Foots twitching/jerking while crouching to backwards

hello there, i got the question,…

im doing TRUEFPS in MP and i got there basics movement - like jumping, sprinting, sliding,…everything works fine,…but when iam crouching - idle is fine, when start moving - its fine, even when you go forward, left or right,…but when you go backwards, foots starts twitching or jerking,…any solution?

Sry for my bad ENG, im from CZE