Knee jerk

automatic, occurring without thought; of the reflex movement that occurs when struck below the kneecap

The patellar reflex or knee-jerk is a stretch reflex.
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Knee jerk n. See patellar reflex. knee jerk Knee-jerk reaction, knee reflex, patellar reflex Neurology A reflex tested by tapping just below the [more]Knee jerk - Community and Resources

Noun
1. a reflex extension of the leg resulting from a sharp tap on the patellar tendon
(synonym) knee-jerk reflex, patellar reflex
(hypernym) reflex, instinctive reflex, innate reflex, inborn reflex, unconditioned reflex, physiological reaction

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