Oh, sorry, i misunderstood. Thought you were doing the 400s at threshold pace to improve threshold.

I don't know. I raced 5ks and 10ks and had all encompassing body death at the end of all of them. I did a 400m race once and felt the same way. Towards the end of an all out effort it's just overwhelming fatigue and saturation of...whatever it is that makes you feel like you're about to physically melt. Same in cycling, though I rarely do efforts like that outside of a finishing hill because of drafting and tactics and all.