The Sun is the source of most of the Vitamin D we receive. That means it also plays a big role in our muscle development. Use this program to test your muscles by testing your reaction times. When you hit the start button it will tell you to wait but as soon as it says hit stop you hit the stop button as quickly as you can. It will time your reaction time from the point where it tells you to hit the stop button to the time when you actually do it.