How to stop biped centre of mass animation.

I have a biped animation made in max. Rather than make him walk on the spot I had him actually move his centre of mass forward at the right speed because it's easier to do for a limping, stuttery walk cycle. But because I use planted keyframes to keep his feet still and utilise IK I can't just go and delete his COM (centre of mass) keyframes to make him stay on the spot.

Result - When the character walks in-game he travels forward WITHIN his forward motion, then when he restarts the loop he flicks back about 3 foot while still travelling forward. Obviously this is because the FBX contains about 3ft of him walking forward but I'm sure there must be a way to just disable that in the import settings so his COM doesn't move? (Sure i saw someone elses anim clips that had the character actually covering distance but staying on the spot as it were in the actual loop). Or do i have to manuall go back to max and fiddle with the keyframes and re-export?

Thanks for reading!