Video: Posterior compartment of forearm muscles

Help! Here, take my hand. I can't reach. You're gonna fall unless you take my hand! I'm not able huhuhuhu... Take it! Use your forearm extensors and reach for my hand! Use my…my…my what? Ahhhhh…

Looks like someone could have done with paying more attention in anatomy class. Brace yourselves – memorizing the names, attachments, and functions of these muscles may certainly feel like trying to summit a mountain. But rest assured – we will guide you along our journey through the posterior compartment of the forearm.

So what's the itinerary for today's tutorial? We'll kick things off by learning all about the extensor muscles in the posterior compartment of the forearm. Next, we'll discover how they are organized into two different subdivisions. Then while we explore all of these important muscles, we'll discuss their attachments, innervation, and action or function.

Before we set off, let's prepare ourselves with an outline of the forearm itself and its muscles we utilize every day for intricate wrist and finger movements.

The forearm is divided into an anterior or flexor-pronator compartment and our topic of focus, a posterior or extensor compartment on the posterior surface, which is represented in this image. This posterior compartment contains 11 muscles that are subdivided into a superficial layer with six muscles and a deep layer housing five muscles – all of which we will look at in detail.

As we proceed along, it's important to recognize that the attachments of these extensor muscles include the origin or proximal area they arise from and the insertion or distal site of attachment. Lastly, from a functional standpoint, the muscles of the posterior compartment at the forearm are mostly, but not exclusively, involved in the extension of the wrist, hand, and fingers. So let's get after it starting with the superficial muscles of the posterior forearm.

Now as previously mentioned, there are six superficial muscles of the posterior compartment of the forearm. These include the brachioradialis, the extensor carpi radialis longus, the extensor carpi radialis brevis, the extensor digitorum, the extensor digiti minimi, and the extensor carpi ulnaris. Notably, the first three muscles of the superficial layer all have radial in their name thus they are sometimes grouped separately as the radial muscles of the forearm. And perhaps the easiest concept to learn before we really jump into all the superficial muscles of the posterior compartment of the forearm is that all of these muscles are innervated by the radial nerve, either directly or via its deep branch and the posterior interosseous nerve.

All right, let's start with the brachioradialis muscle, which we can see isolated in this posterior image of the forearm. This long fusiform muscle is situated close to the surface of the skin and creates the lateral contour of the elbow and forearm. Even though the brachioradialis muscle belongs to the posterior compartment of the forearm, it is actually easier to identify from this anterior perspective.

Arising off the humerus at the lateral supracondylar ridge as well as from the lateral intermuscular septum of the arm, the brachioradialis crosses the elbow anteriorly, courses along the radial side of the forearm, and descends distally until it inserts just proximal to the styloid process of the radius. We can appreciate how the brachioradialis originates laterally and extends along the anterolateral forearm. This contributes to its unique action on the elbow joint producing flexion of the forearm when in a semi-pronated position. The resulting motion is utilized while lifting up and drinking a pint of beer which is why the brachioradialis is also known as the beer-drinking muscle.

Now you might be scratching your head wondering why we're talking about flexion in this tutorial on the extensor muscles of the forearm. Well it so happens that the brachioradialis is a functional exception and serves as the only primary flexor muscle in the posterior compartment of the forearm. So with that straightened out, we'll explore the extensor muscles in the upcoming slides.

Taking a look at the extensor carpi radialis longus, this lengthy muscle spans along the lateral aspect of the forearm and mostly resides underneath the brachioradialis muscle. It also originates off the lateral supracondylar ridge of the humerus and it courses distally to insert onto the posterior aspect of the base of the second metacarpal bone.

Moving onto the action of the extensor carpi radialis longus muscle, we can deduce from its distal attachment that it acts on the wrist joint. When contracted, this muscle primarily produces extension of the wrist as well as abduction or radial deviation of the wrist which is movement of the wrist laterally or towards your radius, seen here in this image of the hand in anatomical position.

What's the opposite of long? Well, it's short, of course. In this case, since we talked about a longus muscle, we now have a brevis muscle – the extensor carpi radialis brevis muscle to be exact. As we can see, this muscle is deep to its longus counterpart and a bit shorter and thicker as well. Originating from the lateral epicondyle of the humerus, it is part of the common extensor tendon that arises from this location which is shared by the rest of the superficial extensor muscles we will discuss in the slides ahead.

The insertion point of the extensor carpi radialis as it descends down the forearm is the posterior aspect of the base of the third metacarpal bone. Functionally, the extensor carpi radialis brevis works in synergy with the corresponding longus facilitating extension and abduction of the wrist.

All right, if you're feeling like the names of these muscles kind of sound like jargon, fortunately, our next muscle – the extensor digitorum – is easier to say and remember. Also originating from the lateral epicondyle of the humerus via the common extensor tendon, the extensor digitorum muscle extends down the forearm and distal to the wrist. It subsequently expands into four separate tendons that split off and insert into the extensor expansions of the second to fifth fingers, hence, the name digitorum.

From a functional or action standpoint, contraction of the extensor digitorum produces extension of the second to fifth fingers at the metacarpophalangeal joint, proximal interphalangeal joint, and distal interphalangeal joints in addition to extension of the wrist joint itself.

Continuing on, we have the extensor digiti minimi. This slender muscle is situated medial to the extensor digitorum and these muscle bellies often blend somewhat with each other. So can you predict where the extensor digiti minimi originates from? That's right – it's another muscle arise from the lateral epicondyle of the humerus via the common extensor tendon and it continues distally until it inserts onto the extensor expansion of the fifth finger.

The primary action of the extensor digiti minimi as the name suggests is extension of the fifth digit although it also contributes to the extension of the wrist. The main action of this muscle is crucial if you fancy sticking out your pinky finger while sipping on tea or if you enjoy imitating the pose of the arch-nemesis of Austin Powers, Dr. Evil.

Lastly, the extensor carpi ulnaris is the most medial muscle of the group. This muscle has two heads, the first of which is known as the humeral head and arises from – you guessed it – the lateral epicondyle of the humerus via the common extensor tendon. The second ulnar head arises from the posterior border of the ulna. The extensor carpi ulnaris then descends along the posteromedial forearm and inserts onto the base of the fifth metacarpal bone. Taking a look at the action of this muscle, it accomplishes extension of the wrist and adduction or ulnar deviation of the wrist, which is movement in the medial direction or towards your ulna.

With the superficial muscles of the posterior forearm all wrapped up, let's dive into the deep group of muscles.

Before we plunge head first into the five deep muscles of the posterior compartment of the forearm, let's go through some basics. The deep muscles include the supinator, the abductor pollicis longus, the extensor pollicis longus, the extensor pollicis brevis, and the extensor indicis, and their innervation is relatively straightforward as all these extensor muscles are supplied by branches of the radial nerve. Specifically, the supinator muscle is innervated by the deep branch of the radial nerve piercing through it while the remaining four extensor muscles receive innervation from the posterior interosseous nerve.

First up – is it a bird? Is it a plane? No – it's supinator! This superhero-sounding muscle resides in the proximal aspect of the posterior forearm. The supinator once again has humeral and ulnar heads which have numerous points of origin arising from the lateral epicondyle of the humerus, the radial collateral ligament of the elbow, the annular ligament of the radioulnar joint as well as the supinator fossa and crest of the ulna.

Phew! That was a load of attachments. Thankfully, there are less insertion points for this muscle. As the supinator spans distally and obliquely, it wraps around the proximal third of the radius and inserts onto its lateral, posterior, and anterior surfaces.

Transitioning to this image, we have just witnessed the action of the supinator muscle. It's all in the name is to supinate the forearm and hand by pulling on and rotating the radius. Supination then is simply lateral or external rotation at the radioulnar joint such that it turns the forearm and hand over so the palm faces upwards. You can remember this handily by thinking that supination is turning your forearm and hand over to hold a bowl of soup.

Next is the abductor pollicis longus. As the name implies, this long muscle involves the thumb which is anatomically known as the pollex. It arises from the posterior surface of the proximal ulna and radius as well as the interosseous membrane between these bones. This muscle then runs distally and inserts on the base of the first metacarpal bone with its tendon forming the lateral border of the anatomical snuff box along the way. It also sometimes attaches to the trapezium bone.

The main action of the abductor pollicis longus is the abduction of the thumb. It also functions in extension and lateral rotation of the thumb and contributes to abduction of the wrist. Abduction itself is movement away from the body and you can think of it here as if the thumb is being abducted by an alien pulling it away from the hand.

Staying with the thumb, we have the extensor pollicis longus. This slim muscle originates from the posterior surface of the middle third of the ulna and the interosseous membrane. It then extends distally into a tendon that inserts onto the base of the distal phalanx of the thumb and forms the medial or ulnar border of the anatomical snuff box.

Taking a peek at the contraction of the extensor pollicis longus in this image, we can see it shortened and performing its primary action of extending the thumb at all three of its joints. Though it might also contribute to some extension of the wrist, this muscle mainly extends the thumb.

Can you deduce what's next? Why, it's the brevis to our longus. The extensor pollicis brevis is a short muscle that arises from the posterior surface of the radius and interosseous membrane. It then runs distally with its tendon forming part of the lateral boundary of the anatomical snuff box before inserting onto the base of the proximal phalanx of the thumb. Focusing now on the action of the extensor pollicis brevis primed to act in synergy with its longus counterpart, it facilitates extension of the thumb at the metacarpophalangeal and carpometacarpal joints.

Finally, we've reached the last of our deep extensor muscles in the posterior forearm. The extensor indicis is a narrow and distal muscle that arises from the posterior surface of the ulna and the interosseous membrane. Its muscle fibers extend distally and converge into a tendon that inserts onto the extensor expansion of the index finger.

Reflecting on its tell-all name, we can figure out its associated action. The extensor indicis primarily extends the index finger at all of its joints as demonstrated in this image of the muscle contracting. So if you ever find yourself in a finger-pointing situation, remember it's your extensor indicis muscle at work and take note that it can also help extend the wrist in concert with the other extensor muscles.

Now that we've learned all about the extensor muscles of the forearm, let's get practical as we explore important clinical notes.

A common condition that impacts the superficial muscles of the posterior forearm is lateral epicondylitis or tennis elbow. This painful condition is caused by injury to the common extensor tendon specifically from overuse, repetitive extension, or contractile overload of the extensor muscles we looked at which are part of this tendon attaching to the lateral epicondyle of the humerus. Common symptoms of tennis elbow include sharp pain and burning at the site of the common extensor tendon along with weakened grip strength.

Tennis elbow is diagnosed clinically, often onsets gradually, and can worsen over the course of weeks to months. Treatments include non-invasive interventions such as rest or activity modification, icing, anti-inflammatory medications, physical therapy, bracing, and injections of a steroid medication or platelet-rich plasma. If symptoms don't improve with these treatments, surgical release and debridement of the common extensor tendon can be completed.

You can go ahead now and extend your arms above your head in celebration because we've completed our tutorial on the extensor muscles of the forearm. Now before you jet off, let's quickly summarize what we've learned today.

To kick things off, we looked at the six superficial muscles of the posterior forearm and discussed all of their attachments and actions. Starting with the brachioradialis, we looked at its ability to flex the forearm from a partially pronated position. Next, we learned about the extensor carpi radialis longus muscle and its shorter counterpart, the extensor carpi radialis brevis muscle, which works synergistically to extend and abduct the wrist.

Subsequently, we discussed the extensor digitorum muscle and its expansion onto digits 2 to 5 allowing for extension of the index finger through the little or pinky finger. Then we moved to its neighbor, the extensor digiti minimi, which as its name implies extends the little finger.

Finally, we reviewed the last superficial extensor muscle of the posterior forearm – the extensor carpi ulnaris – and we covered how this muscle functions to extend and adduct the wrist.

Transitioning to the deep layer of the posterior forearm, we learned about the five deep extensor muscles. Beginning with the supinator, we learned about its prolific attachments and dynamic action of supinating the forearm and hand. Continuing on, we covered the three deep muscles that act on the thumb. First we saw the abductor pollicis longus and how it abducts or moves the thumb away from the hand. Then we looked at the extensor pollicis longus and its complementary extensor pollicis brevis which work together to extend the thumb. We then wrapped up things by learning about the extensor indicis muscle that attaches to and extends the index finger or second digit.

And to conclude our tutorial, we looked at lateral epicondylitis or tennis elbow. We discussed how this is an overuse injury to the common extensor tendon arising from the lateral epicondyle itself and we also reviewed the associated symptoms along with the indicated treatments both non-invasive and surgical.

That's all for today. Hope you enjoyed our tutorial and happy studying!