A sudden reflexive movement of the leg below the knee, as a reaction to a tap to the tendon just below the patella (kneecap)
disapproval If you call someone's response to a question or situation a knee-jerk reaction, you mean that they react in a very predictable way, without thinking. The knee-jerk reaction to this is to call for proper security in all hospitals. a knee-jerk reaction, answer etc is what you feel or say about a situation from habit, without thinking about it = automatic
A reflex contraction of the quadriceps muscle resulting in a sudden involuntary extension of the leg, produced by a sharp tap to the tendon below the patella; patellar reflex