Superficial Layer of the Posterior Compartment of the Forearm

00:01 So, now, let's turn our attention to the posterior compartment of the forearm.

00:06 So, again, within the posterior compartment of the forearm, we can look at a number of different layers that are formed by numerous muscles.

00:15 So, here, on the superficial layer, we can see the brachioradialis muscle.

00:20 We can see extensor carpi radialis longus. We can see extensor carpi radialis brevis, so, a long and a short version of these two equivalent muscles.

00:31 Don't get confused by all the different names. Extensor means it's going to extend.

00:36 Carpi means it's going to the carpal bones. Radialis means it's running along the radial aspect of the forearm and longus and brevis indicates the length of the muscle itself.

00:48 So, essentially, these two muscles are going to extend the carpal bones on the radial aspect.

00:54 Extensor digitorum associated with extending the digits.

00:57 Extensor digiti minimi, extensor carpi ulnaris.

01:02 And these muscles form that superficial layer on the posterior compartment of the forearm.

01:08 So, let's have a look at these muscles in more detail. We have brachioradialis.

01:12 Brachioradialis coming from the lateral supracondylar ridge of the humerus.

01:17 And it passes all the way down to the lateral surface of the distal radius and the pre-styloid process.

01:24 So, brachioradialis forming really on the most lateral aspect of the forearm.

01:30 It's innervated via the radial nerve and its important function is to help flex the elbow, but only does this weakly, so, it's a weak flexor of the elbow.

01:41 But it's much stronger at flexing when the forearm is in this mid-pronated position.

01:47 So, we've got a supinated forearm. We've got a pronated forearm. So, it helps to flex the elbow.

01:54 Yes. But it's strongest at flexing the forearm within this mid-pronator position.

01:59 So, the position when you're having a drink, that's a primary flexor of the elbow in this mid-pronated position.

02:07 Let's have a look at extensor carpi radialis longus.

02:10 Extensor capri radialis longus, again, comes from the lateral supracondylar ridge of the humerus and it passes through the dorsal aspect of the second metacarpal.

02:21 It's also innervated via the radial nerve. Its function is very much abduction of the wrist and it also helps to extend the wrist as well.

02:33 Flexor carpi radialis brevis is very similar. It's coming away from the lateral epicondyle of the humerus.

02:39 So, further distal from the humerus compared to the longus version.

02:43 And it goes and attaches the dorsal aspect of the third metacarpal.

02:48 It's also supplied by the radial nerve but as it's a deeper structure, it's the deep branch of the radial nerve.

02:54 And its function is similar to its sibling in that it helps to abduct the wrist and also helps to extend the wrist.

03:04 Extensor digitorum, extensor digitorum comes away from the lateral epicondyle and it passes all the way through the forearm to attach to the extensor expansion hoods.

03:14 We'll come back to those later on of the medial four digits.

03:18 So, not the thumb. It's attaching to the medial four digits extensor expansion hoods.

03:24 It's supplied by the radial nerve and also, has contribution from the posterior interosseous nerve as well which we can see. The function of the extensor digitorum muscle is to help extend the wrist.

03:37 It also helps with extension of the medial four digits via those extensor expansion hoods.

03:45 Extensor digiti minimi, this is a small slender muscle that's coming away from the lateral epicondyle and it passes all the way to the specific extensor expansion hood of the fifth digit.

03:58 So, as its name implies, it's just going to the final, the small little finger.

04:02 The extension expansion of that fifth digit. It's supplied by the deep radial nerve which is coming away from the radial nerve as its name suggests, the deep branch from it.

04:15 It also has contributions from that posterior interosseous nerve.

04:19 The function of this muscle is very much to extend the wrist.

04:21 But it also helps with specific extension of the fifth digit.

04:27 If we look at extensor carpi ulnaris, extensor carpi ulnaris comes again, away from the lateral epicondyle of the humerus and it runs alongside the lateral aspect of the forearm, the running from the posterior surface of the ulna which we can see here.

04:42 And again, this goes and attaches to the dorsal aspect of the fifth metacarpal.

04:48 So, as it attaches in this location, its very similar innervation coming away from the radial nerve, we've got the deep branch of the radial nerve and the posterior interosseous nerve, we can see here.

04:59 But its function is very similar, I should expect, to other muscles in this region.

05:03 It helps to extend the wrist and because of its position, it also helps to adduct the wrist which we can see in this position here. So, it helps with adduction of the wrist.