Definition for Achilles reflex
From Biology Forums Dictionary
Also known as the ankle jerk reflex, the Achilles reflex is a response to a strike to the back of the ankle. It can be demonstrated by hitting the Achilles tendon when the foot is flexed upward at the ankle joint so that the top of the foot is brought toward the shin. This reaction is seen as a sudden downward jerk of the foot from the dorsiflexed position as caused by a contraction of the calf muscles. The reflex may be tested in a medical setting to determine whether the sciatic nerve, which crosses the back of the ankle just deep to the Achilles or calcaneal tendon, is functioning properly.
All reflexes are simply a reaction to a physical stimulus by the nervous system. It is a type of reflex known as a stretch reflex, in which a muscle is placed into a stretched position before the stimulus causes it to contract, thereby returning to a shortened position. A stretch reflex can be observed by tapping the anatomical structure in question, then noting the reaction time. A reflex should be both involuntary, meaning that it happens without conscious thought, and immediate. Therefore, if no response occurs, or if that response is delayed, it may indicate a nervous system problem.