Hello all this is Dr Alsup and in this learning objective video we will be discussing the muscles
of the posterior compartment of the forearm. So this will be a very similar format to how we
discussed the anterior compartment and there are even more muscles in the posterior compartment
than the anterior, 11 and all, so again we will go into the big picture concepts of this compartment
with a particular emphasis on actions. As always I like to start with generalizations.
So there are only two layers in this compartment in this posterior compartment but more muscles per
layer than what we saw in the anterior compartment. This compartment is also known as the extensor/supinator
compartment which gives you a general idea regarding the main actions
which include extension of joints of the wrist and hand as well as one muscle playing a role
in supination of the radioulnar joints. Now what a really good thing here is that all of these
muscles are innervated by the radial nerve and its branches, so we don't have to focus on any unique
innervations here and as I have said 100 times and we'll say 100 more, radial nerve = posterior.
Let's get into a bit more detail regarding the superficial layer. There are six muscles
in this superficial layer so a lot going on. There is my personal favorite brachioradialis
which is our oddball in this region. This is not an extensor, its muscle belly is seen on the anterior
surface as well as the posterior, and its distal tendon doesn't even reach the wrist. But like
all the other muscles here it's innervated by the radial nerve so it is included in this compartment.
There are two extensor carpi radialis muscles so there is the longus and the brevis.
And so the longus and brevis indicates the length of
each and these will work together we'll talk about that in a bit more detail.
There is only one digitorum muscle so there's only one extensor digitorum, so not as superficialis and
profundus so here is your digitorum here. There's an itty-bitty little muscle that is going to
affect the fifth digit so the digiti minimi so the extensor digiti minimi and it's really just an
offshoot of the extensor digitorum so I'm not really going to go into too much detail regarding
the digiti minimi muscle even though it's got a fun name. And on the medial side or the medial most
side there is one extensor carpi ulnaris so no longus or brevis on for the ulnaris just the one.
Four of the six muscles will share a proximal attachment on the lateral epicondyle
of the humerus via the common extensor tendon. The other two which are brachioradialis and the ECRL
will attach just a little proximal to the lateral epicondyle.
People who repetitively use their superficial extensor muscles
in say like tennis if you think about the backhand, if they use this repetitively, they may feel pain
over the lateral epicondyle region as you can kind of see right here. Particularly they feel
the pain when they're opening a door or lifting a glass or kind of in that extension movement
and this pain can be caused by micro tears in that common extensor tendon or even inflammation
of the periosteum of the lateral epicondyle. This is referred to as lateral epicondylitis
also referred to as elbow tendinitis and colloquially it's referred to as tennis elbow
because individuals that play tennis are more prone to this type of injury.
Now obviously there are all kinds of distal attachments but what I want to draw your
attention to are those of the four long tendons of the extensor digitorum, which will flatten and
form what are called extension extensor expansions. And sometimes
you hear these referred to as dorsal expansions or dorsal hoods. These will wrap around the
posterior portions and sides of the head of the metacarpals as well as the proximal phalanges
so you can kind of see them on either side here. It'll serve to kind of tether down these tendons
there's one here too on the fifth digit it's just showing you the bones here on this model.
Some of the muscles we will discuss in the hand will also have an
attachment on these expansions which will help us understand those actions, sometimes it seems
counterintuitive but understanding where these extensor expansions are located will help give an
indication of what's going on. So these will play a role with extending the digits of 2-5
particularly in terms of the interphalangeal joints. These extensor expansions and various
ligaments throughout the posterior hand really give a very distinctive look to the posterior
portion of the digits. So we'll come back to these extensor expansions when discussing the hand.
Okay in terms of actions of the muscles in the superficial layer recall that that brachioradialis
is the odd one out. It is a flexor oops excuse me it's a flexor so it is unique
and it will flex particularly that elbow joint, so synergistic inflection of the elbow joint.
The extensor carpi muscles will extend the wrist which is of particular importance in
making a fist so if you think about making a fist a lot of times you need that very secure
extension of the wrist. The carpi radialis muscles so both the longus and the brevis will also
AB duct or abduct the wrist so if you think about your palm in anatomical position
that would have the hand moving away from the body. In the extensor carpi ulnaris
so in addition to the radialis muscles we'll extend the wrist and it will do the opposite
action in regards to adduction or kind of adding back the hand towards the body at the wrist joint.
And as mentioned when we were talking about the extensor expansions the extensor digitorum muscle
will extend the digits so at those interphalangeal joints.
And specifically I should have put this here it will extend the digits of 2-5.
The deep layer is an interesting one as three of the five muscles will solely affect
the first digit, and it makes sense your first digit or your thumb can do
some complex movements so you're going to have lots of muscles that play a role here.
Additionally these pollicis muscles will start deep and then pop through to a more
superficial position so here we're looking at a more superficial view of the forearm extensors
so they are all those muscles that we just talked about but you can see in this distal portion you
can see those pollicis muscles kind of popping out and becoming a bit more superficial. These are
sometimes called the outcropping muscles of the deep layer. All right keeping on with the pollicis
muscles when you see pollicis I want you to think first digit always think first or the thumb,
and these muscles are very fortunately named. They tell you exactly which action they play a role
in, so the abductor pollicis longus will abduct the first digit this action will specifically occur at
the carpometacarpal first...the first medic or excuse me the car...first carpometacarpal joint.
These are hard to say sometimes so you kind of have to take your time and get that word out.
These other two muscles extensor pollicis longus and brevis will extend the thumb
through the various joints of the first digit. I always describe the action of extension of
the thumb as like a hitchhiker's thumb so kind of jutting it out towards the side
that's going to be extension of the thumb. And we'll talk a bit more about thumb movements
when we talk about the hand as they're going to be unique in terms of the plane in which they occur.
There is a very small muscle called the extensor indicis, you can see a little bit of it right here,
that serves a similar role to the extensor digitorum but only to the second digit or the
index finger that's where the indicis comes from, this allows independent extension from
the other digits. So while I kind of want you to be aware that this occurs that this
muscle is here I don't want you to go into too much detail in terms of the extensor indicis.
Lastly and more proximally located you will have the supinator muscle which as you might guess
will cause supination of the radial ulnar joints particularly supination and slow unopposed
supination. And do you remember what the other main muscle that causes supination to occur?
I hope you remember it's the biceps brachii, remember that biceps brachii
in the anterior compartment of the arm. And biceps brachii is
particularly important in terms of rapid or forceful supination against resistance.
Excellent we have made it to the summary slide. I always suggest you you start your
review with these generalizations I think that's a good place to start always and
then you can kind of add in detail from there. And as you review always feel free to reach
out with any questions. Thanks for your time and attention here and have a great rest of your day