Lumbrical muscles of the hand
Anatomy and supply
The lumbrical muscles (lumbricidae - Latin = earthworm) are four short hand muscles located in the metacarpus deep to the palmar fascia. One feature of these muscles is that they originate from tendons instead of bony structures, making their origin surfaces quite moveable. Usually they arise from the radial side of the tendons of the flexor digitorum profundus. In addition, the third and fourth lumbrical muscles have a second head attached to the ulnar side of the adjacent tendon. Distally, their insertion tendons attach to the dorsal aponeurosis of the respective finger. The first two lumbricals are supplied by the median nerve (C8-Th1), whereas the ulnar nerve (C8-Th1) is responsible for the innervation of the third and fourth.
The lumbricals fulfill movements of the second to fifth finger. Their contraction leads to flexion in the metacarpophalangeal joints (MCP) and extension in both the proximal (PIP) and distal interphalangeal joints (DIP). The reason for the opposite actions is that the tendons cross the MCP on the palmar side, but distally insert at the dorsal side of the finger. These combined movements support a strong hand grip (e.g. holding a pen).
When the tendons of the flexor digitorum profundus detach distal from the origin surfaces of the lumbricals an interesting phenomenon occurs: when trying to close the fist, the fingers strangely extend instead. The reason: after detachment of the distal tendons, the lumbricals now serve as the new insertion surface of the flexor digitorum profundus. This means, even though the person consciously activates the flexor muscle, he/she actually moves the lumbricals instead. And since the flexor digitorum profundus and the lumbricals are antagonists in the PIP and DIP the intended fist closure paradoxically leads to an extension of the fingers. This oddity is clinically referred to as the lumbrical-plus finger and can occur after injuries or amputations.