Muscles and Tendons
There are two muscle groups that work in the hand, the intrinsic and the extrinsic. The intrinsic muscles include the thenar muscle group and the hypothenar muscle group in the thumb and the pinky, the interossei muscles (dorsal and palmar), which originate between the metacarpal bones; and the lumbrical muscles, which originate at the deep flexor tendon. Intrinsic muscles only act on the fingers.
The extrinsic muscles are the long flexors and extensors, which run from the forearm into the hand, providing strength to the hand and wrist. The flexors enable the fingers to bend while the extensor muscles are used to straighten the fingers.
The hand contains the extensor digitorum, extensor indicis, extensor digiti minimi, flexor digitorum superficialis, and the flexor digitorum profundus. There are also extrinsic muscles that work only in the thumb; these include the flexor pollicis longus, extensor pollicis longus, extensor pollicis brevis, and adbuctor pollicis longus.
In addition to the intrinsic and extrinsic muscles of the hand, the wrist also has its own set of muscles. They provide strength and motion to the wrist, and in some cases also the elbow. These muscles include the extensor carpi radialis longus, extensor carpi radialis brevis, extensor carpi ulnaris, flexor carpi radialis, flexor carpi ulnaris, and Palmaris longus.
A tendon is part of the muscle that attaches muscle (it) to bone. The flexor tendons and extensor tendons are strong, fibrous tissues that are responsible for transferring the forces generated by the muscle to the bone, thus producing movement at the joint. The flexor tendons play a role in helping the fingers and wrist to bend downward, while the extensor tendons help the fingers straighten and the wrist to bend upward. When a tendon becomes irritated or inflamed it becomes painful, especially with movement. Inflammation of the tendon is called tendinitis.