Knee-jerk reflex

How it works

The patellar tendon connects the big muscle on the front of the thigh, the quadriceps, to the shin bone. It starts at the lower tip of 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 second¹. The brain is not involved. You feel the kick after it has already happened.

This is a stretch reflex. It helps you keep your posture and balance with little effort or thought¹.

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.

How common it is

Everyone with an intact reflex loop has this reflex, so there is no rarity to measure. What varies is how strong the kick looks, which can change if the person holds it back or exaggerates it on purpose¹. Doctors grade the strength on a scale from absent to very brisk, and a weak or hard-to-find kick in an otherwise healthy, relaxed person is a common, harmless finding.

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. The tension in your arm muscles 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.

Is it a problem?

No. Testing your own reflex, or having a doctor test it, is not harmful, and a single weak result in a tense muscle means little on its own¹.

Mention the reflex to a doctor if it:

• is completely absent on both sides,
• is much stronger on one side than the other,
• comes with numbness or weakness.

Those patterns can point to a problem in the nerve, the spinal cord, or the brain¹.

History and culture

The reflex was described at the same time in 1875 by two German doctors, Wilhelm Erb and Carl Westphal¹. The phrase “knee-jerk reaction” entered everyday language to mean a response made without thought.