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.
So 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.
So in the posterior compartment, you can separate the muscles into a superficial layer and a
deep layer.
So 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.
So 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.
So 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.
So it originates on the lateral supracondylar ridge of the humerus and it runs down the
forearm and inserts laterally on the distal radius.
So 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.
So when it contracts, it pulls the wrist back this way.
It can also pull the wrist up like this, abducting it.
So 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.
So 'brevis' means short in Latin.
So it refers to the shortness of the tendon of this muscle.
So this muscle originates on the lateral epicondyle and it inserts on the base of the second and
third metacarpals.
So this muscle has the same action as the extensor carpi radialis longus in that it
extends and abducts the wrist.
So 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.
So 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.
So 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.
So 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.
So just to show you that a bit more clearly.
That's the extensor digitorum.
So 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.
So 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.
So 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.
So again, this muscle originates on the lateral epicondyle and it inserts on the dorsal hood
of the little finger.
So 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.
So 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.
So 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.
So 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.
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.
So 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.
So 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.
So 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.
So 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.
So this muscle supinates the forearm.
There are two supinators of the forearm.
You've got the biceps as well.
So remember, the biceps flexes the elbow and it also supinates the forearm.
So two supinators you've got.
So 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.
So it winds down the forearm and it inserts here on the base of the first metacarpal.
So what this muscle does when it contracts is that it abducts the carpometacarpal joint,
so this joint between the carpal bones and the metacarpals.
So this thumb joint, it abducts the thumb.
The word 'pollicis' refers to thumb.
In Latin, 'pollicis' means 'thumb'.
So it's the abductor of the thumb.
So 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.
So you've got the proximal and distal phalanx in the thumb.
It inserts at the base of the proximal phalanx here.
So 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.
So that's the extensor pollicis brevis.
So 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.
So 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.
So 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.
So that's the extensor pollicis longus because it's got a longer tendon, which inserts more
distally.
So 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.
So there you have the five muscles of the deep layer of the posterior compartment of
the forearm.
So 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.
So you've got the abductor pollicis longus, the extensor pollicis brevis, the extensor
pollicis longus and the extensor indicis.
There's 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.