Muscle · Forearm

Flexor digitorum profundus

Musculus flexor digitorum profundus

The deep finger flexor, lying directly on the ulna and the membrane between the forearm bones. It is the only muscle that can bend the fingertip, so it is the muscle of a hook grip — hanging from a bar, carrying a bag by the handles. It has a split nerve supply that is a useful piece of clinical logic: the ulnar nerve drives the half going to the ring and little fingers while the median nerve drives the half going to the index and middle, so which fingertips can still curl tells you which nerve is in trouble. Its tendons pass through the carpal tunnel.