O-I-A-N. What does that spell? Nothing really, but nonetheless this sequence of letters strikes fear into the hearts of anatomy students around the world, whether they’re preparing for dental school, medical school, chiropractor school, nursing school, physical therapy school, physician’s assistant school, or any number of health professions programs.
These are the initials for Origin, Insertion, Action, and Nerve (Supply) and depending on the anatomy course, students need to know this information for some or all of the 300+ muscles found in the human body.
Based on my experience as an anatomy instructor and tutor, no area of the body causes as much stress to budding health professionals as the area between the elbow and the wrist, known anatomically as the antebrachium, or more informally as the forearm. Every semester, I work with at least one student who is overwhelmed or intimidated by this group of small, parallel muscles, many of which have three or four words in their names.
However, when we break this task down into its basic elements, it turns out that these muscles need not be as difficult to learn as their reputation makes them out to be. There are several reliable and easily remembered techniques that can make this task far easier than it seems.
In learning musculoskeletal anatomy, an important rule that applies especially to the forearm muscles is as follows: learn to recognize the general pattern and identify any exceptions.
We’ll be returning to this idea as we work our way through the forearm muscles. Some other rules we’ll be using that apply generally to the study of anatomy are that we want to avoid memorization whenever possible and instead apply our knowledge of patterns and vocabulary. Additionally, when we have a long list of things to learn we want to do our best to group that list into chunks or categories.
Before we dive in, let’s set the terms of our discussion. We’ll be focusing on muscles that are found in the anterior and posterior compartments of the forearm. Most of these muscles cross the wrist joint and therefore move the hand, but (of course) there are a few exceptions. The brachioradialis and anconeus have action at the elbow — the anconeus is sometimes classified as an arm (as opposed to forearm) muscle, but we’ll include it in our discussion here,. We’ll look at some other exceptions later on. We won’t be dealing with the intrinsic muscles of the hand. This gives us 20 muscles – so does that mean we have 80 OIAN’s to memorize? Far from it.
(Note that you'll get the most out of this post if you follow along with diagrams from your textbook as you read through it)
EASY FIRST – INNERVATION AND ACTION
Whenever I’m working on a big task or have a lot of things to do, I like to get the easy stuff out of the way first. Unlike in some other regions of the body, innervation is quite simple when it comes to the forearm. Posterior muscles are innervated by the radial nerve. All of them. That was easy. These muscles include anything that has the word “extensor” in it, the supinator, abductor pollicis longus, anconeus, and brachioradialis.
We’re off to a good start. How about the muscles of the anterior compartment? Almost all of them are innervated by the median nerve. The other nerve serving the forearm and hand is the ulnar nerve. One of the exceptions is easy to remember because it’s the only anterior forearm muscle with the root “ulnar” in it – flexor carpi ulnaris. The other is the flexor digitorum profundus which is innervated by both the median and ulnar nerves. In a way this makes sense, because it serves both the medial and lateral aspect of the hand (digits 2-4). As far as which flexor digitorum is involved, just think that superficialis should be easy to remember because it’s superficial (only innervated by the median nerve) whereas “profound” implies depth and difficulty so profundus is the one innervated by two nerves.
Done with innervation. 20 down, 60 to go.
The most helpful thing about the forearm muscles is that almost all of them give you their action in the names. Most of these muscles have the word “flexor” or “extensor” in them followed by the name of what they flex or extend – wrist, fingers, thumb, index finger, pinky. The supinator and the pronators also have their actions in their names.
The main exceptions are the anconeus, brachioradialis, and palmaris longus. As long as we can visualize these muscles we can remember their actions. Anconeus crosses the elbow posteriorly and is therefore mostly involved in elbow extension. Brachioradialis is the most confusing because it is technically a posterior muscle but functions as an elbow flexor, but it’s the only major exception.
Palmaris longus is a bit of an oddball — so much so that it’s an optional muscle. 15% or more of people don’t have this muscle on one or both sides. Try this test to see if you’re in the 15% of people missing this muscle. In any case, while serving as a weak wrist flexor, it also has the unique function of tensing the fascia of the palm.
Two other notes on actions. 1) All of the muscles that move the digits cross the wrist joint and therefore also have actions on this joint. This includes muscles with the words digitorum, indicis, pollicis, and digiti minimi. Typically, this will be the same action the muscle has on the finger or fingers for which it’s named (flexion and extension).
2) In addition to flexion and extension, the wrist can abduct and adduct. However it’s easy to know which muscles are involved in these actions. If it has the words “carpi ulnaris” in it, it adducts the wrist which makes sense because the ulna is the forearm bone on the pinky side whereas the radius is on the thumb side. With that said, any muscle containing the name “carpi radialis” is involved in wrist abduction. The only somewhat tricky one is abductor pollicis longus – in addition to abducting the wrist, by virtue of being a posterior compartment muscle it also extends the wrist.
That’s all for actions so now we’re halfway through our OIAN’s!
Of the remaining things to identify, I find the origins easier so we’ll start with those. We’ll start with the anterior muscles, and we’ll follow our rule of breaking a long list into categories to make our task easier. In this case, we’ll categorize anterior muscles into superficial and deep layers.
The superficial anterior muscles (pronator teres, flexor carpi radialis, palmaris longus, flexor carpi ulnaris, flexor digitorum superficialis) have a common origin on the medial epicondyle of the humerus. Additionally, pronator teres and FD superficialis have a common origin on the coronoid process of the ulna which makes sense for anterior muscles given our knowledge of the bony landmarks of the ulna. FD superficialis also originates on the radius – again spanning both sides of the hand, it makes sense that both bones of the forearm are involved. The other exception here is FC ulnaris which, as the name suggests, also originates on the ulna (olecranon).
What the deep anterior muscles (flexor pollicis longus, flexor digitorum profundus, pronator quadratus) have in common is originating on the bones of the forearm. Of these three which is most likely to originate on the radius? Of course it’s FP longus because the radius is on the thumb side. The two flexors also share an origin on the interosseus membrane. Given the action of the pronator and its need to go from one forearm bone to the other, it also makes sense that this would be the one of the three that doesn’t originate on this membrane.
On to the posterior muscles, which we’ll again break down into superficial and deep.
In a parallel to what we saw on the anterior side, the superficial muscles (extensor carpi radialis longus & brevis, extensor digitorum, extensor carpi ulnaris) originate from the lateral epicondyle of the humerus. In case you have trouble remembering which epicondyle each side is associated with, remember that the posterior muscles are innervated by the radial nerve, and the radial side of the forearm is lateral to the ulnar side, hence these originate on the lateral epicondyle.
Just like its flexor counterpart, EC ulnaris also originates on the ulna, so any muscle with “carpi ulnaris” originates on its characteristic epicondyle as well as the ulna. The main exception is EC radialis longus. But we can use the name to our advantage. The fact that it has the word “longus” in the name suggests that it’s longer than its neighboring muscles. The origin of this muscle is slightly more proximal, on the lateral supracondylar ridge of the humerus.
The deep muscles (supinator, abductor pollicis longus, extensor pollicis longus & brevis, extensor indicis, extensor digiti minimi) share some similarities with their anterior counterparts in that most of them originate on the ulna and/or radius. It’s helpful that the three pollicis muscles originate from the trifecta of ulna, radius, and interosseus membrane. While having an origin on the lateral epicondyle, the supinator is similar to the pronators in originating on the ulna and inserting on the radius.
One major exception is extensor indicis which somewhat unexpectedly originates on the ulna (and interosseus membrane) even though it’s closer to the thumb side. Although it’s a common origin for extensor muscles, it’s also unexpected that the extensor digiti minimi originates on the lateral epicondyle as the pinky is on the medial side of the hand. However, the pinky and index finger can be seen as opposites so we just need to remember that the extensors dedicated to these fingers have origins located on opposite sides of the fingers in question.
We haven’t mentioned brachioradialis yet, but this is one of those helpful muscle names that give us both the origin and insertion – the humerus (i.e. the brachium or upper arm) and the radius. Our other oddball, anconeus, originates on the lateral epicondyle and inserts on the olecranon which should be readily apparent if we can visualize the location of this muscle or identify it on a diagram. For a fun memory tip, the windy spiral appearance of the anconeus has a snake-like appearance which reminds me of a snake – such as an anaconda which is a somewhat similar word.
Which leaves us with the real challenge with these muscles – the insertions. Luckily, many of them are given to us such as all the “finger” muscles. The pollicis longus muscles insert into the distal phalanx which makes sense, with the exception of abductor pollicis longus which inserts into the first metacarpal and trapezium. Extensor pollicis brevis inserts into the thumb's proximal phalanx.
The three “digitorum” muscles insert into connective tissue and the phalanges. The main trick is that FD superficialis inserts into middle phalanges while the others insert into the distal phalanges. But remember that superficialis is entirely innervated by the median nerve so we can think of median going with middle phalanges.
As we’ve discussed already, pronators and the supinator insert into the radius so we don’t have to worry much about those. The palmaris longus inserts into the palmar fascia. Which leaves us with the “carpi” muscles.
One thing we have going for us is that all of them insert into metacarpals. Here we can remember that 2+3 = 5. What does this mean? The “radialis” muscles insert into the 2nd, 3rd, or both metacarpals while the “ulnaris” muscles insert into the 5th. It makes sense for the ulnaris muscles to be associated with the 5th digit.
Since there’s only one flexor carpi radialis muscle, it should be easier to remember that it inserts into both the 2nd and 3rd metacarpals, while the two EC radialis muscles insert into the 2nd (longus) and 3rd (brevis). It would be nice if longus went with the longer digit (the middle or 3rd finger), but unfortunately this isn’t the case. However, when we look at the number 2 we see a long diagonal line whereas the number 3 consists of many short line segments; therefore we can remember that EC radialis brevis goes with 3.
Leaving us with our last exception – the one “carpi” muscle that inserts into carpal bones. Flexor carpi ulnaris inserts into the pisiform and hamate as well as the 5th metacarpal as we’d expect. As long as we know that pisiform and hamate are the most medial of the proximal and distal carpal bones, it should be relatively straightforward to remember that these are the bones involved.
By now, you should have mnemonic device that allows you to remember the order of the carpal bones. Using an acrostic based on the letters S-L-T-P-T-T-C-H (if you prefer proximal before distal and radial to ulnar side) is a great way to help with this. This might look something like Some Like To Pull The Top Cover Higher.
So there you have it. There’s no way around the fact that you’ll have to put in some serious work to learn the OIAN’s for the forearm muscles. But we’ve made our task much easier by focusing on commonalities and exceptions, using the information supplied to us by the name of the muscle, applying general knowledge we have about anatomy of the region, and mixing in a hefty dose of memory techniques.
These are the types of techniques you should be using as you work on OIAN’s for other regions of the body. Remember that you’ll be doing one region at a time so you can work your way through it slowly. Also know that like with anything else, with repetition (or perhaps a lot of repetition!), it will become second nature for you.
Visualize! Look at diagrams and test yourself often. Don’t underestimate the value of tables and other ways to summarize large amounts of information in a compact form.
Categorize and group. Turn one long list into a series of shorter ones. Look for connections and patterns. The study of anatomy is highly pattern oriented so this is an important mindset to get into.
Use memory tips and tricks to boost your ability to retain and recall.
Finally, it’s key not to be daunted or overwhelmed. Mindset makes a huge difference and confidence can take you a long way while anxiety can cloud your mind and hinder you in your task.