---
title: "Knee-jerk (patellar) reflex: 22 ms"
description: "Knee-jerk (patellar) reflex: typically 22 ms, really 18 ms to 24 ms. What is actually being measured, and why it counts as a reflex rather than one of the other two."
url: https://atoneplace.net/reaction-time/knee-jerk-reflex/
site: "At One Place"
---

# Knee-jerk (patellar) reflex

**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.

## Related pages

- [Vestibulo-ocular reflex](https://atoneplace.net/reaction-time/vestibulo-ocular-reflex/)
- [Blink reflex](https://atoneplace.net/reaction-time/blink-reflex/)
- [Withdrawal reflex](https://atoneplace.net/reaction-time/withdrawal-reflex/)
- [A reflex is not a reaction](https://atoneplace.net/reaction-time/reflex-vs-reaction/)
- [Stopping distance at every speed](https://atoneplace.net/reaction-time/stopping-distance/)
- [Measure your own reaction time](https://atoneplace.net/tools/reaction-time-test/)

## Sources

- Typical range compiled from manufacturer specifications — At One Place (/about/)