The secret to running a good 400m is the effort in running the 2nd 200m must be faster than the first half. It doesn't mean the time is necessarily faster, it just means that in the 3rd segment, you must make a concerted effort to pick up speed even if you are actually slowing down or maintaining speed according to the stopwatch.
That being said, Clyde Hart has his "event 300m" workouts to help practice the 400 meters by running the 3rd 100m segment a little harder.
If you know Clyde, he loves his beeper machine. In this example, he has it set at 7 second intervals. (You can set one to 8 seconds for youth or JD athletes)
The workout is a total of 300 meters. First 50m "hard" followed by a relaxed 150m, then "pick up speed" for the last 100m.
While this workout requires a fast first 50m, usually Hart recommends a fast 60 meters at the start during a 400 meter race. You might as well use up that ATP/CP while you have it.
The splits (in his example) is 200m in 28 seconds (hard 50m followed by relaxed 150), then the next 100m in 12, for a total 300 meters of 40 seconds.
At 7 sec intervals, you hit the splits in 14, 28, then quicken the pace, and get to the 300m line 2 sec before the beeper goes off at 42 seconds. You can count Mississippi's or Steamboats after crossing the line.
You'll have to adjust the number of sets, reps and recovery based on their top running speed.
Most stopwatches have a countdown, chronograph, or a repeat timer function. If you are looking for a timer that can beep at irregular intervals (i.e. 20 seconds ON, 10 sec OFF), then take a look at The Gymboss. Highly recommended for those who love circuit training, Tabata workouts, Speedbag boxing workouts, or HIIT fanatics. Under $20.
