Muscles of the hand

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Introduction0:00–0:36

The hands are the most distal part of our upper limbs, and they’re quite marvelous! They’re strong enough to lift heavy boxes, and at the same time delicate and precise enough to thread a needle, or type on a keyboard.
The diverse range of movements our hands are capable of is made possible by a variety of muscles and their arrangement within the hand.
The hand is composed of intrinsic muscles, which are entirely located within the hand; and extrinsic muscles, which are forearm muscles, but act on the hand and fingers through their long tendons.
The intrinsic muscles of the hand are covered by the fibrous palmar fascia, which divides them into 4 main compartments: the thenar compartment located below the first digit, known as the thumb, the hypothenar compartment located below the fifth digit also known as the little finger, the adductor compartment located in the lateral part of the hand, and finally, the central compartment that’s found in the center of the palm.

Hand fasciae0:36–2:16

There are also several interosseous compartments between the metacarpals. The palmar fascia , which is the continuation of the antebrachial fascia in the forearm, forms the palmar aponeurosis centrally.
The palmar aponeurosis is a strong, thickened portion of the palmar fascia covering the central compartment and overlies the flexor tendons.
Proximally, the palmar aponeurosis blends with the tendon of palmaris longus and the flexor retinaculum, which if you remember is a transverse ligament spanning over the carpal bones creating the carpal tunnel.
Distally, the palmar aponeurosis divides into four longitudinal bands that radiate distally to attach to the bases of the proximal phalanges, where they become continuous with the fibrous digital sheaths which enclose flexor tendons going to the digits.
Within the hand, the medial border of the palmar aponeurosis becomes continuous with the thin hypothenar fascia, which covers the muscles of the hypothenar compartment.
The palmar aponeurosis extends laterally where it continues with the thin thenar fascia, which covers the muscles of the thenar compartment.
Now, deep to the flexor tendons there are two potential spaces; the thenar space, and the midpalmar space. These spaces are important as they are potential closed off spaces that can become affected during a pathologic process like an infection.

Thenar space & Midpalmar space2:16–2:50

The thenar space underlies the thenar compartment and is located anterior to the adductor pollicis muscle deep to the central and thenar compartments, while the midpalmar space is deep to the central compartment of the palm and anterior to the deep hand muscles and the third, fourth, and the fifth metacarpal bones.
Before we take a look at the muscles of the hand, let's take a quick look at some of the movements of the hand, particularly the thumb.

Hand movements2:50–3:19

The thenar muscles are mainly responsible for the various movements of our thumb. The thumb can undergo flexion and extension, and these movements are in the same plane as the palm, abduction and adduction, where the movement is in a plane perpendicular to the palm, and finally, opposition and reposition where the tip of the thumb can come in contact with the tip of the 5th digit.
Now, let’s take a look at each of the muscle compartments of the hand, starting with the thenar compartment. The thenar compartment is located in the lateral part of the palm, and forms the thenar eminence of the hand.

Thenar compartment3:19–5:21

It contains three muscles which mainly act on the thumb. These are the abductor pollicis brevis, the flexor pollicis brevis, and the opponens pollicis muscle.
The abductor pollicis brevis makes up the lateral part of the compartment. This muscle attaches proximally to the flexor retinaculum and the tubercles of the scaphoid and the trapezium, and then extends distally to attach to the lateral base of the proximal phalanx of the thumb.
The abductor pollicis brevis is innervated by the recurrent branch of the median nerve, and when it contracts, it abducts the thumb and also assists in opposition.
Next up, there’s the flexor pollicis brevis, which is medial to the abductor pollicis brevis. The flexor pollicis brevis has a superficial and a deep head.
Both heads attach proximally to the flexor retinaculum and the tubercles of the scaphoid and trapezium, and fuse distally to form a single tendon with a sesamoid bone within it, which attaches to the lateral base of the proximal phalanx of the thumb.
The superficial head of the flexor pollicis brevis is innervated by the recurrent branch of the median nerve, while the deep head receives its innervation from the deep branch of the ulnar nerve.
When they contract, they both flex the thumb. Finally, there is the opponens pollicis, which lies deep to the abductor pollicis brevis.
This muscle also originates on the flexor retinaculum and tubercles of the scaphoid and trapezium, and it extends distally to attach to the lateral side of the shaft of the first metacarpal.
The opponens pollicis receives its innervation from the recurrent branch of the median nerve, and it opposes the thumb, by pulling the first metacarpal medially, towards the 5th digit.
Alright, next is the adductor compartment, which is located below the thenar muscles. This compartment has only one muscle, called the adductor pollicis.

Adductor compartment5:21–6:35

The adductor pollicis muscle also has two heads: an oblique head and a transverse head. T he oblique head attaches to the base of the second and the third metacarpals, as well as the capitate carpal bone.
The proximal attachment of the transverse head is to the anterior shaft of the third metacarpal. Distally, both heads of this muscle form a single tendon, which contains a sesamoid bone within it, and attaches to the medial base of the proximal phalanx of the thumb.
The adductor pollicis is innervated by the deep branch of the ulnar nerve, and when it contracts, it adducts the thumb. Now, besides abduction, adduction, flexion and opposition, the thumb can also perform movements such as reposition, which is moving the thumb in the opposite direction to the opposition, and extension.
Thumb reposition is done by the extensor pollicis longus muscle. In order for extension to happen, the extensor pollicis brevis, the extensor pollicis longus, and the abductor pollicis longus work together.
Now, before we move on, feel free to pause and see if you remember the nerve to the deep head of the flexor pollicis brevis?

Quiz6:35–6:51

Alright, now let's look at the muscles of the hypothenar compartment. The hypothenar compartment also consists of three muscles, which move the little finger.

Hypothenar compartment6:51–8:10

Inside the hypothenar compartment, we have the abductor digiti minimi, which attaches to the pisiform carpal bone proximally, and extends distally to attach to the base of the proximal phalanx of the little finger.
This muscle mainly abducts the 5th digit. There is also the flexor digiti minimi brevis, which proximally attaches to the hook of the hamate, as well as the flexor retinaculum.
Distally, the flexor digiti minimi brevis also attaches to the base of the proximal phalanx of the little finger and, obviously, it flexes the 5th digit.
Finally, there is the opponens digiti minimi, which attaches proximally to the hook of hamate and the flexor retinaculum, just like the flexor digiti minimi brevis.
The opponens digiti minimi extends distally and attaches to the fifth metacarpal. It pulls the metacarpal towards the thumb, helping to oppose the 5th digit.
All three muscles of the hypothenar compartment share a common innervation, which is the deep branch of the ulnar nerve.

Palmaris brevis8:10–8:44

Now, outside the hypothenar compartment, just superficial to it, there’s another muscle called the palmaris brevis. Proximally, the palmaris brevis attaches to the palmar aponeurosis, and distally, it attaches to the dermis of the skin of the hypothenar eminence.
This muscle is innervated by the superficial branch of the ulnar nerve, and when it contracts, that tightens the palmar aponeurosis to wrinkle skin of the hypothenar eminence.
This helps form a stronger palmar grip, like when holding on to the rope during tug of war. Finally, there are what are called the “short muscles” of the hand - the lumbricals and interossei.

Short muscles8:44–9:07

The lumbricals attach to forearm flexor tendons, so they are found within the central compartment of the hand, which is located between the thenar and the hypothenar compartments.
The interossei are located between the metacarpals, which can be called interosseous compartments. Now, there are four lumbricals which attach proximally to the tendons of the flexor digitorum profundus muscle.

Central compartment9:07–10:26

The first and the second lumbricals have one head each, so they’re unipennate muscles, while the third and the fourth ones have two heads each, which makes them bipennate muscles.
The proximal attachment of the first and the second lumbricals is to the two lateral tendons of the flexor digitorum profundus, while the two medial lumbricals attach to the medial three tendons.
Distally, all four lumbricals attach to the lateral sides of the extensor expansions of the second to the fifth digits. Extensor expansions are hood-like aponeuroses formed by the extensor tendons as they insert onto the phalanges, and serve as attachment points for muscles such as the lumbricals and interossei.
The first and the second lumbricals are innervated by the median nerve, while the third and the fourth lumbricals are innervated by the deep branch of the ulnar nerve.
All four lumbricals flex the metacarpophalangeal joints and they also extend the interphalangeal joints of the second to the fifth digits.
This mixture of proximal flexion and distal extension creates the ‘bye bye’ motion showcasing the action of the lumbricals!
And then we have the interosseous muscles - or interossei. There are four dorsal interossei, and three palmar interossei.

Interosseous compartments10:26–12:59

Before we move on, a good mnemonic to remember these muscles and their actions is DAB and PAD. DAB stands for Dorsal interossei ABducting the digits, and PAD stands for the Palmar interossei ADDucting the digits.
Remember that abduction and adduction of the fingers are in reference to the axial line of the hand that passes through the 3rd digit.
Let's take a closer look at the four dorsal interossei located on the dorsal aspect of the hand between the metacarpals.
Proximally, they attach to the adjacent sides of the five metacarpals, for example the first dorsal interosseous attaches proximally to the adjacent sides of digit 1 and 2.
Distally, the dorsal interossei attach to the bases of the proximal phalanges, along with the extensor expansions of digits 2-4.
The first and second interossei attach to the lateral sides of the bases of the second and the third proximal phalanges respectively, while the third and the fourth dorsal interossei attach to the medial sides of the bases of the third and fourth proximal phalanges.
If you notice, the 2nd and 3rd dorsal interossei both attach to the third digit on opposite sides to abduct it either to the left, or to the right.
And then, there are three palmar interossei, which are located on the palmar side of the hand. The proximal attachment of the first palmar interosseous is along the medial part of the second metacarpal, while the second and the third palmar interossei attach to the lateral parts of the fourth and the fifth metacarpals respectively.
Distally, the first palmar interosseous muscle attaches to the medial part of the second proximal phalanx, while the remaining two palmar interossei attach to the lateral parts of the bases of their respective proximal phalanges while also attaching to the extensor expansions of digit 2, 4 and 5.
In addition to adducting and abducting the digits, when all the interossei act together, they help the lumbricals flex the metacarpophalangeal joints and extend the interphalangeal joints.
Remember, this looks like the “bye-bye” movement. Both the dorsal and the palmar interossei have a common innervation, which is the deep branch of the ulnar nerve.

Quiz12:59–13:18

Okay, what if before we recap everything, we press the pause button, andtry to remember the function of the palmar interossei, as opposed to the dorsal interossei?

Review13:18–14:36

##Summary Alright, as a quick recap… the major muscle compartments of the hand are the thenar compartment, which has the flexor pollicis brevis, the abductor pollicis brevis, and the opponens pollicis; the adductor compartment which only contains the adductor pollicis; the hypothenar compartment which has the abductor digiti minimi, the flexor digiti minimi brevis, and the opponens digiti minimi; the central compartment which contains the four lumbricals, and finally in the interosseous compartments there are seven interossei.
Each set of muscles is very important for the different movements of our hands. The muscles of the thenar compartment allows for flexion, abduction and opposition of the thumb, while the adductor pollicis permits adduction of the thumb.
The hypothenar compartment allows for flexion, abduction, and opposition of the 5th digit. The lumbricals act on digits 2-5 and allow for flexion of the metacarpophalangeal joints and extension of the interphalangeal joints, while the dorsal interossei abduct digits 2, 3 and 4, and the palmar interossei adduct digits 2,