How do some

…big leaguers get such a high leg kick

flexibility and practice.

what muscles would i have to stretch

What muscles keep you from raising your leg that high and what muscles help you raise your leg that high?

Why do you want a high leg kick?

I’m guessing because guys with high leg kicks generally seem to throw hard.

no its not that my leg kick is below average and I would like to get it just a little above my waist

I don’t think a leg kick could be considered below average, but do hip and hamstring stretches to get your legs more flexible

what would be some hip stretches I could do.

Higher leg kick is fine. More importantly, stretching will allow you to have a longer stride while maintaining momentum to the plate.