The Jendrassik maneuver is used in conjunction with a patellar reflex, or "knee-jerk," test to distract the patient and facilitate the automatic muscle response when the tendon below the patella, or knee cap, is tapped. The distraction is accomplished by requesting the patient to clench their teeth and attempt to pull their interlocked fingers apart while the tendon below the patella is tapped, as noted in an article published on the National Institutes of Health (NIH) website.
The Jendrassik maneuver elicits a more pronounced response to the patellar reflex test than when it is not used. It has also been shown to be more effective than incorporating a mental distraction, according to a study published on the United States Library of Medicine National Institutes of Health website. It is believed that the Jendrassik maneuver inhibits the patient's ability to voluntarily affect the response to the tap made to the tendon even if they are fully aware of why the distraction is being incorporated into the test.
The maneuver is named after the 19th-century Hungarian physician, Erno Jendrassik, who first noticed its effect on the patellar reflex test.