Muscles of the Forearm

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Anterior Compartment

Okay so this a tutorial on the muscles of the flexor compartment of the forearm. Just like the upper arm, the muscles of the forearm can be split into anterior and posterior compartments - so flexor and extensor compartments. The flexor compartment is separated from the extensor compartment by bones, an interosseus membrane and a lateral intermuscular septum.

Here I'm just showing you a cross section of the forearm, so this is anterior, posterior, lateral and medial, and you've got the ulna and radius we are looking at. In between the radius and ulna, you've got this membrane called the interosseus membrane. "Inter" meaning "between", "os" Latin for "bone", so "between the bone membrane" - "inter-osseous membrane". Connecting to the radius you've got the lateral intermuscular septum, which connects to the deep fascia, which surrounds the muscles of the forearm.

 

You’ve got these compartments, the anterior flexor compartment, and the posterior extensor compartment.

 

This interosseus membrane runs between the radius and ulna in this gap here, separating the anterior and posterior compartments.

 

Common Origin

The anterior compartment, the flexor compartment, which I'm talking about in this tutorial, the muscles are generally supplied by the median nerve, and these muscles have a common origin on the medial epicondyle of the humerus. This part here. And if these muscles are overused, then you can get inflammation at this origin, so you get medial epicondylitis which is also known as "golfers elbow".

 

Action

The muscles of the anterior compartment flex the wrist and digits, and pronate the hand.

 

Pronation is bringing the hand round so that the palm faces away from you, sorry towards you.

 

In this position here, in the anatomical position, the palm is facing away from you, so pronation would bring the palm to face towards you, so that you show the back of the hand, so it would rotate the hand around.

 

The muscles of the posterior compartment do the opposite to the anterior compartment muscles - they extend the wrist and digits, and they supinate the hands, and they have a common origin on the lateral epicondyle of the humerus.

 

Overuse of the posterior compartment, the extensors of the forearm, you get lateral epicondylitis, which is also known as tennis elbow.

 

Layers of the anterior compartment

You’ve got three layers of muscle in the anterior forearm; you've got the superficial layer, the intermediate layer, and the deep layer.

 

I'm just going to talk you through from superficial to deep, these muscles.

 

Superficial Layer

In the superficial layer you've got four muscles, you've got the flexor carpi ulnaris, the palmaris longus, the flexor carpi radialis, and the pronator teres.

 

Palmaris Longus

I’ll just start with this muscle here, which is the pronator, sorry, the palmaris longus, so this muscle like the rest of the muscles of the anterior compartment, it originates on the medial epicondyle of the humerus and it runs down the length of the forearm to insert onto this flat fibrous sheath, known as the palmar aponeurosis, so this is a thick layer of deep fascia that lies beneath the skin.

You can see this tendon in yourself, if you flex your wrist, it's quite superficial, you can see that tendon. What this muscle does, the palmaris longus is that it flexes the wrist joint.

 

This palmar aponeurosis actually attaches to the overlying skin of the palm and the fingers. When you contract the palmaris longus, it flexes the wrist.

 

Flexor Carpi Ulnaris

Medial to the palmaris longus, you've got this muscle called the flexor carpi ulnaris.

 

It’s useful to try and think of what the words actually mean, so flexor obviously means it flexes, causes a flexing action at the wrist, carpi is Latin, "carpus" means wrist, and ulnaris refers to the ulna side. It flexes the wrist at the ulna side.

 

This muscle actually has two heads, so I'll just rotate this model round, and you can see one head inserting on the medial epicondyle of the humerus, and the other head inserting posteriorly on the ulna and at the olecranon - so its got two heads, the flexor carpi ulnaris.­­ What this muscle does is it runs down and inserts at the wrist, down here.

 

It actually, it's not very clear on here, but it inserts on this little bone here - the pisiform bone, and this bone has ligaments which attach to this metacarpal, so this fifth metacarpal bone. It inserts on this little bone here and this pisiform bone, which I'll talk about in another tutorial, has ligaments which attach onto this fifth metacarpal, so what this muscle does when it contracts, is that it flexes the wrist and adducts the wrist.

 

Flexor Carpi Radialis

Just lateral to the palmaris longus, which I just talked about, so this muscle here which attaches to the palmar aponeurosis, you've got the flexor carpi radialis. Remember what I just said about the word "carpi" meaning "wrist", and looking at the name of the muscle to get an idea of its function and its structure. Flexor - flexes, at the wrist - carpi, on the radial side.

 

You can see here it inserts at the medial epicondyle of the humerus, sorry, originates there, and it runs down the forearm to insert onto the metacarpals.

 

I'll just remove the palmar aponeurosis, and you can see that it has this insertion point at the base, so you've got these metacarpals here and these phalanges, so these are the metacarpals, so it inserts onto the base of the second and third metacarpal. And when this muscle contracts, it flexes and adducts the wrist, sorry abducts - AB-ducts.

 

Pronator Teres

The fourth and final muscle in the superficial layer is this muscle, which lies lateral to the flexor carpi radialis, it's the pronator teres. As the name suggests it pronates the forearm.

 

It has two origins, it originates on the medial epicondyle of the humerus and also on the ulna bone, and it inserts onto the lateral part of the mid-shaft of the radius.When this muscle contracts, it pronates the forearm.

 

You’ve got those four muscles of the superficial layer of the anterior compartment. You've got the flexor carpi radialis, and ulnaris, which lie either side of the palmaris longus, and then you've got the pronator teres which lies lateral.

 

Intermediate Layer

Okay so now I'm just going to dissect away the superficial muscles and we'll look at the intermediate muscle layer, so there's one muscle in the intermediate muscle layer - the flexor digitorum superficialis, so the name gives you some idea of its function.

 

Obviously it's a flexor muscle, because it's in the anterior compartment, so the word digitorum refers to fingers, so it flexes the fingers. And it's called superficialis, because it's superficial to the other muscle which acts to flex the fingers, which I'll show you in a moment.

 

The flexor digitorum superficialis lies in the intermediate muscle layer, of the anterior forearm, and it has two origins: it's got this humero-ulna origin, and it's got this other origin on the radius bone. I'll just get rid of that quickly.

 

It originates on the medial epicondyle of the humerus also on the ulna and it also originates on this radial, this oblique line on the radius. It then runs down and you've got these four tendons which pass through the carpal tunnel, so the carpel tunnel is this space between the flexor retinaculum and the carpal bones, and the median nerve passes through this, and when it, when the contents of the carpal tunnel get compressed, you get muscle weakness and loss of sensation, and I'll talk a bit about that in another tutorial.

 

You’ve got these four tendons of the flexor digitorum superficialis which run through the...underneath the flexor retinaculum, and they insert onto the middle phalanx of the index, middle, ring and little fingers. This tendon is quite interesting because it actually split and inserts onto the lateral margins of the middle phalanx, and it allows this other tendon to pass through, so the tendon of the flexor digitorum profundus, to pass through.

 

The tendons of the flexor digitorum superficialis insert onto the middle phalanx, so they split to allow the passage of this flexor digitorum profundus.

 

An easy way of remembering this, is that "superficialis splits; profundus passes through", so "s" - superficialis splits, so beginning of the word begins with "s" so "superficial splits; profundus passes through". It inserts on the lateral margins of the middle phalanx.

 

Just to orientate you a bit, you've got the metacarpal bones here; you've got the proximal phalanx, middle phalanx, and distal phalanx. You’ve got your proximal interphalangeal joint, and your distal interphalangeal joint.

 

The action of the flexor digitorum superficialis is to flex at the proximal interphalangeal joint, of the index, middle, ring and little fingers, and it can also flex at this joint, so the metacarpophalangeal joints. It can flex at the MCP joints and the PIP joints of the index to little fingers.

 

Deep Layer

I'm just going to move onto the next muscle layer, the deep muscle layer. Okay so I've rid of the flexor digitorum superficialis, and now we're looking at the flexor digitorum profundus, so this muscle lies underneath the flexor digitorum superficialis, and it originates on the anterior and medial surfaces of the ulna bone, and it runs down, and again you've got these four tendons, which pass through the carpal tunnel, underneath the flexor retinaculum, and they run along to insert onto the base of the distal phalanges.

 

I'll just bring back those other tendons, so you can see how they split, you can see the tendon of the flexor digitorum superficialis splitting and the tendon of the flexor digitorum profundus passing through to insert onto the distal phalanx.

 

You can probably guess that this muscle, when it contracts, it flexes the distal interphalangeal joint. The flexor digitorum superficialis flexes the proximal interphalangeal joint, and the flexor digitorum profundus flexes the distal interphalangeal joint. And it can also flex the metacarpophalangeal joints, so the MCP joints. Just bring you back up here.

 

Flexor Pollicis Longus

The next muscle is the flexor pollicis longus, so it's this muscle here, which is actually shown a little out of place on this model, because it inserts more medially and anteriorly on the radius bone, so more in this sort of region, so this is the flexor pollicis longus.

 

Pollicis refers to the thumb, so we'll go through a lot of other muscles which act on the thumb, and they all have the words "pollicis, so "pollicis" is Latin, referring to thumb.

 

What this muscle does is it flexes the thumb and the interphalangeal joint, so it runs down. Again it's shown a bit out of place, it actually runs underneath the flexor retinaculum in the carpal tunnel, so it runs through the carpal tunnel and it inserts on the distal phalanx.

 

When it contracts it flexes the thumb at this interphalangeal joint, and it can also flex at the metacarpophalangeal joint.

 

Pronator Quadratus

The final muscle of the deep layer is the pronator quadratus, which lies underneath the tendons of the flexor pollicis longus and the tendons of the flexor digitorum profundus.

 

From its rectangular shape, so "quadratus" referring to its four sides, so I've just removed the flexor pollicis longus and the flexor digitorum profundus, and you've got this pronator quadratus, so as the name suggests it's a pronator muscle, so it pronates the wrist, and it's shown a bit funny here, but I'll just isolate it.

 

It’s actually this rectangular shape. It originates on this distal anterior surface of the ulnar, and it inserts distally on the anterior surface of the radius, and it contracts, when it contracts, it pronates the forearm.

 

Those are the muscles of the anterior compartment, the flexor compartment of the forearm. You've got the superficial, intermediate and deep layers, and you've got muscles which flex the wrist, which flex the fingers, and which pronate the forearm.

 

Posterior Compartment

This is a tutorial on the muscles of the posterior compartment of the forearm. These muscles are extensors. It’s also called the extensor compartment. Hopefully, you’ve watched the tutorial I did on the muscles of the anterior compartment. The muscles of the flexor compartment have a common origin on the medial epicondyle of the humerus and the muscles of the extensor compartment have a common origin on the lateral epicondyle of the humerus.

 

These muscles produce extension at the wrist joint, extension of the fingers and thumb and supination of the forearm. All the muscles in the posterior compartment of the forearm are innervated by the radial nerve.

 

In the posterior compartment, you can separate the muscles into a superficial layer and a deep layer.

 

Superficial Layer

First, I’m going to talk about the muscles of the superficial layer and there are seven muscles in the superficial muscle. The tendons, as you can see here, pass through this retinaculum, which is called the extensor retinaculum. This is a fibrous band of connective tissue, which holds these tendons in place.

 

I’ll just remove that and we can take a look at these muscles now. There are seven muscles that you’ve got in the superficial layer. I’m just going to work lateral to medial. We’re looking posteriorly at the right arm here. This side is lateral.

 

The most lateral muscle of the posterior compartment is this muscle here, the brachioradialis muscle. This muscle, as the name suggests, originates on the arm. ‘Brachio’ or ‘brachium’ is Latin for ‘arm’. It originates on the humerus and it inserts onto the lateral surface on the distal radius, so brachioradialis.   It originates on the lateral supracondylar ridge of the humerus and it runs down the forearm and inserts laterally on the distal radius.   That’s the brachioradialis. What this muscle does is that it can act as an accessory flexor of the elbow.

 

Just working medially, the muscle just next to the brachioradialis here, which inserts slightly low down on the lateral supracondylar ridge, this is the extensor carpi radialis longus. This muscle originates a little bit below the brachioradialis on the lateral supracondylar ridge and it inserts down here on the base of the second metacarpal – so this is the dorsal surface – on the base of the second metacarpal.

 

What this muscle does is it extends the wrist and it also can abduct the wrist.   When it contracts, it pulls the wrist back this way. It can also pull the wrist up like this, abducting it.

 

Medial to this muscle, you’ve got the extensor carpi radialis brevis. We’ve got the longus here and the brevis here. ‘Brevis’ in Latin means short, so you get the English derivative word, ‘brevity’ referring to shortness.   ‘Brevis’ means short in Latin.   It refers to the shortness of the tendon of this muscle.

 

This muscle originates on the lateral epicondyle and it inserts on the base of the second and third metacarpals.   This muscle has the same action as the extensor carpi radialis longus in that it extends and abducts the wrist.

 

Medial to this muscle, we’ve got the extensor digitorum, which originates on the lateral epicondyle just medial to the extensor carpi radialis brevis. You can see this muscle here. This is the outline of the muscle. And if we follow it down the forearm, you can see it gives off four tendons.   You’ve got these four tendons coming off the extensor digitorum. They run on the dorsum of the hand and insert onto the base of the middle and distal phalanx of these four digits – the index, middle, ring and little fingers.

 

I’ve just switched over to a diagram to show you this muscle. You’ve got the extensor digitorum tendons here. You can see at the dorsum of the hand, so over the metacarpals, [inaudible 00:05:20]. But over the phalanges, you can see how the tendons splits around itself.   One part of the tendon inserts at the base of the middle phalanx and then it splits around to insert at the base of the distal phalanx.   Just to show you that a bit more clearly.

 

That’s the extensor digitorum.   Remembering, digitorum refers to the fingers, so that’s ‘extensor of the fingers’. This muscle extends the index, middle, ring and little fingers and it can also extend the wrist.

 

Just going back up to look at the next muscle. We’ll just quickly recap. We’ve gone through the brachioradialis, the extensor carpi radialis longus, the extensor carpi radialis brevis, the extensor digitorum muscle. And next, we’ve got this muscle here, the extensor digiti minimi.

 

This muscle lies medial to the extensor digitorum. As the name suggests, it’s the extensor of the smallest finger. ‘Minimi’ is Latin for ‘smallest’, ‘digiti’, obviously ‘finger’, so it extends the little finger.

 

Again, this muscle originates on the lateral epicondyle and it inserts on the dorsal hood of the little finger.   It extends the little finger as the name suggests.

 

Just next to this one, medial again, we’ve got the extensor carpi ulnaris. This is the antagonist of the flexor carpi ulnaris, which you can see just here with its ulnar and humeral heads in the flexor compartment.

 

This is the extensor carpi ulnaris and what this does is it extends and adducts the wrist. It originates here on the lateral epicondyle and it inserts on the medial surface of the base of the fifth metacarpal.   You can see that there.

 

You shouldn't get really too confused about whether it adducts or abducts because if you know where it inserts, you can imagine the muscle contracting and try to visualize its action. You’ve got the extensor carpi ulnaris and you’ve got the extensor carpi radialis and you’ve got the longus and brevis. Because they insert on the radial side, they’re going to abduct the wrist because they insert on the radial side (away from the midline). They’re going to pull it up this way, abducting the wrist. The extensor carpi ulnaris inserts on the ulnar side, so it’s going to adduct the wrist.

 

The final muscle of the superficial area of the posterior compartment is this little muscle here, the anconeus muscle. This muscle originates on the lateral epicondyle of the humerus again and it inserts posteriorly on the ulna and on the olecranon.

 

There are seven muscles to remember here.   Again, you’ve got the brachioradialis, you’ve got the extensor carpi radialis longus, the extensor carpi radialis brevis, you’ve got the extensor digitorum muscle, the extensor digiti minimi, the extensor carpi ulnaris and the anconeus muscle. These muscles form the superficial layer.

 

If I remember all these seven superficial muscles of the posterior compartment, we can take a look at the muscles of the deep layer of the posterior compartment.

 

Deep Layer

There are five muscles in the deep layer of the posterior compartment of the forearm. These muscles, apart from the supinator, which has one head that attaches to the lateral epicondyle, these muscles all originate on the posterior surfaces distally on the ulna and radius and interosseous membrane.

 

All these muscles, all five of these muscles are innervated by the posterior interosseous nerve. This nerve is a continuation of the deep branch of the radial nerve.

 

First, we’ve got this muscle up here, the supinator muscle. As its name suggests, this muscle supinates the forearm and it has two heads. It’s got a superficial and a deep head.   It’s got one head which attaches to the lateral epicondyle of the humerus, which is the superficial head and it’s got this other head, the deep head, which attaches to the posterior aspect of the ulnar bone.

 

This muscle wraps around the lateral edge of the radius on the shaft (so just below the head and neck of the radius) and it inserts laterally on the radius.   This muscle supinates the forearm.

 

There are two supinators of the forearm. You’ve got the biceps as well.   Remember, the biceps flexes the elbow and it also supinates the forearm.   Two supinators you’ve got.

 

That’s the most proximal muscle. Now just working our way a little bit more distally, we’ve got this muscle here, the abductor pollicis longus. This muscle originates distal to the supinator and it originates on the posterior surfaces of the ulnar and the radius and also, it attaches to the interosseous membrane in between.   It winds down the forearm and it inserts here on the base of the first metacarpal.

 

What this muscle does when it contracts is that it abducts the carpometacarpal joint, so this joint between the carpal bones and the metacarpals.   This thumb joint, it abducts the thumb. The word ‘pollicis’ refers to thumb. In Latin, ‘pollicis’ means ‘thumb’.   It’s the abductor of the thumb.

 

Just distal to the abductor pollicis longus, you’ve got this muscle here, which is the extensor pollicis brevis. This muscle originates on the posterior surface of the radius bone and it inserts on the dorsal surface of the base of the proximal phalanx.   You’ve got the proximal and distal phalanx in the thumb. It inserts at the base of the proximal phalanx here.

 

When this muscle contracts, it extends the metacarpophalangeal joint, this joint here between the metacarpal and the phalanx, it extends this joint and it can also extend the carpometacarpal joint of the thumb.   That’s the extensor pollicis brevis.

 

The next muscle is the extensor pollicis longus. It’s this muscle here. This muscle originates a bit higher up on the posterior surface of the ulnar bone.   The brevis muscle, the brevis counterpart originated on the posterior surface of the radius, the extensor pollicis longus originates on the posterior surface of the ulnar bone. This muscle has a longer tendon, so it’s called ‘longus’. ‘Brevis’ means ‘short’ in Latin, ‘longus’ means ‘long’ obviously. This muscle has a longer tendon so it inserts at the base of the distal phalanx of the thumb, so here on the dorsal surface again.

 

When this muscle contracts, you can see that it will extend the interphalangeal joint of the thumb. And again, it can also extend the carpometacarpal and the metacarpophalangeal  joints, so this joint here and this joint here, but its prime purpose is to extend the interphalangeal joint of the thumb.   That’s the extensor pollicis longus because it’s got a longer tendon, which inserts more distally.

 

Finally, we’ve got this muscle here, which originates more distal to the extensor pollicis longus and it’s called the extensor indicis. ‘Indicis’ refers to the ‘index’ finger. It’s the extensor of the index finger. This muscle originates on the posterior surface of the ulna distal to the extensor pollicis longus. And then it inserts onto the extensor hood of the index finger.

 

Do you remember that muscle I showed you in the superficial area, the extensor digitorum, which extends all four fingers? Well, this muscle joins the insertion point of the extensor digitorum to extend the index finger.

 

There you have the five muscles of the deep layer of the posterior compartment of the forearm.   You’ve got the supinator up here, which supinates the forearm. And then you’ve got the muscles of the thumb and the index finger.   You’ve got the abductor pollicis longus, the extensor pollicis brevis, the extensor pollicis longus and the extensor indicis.

 

There are quite a lot of muscles in the extensor compartment, but they’re all logically named, so have a look through that tutorial again and hopefully that will help things.