You'll see a lot of fatigue with enlongated acceleration, unless you are a talented runner who has unbelievable force capabilities along with some musclar endurance to go along with it or else your start is too slow and your not reaching your potential. Also, everything Mike said, when I tell my kids to put the hammer down, I know it can only last a second or two, then I am all about relax and knee drive. In fact, my runners (sprinters and mids) say those are the only thing I say at meets or in hard practices.
With the 400m race, depending on the athlete and fitness level I may tell them to slow their start a bit and work on progressing through their race distribution.