The faster the 400, the more even the two halves have to be. To run a WR 400, the first 200 has to be speedy, and to run a WR 400, the deceleration in the second half has to be slower/controlled, or else no WR. The "wall" for a first 200 is obviously 19.32 seconds today--no one could run it faster. As that wall is approached in the first half of the 400m race, the physical cost increases--so the first 200 has to fast (doh...) but not so fast that the runner can't almost sustain it (while inevitably decelerating). The result will have to be closer to even splits as the first 200 is "balanced" between an all-out 200m and loafing (both strategies being ridiculous), while the second half depends on highly controllable, minimized deceleration.
Mitch
