Dr. Ebraheim’s educational animated video describes the anatomy of the lumbrical Plus Finger in the hand, with simple images; this video also provides you with all you need to know.

What is lumbrical plus finger? Lumbrical plus finger is a paradoxical extension of the IP joint when attempting finger flexion. It is usually caused by disruption of the flexor digitorum profundus tendon distal to the lumbrical origin.
Anatomy: the lumbrical muscles in the palm of the hand arise from the tendons of the flexor digitorium profundus and are inserted into the dorsal extensor expansions on the backs of the proximal phalanges of the fingers. The flexor digitorum profundus tendon to the index finger is most commonly separate from the conjoined tendons to the long, ring, and little fingers. The flexor digitorum profundus for the long, ring and the 5t finger share a common muscle belly. The patient cannot independently flex two digits without flexing the third digit.
Function of the four lumbricals (with help from interosseous muscles) is to flex the metacarpophalangeal joints and extend the interphalangeal joints. Contraction of the lubmricals relaxes the flexor pull on the DIP. In a normal finger, the lumbrical muscle relaxes to allow flexion of the interphalangeal joint. The lumbricals coordinate flexor and extensor systems. The middle finger is most commonly involved in paradoxiacal extension of the IP joint. Extension of the IP joint paradoxically occurs through the lateral bands.
Why does paradoxical extension of the finger occur? The transected flexor digirtorum profundus tendon retracts with active flexion. When the tendon is cut or lax, the lubmrical is drawn proximally by the retracted FDP tendon. The lubmrical will pull on the lateral bands, creating PIP extension.
Causes:
•FDP disrupted tendon distal to the origin of the lumbrical muscle.
•Distal amputation of the finger
•Flexor tendon graft lax and long
•Avulsion of the FDP tendon.
Test for lumbrical plus: when the patient is relaxed, the affected finger can be flexed passively. With the patient gripping all of the fingers, the affected finger extends actively at the PIP joints. This is known as paradoxical extension.
Treatment: lumbrical release from the flexor digitorum profundus tendon.  

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Background music provided as a free download from YouTube Audio Library.
Song Title: Every Step