Lumbricals are small muscles in the hands and feet. They are intrinsic, meaning that they are a part of the appendage on which that they perform work. They were named after the scientific family Lumbricidae, which includes earthworms, because of the long, worm-shaped appearance of the muscles. They are responsible for flexion of certain joints and extension of other joints in the fingers and toes. Unlike most other muscles, the lumbricals aren't anchored on bone but instead are attached to tendons that extend from other muscles.
The lumbricals in the hands are attached to the extension of the tendon of the flexor digitorum profundus muscle in the forearm. They cause flexion, or bending, of the metacarpophalangeal joints, the larger joints at the base of the four fingers, because they lie underneath the joint. In the interphalangeal joints, they pass over the top of the joint and cause extension or straightening. The interphalangeal joints are the ones at the midpoint of each digit and the ones just before the fingertip.
In the foot, the lumbrical muscles attach to tendons that run from the last joint in each of the four smaller toes up to a larger tendon, associated with the flexor digitorum longus muscle in the lower calf. As with those in the foot, these lumbricals are active in flexing the larger joints at the base of the digits, the metatarsal phalangeal joints of the toes. They also create extension in the interphalangeal joints of the toes.
The lumbricals of the hand receive signals from the brain through both the median and ulnar nerves. The median nerve supplies sensation and movement messages to the lumbricals of the pointer and middle fingers, and the unlnar nerve connects the lumbricals of the ring and pinky fingers to the central nervous system. In the foot, nerve signals are carried by the medial and lateral plantar nerves. The medial plantar nerve supplies only the most medial lumbrical, the one that controls the first small toe next to the big toe, and the other three toes are supplied by the lateral plantar nerve.