The tendon reflex, also known as the T-reflex, is found in humans and occurs when pressure on a tendon causes it to relax and snap back immediately afterward. Like all reflexes, this is an involuntary reaction to a stimuli, which in this case is pressure placed on the tendon. Tendons are thick lengths of tissue that connect muscles to bones in the body and in response to tightening or lengthening of the muscle the body compensates for this by performing the reverse action. When a doctor strikes the patient's knee with a reflex hammer, it forces the tendons to relax and in response the body tells the tendons to contract, which causes the involuntary kick from the patient.
When pressure is applied to the tendon, it causes it to constrict and shorten. This tension triggers a response from the nervous system which alerts the body that the tendon is too tight. The nervous system then sends out signals to relax the muscle. It blocks some of the signals telling the muscle to constrict and thus the tendon relaxes.
This type of reflex is known as a myotatic reflex, meaning it deals with the natural reflex of the body's muscles to tighten in response to stretching. When the body senses the tendons are stretching and elongating, it tightens them to keep from stretching too far. To cause stronger contractions, the tendon reflex must overcompensate or the muscle must stretch quickly. In cases where the muscle stretches too far, such as lifting a heavy object, the body first sends a relaxation response to instruct the muscles to drop the heavy weight and then contracts to pull the muscles back into place.
A strong tendon reflex can cause the muscles to snap back so hard it tears the tendons. This may occur during exercise or heavy lifting when the person lifts much more than he can handle. Sports also present opportunities for injury when a fall twists part of the body, such as the ankle, and the muscle snaps back too hard.
Doctors examine the tendon reflex to evaluate how the body's nervous system is functioning. A person whose nervous system isn't sending the proper signals may have a low tendon reflex. This person may kick very little when the doctor hits the knee with a reflex hammer. Reflexes are designed to protect the body from harm, and lower reaction times not only raise the risk of injury, but also may indicate a serious underlying problem with the nervous system.