Video: Muscles of the hand

Ahhhh! Isn't that lovely? So peaceful. Look how quickly her fingers are moving. Uh-oh…I think she stretched her little finger out too far and might have pulled a muscle. But which muscle? Maybe her abductor digiti minimi muscle? I'm not too sure. Stay tuned to find out because today we're going to be learning about the muscles of the hand.

There are two muscle groups which create movement at the hands and fingers. These are the extrinsic and intrinsic muscles of the hand. The extrinsic muscles of the hand are the muscles which originate within the forearm and extends to insert onto the structures of the hand. These muscles facilitate the gross motor movements of our hands but we aren't going to be focusing on this muscle group today.

In today's tutorial, we're going to take a look at the intrinsic muscles of the hand. These are the muscles which originate and insert within the hand. These muscles are responsible for our fine motor skills helping us to hold a pen, play the piano, and even button up our jackets.

The intrinsic muscles of the hand can be divided into three groups – the metacarpal muscles, the thenar muscles, and the hypothenar muscles. We'll take a look at each of the muscles which make up these groups and explore their attachments, functions, and innervation. We'll then finish off today's tutorial with a quick look at some clinical notes on the muscles of the hand to help consolidate our knowledge.

Let's begin with the muscles associated with the metacarpal bones.

The metacarpal muscles are located between the metacarpal bones of the hand. They are made up of three groups of muscles – the dorsal interossei, the palmar interossei, and the lumbricals. So without further ado, let's begin by taking a look at the dorsal interossei. Note that we are now looking at a dorsal view of the hand, otherwise, known as the back of the hand. The dorsal interosseous muscles comprise four short muscles. They can be found in the spaces between the metacarpal bones and are usually numbered from 1 to 4, starting at the thumb.

Each dorsal interosseous muscle has two heads, which attach to the near sides of the adjacent metacarpals. Each muscle then converges onto a central tendon which extends to insert onto the base of the associated proximal phalanx and extensor expansion. They are bipennate muscles. This means that muscle fibers come towards the inserting tendon at an angle from two directions, similar to the barb and quill of the feather.

Let's take a closer look at the insertions of each individual dorsal interosseous muscle now.

The first and second dorsal interossei insert into the index and middle fingers and their extensor expansions while the third and fourth dorsal interossei insert onto the middle and ring fingers and their extensor expansions. Each dorsal interosseous muscle inserts onto the base of the associated proximal phalanx. Taking an even closer look, the first and second dorsal interossei insert onto the radial aspect of the proximal phalanges, while the third and fourth dorsal interossei insert onto the ulnar aspect of the associated proximal phalanges.

Here we are using the radius and ulna as a guide to describe the location of the structures within the hand. This is a common way of describing the positions of hand structures as it's more intuitive than using lateral and medial. If you need help remembering, remember that ulna has an L in it so it is on the same side as the Little finger. Alternatively, hold your arm out and trace your ulna from your elbow.

As a brief aside, you might be wondering what the extensor expansions are. They are fibrous connective tissue fibers which are formed by the flattened extensor tendons of the hand. They surround the distal metacarpals and proximal phalanges facilitating extension of the fingers at the interphalangeal joints, often abbreviated to the IPJs. Have you noticed the dorsal interossei here? And this is a lumbrical, which we'll look at properly soon.

Okay, back to the dorsal interossei. These four interosseous muscles work together to spread or abduct the fingers away from the midline of the hand. The dorsal interossei also contributes to flexion of the fingers at the metacarpophalangeal joints and extension at the IPJs via the extensor expansions. All of these movements occur through the innervation provided by the deep branch of the ulnar nerve which stems from the brachial plexus.

Let's flip the hand around and take a look at the next group – the palmar interossei muscles – which are located on the palmar, a.k.a. volar, surface of the hand. Similar to their dorsal counterparts, the palmar interossei occupy the spaces between the metacarpal bones. The palmar interossei are unipennate; however, their muscle fibers are arranged obliquely arising and attaching onto only one side of the tendon, like one-half of a feather.

The palmar interossei usually consists of three muscles which attach to the index, ring, and little fingers. They can sometimes be classified as four muscles with the inclusion of the palmar interosseous muscle attached to the metacarpal of the thumb. This muscle is rudimentary, however, and is often considered part of the adductor pollicis muscle.

As you will have noticed, the middle finger does not have a palmar interosseous muscle. These muscles are numbered 1 to 3, starting from the one closest to the thumb. Literature often differs on the numbering of the palmar interossei. If a fourth interosseous muscle was present, the interosseous muscle attached to the metacarpal of the thumb would be labelled number one. However, we will view the palmar interossei as three muscles.

Much like the dorsal interossei, the palmar interossei also originate from the metacarpals of the hand. The first palmar interosseous muscle arises from the ulnar side of the metacarpal of the index finger and extends to insert into the extensor expansion of the index finger. The second and third interosseous muscles arise from the radial side of the metacarpals of the ring and little fingers and extends to insert into their respective proximal phalanges and extensor expansions.

The main function of the palmar interossei muscles is to adduct the fingers bringing the fingers back in towards the middle finger. In addition, the palmar interossei greatly contribute to the flexion at the metacarpophalangeal joints and extension at the interphalangeal joints of the index, ring, and little fingers. Similar to their dorsal counterparts, all of these movements are possible because of the innervation by the deep branch of the ulnar nerve.

It can be a little confusing to try to remember the actions of the interosseous muscles, but here's a little tip. Just remember PAD and DAB – the Palmar interosseous muscles ADduct or PAD while the Dorsal interosseous muscles ABduct or DAB.

Also located in the metacarpal region are the lumbrical muscles, often simply called the lumbricals of the hand. These four muscles lie just deep to the palmar fascia and between the metacarpal bones of the hand. Moving from radial to ulnar, the lumbricals are numbered 1 to 4. Unlike the interosseous muscles, the lumbricals do not originate from bone, but arise from the tendons of the flexor digitorum profundus muscle. Lumbricals 1 and 2 are usually unipennate while lumbricals 3 and 4 are bipennate.

From their origins, the lumbrical muscles pass distally along the radial side of the corresponding finger. They extend obliquely in a dorsal direction to attach onto the radial margin of the extensor expansion at the proximal phalanges of the index, middle, ring, and little fingers.

In terms of function, the lumbrical muscles can be thought of as refinement muscles. They originate and insert into soft tissue structures creating mobile attachment points capable of two different motor actions. These muscles can both flex and extend. They flex the fingers at the metacarpophalangeal joints and extend the fingers at the interphalangeal joints via the extensor hoods.

These muscles are weak on their own so we use our lumbrical muscles for fine movements such as when we're holding a pen to write or doing up the buttons on our shirt. They also monitor the balance of flexor and extensor forces. These muscles have a high spindle density. Spindles are responsible for sending proprioceptive feedback from muscles to the central nervous system, therefore, they give us the sensory and motor abilities necessary to fine-tune movements.

Despite being thought of as one group of muscles, the lumbricals actually have split innervation. The radial two lumbricals receive their innervation from the median nerve while the third and fourth lumbricals are innervated by the ulnar nerve – unsurprising when you consider lumbricals 3 and 4 are on the ulnar side.

Now that we have met the muscles of the metacarpals, let's take a look at the next group of muscles – the thenar group.

Thenar stems from the Greek word thenin which means to strike. When you hit or strike something with an open palm, the area of your hand that usually makes contact first is the fleshy pad at the base of your thumb on the palm of the hand, thus giving the fleshy pad the name thenar. This fleshy pad known as the thenar eminence is located on the radial aspect of the palm of the hand and is formed by the bulk of four short muscles – the thenar muscles. These are the abductor pollicis brevis muscle, the flexor pollicis brevis muscle, the adductor pollicis muscle, and the opponens pollicis muscle. Pollicis is derived from the Latin word pollex which means thumb, and as you can guess, all these muscles do in fact act on and attach to the thumb.

If you're an avid anatomy textbook reader, you may have noticed that some texts do not classify the adductor pollicis muscle as part of the thenar muscle group. This is because the adductor pollicis muscle does not directly contribute to the bulk formation of the thenar eminence. However, for the purpose of this tutorial, we will classify adductor pollicis as a muscle of the thenar group due to its action on the thumb and proximity to the other thenar muscles.

Now let's take a look at each muscle individually starting with the abductor pollicis brevis muscle. This muscle is the most lateral and most superficial when compared to the other three thenar muscles. It originates from the scaphoid and trapezium carpal bones with some muscle fibers also attaching to the fibrous flexor retinaculum band which arches over the carpal bones. It then extends distally to insert onto the lateral aspect of the base of the proximal phalanx of the thumb.

This muscle, as its name suggests, abducts the thumb at its carpometacarpal joint bringing the thumb away from the midline of the hand. Allowing this muscle to carry out its associated motor movements is a branch of the median nerve, specifically, the recurrent branch of the median nerve.

Just deep to this muscle is the flexor pollicis brevis muscle. This muscle arises as two heads – a superficial and a deep head. The superficial head is much wider than its deep counterpart making it a little tricky to see the deep head. The superficial head attaches to the tubercle of the trapezium bone and the overlying flexor retinaculum while the deep head arises from the trapezoid and capitate carpal bones.

The muscle fibers from the two heads converge onto one muscle belly and extend distally to insert at the lateral aspect of the base of the proximal phalanx of the thumb. This muscle works together with the flexor pollicis longus muscle flexing the thumb at the carpometacarpal joint. This muscle also receives the majority of its nerve supply from the recurrent branch of the median nerve. The deep head will also receive additional nerve fibers from the ulnar nerve.

The next muscle in the thenar muscle group is the opponens pollicis muscle. It's hidden deep to the other muscles of the thenar eminence so we've removed them to show it better. This small muscle originates from the tubercle of the trapezium bone and the overlying flexor retinaculum. It extends distally over the palmar aspect of the body of the metacarpal of the thumb inserting onto its radial border of the thumb.

The main function of the opponens pollicis is to produce opposition of the thumb. Opposition is a complex movement made up of a combination of adduction, flexion, and medial rotation of the thumb at the carpometacarpal joint. Practically speaking, the action of bringing the tip of the thumb in contact with any other fingertip on the same hand is opposition. Why don't you try it out for yourself? This movement is often used when picking up something small or when holding a pen. The opponens pollicis muscle usually receives its innervation through the recurrent branch of the median nerve, however, it can also sometimes receive contribution from the deep terminal branch of the ulnar nerve too.

We have arrived at the final muscle of the thenar muscle group – the adductor pollicis muscle. Remember, the adductor pollicis is not always included as a muscle of the thenar group. This muscle has two heads – a transverse head and an oblique head – which are named after the direction in which their muscle fibers lie.

The transverse head originates from the palmar base of the metacarpal bone of the middle finger. Meanwhile, the oblique head originates from the capitate bone and the palmar bases of the metacarpals of the index and middle fingers. The two muscle bellies converge to form one muscle which extends laterally towards the first metacarpal inserting onto the base of the proximal phalanx of the thumb as well as the extensor hood.

As its name suggests, this muscle is a strong adductor of the thumb at the carpometacarpal joint. On contraction, it draws the thumb in towards the index finger from an abducted position. This muscle also works together with the opponens pollicis to facilitate the later stages of opposition. Making these movements possible is the deep terminal branch of the ulnar nerve which innervates both heads of this muscle.

Moving on to the final muscle group of the hand, we meet the hypothenar muscles.

The hypothenar muscles are also found on the palm of the hand but on the ulnar side proximal to the little finger. Located in the medial palmar compartment of the hand is the hypothenar eminence. This eminence is created by the four short hypothenar muscles – the palmaris brevis, the abductor digiti minimi, the flexor digiti minimi, and the opponens digiti minimi muscles. Let's take a closer look at them now.

The palmaris brevis muscle is the most superficial of all the hypothenar muscles. While this muscle is located in the hypothenar region, it is not always considered to be part of the hypothenar muscle group as it is not attached to the little finger nor does it act on the little finger, therefore, it may be classed as a separate superficial muscle of this region. This small quadrangular muscle originates from the palmar aponeurosis of the palm of the hand with some fibers also arising from the flexor retinaculum on the palmar surface of the wrist. This muscle travels medially to insert into the skin of the hypothenar region.

This little muscle is very helpful when we're doing pull-ups at the gym or holding a ball as it tightens the palmar aponeurosis on contraction. This tightening wrinkles the hypothenar skin and deepens the palmar fossa, helping us to position and stabilize the palms of our hand as we grip onto something. The innervation of this important little muscle is provided by the superficial branch of the ulnar nerve.

Immediately deep to the palmaris brevis muscle is the abductor digiti minimi muscle. Digiti minimi literally translates to mean smaller finger, therefore, the name of this muscle tells us exactly what it does and where it's located. This short muscle of the hypothenar region mainly originates from the pisiform carpal bone, however, some muscle fibers also arise from the pisohamate ligament which stretches between the pisiform and hamate bones and from the tendon of the flexor carpi ulnaris muscle. The muscle fibers converge into a single tendon traveling distally to insert onto the ulnar side of the extensor expansion of the proximal phalanx of the little finger and the base of the proximal phalanx of the little finger.

The main function of this muscle is to abduct the little finger, moving the digit away from its neighbor, the ring finger. This muscle also flexes the little finger at both its metacarpophalangeal and proximal interphalangeal joints. This muscle is important in gripping large objects such as when you hold a basketball. This is the muscle that the poor pianist may have strained, abducting that little finger out too far.

Facilitating the movements of the little finger is the deep branch of the ulnar nerve which provides the motor innervation to the abductor digiti minimi muscle.

The next muscle of the hypothenar region is the flexor digiti minimi brevis muscle. This muscle is situated just inferior and lateral to the abductor digiti minimi muscle. This muscle arises from the hook of the hamate bone and medial aspect of the flexor retinaculum. It then extends distally to insert together with the abductor digiti minimi muscle onto the medial side of the base of the proximal phalanx of the little finger.

This muscle, as its name suggests, is a flexor of the little finger. It flexes the little finger at the metacarpophalangeal joint and also contributes to slight lateral rotation of the little finger. All these movements couldn't occur without that little electric current that supplies the muscle. The ulnar nerve travels all the way down the forearm to give off the deep branch of the ulnar nerve which supplies this muscle.

The final and deepest muscle of the hypothenar region is the opponens digiti minimi muscle. This small muscle is located just deep to the flexor digiti minimi brevis and abductor digiti minimi muscles. It arises from the hook of the hamate bone with some fibers radiating from the flexor retinaculum of the hand. The muscle fibers of the opponens digiti minimi muscle extend distally to insert onto the ulnar and palmar surfaces of the metacarpal of the little finger.

For such a small muscle, the opponens digiti minimi muscle actually has quite a few actions. Its main action is to flex and laterally rotate the little finger at its carpometacarpal joint. This combined movement deepens the hollow of the palm and helps to improve the grip function of the palmar surface of the hand. As its name suggests, this little muscle also aids in facilitating opposition of the little finger. When combined, flexion and lateral rotation at the metacarpophalangeal and flexion at the interphalangeal joints move the tip of the little finger to meet the tip of the thumb. All of these movements are facilitated by innervation from – you guessed it – the deep branch of the ulnar nerve.

And there we have it, all the little intrinsic muscles of the hand that work so hard to help us type, write, and get dressed in the morning. Before we finish up though, let's take a quick look at the clinical notes on the muscles of the hand.

As we know, the lumbrical muscles arise from the long tendons of the flexor digitorum profundus muscle. As a result of injury or surgery, the flexor digitorum profundus tendons may become detached from their point of insertion. If this detachment arises distal to the origin points of the lumbricals, an interesting phenomenon known as lumbrical-plus finger occurs. The insertion point of the lumbrical now serves as the insertion surface of the flexor digitorum profundus muscle.

If we remember from previously, the insertion of the lumbricals is the extensor mechanism. This means that even though a person consciously activates the flexor muscle, that force is transmitted through the extensor mechanism rather than through the insertion of the flexor tendon.

Upon trying to flex the finger at the distal interphalangeal joints, the intended fist closure paradoxically leads to extension of the distal interphalangeal joint of the affected finger. It is most commonly identified at the second lumbrical, thereby, affecting the middle finger.

Physical examination usually involves testing a patient's ability to make a fist or to hold a can looking for any signs of paradoxical extension of the finger at the distal interphalangeal joints. Treatment surrounding this unusual injury involves both surgical and conservative interventions. Surgical repair of the damaged flexor digitorum profundus may be carried out followed by regular physiotherapy as a method of restoring elasticity and improving strength.

We have now arrived at the end of this tutorial. Before we finish up though, let's go over a quick summary to wrap up all that we learnt today.

In today's tutorial, we learned about the intrinsic muscles of the hand. We explored the three groups of intrinsic muscles, namely, the metacarpal muscles, the thenar muscles, and the hypothenar muscles. We identified the metacarpal muscles first beginning with the dorsal interossei. These four muscles attach to the adjacent sides of the metacarpals of the index, middle, ring, and little fingers, and extends to insert at the proximal phalanx and extensor expansions of the associated digits. These muscles abduct the fingers and are innervated by the deep branch of the ulnar nerve.

On the palmar surface of the hand, we meet the palmar interossei. These are three muscles which arise from the metacarpals of the index, ring, and little fingers. The middle finger does not have a palmar interosseous muscle. These muscles extend to insert onto the associated proximal phalanges and extensor expansions. In opposition to the dorsal interossei, these muscles adduct the fingers and are similarly innervated by the deep branch of the ulnar nerve.

The final muscle group of the metacarpal muscles is the lumbrical muscles. These four muscles arise from the tendons of the flexor digitorum profundus muscles and insert into the extensor expansions of the fingers but not the thumb. These four muscles flex the metacarpophalangeal joints and extends the interphalangeal joints on contraction. On top of that, they refine movements of the fingers. They receive a split innervation from the median and ulnar nerves.

Next, we had a look at the four muscles which make up the thenar muscle group. On the radial aspect of the palm of the hand, we examined the abductor pollicis brevis muscle first. This muscle arises from the scaphoid, trapezium, and flexor retinaculum. It then travels distally to insert onto the base of the proximal phalanx of the thumb. This muscle abducts the thumb at the carpometacarpal joint and is innervated by the recurrent branch of the median nerve.

Next, we came to the flexor pollicis brevis muscle which has a superficial head and a deep head. This muscle arises from the trapezium, trapezoid, and capitate carpal bones with some fibers arising from the flexor retinaculum. It inserts onto the base of the proximal phalanx of the thumb. This muscle flexes the thumb at the carpometacarpal joint and is innervated by the recurrent branch of the median nerve.

Just deep to it, we met the opponens pollicis muscle. This small muscle arises from the trapezium bone and flexor retinaculum. It extends to insert onto the lateral border of the metacarpal of the thumb. This muscle causes the composite movement of opposition of the thumb. It is innervated by the recurrent branch of the median nerve.

The final muscle of the thenar muscle group is the adductor pollicis muscle. This muscle has an oblique head and a transverse head which arise from the metacarpal bones of the index, middle, and ring fingers as well as the capitate carpal bone. This muscle extends to insert onto the base of the proximal phalanx of the thumb and its extensor hood. As its name suggests, this muscle adducts the thumb and it is innervated by the deep branch of the ulnar nerve.

Moving on to the final muscle group of the hand, we come to the four hypothenar muscles which lie proximal to the little finger. The first hypothenar muscle is the palmaris brevis muscle. This muscle arises from the palmar aponeurosis and flexor retinaculum and inserts onto the skin of the hypothenar. This muscle tightens the palmar aponeurosis, improving grip strength. It receives its motor innervation from the superficial branch of the ulnar nerve.

Just deep to the palmaris brevis muscle is the abductor digiti minimi muscle. This muscle arises from the pisiform bone, the pisohamate ligament, and from the tendon of the flexor carpi ulnaris muscle. It extends to insert at the proximal phalanx of the little finger. It abducts the little finger and receives its motor innervation from the deep branch of the ulnar nerve.

Next, we met the flexor digiti minimi muscle which arises from the hook of the hamate bone and flexor retinaculum. It extends to insert onto the proximal phalanx of the little finger. This muscle flexes the little finger and is innervated by the deep branch of the ulnar nerve.

The last of the hypothenar muscles is the opponens digiti minimi muscle. This muscle arises from the hook of the hamate bone and the flexor retinaculum. It travels distally to insert onto the metacarpal bone of the little finger. This muscle flexes and laterally rotates the little finger, thereby, facilitating opposition and is innervated by the deep branch of the ulnar nerve.

To finish off, we explored an unusual phenomenon – lumbrical-plus finger. Damage to the flexor retinaculum profundus tendon of a finger distal to the origin of the lumbrical results in the lumbrical becoming the insertion point for FDP. We had a quick look at the signs and symptoms, clinical examination, and treatment regimens associated with this condition.

And that's it! We hope you enjoyed learning about the muscles of the hand. See you next time and happy studying!