The Achilles reflex, also called the ankle jerk reflex, is an abrupt bending of the foot when a doctor strikes a person's Achilles tendon, which is located just above the heel. In a positive response, the foot moves as if the person is pointing his toes.
A reflex is a movement in a muscle when a tendon is tapped, usually with a small hammer. If a reflex is decreased or absent, a nerve supplying the muscle might be impaired or compressed. The Achilles tendon connects the muscle at the back of the calf to the heel bone. When the Achilles reflex is tested, the Achilles tendon is tapped while the foot is relaxed at a right angle to the rest of the leg.
The reflex is graded on a scale of zero to four, with zero meaning that the reflex is absent. A score of zero or one for the Achilles reflex often indicates a compression in the S1 or S2 region of the spine, which are at the base of the spine around the tailbone. This type of compression is associated with sciatica, back pain that travels through the buttock and down the sciatic nerve of one leg.
Other common causes of a reduced or absent Achilles reflex include a herniated disk, hypothyroidism and hypothermia. A qualified health professional must test the Achilles reflex to make a diagnosis.