Bodily Oddities

Knee-jerk reflex

Also called: Patellar reflex · Patellar tendon reflex

A tap on the tendon below the kneecap makes the lower leg kick. The signal goes to the spinal cord and straight back through one connection. It is the simplest reflex in the body.

Nerves Muscle Reflex Reflex
Try it

How common

Everyone Universal · ~100% The strength varies a lot between healthy people.

Does this happen to you?

These are answers from visitors, not a study.

How it works

The patellar tendon connects the big muscle on the front of the thigh, the quadriceps, to the shin bone. It passes over the kneecap. A sharp tap on the tendon gives the quadriceps a tiny, sudden stretch1.

Inside the muscle are sensors called muscle spindles. They fire when the muscle is stretched3. Their signal runs up a sensory nerve to the spinal cord, at the level of the second to fourth lumbar segments1. There it connects directly to the motor neuron that controls the quadriceps. One connection, one synapse. The motor neuron fires, the quadriceps contracts, and the lower leg swings forward2.

At the same time a second branch of the sensory signal switches off the hamstrings at the back of the thigh, so that they do not fight the kick2. The whole loop takes a fraction of a second. The brain is not involved. You feel the kick after it has already happened.

This is a stretch reflex. It exists to keep you upright. If your knee starts to buckle, the quadriceps is stretched, and the same loop straightens the leg before you have time to notice2.

Doctors tap the knee to test the nerves and spinal cord. A missing reflex can mean a problem in the nerve loop. A very exaggerated reflex can mean a problem in the brain or spinal cord above the loop1.

Try it

  1. Sit on a table or a high stool so that your lower legs hang free and relaxed.
  2. Find the soft spot just below your kneecap. That is the tendon.
  3. Tap it firmly with the edge of your hand or the handle of a wooden spoon.
  4. The lower leg should kick forward on its own.
  5. If nothing happens, hook the fingers of both hands together and pull hard while a friend taps. This is the Jendrassik manoeuvre. It takes the brain’s attention away and makes the reflex stronger4.

A weak or absent kick in a relaxed, healthy person is common. Getting the spot and the timing right takes a few tries.

History and culture

The reflex was described in 1875 by two German doctors, Wilhelm Erb and Carl Westphal, working separately1. The phrase “knee-jerk reaction” entered everyday language to mean a response made without thought.

Sources and further material

  1. Encyclopedia Patellar reflex First described by Wilhelm Erb and Carl Westphal, 1875.
  2. Encyclopedia Stretch reflex
  3. Encyclopedia Muscle spindle The sensor inside the muscle that detects the stretch.
  4. Encyclopedia Jendrassik maneuver A trick that makes a weak reflex stronger.
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