Reaction Time vs Reflexes: What's the Difference?
Commentators love the phrase "lightning reflexes", and people say it about goalkeepers, gamers and anyone who catches a falling glass. In everyday speech it just means "reacts fast". To a doctor or a physiologist, though, a reflex is something quite specific, and it's not what a reaction test measures.
A reflex skips the decision
A reflex is an automatic response that's wired into your nervous system. The signal from a sensor travels to your spinal cord or brainstem and straight back out to a muscle, without waiting for the thinking parts of your brain to weigh in. You don't choose to do it, and usually you can't stop it.
The classic example is the knee-jerk. A doctor taps the tendon just below your kneecap, stretch sensors in the thigh muscle fire, and the muscle contracts so your lower leg kicks. The whole loop takes a few hundredths of a second. Pulling your hand off something hot works the same way, and so does blinking when something touches your eye. Your hand is already moving before you consciously feel the heat.
A reaction goes through the brain
A reaction is a voluntary response. You have to notice the signal, recognise it, decide what to do and send the command to your muscles. That loop runs through your brain, which is why it takes longer: around 200 to 300 milliseconds for a simple visual signal, and more when there's a choice to make (see simple versus choice reaction time).
This game measures reactions. Nothing wired into your spinal cord tells your finger to tap a green circle on a screen; you decide to do it, very quickly, and that decision is the part being timed.
Why it matters which one you mean
Reflexes are mostly set by how your nervous system is built, which is why doctors check them as a sign of nerve health rather than as a skill. Nobody trains their knee-jerk to be quicker.
Reactions are different. They get faster with practice, with sleep, with focus, and with good equipment, and they get slower when you're tired, distracted or have been drinking. Better still, a response you practise often enough starts to feel automatic. An experienced driver's foot moves to the brake with very little conscious thought. It's still a reaction going through the brain, but practice has made the decision part quicker and smoother.
So what makes a goalkeeper "sharp"?
Mostly reactions plus anticipation. Elite goalkeepers, batters and tennis players aren't running on spinal reflexes; they're reading cues early and predicting what happens next, so they can start moving sooner. That's covered in reaction time in sport and in how batters hit a 150 km/h delivery.
If you want to see the difference for yourself, try both games here. The reaction test measures how fast you respond to something appearing. The cricket challenge measures whether you can predict when something will arrive. Most people find they're noticeably better at one than the other.