The Achilles tendon is the large band of tough tissue that attaches the calf muscles to the heel bone. It is found at the back of the ankle. The tendon is named after the Greek hero Achilles whose heel was supposed to be the only vulnerable part of his body. The Achilles tendon reflex, also known as Achilles jerk, or ankle jerk, is a contraction of the calf muscles in response to a sharp tap on the Achilles tendon. This response is used as one of the standard tests in neurological examinations and is a valuable diagnostic tool.
For example, the ankle reflex is absent in spinal shock resulting from spinal injury. It reappears as the shock wears off. Decreased response is often associated with many diseases, such as intervertebral disc disease, poliomyelitis, and peripheral neuritis, that affect the nerves that control the calf or gastrecnemius muscle. It can also be decreased in severe disease of the calf muscle itself, such as occurs in muscular dystrophy.
The Achilles tendon reflex time, or the time before relaxation occurs after contraction has been induced, is also used in detecting disorders of the thyroid. As a rule, most patients with an underactive thyroid gland show considerable delay in the contraction and relaxation phase, while the duration of the reflex is shortened in patients with an overactive thyroid.
A special apparatus is used to measure accurately the duration of the reflex, but many doctors question the value of this test in determining thyroid function, since several other conditions, such as pregnancy, anemia, and lowered body temperature, will also affect the duration of the tendon reflex. Contraction and lengthening of the Achilles tendon will result in deformities of the foot. In patients paralyzed after a stroke, downward dropping or flexure of the foot associated with paralysis of the leg results in shortening or contraction of the Achilles tendon. It can be partially or wholly prevented by early use of a bed cradle and footrest support to keep the patient’s ankle at a 90 degree angle.