---
title: "A reflex is not a reaction — 20 ms against 250"
description: "A knee-jerk comes back in 20 milliseconds and never reaches the brain. A reaction takes 250 and is a decision. Why training cannot close the gap."
url: https://atoneplace.net/reaction-time/reflex-vs-reaction/
site: "At One Place"
---

# A reflex is not a reaction

**The difference: A reflex turns round in the spinal cord. A reaction goes through the brain.**

Roughly 20 milliseconds against roughly 250 — an order of magnitude, for structural reasons

These get printed in the same column constantly, and the result is that people think their reactions are unusually slow and that reflex speed is something you could train towards. It is not. The two travel different circuits and the shorter one does not involve you at all.

## What a reflex does

A stretch receptor in the patellar tendon connects, through exactly one synapse in the spinal cord, to the motor neuron that contracts the quadriceps. That is the whole circuit. About 20 milliseconds is almost entirely the time the signal takes to travel down the leg and back, and no part of it passes through anything that could be called a decision. You learn your leg has moved after it has moved.

## What a reaction does

Light hits the retina, is transduced chemically, travels the optic nerve, is processed in visual cortex, a decision is formed, motor cortex issues a command, it travels the spinal cord, and a muscle contracts. Roughly 250 milliseconds, and the floor for a healthy human is around 150. Nothing in training moves that floor much — what training moves is prediction, which is a different thing entirely and is how every fast-looking athlete actually works.

## And the third category nobody separates

Road design uses 2.5 seconds as the driver perception-reaction time, and that figure is not a measurement of a driver. It was chosen to cover nearly all drivers under nearly all conditions, deliberately conservative because a road built for the average driver kills the slow half. It is quoted as "average driver reaction time" all the time, including by people teaching driving, and it overstates the real figure by roughly three times.

## Side by side

| What | Typical | Circuit |
| --- | --- | --- |
| Knee-jerk (patellar) reflex | 22 ms | Spinal cord or brainstem — no conscious processing |
| Withdrawal reflex | 90 ms | Spinal cord or brainstem — no conscious processing |
| Vestibulo-ocular reflex | 10 ms | Spinal cord or brainstem — no conscious processing |
| Blink reflex | 40 ms | Spinal cord or brainstem — no conscious processing |
| Simple visual reaction time | 250 ms | Full sensory-to-motor path through the brain |
| Simple auditory reaction time | 170 ms | Full sensory-to-motor path through the brain |
| Simple tactile reaction time | 155 ms | Full sensory-to-motor path through the brain |
| Choice reaction time | 380 ms | Full sensory-to-motor path through the brain |
| Recognition reaction time | 340 ms | Full sensory-to-motor path through the brain |
| Driver reaction to an expected event | 850 ms | Full sensory-to-motor path through the brain |
| Road design figure | 2.5 s | Not a circuit — a conservative value chosen for safety margin |

> The practical consequence: if an event gives you less than about 150 milliseconds, reacting is not available and only anticipation is. That is why a boxer slips a jab by reading the shoulder, why a goalkeeper dives before the penalty is struck, and why a batter watches the pitcher's arm rather than the ball.

## Common questions

### What is the difference between a reflex and a reaction time?

A reflex turns round in the spinal cord or brainstem without reaching the brain, and takes about 20 to 90 milliseconds. A reaction is a decision made in the brain and takes about 150 to 300. They are different circuits, and you cannot train one into the other.

### What is the fastest possible human reaction time?

Around 150 milliseconds for a simple visual response. Anything measured much below 100 ms is anticipation rather than reaction, which is why athletics treats a sub-0.100-second start as a false start by rule.

### Can you improve your reaction time?

Marginally. Practice, sleep and low-latency equipment all help at the edges, but the floor is set by nerve conduction and processing time. What improves substantially with training is prediction — reading the cue that comes before the event — which is what makes skilled athletes look fast.

## Related pages

- [Knee-jerk reflex](https://atoneplace.net/reaction-time/knee-jerk-reflex/)
- [Simple visual reaction time](https://atoneplace.net/reaction-time/visual-reaction-time/)
- [The 2.5-second road design figure](https://atoneplace.net/reaction-time/aashto-perception-reaction/)
- [Measure your own reaction time](https://atoneplace.net/tools/reaction-time-test/)

## Sources

- Typical range compiled from manufacturer specifications — At One Place (/about/)
- A comparative study of visual and auditory reaction times on the basis of gender and physical activity levels — Jain et al., International Journal of Applied and Basic Medical Research 5(2), 2015 (https://doi.org/10.4103/2229-516X.157168)
- Response Times: Their Role in Inferring Elementary Mental Organization — R. Duncan Luce, Oxford University Press, 1986 (https://global.oup.com/academic/product/response-times-9780195036428)
- A Policy on Geometric Design of Highways and Streets (the Green Book) — American Association of State Highway and Transportation Officials (https://store.transportation.org/)