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Medical Definition of Knee jerk

Knee jerk: The reflex that is tested by tapping just below the bent knee on the patellar tendon. Normally this causes the quadriceps muscle to contract and bring the lower leg forward. It has given rise to the saying 'a knee-jerk reaction.' Also known as patellar reflex.

Source: MedTerms™ Medical Dictionary
Last Editorial Review: 5/13/2016

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