Lattice vertices was lost

I make an animation of a cube that makes karate :grinning_face_with_smiling_eyes: I have to bend it’s leg one side to make kicking movement. I have to move lattice vertices but it doesn’t moving. Also there is no point on lattice. I used “simple deform modifier”. is the problem about simple deform?
Or was there any easy way to manipulating object smoothly?