Achilles Reflex Test

The Achilles Reflex or ankle jerk is obtained by striking the Achilles tendon just above its insertion on the calcaneus. The Achilles reflex is mediated by the tibial nerve (S1), when the pathology of the S1 nerve root is in question, the Achilles tendon reflex test is the initial assessment of choice.

See Also: S1 Nerve Root

How to perform the Achilles Reflex?

The ankle jerk is by far the most difficult reflex to master. There are two critical variables: proper stretch and efficient striking. Of the two, proper stretch is the more difficult to learn. Too little dorsiflexion leaves the tendon slack and able to absorb the blow without stretching the muscle. Too much passive dorsiflexion makes the tendon too taut and unable to be stretched.

If the patient is seated or lying in bed, the thigh should be held in moderate abduction and external rotation and the knee flexed. If the patient is supine, access to the tendon requires placing the legs into a frog-leg position with the knees apart and the ankles close together. Some prefer to have the patient cross the leg to be examined atop the other shin or ankle (“figure-four position,” as the legs form a 4). The examiner should place one hand under the foot and pull upward slightly to passively dorsiflex the ankle to about a right angle.

See Also: Thompson Test

If the Achilles reflex is difficult to obtain, the patient may be asked to press the foot lightly against the examiner’s hand in order to tense the muscle and reinforce the reflex. Using a driving analogy and asking the patient to imagine pressing on an accelerator enough to go “17 mph” communicates the need for a low-level but precisely graded contraction, which is then easy to adjust up or down to the proper level.

The Achilles reflex may also be elicited by having the patient kneel on a chair or similar surface, with the feet projecting at right angles; the Achilles tendons are percussed while the patient is in this position. This method, introduced by Babinski, is particularly useful for comparing reflex activity on the two sides. Another method for supine examination is to strike the ball (sole) of the foot or strike the examiner’s hand placed flat against the sole. This plantar stretch reflex is considered equivalent to the ankle jerk for clinical purposes.

See Also: Deep Tendon Reflex Testing

Results of the Reflex

The resulting contraction of the posterior crural muscles, the gastrocnemius, soleus, and plantaris, causes plantar flexion of the foot at the ankle.

If the ankle jerk is hyperactive, it may be elicited by tapping other areas of the sole of the foot, the medioplantar reflex, or by tapping the anterior aspect of the ankle, the paradoxical ankle reflex. A hyperactive reflex may also result in extra beats or even clonus when the tendon is percussed. When there is reflex spread, striking the Achilles tendon may cause flexion of the knee.

See Also: Gastrocnemius Muscle

Although the Achilles reflex, when carefully elicited, should be present in normal individuals, it tends to diminish with age, and its bilateral absence in elderly individuals is not necessarily of clinical significance. Although it is common wisdom that the normal elderly may have absent ankle reflexes, of 200 consecutive patients admitted to a geriatric unit, 188 had ankle jerks using plantar rather than Achilles tendon strike; only 1.5% had absent ankle jerks attributable only to age.

Another study comparing plantar strike and tendon strike in elderly patients found better intraobserver and interobserver agreement with the plantar strike. Differences in technique may explain some of the discrepancy between studies examining the prevalence of absent ankle jerks in elderly people.

See Also: Silfverskiold Test Interpretation

The patellar reflex and Achilles reflex are the most important Deep Tendon Reflexes in the lower extremities. The response may be minimal or even absent in normal individuals. Because the responses may be difficult to elicit even in normal subjects, side-to-side comparison is critical. To be significant, absence must be unilateral. Exaggeration of these difficult-to-obtain reflexes suggests the presence of corticospinal tract disease.

References & More

1. DeJong’s The Neurologic Examination – 8th Edition
2. Figliuzzi A, Alvarez R, Al-Dhahir MA. Achilles Reflex. [Updated 2023 Jun 5]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: Pubmed