The lumbricals are large, deep muscles of the upper arm that extends the glenohumeral joints to the scapula and flex the metatarsal joints at the wrist. It has two, small, ring shaped muscles in each of the forearms, located on each side of the ulnar loop. These muscles are important for maintaining proper posture and good movement. When arthritis causes the formation of bone spurs (calcaneus) on the ulnar loop of the wrist, the lumbricals become tight and pinch the nerve and cause severe pain.
Anatomy and physiology of the lumbricals reveal a variety of complicated details of the lumbricals. The lumbricals are composed of two types of muscles, both of which contribute to joint motion. One of them is the superficial lumbricals, lying adjacent to the tendon. The second type of muscle is the deep flexor lumbricals, extending down the medial epicondyle of the ulnar loop and joining the medial epicondyle of the tibia. This muscle originates near the tendon and inserts on either side of the joint.
The location of the lumbricals along the radial side of the ulnar loop provides information concerning functionally important sites of the lumbricals. At the top of the humerus is the glenohumeral crest, which extends down the lateral epicondyle of the tibia. It meets the acromion along the medial border of the radius on the anterior side of the arm. The nerves exit the acromion and pass through the bulbous tissue of the lumbrical muscles to the nerve bundle that extends distally along the radial side of the ulnar loop.
The location of the lumbricals and their orientation with respect to the angle of the elbow bend make important identification clues. Distally along the radial side of the ulnar loop and inside the elbow joint (the epicondyle) are two prominent nerve roots that are important in identifying the function of the lumbricals. On the medial aspect of the elbow there are two large muscles, the levator scapulae and the levator minor. The nerve roots that exit these muscles and extend into the radius of the elbow are called the nerve root system.
The nerve supply from the lumbricals to the ulnar groove and the nerves exiting this area can be visualized by looking at where the nerve exits the elbow joint. At the level of the elbow cap is a small fingerlike nerve called the iliopsoas muscle. It passes posteriorly along the radial side of the ulnar loop through the glenohumeral crest to the epicondyle, which is situated at about midway between the elbow cap and the ulnar loop. The nerve supplies the extended portions of the thumb and fingers as well as the palm.
The largest and most complicated nerve of the limb is the median nerve, which extends from the inferior cervical spine to the scalp. This broad nerve is subdivided into five groups: median nerve roots, cranial nerve, base of the thumb, ulnar nerve, and metacarpalangeal joint flexion. Each group has its own specific nerve supplying organs and tissues. The median nerve passes through the spinal cord and branches off into various areas including face, neck, hands and feet.
Specific nerves may be innervated in the lumbricals or not. For example, the median nerve may not innervate the phalanges because the lumbricals itself are underdeveloped and not close to the phalanges. If the lumbricals fail to innervate the phalanges, then the nerves will compensate by over stimulating the Phalanges to a point where they contract and lose their supply of blood. This results in a loss of sensation and makes the person unable to function properly. In this type of situation it is very important to correct the problem before it gets out of hand.
The most common condition that affects the lumbricals is called carpal tunnel syndrome. This syndrome is caused when the median nerve becomes pinched between the carpal bones. This is caused by either poor mechanics of the hand (such as bending the wrist too much) or from damage to the bone, ligaments or the cartilage. This condition results in a loss of feeling along with numbness and weakness in the thumb and fingers.