Reaction time is measured by determining the amount of time between when an event occurs and when the body reacts to it. This is done using a stopwatch, electronically with a computer or using measured distance. Reaction times vary based on age, health, mental state and other factors.
The ruler test is an accurate way of measuring reaction time. Have a person sit down with an arm on a flat surface and hand extending past the edge. Hold a meter stick just above the thumb and forefinger, and have the person grab it as soon as it falls. Record the distance the ruler fell before the person caught it, and plug it into the formula t = square root of 2d/g, where "t" is time, "d" is distance, and "g" is 9.8.
When there is a stimulus, such as a ball flying at a person, the eyes are the first to recognize it and send the image to the brain. The brain then tells the arms to come up and cover the face. The faster the reaction time, the less chance of the ball hitting the person in the face. Athletes often have high reaction times, which is part of what makes them good at the sport.