Muscles flexing the fingers

Flexor digitorum superficialis

Flexor digitorum profundus

Lumbricals

Flexor digiti minimi brevis

Lumbricals

The lumbricals are four small, round muscles found in the palm in association with the deep flexor tendons of the fingers. The lateral two lumbricals are frequently unipennate arising from the lateral side of the flexor digitorum profundus tendons, passing to the index and middle fingers. The medial two muscles are bipennate, arising from the adjacent sides of the profundus tendons of the middle and ring fingers, and the ring and little fingers.

From this proximal attachment the muscles pass distally, anterior to the deep transverse metacarpal ligament and then obliquely on the lateral side of the metacarpophalangeal joint of the finger concerned. They then attach to the lateral edge of the dorsal digital expansion at the side of the proximal phalanx. A few fibres make their way to the bone of the middle phalanx, but the majority can be traced to the base of the distal phalanx via the extensor expansion.

Nerve supply

The nerve supply varies, but the most common arrangement is for the lateral two lumbricals to be supplied by the median nerve, root value T1, and the medial two by the ulnar nerve, root value T1.

Action

The lumbricals in both hands and feet are unique muscles as they pass between the flexor and extensor tendons. Their anatomical position means that they flex the metacarpophalangeal joint, and extend both interphalangeal joints of the corresponding finger, the “lumbrical action”. In theory the lumbricals should be able to rotate their respective fingers at the metacarpophalangeal joint. However, this appears to be well marked only in the index finger so that the pad faces medially. The first dorsal interosseus muscle is also involved in producing this movement. Functionally, the lumbricals are involved in the coordination of complex activities of the fingers which involve movements of both flexion and extension, for example as in writing. As a group they have major functional significance in the dexterity of the hand, which is further enhanced by their rich sensory innervation. 

Flexor digiti minimi brevis

Flexor digiti minimi brevis is a muscle which is sometimes absent. When present it lies on the lateral side of abductor digiti minimi, arising from the hook of the hammate and the adjacent flexor retinaculum. It inserts, together with abductor digiti minimi, into the base of the proximal phalanx of the little finger on its ulnar side.

Nerve supply

Flexor digiti minimi brevis is supplied by the deep branch of the ulnar nerve, root value T1.

Action

Flexor digiti minimi brevis flexes the metacarpophalangeal joints of the little finger.