The race starts with a command: "on your mark" followed by "set" and then the gun is fired.
The runners accelerate up to between the 40m and 60m marks, when they reach top speed.
In the 200m races, runners only use 92 to 93 percent of their maximum speed.
They reach their highest speed between the 100m and 150m marks.
The time is registered when the runner's torso (chest) passes the finish line.
That is the reason why most athletes lunge forward when they approach the finish line.