumb. When this muscle contracts, it extends the proximal phalanx of the thumb.
Next, there's the extensor pollicis longus. Its proximal attachment is to the middle third of the ulna and the interosseous membrane.
It then attaches to the base of the thumbs, distal phalanx. After it crosses medial to the dorsal tubercle of the radius to create space between it and the extensor pollicis brevis.
When the extensor poly, it longus contracts, it extends the thumb at each of its joints, including the interphalangeal joint.
Finally, there is the extensor indices, muscle. The proximal attachment of this muscle is to the posterior surface of the distal third of the ulna and interosseous membrane.
At the wrist, the extensor indices forms a tendon that crosses the joint to attach to the extensor expansion of the second digit aka the index finger.
The extensor indicis muscle is innervated by the posterior interosseous nerve, which is the continuation of the deep branch of the radial nerve.
When this muscle contracts, it extends the index finger and has a minor role in extending the wrist. Now, back to the outcropping muscles as the tendons of the extensor pollicis Longus and the extensor pollicis Brevis cross the wrist to their attachment points.
Anatomical snuffbox17:21–18:32
The gap between them forms a triangular shaped depression over the trapezium and scaphoid bone. This depression is often referred to as the anatomical snuffbox.
The abductor pollicis longus and the extensor pollicis. Brevis makes the anterior boundary of the anatomical snuffbox.
While the extensor pollicis longus makes the posterior border to remember the anterior boundary. Think of the bee in extensor pollicis, brevis being a buddy to the abductor pollicis Longus where the extensor pollicis longus on its own forms the posterior boundary.
You can also think of it as a Brevis sandwich between the two longus muscles. Hm Brevis sandwich.
But anyway, the anatomical snuffbox can be seen when the thumb is fully extended. And it's an important landmark because the radial artery can be palpated against the floor of the snuffbox, which is made up by the trapezium and the scaphoid bones.
Now, for a quick break, can you remember the muscles of the superficial layer of the posterior compartment? All right, it's a quick recap.
Quiz18:32–18:42
Review18:42–20:10
The anterior compartment of the forearm has eight muscles divided into a superficial intermediate and deep layer. There are four muscles in the superficial layer.
The pronator teres, the flexor carpi radialis, the palmaris longus and the flexor carpi ulnaris. In the intermediate layer, there is only one muscle, the flexor digitorum superficialis in the deep layer, there are three muscles, the flexor digitorum profundus, the flexor pollicis, Longus and the pronator quadratus.
In the posterior compartment. There are 11 muscles divided into a superficial and deep layer.
The superficial layer has six muscles, the brachial radialis, the extensor carpi, radialis, longus and the extensor carpi radialis brevis, the extensor digitorum, the extensor digiti minimi and the extensor carpi ulnaris.
In the deep layer. There are five muscles, the supinator, the three outcropping muscles which are the abductor pollicis, longus, the extensor pollicis, brevis and the extensor pollicis, longus and last but not least the extensor indices.
Finally, the anatomical snuffbox is a superficial landmark on the posterolateral wrist of which the abductor pollicis Longus and the extensor pollicis brevis make the anterior boundary.
While the extensor pollicis longus makes the posterior border.
- "Human Anatomy & Physiology, 11th edition" Pearson (2018)
- "Costanzo Physiology, 7th edition" Elsevier (2021)
- "Moore’s Clinically Oriented Anatomy, 9th edition" Wolters Kluwer (2023)
- "Fractional Lengthening of the Forearm Flexor Muscles: Anatomic Study" J Hand Surg Am (2022)
- "Use of free radial forearm and pronator quadratus muscle flap: Anatomical study and clinical application" J Plast Reconstr Aesthet Surg (2022)
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