There's a cue in many leg exercises that often gets over looked.
It was highlighted to me several years ago by Mike Robertson in one of his videos / articles and I've been using with my clients to help their technique.
The cue is "feel the whole foot"
This applies to leg exercises such as Squats, Split Squats, Lunges, Rear Foot Elevated Split Squats, Leg Press, Deadlifts, Romanian Deadlifts and I'm sure several more.
The idea is that during these sort of exercises, many will roll onto the ball of the foot / toes because of a balance issue or because it will often feel easier to push off through the ball of the foot than through the heel.
I find that pushing through the ball of the foot uses the muscles at the front of the leg more whereas pushing through the heel brings in the butt muscles more. When I cue my clients to push through the whole foot, rather than just the toes, they find the same too.
Here's what to do:
1) When you start a leg exercise, spread your weight across the heel of the foot and the ball of the foot / toes
2) Then make sure both sides of your foot are on the floor. You want to feel the weight of the foot from the ball under the big toe across to the little toe and across the heel too
3) As you perform a leg exercise, focus on feeling your whole foot keep in contact with the floor / platform. This will help to keep the foot stable so you do not roll it - front to back or side to side
4) As you push through your foot for the exercise, focus on pushing through the ball and the heel.
For exercises that use both feet at the same time - Leg Press, Squats, Deadlift, Romanian Deadlift - you will want to do this through both feet at the same time.
For exercises that use 1 foot at a time - 1 - Leg Romanian Deadlifts, Split Squats, Lunges, Rear Foot Elevated Split Squat (Bulgarian Split Squats). You will only be focusing on 1 foot at a time doing this.
This is my, mostly, Personal Trainer musings and information which I hope you'll find helpful!