Patellar Reflex Test

The patellar reflex is contraction of the quadriceps femoris muscle, with resulting extension of the knee, in response to percussion of the patellar tendon. A firm tap on the tendon draws the patella down, stretching the quadriceps and provoking reflex contraction. The patellar reflex is mediated by the femoral nerve (L2-L4).

If the reflex is brisk, the contraction is strong, and the amplitude of the movement is large. If the examiner places one hand over the muscle, and with the other hand taps the patellar tendon just below the patella, the examiner can palpate the contraction as well as observe the rapidity and range of response. When the reflex is hypoactive, it may be better felt than seen. Palpation helps in judging the latency between the time of the stimulus and the resulting response.

See Also: Deep Tendon Reflex Testing

How to perform the Patellar Reflex?

The knee jerk can be elicited in various ways. The patient may sit in a chair with the knees slightly extended and the heels resting on the floor or sit on an examination table with the legs dangling. If the patient is lying in bed, the examiner should partially flex the knee by placing one hand beneath it and then tap the tendon. The responses on the two sides can be compared by lifting both knees simultaneously, supporting them on one forearm as the patient’s heels rest lightly on the bed, before tapping the tendons.

If the patient is wearing loose pajamas, the examiner can suspend both legs by holding the pajamas, as she uses the other hand to strike the tendon. Another technique is having the patient sit with one leg crossed over the other and tapping the patellar tendon of the uppermost leg, but this method does not facilitate side-to-side comparison.

If there is reflex spread, extension of the knee may be accompanied by adduction of the hip, which on occasion is bilateral, or there may be bilateral knee extension. If the reflex is exaggerated, the response may be obtained by tapping the tendon not only in the usual spot but also just above the patella (suprapatellar reflex); the tendon can be tapped directly, or, with the patient recumbent, the examiner can place her index finger on the upper border of the patella and tap the finger to push down the patella.

See Also: Quadriceps Femoris Muscle Anatomy

Contraction of the quadriceps causes a brisk upward movement of the tendon, together with extension of the leg. Marked exaggeration of the patellar reflex may be accompanied by patellar clonus.

An inverted patellar reflex may be seen with lesions of the nerve or nerve roots supplying the quadriceps: Tapping the patellar tendon results in contraction of the hamstrings and flexion of the knee.

References & More

1. Zimmerman B, Hubbard JB. Deep Tendon Reflexes. [Updated 2023 Jul 24]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: Pubmed
2. DeJong’s The Neurologic Examination – 8th Edition