Forearm Muscles Part 2 – Posterior (Extensor) Compartment – Anatomy Tutorial


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.