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Ankles jerk — yes, but how?

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Eliciting an ankle jerk is not a trivial task. It requires the skills of an experienced bell-ringer, an expert golfer and a mechanical engineer. The experience of the examiner and the positioning of the patient are two factors contributing to the accuracy of reported findings.1,2 Accurate findings can provide invaluable clinical information.

When an ankle jerk is elicited, the response can be seen, felt and heard. When the response is present, the tendon produces an undamped oscillation heard as a “boing”, and when it is absent, the damped oscillation is heard as a dull “thud”.

Percussion on the Achilles tendon activates muscle spindles — specialised sensory receptors within the muscle that respond to stretch (lengthening) and vibration. Action potentials move centripetally via large diameter, myelinated, fast conducting fibres, and trigger responses in the spinal cord from α motor neurones that mediate muscle contraction. The magnitude and rapidity of the reflex response is modulated by both autonomic sympathetic tone and descending (upper motor neurone) inhibition.3

Reflex activation is faster and stronger when sympathetic nervous system tone is high. Patients who are anxious, thyrotoxic or withdrawing from alcohol will have stronger, brisker…