When the doctor hits your kneecap or patella your knee kicks out, this is called the knee jerk reflex. If you want to try the knee jerking reflex your will need a partner. Have your partner sit on a chair so that their feet do not touch the ground, your partner also needs to have their feet and legs relaxed. Hit just below your partners knee with the side of your hand, when you perform this, your partners leg will kick out immediately. If you want to try this have your partner do the same thing as you did. The knee jerking reflex takes less than 50 milliseconds for the knee to take the pressure then send a signal to the brain and the brain to send different signal back to the knee.