Flexor digitorum profundus is the most powerful and the bulkiest of the forearm muscles. It arises by fleshy fibers from the medial surface of the olecranon, from the upper three- fourths of the medial and anterior surfaces of the ulna as far distally as pronator quadratus, and from a narrow strip of interosseous membrane. The tendon for the index separates in the forearm; the three other tendons are still partly attached to each other as they pass across the carpal bones in the flexor tunnel and do not become detached from each other until they reach the palm. At this point of separation the four lumbricals take origin. They are described in the section on the hand.
Nerve supply of flexor digitorum profundus:
By the anterior interosseous branch of the median nerve and by the ulnar nerve (C7 and 8). Characteristically these nerves share equally; the bellies which merge into the tendons for index and middle fingers being supplied from the median, the ring and little fingers from the ulnar nerves. The corresponding lumbricals are similarly supplied.
This distribution of 2: 2 between median and ulnar nerves occurs in only 60% of individuals. In the remaining 40% the median and ulnar distribution is 3:1 or 1:3 equally (20% each). Whatever the variation, however, the rule is that each lumbrical is supplied by the same nerve which innervates the belly of its parent tendon.
Action of flexor digitorum profundus:
The muscle flexes the terminal interphalangeal joints and, still acting, rolls the fingers and wrist. It is the great gripping muscle. Extension of the wrist is indispensable to the full power of contraction of the muscle.