How long is knee-jerk (patellar) reflex?

Typically 22 ms, with a real range of 18 ms to 24 ms. Tap the patellar tendon; time to the quadriceps contracting.

Knee-jerk (patellar) reflex

18 ms–24 ms

Reflex — does not reach the brain

The patellar reflex is monosynaptic — the stretch receptor connects to the motor neuron with exactly one synapse in the spinal cord, and no signal goes to the brain before the leg moves. Roughly 20 milliseconds is almost entirely nerve conduction time over the round trip from knee to spine and back. It is ten times faster than any reaction time and it is not a reaction: you find out your leg has moved after it has moved.

The numbers

Typical: 22 ms
Range: 18 ms to 24 ms
What it is: Reflex — does not reach the brain
What is measured: Tap the patellar tendon; time to the quadriceps contracting.

Why this is a reflex

A reflex arc turns round in the spinal cord or brainstem and never involves conscious processing. That is why reflex latencies are an order of magnitude shorter than reaction times, and why you cannot train a reaction down to a reflex figure.

Where it sits against everything else

Reflexes at the top, reactions in the middle, design values off the end of the scale.

Vestibulo-ocular reflex — Reflex 10 ms

Knee-jerk (patellar) reflex — Reflex 22 ms

Latency added by your screen and mouse — Physical limit 35 ms

Blink reflex — Reflex 40 ms

Withdrawal reflex — Reflex 90 ms

Time a jab takes to land — Physical limit 110 ms

Sprint start reaction — Reaction 140 ms

Simple tactile reaction time — Reaction 155 ms

Simple auditory reaction time — Reaction 170 ms

Professional gamer reaction time — Reaction 190 ms

Formula One start reaction — Reaction 200 ms

Simple visual reaction time — Reaction 250 ms

How reaction time changes with age — Reaction 250 ms

What sleep loss does to reaction time — Reaction 300 ms

What alcohol does to reaction time — Reaction 320 ms

Recognition reaction time — Reaction 340 ms

Choice reaction time — Reaction 380 ms

Time to hit a 95 mph fastball — Physical limit 400 ms

Time a goalkeeper has on a penalty — Physical limit 450 ms

Time to play a 90 mph delivery — Physical limit 500 ms

Driver reaction to an expected event — Reaction 850 ms

Reflexes

What	Typical	Range	Kind
Vestibulo-ocular reflex	10 ms	7 ms–15 ms	Reflex
Blink reflex	40 ms	30 ms–60 ms	Reflex
Withdrawal reflex	90 ms	70 ms–150 ms	Reflex

Common questions

How long is knee-jerk (patellar) reflex?: Typically 22 ms, with a real range of 18 ms to 24 ms. Tap the patellar tendon; time to the quadriceps contracting.
Is that a reflex or a reaction?: Reflex — does not reach the brain. A reflex arc turns round in the spinal cord or brainstem and never involves conscious processing. That is why reflex latencies are an order of magnitude shorter than reaction times, and why you cannot train a reaction down to a reflex figure.

Vestibulo-ocular reflex 10 ms
Blink reflex 40 ms
Withdrawal reflex 90 ms
A reflex is not a reaction Why one is 20 ms and the other 250
Stopping distance at every speed Reaction distance plus braking distance, computed
Measure your own reaction time Five attempts, median reported

Sources

Typical range compiled from manufacturer specifications — At One Place

How these figures are compiled and checked