The Achilles reflex test is also called the ankle reflex test. It is a deep tendon reflex that can be elicited easily with the use of a standard reflex/neurological hammer. The Achilles reflex test is a simple and effective part of a complete lower extremity examination. It is also part of the nervous system portion of a complete history and physical examination. The Achilles tendon can be palpated easily on most patients and is easy to find just superior to the posterior aspect of the calcaneus. There are various ways to conduct the Achilles reflex test, but the basic method is what examiners use most. The Achilles reflex test is to primarily assess the S1 nerve root, which innervates the area. All deep tendon reflexes are graded on a scale of 0 to 4+ with 0 being no response, 1+ being somewhat diminished/low normal, 2+ being average/normal, 3+ being brisker than average/possibly indicative of disease, and 4+ being very brisk, hyperactive, with clonus.
The Achilles tendon attaches the soleus muscle and gastrocnemius muscle to the posterior aspect of the calcaneus on the calcaneal tuberosity. It is a tendon that does not have a tendon sheath but has a paratenon. A paratenon is a thin fascial covering that decreases friction and provides some blood supply. The tendon exhibits a watershed area that is about 1.5 inches superior to its insertion and receives minimal blood supply. Much of the Achilles tendon’s blood supply comes from muscle bellies above and bone below. The Achilles tendon is also referred to as the Tendo Calcaneus. The Achilles tendon twists internally from proximal to distal leaving the Gastrocnemius fibers to insert more lateral and the Soleus fibers to insert more medial onto the calcaneus. There are two bursae associated with the Achilles tendon, the deep retrocalcaneal bursa, and the superficial retrocalcaneal bursa. 
The Achilles reflex test is completed with any complete history and physical or lower extremity examination. It can also be used to aid in diagnosing certain conditions such as hypothyroidism, peripheral neuropathy, or Charcot Marie Tooth disease among others. When damage or dysfunction of the deep tendon reflex of the S1 nerve root is in question, the Achilles reflex test is the main indicator of its function. 
Some contraindications for performing an Achilles reflex test are the presence of an open wound in the area, severe pain from a medical condition such as arthritis, history of acute trauma or patients who are unable to relax their muscles because of a medical condition physically.
There is one standard piece of equipment needed to perform the Achilles reflex test, and it is called a reflex or neurological hammer. The top of the standard reflex hammer is shaped like a triangle, and the flat part of the triangle is used to elicit the reflex on the Achilles tendon. A practitioner holds a standard reflex hammer by the handle in a firm manner, typically with their dominant thumb and forefinger holding most of the weight with the flat part of the top toward the patient’s Achilles tendon area. Other, larger, more advanced reflex hammers, like a pendulum hammer, can be used to assess the Achilles reflex as well.
There is minimal preparation needed to complete the Achilles reflex test. The patient should be relaxed, laying on an examination table and their Achilles tendon area well exposed from superior calf to plantar aspect of the foot. The practitioner should have washed hands, reflex hammer in hand and should be in an adequate position to perform the exam.
There are various techniques for performing the Achilles reflex test, but the standard technique is most often performed. With the standard technique, the examiner strikes the Achilles tendon with the patient in a supine position with the knee flexed and hip externally rotated on the examination table. One hand is slightly dorsiflexing the foot from the plantar aspect, as the other hand holds the reflex hammer. The Achilles tendon is struck directly with the reflex hammer, and a positive result is when the calf muscle contracts and the foot plantarflexes. The degree of the positive result is graded on a scale as explained previously. The other techniques for performing the Achilles reflex are as follows: the plantar strike, with leg dangling, and with leg elevated. These techniques are mostly done by neurologists and are not commonly performed by other practitioners. For elderly patients, studies show that the examiner should start with the plantar strike method, and if negative, move on to the Achilles strike with a leg elevated method to ensure most accurate results. The plantar strike method is performed by striking the examiner’s hand on the plantar forefoot of the patient with the patient in the supine position with legs extended. The examiner should place slight tension on the calf muscles when pulling up on the foot. The Achilles strike with leg elevated is the same technique, but with the leg elevated, knee flexed and placed between the examiner’s torso and arm. The Achilles strike with the leg dangling exhibits the same technique of slightly dorsiflexing the foot while striking the Achilles tendon, but with the patient sitting and dangling their leg, at the knee, off the table. Positive and effective results have also been found when using a pendulum hammer on a patient while standing to assess the Achilles reflex.
There are minute and rare complications to performing the Achilles reflex test. There could be post-test bruising, pain or soreness, but that mostly occurs with poor technique or an underlying condition.
The Achilles reflex test is important when assessing for various diseases and neurological deficits. An absence of the Achilles reflex has been shown to be normal for aging patients but also exhibits controversial results in various studies. An absent or decreased Achilles reflex could also reveal certain conditions such as a first sacral radiculopathy, hypothyroidism, peripheral neuropathy secondary to diabetes mellitus, or Charcot Marie Tooth disease just to name a few. So, with a negative result, further testing is warranted with an absent or decreased Achilles reflex to rule out a more serious condition.
Anyone who has been properly trained to complete the Achilles reflex test can complete the test. Medical students, nurses, doctors, physician assistants, and medical assistants, just to name a few, are all capable of performing the test with accuracy. An accurate technique is important to prevent a false negative result. Practice and experience decrease the likelihood of obtaining a false negative when testing the Achilles reflex.