i am more of a basketball player, but i also ran the 400 in track last year. i have found that many of the workouts i did to increase jumping also helped my ability to run the 400 by increasing muscle and stamina. one drill that worked especially well was very simple. just stand and jump as high as u can 50 times. when u are done, your legs (mainly thighs) feel like jello, much like after a 400. after that run a 100 or 200 and repeat the jumps. each week, add 15 jumps.