Flexor digitorum profundus

What it is

A large deep muscle covering the front of the ulna, sending four tendons through the carpal tunnel and along the fingers, passing through the split tendons of flexor digitorum superficialis to reach the last bone of each finger.

It is the only muscle able to flex the distal interphalangeal joint — the fingertip joint. Nothing else can do it.

Its muscle belly is only partly divided. The portion serving the index finger is usually separate; the portions serving the middle, ring and little fingers share a common belly.

What it does

Flexes the fingertips, and with them the whole finger, and assists wrist flexion.

That shared muscle belly has a consequence you can demonstrate on yourself. Put your hand flat on a table, fold your middle finger under so its middle knuckle rests on the surface, and try to lift each other finger in turn. The ring finger will barely move. It is not weakness — the tendon is tethered by the shared belly and by connections between the tendons, and this is why the ring finger is the least independent digit.

What loads it

Every grip. Hanging, deadlifting, carrying, climbing. Crimping holds in climbing load it particularly hard, which is why finger injuries are the characteristic injury of that sport.

What is commonly got wrong about it

That finger independence is trainable without limit. It improves with practice, and there is a structural ceiling set by the shared muscle belly and the tendon interconnections. Musicians run into this and no amount of practice fully removes it.

"Grip strength" as one quality. Crushing, pinching and holding are measurably different, they respond differently to training, and a strong handshake does not predict a strong hang.

That tendon injuries in the fingers heal like muscle strains. They do not, and they are among the more consequential injuries in climbing. This site does not cover injury.