The Functional Anatomy of the Hand

Muscles that articulate the hand

Two groups of muscles articulate the hand. Muscles that originate on the arm and cross the wrist are called extrinsic muscles while those that originate on the hand itself are called intrinsic muscles.

Extrinsic muscles of the hand

It's not as complicated as it may look.

• Flexors are on the inside of the forearm and extensors are on the back.
• The larger extrinsic muscles branch in the hand to connect to all four fingers. Individual fibres within the muscle control particular fingers.
• Two muscles flex the fingers. A deep one connects to the end of each finger, it does most of the work and acts on the whole finger. A more superficial one connects to the base of each finger and helps out when the wrist is flexed or when you don't need to flex the whole finger.
• Only one extrinsic muscle flexes the thumb
• There is only one main muscle that extends the fingers, it attaches on the middle bone of the finger and to the end of the finger via a mechanism of tendons. Two other extensors attach to the index and little fingers giving them greater independance.
• Three muscles extend the thumb at different points, one of them is also an abductor.

Muscle	Location	Action
Flexor digitorum superficialis	On the inside of the forearm, attaches to the base of the finger (proximal phalanx)	Flexes the fingers, does not act on the distal joint
Flexor digitorum profundus	On the inside of the forearm, attaches closer to the tip of the finger (on the distal phalanx)	Flexes the fingers, including the distal joint
Flexor pollicis longus	On the inside of the forearm, attaches to the distal phalanx of the thumb	Flexes the thumb
Extensor digitorum	On the back (dorsal surface) of the forearm, from the elbow (on the lateral epicondyle) to the back of the middle phalanx	Extends the fingers, an extensor mechanism attaches to the distal phalanx ensuring that both joints of the finger extend together
Extensor indicis	On the back of the forearm from the ulna to the extensor digitorum tendon on the index finger	Allows more independant extension of the index finger
Extensor digiti minimi	At the back of the forearm from the lateral epicondyle to the tendon of the extensor digitorum	Allows more independant extension of the little finger
Extensor pollicis longus	On the radial side of the back of the forearm connecting on to the distal phalanx of the thumb	Extends the thumb
Extensor pollicis brevis	On the radial side of the back of the forearm to the proximal phalanx of the thumb	Extends the thumb
Abductor pollicis longus	On the radial side of the back of the forearm to the base of the first metacarpal	Extends and abducts the thumb.

Intrinsic muscles of the hand

The interossei lie on either side of the hand between the metacarpals and the lumbricals are on the palmar side between the deep flexor tendons of the fingers. Their primary function is in adduction/abduction of the fingers and to contribute to fine motor control.

• Adduction and abduction of the fingers. Positioned between the metacarpals they attach onto the proximal phalanges enabling them to spread the fingers.
• Assisting in flexion and extension of the fingers. Because they lie in between the metacarpals the position of the fingers, as determined by the extrinsic muscles will sometimes put them closer to the front (making them flexors) and sometimes closer to the back of the hand (making them extensors).
• Acting as synergists to control the finer positioning of the fingers. For example your fretting hand is sometimes extended at the first knuckle (metacarpophalangeal joint) while the fingers themselves are flexed. In this position the larger extrinsic flexors and extensors compete with each other, passive tension in the interossei helps you do this with a greater degree of control.

The other intrinsic hand muscles control the thumb and the little finger, allowing the hand to close in and grip objects. Look at your hand and you'll see two fleshy pads at the base of the palm. On the thumb side (the thenar eminence) are the three thumb muscles, there are three more on the ulnar side controlling the little finger and one in the middle.

Muscle	Location	Action
Abductor pollicis brevis	From the carpals to the first phalange of the thumb (on the thenar eminence)	Abduct and assist in opposition of the thumb
Opponens pollicis	From the lateral wrist to the first metacarpal (on the thenar eminence)	Opposition of the thumb
Adductor pollicis	From the centre of the hand to the thumb (also on the thenar eminence)	Adduction of the thumb
Palmaris brevis	In the palm of the hand	Assist in closing the palm of the hand
Abductor digiti minimi	On the ulnar side of the hand from the medial carpals to the first phalange of the little finger	Abduction of the little finger
Flexor digiti minimi brevis	On the ulnar side of the hand attaching on the medial carpals just inside the abductor attachment to the first phalange of the little finger	Flexion of the little finger
Opponens digiti minimi	Attaching just inside the flexor to insert onto the outside of the fifth metacarpal	Opposition of the little finger (roll it in to the middle)