automatic, occurring without thought; of the reflex movement that occurs when struck below the kneecap
A jerk or kick produced by a blow or sudden strain upon the patellar tendon of the knee, which causes a sudden contraction of the quadriceps muscle
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
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