A reflex skips your brain entirely
When a doctor taps your knee, the stretch signal travels up to the spinal cord, connects straight to a motor neuron, and comes back down to the muscle. Your brain finds out afterwards. The whole loop takes roughly 30 to 50 milliseconds and you have no say in it — you cannot decide not to have a knee jerk.
The withdrawal reflex that pulls your hand off a hot pan works the same way: the hand is already moving before you consciously feel the heat. That is not you being fast. That is a circuit that does not consult you.
A reaction goes the long way round
What this site measures is a voluntary reaction. The signal reaches your retina, is converted and passed to the visual cortex, is recognised as the thing you were waiting for, a response is selected, the motor cortex issues a command, and your finger moves. Each stage costs time, and the total comes to around 250 ms for a simple visual task.
That is five to eight times longer than a reflex, and the gap is almost entirely brain. Nothing about it is a failing; it is the price of being able to choose your response, which is what makes a reaction useful and a reflex fixed.
| Stage | Approximate time |
|---|---|
| Light hits the retina and becomes a nerve signal | 20 to 40 ms |
| Signal reaches and is processed by visual cortex | 40 to 60 ms |
| Response selected and motor command issued | 50 to 100 ms |
| Command travels to the muscle and movement begins | 30 to 50 ms |
| Movement completes and the device registers it | 20 to 50 ms |
Why the confusion is harmless until it is not
"Quick reflexes" is ordinary English for "responds fast", and correcting people about it is tiresome. The distinction only matters when numbers get compared. Someone who reads that a reflex takes 40 ms and then scores 250 ms on a browser test can conclude they are six times slower than a normal human, which is nonsense produced entirely by comparing two different measurements.
The rule of thumb: if a number is in the tens of milliseconds, it is a reflex or a single nerve pathway. If it is in the hundreds, it is a voluntary reaction, and it is the one you can do something about.
A sprint start under 100 ms is disqualified as a false start precisely because no voluntary reaction can be that fast.
Trained responses sit in between
Practice does not turn a reaction into a reflex, but it does shorten the middle stages. A heavily rehearsed response — a boxer's slip, a driver's emergency brake, a keyboard shortcut you have used ten thousand times — skips most of the deliberation and runs closer to automatic. That is why experienced people look faster without being physically faster, and it is the part of your performance that repays training.