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Origins, insertions, innervation, functions and related clinical anatomy of the muscles of the anterior compartment of the forearm.
Whether you’re solving a puzzle, playing your heart out on the piano, or typing frantically in the early hours of the morning to meet a deadline, you need a complex group of muscles to help you coordinate the movements of your hands and fingers. Some of these muscle are actually located in the forearm, and in today’s tutorial, we’re going to be featuring the muscles of the anterior compartment of the forearm, many of which help to bend and flex the joints of your wrist and fingers.
Before we begin, however, I’d like to give you a quick overview of what we’re going to be talking about in this tutorial. So, in this image here, we can see the muscles and the bones of the forearm from an anterior perspective, and that’s exactly what we’ll be focusing on today. We’ll start off by talking about the bones of the upper limb – about the bones we can see in this illustration – as they form the bony framework that our muscles attach to. Then we’re going to move on to talk about the muscles of the anterior compartment of the forearm which can be divided into 2 groups – the superficial layer and the deep layer – and these muscles are also collectively known as the flexors of the forearm, and as we talk about their functions, you’ll see why.
For each muscle we’ll look at today, we’ll learn their origin, their insertion, their function as well as their innervation. And lastly, we’ll wrap up our tutorial with some clinical notes relevant to these muscles.
Okay, so let’s get started with the bones of the upper limb.
So here we can see the image I just showed you, but I have just dissected away the musculature so that you can see the bones a little bit better. So, this bone that we can see up here is the bone of the arm, and this bone is known as the humerus. Moving distally, we can see the bones of the forearm which are the radius and the ulna, and next, we have the bones of the hand which can be divided into three groups.
First, we have the carpals – there are eight of these – and they’re located in the wrist area; then there are the metacarpals and we have five of these – one for each digit; and located most distally are the phalanges, which are the bones of your digits, and as you can see, the thumb has a proximal phalanx and a distal phalanx whereas the fingers each have three phalanges – the proximal, the middle, and the distal phalanges.
So now that we’re familiar with the bony framework our muscles attach to, let’s move on to have a look at the muscles of the anterior compartment of the forearm. And we’ll start with our first group, which are the muscles of the superficial layer.
The first muscle of the superficial layer that we’re going to be talking about today is the pronator teres muscle, and this muscle originates from the medial epicondyle of the humerus and the coronoid process of the ulna, and it then goes on to insert here at the lateral surface of the radius. As its name suggests, one of the functions of the pronator teres is pronation of the forearm. And pronation is a movement that involves rotation of the hand and forearm so that the palm faces downwards or backwards as demonstrated by our image here. And the pronator teres is a flexor of the forearm, therefore, it also assists with the flexion of the forearm at the elbow joint. And flexion of the forearm decreases the angle between the arm and the forearm at the elbow joint.
So in order for this muscle to perform its functions, it requires innervation, and the pronator teres is innervated by the median nerve which arises from the brachial plexus.
Located the most medially is the flexor carpi ulnaris, and this muscle originates from the medial epicondyle of the humerus and the olecranon of the ulna. It then goes on to insert onto two of the carpals – the pisiform and the hamate – as well as the base of the fifth metacarpal. So, when the flexor carpi ulnaris contracts, it causes adduction and flexion of the hand at the wrist joint, and as we can see in our image, adduction is the movement of the hand towards the midline of the body, whereas flexion of the hand involves decreasing the angle between the forearm and hand at the wrist joint.
In terms of innervation, the flexor carpi ulnaris is innervated by the ulnar nerve, which is located medial to the median nerve. And like the median nerve, this nerve arises from the brachial plexus.
Located between the flexor carpi ulnaris and the flexor carpi radialis is this muscle that we can see here called the palmaris longus. And the palmaris longus is absent in about fifteen percent of the population, and if you’re curious, you can actually check if you have this muscle by touching the pads of your pinky and thumb while flexing your wrist, and if this muscle is present, then its tendon will be visible like you can see in this image just here.
As for attachments, the palmaris longus originates from the medial epicondyle of the humerus and inserts into the palmar aponeurosis of the hand. This muscle is responsible for the flexion of the hand at the wrist joint and it also tightens the palmar aponeurosis. So when it comes to the innervation of the palmaris longus, this muscle is innervated by the median nerve, which we can see here highlighted in green.
Okay, so let’s move on to look at the muscle you can now see on your screen, which is the flexor carpi radialis. And this muscle originates from the medial epicondyle of the humerus and inserts at the bases of the second and third metacarpals. So, what does the flexor carpi radialis do? Well, it’s actually responsible for two movements of the wrist joint, and these two movements are contraction which causes abduction and flexion of the hand at the wrist joint, and as we can see in our image, abduction is the movement of the hand away from the midline of the body. And the innervation of the flexor carpi radialis is supplied by the median nerve.
So, the next muscle we’re going to be talking about is a little bit controversial. Some people say that it’s a muscle of the superficial layer while others think that it deserves a layer of its own – an intermediate layer. But to keep things simple, we’re just going to class it as a superficial muscle. And the muscle I’m talking about is this one just here – the flexor digitorum superficialis.
Now, this muscle originates from the medial epicondyle of the humerus, the coronoid process of the ulna, and the anterior surface of the radius, and then it splits into four tendons at the wrist and inserts into the middle phalanges of the four fingers.
As always, its name gives us a clue about its functions, and flexor implies that its main function is flexion and digitorum means fingers in Latin, so it must be responsible for the flexion of your fingers. More specifically, contraction of the flexor digitorum superficialis causes flexion of the fingers at the proximal interphalangeal joints and the metacarpophalangeal joints. And flexion or bending of these joints allows you to carry out various activities like playing guitar, for example. And the flexor digitorum superficialis is also responsible for flexion of the hand at the wrist joint. In terms of innervation, the flexor digitorum superficialis is innervated by the median nerve.
So now that we talked about the superficial muscles of the anterior compartment of the forearm, let’s dig a little bit deeper and take a look at the muscles found in the deep layer.
The first muscle of this layer that we’re going to be talking about is the flexor digitorum profundus. And this muscle originates from the anterior margin of the ulna and the interosseus membrane of the forearm. At the wrist, it splits into four tendons that insert onto the distal phalanges of the four fingers. When the flexor digitorum profundus contracts, it causes flexion of the fingers at the distal interphalangeal joints and the metacarpophalangeal joints. It is also responsible for flexion of the hand at the wrist joint.
The innervation of the flexor digitorum profundus is supplied by two nerves. The medial half of this muscle which moves the pinky and the ring finger is innervated by the ulnar nerve whereas the lateral half which moves the middle finger and the index finger is innervated by the median nerve.
Lateral to the flexor digitorum profundus, we can see this muscle here which is called the flexor pollicis longus. And in terms of origin points, this muscle originates from the anterior margin of the radius and the interosseus membrane of the forearm. It then goes on to insert at the distal phalanx of the thumb. And this muscle is responsible for flexion of the thumb at the interphalangeal and metacarpophalangeal joints.
So when it comes to the innervation of the flexor pollicis longus, this muscle is innervated by the median nerve which we can see here highlighted in green.
And if we move the flexor digitorum profundus and the flexor pollicis longus, we can see the last muscle of the deep compartment, which is the pronator quadratus. And this muscle originates from the anterior surface of the ulna and extends horizontally to insert onto the anterior surface of the radius.
So what does the pronator quadratus do? Well, as the name suggests, this muscle works with the pronator teres to perform pronation of the forearm. And the innervation of the pronator quadratus is supplied by the median nerve.
Okay, now that we’re familiar with the muscles of the anterior compartment of the forearm, let’s get clinical.
So, you may have noticed that most of the muscles we saw today extended from the forearm into the hand, and as they do this, some of these muscles pass through a structure known as the carpal tunnel. And if we take a look at this image here, which is an MRI scan of the wrist in the axial plane, we can see that this tunnel is formed by the carpal bones and a band of fibrous tissue known as the flexor retinaculum. And the contents of the carpal tunnel include the tendons of the flexor pollicis longus, flexor digitorum profundus, and flexor digitorum superficialis as well as the median nerve. And compression of these structures specifically the median nerve can cause carpal tunnel syndrome. And this is usually as a result of swelling, but the cause of this condition is often idiopathic or unknown.
However, individuals are more at risk if their work or hobbies require repetitive motions of the wrist if they have conditions like hypothyroidism, obesity, rheumatoid arthritis, and diabetes, or if they are pregnant. Symptoms include pain in the fingers, hand or arm; numbness of the hand, pins and needles, and weakness of the thumb or difficulty gripping. And symptoms normally start slowly, are intermittent, and are usually worse at night.
Carpal tunnel syndrome can resolve by itself and treatment usually takes a conservative approach. It involves wearing a wrist splint to relieve pressure on the nerve and rest and pain medication. If this doesn’t work, then a steroid injection might be required, and if all other treatments are proven insufficient, surgery may also have to be considered. And it usually takes about one month to recover from surgeries and get back to normal activities.
So before we bring our tutorial to a close, let’s quickly summarize what we’ve learned today.
So we started our tutorial with the bones of the upper limb which are the humerus, the radius, the ulna, the carpals, the metacarpals, and the phalanges. We then moved on to the muscles of the anterior compartment of the forearm which we divided into two groups – the superficial layer and the deep layer. And for each muscle, we discussed their origin, insertion, function, and innervation. Lastly, we concluded our tutorial with some clinical notes about carpal tunnel syndrome.
And that brings us to the end of our tutorial on the muscles of the anterior compartment of the forearm. I hope you enjoyed it, thanks for watching, and see you next time!