The split squat is a staple lower body exercise that has its place in virtually every training program. There are several variations of the split squat, and one of the most useful is the front foot elevated version. Propping up the working leg makes any split squat more accessible, but in a way that can be difficult to intuit: the purpose of elevating the front foot is to relieve pressure through the back leg.
This forward direction is key because, without it, your working leg simply can’t bend at the knee very deeply. But traveling forward in this way puts a big stretch on the front side of your back leg, because your femur of that non-working leg needs to reach back behind you. Very few of us get exposure to this level of hip extension regularly, so it’s common for this to feel very tight and uncomfortable. If the hip can’t extend to the degree the split squat requires, arching at the back and/or leaning forward are likely compensations… neither of which are good.
However, when you prop up your front foot, your back knee has some extra room to work with. You don’t need to keep your back leg quite so straight to keep your knee clear of the floor, meaning that your femur doesn’t need to point backward quite as far. Taking pressure off the back leg means better depth for the front leg, plus a neutral and upright trunk.
One thing to remember is that the stretch through your back leg isn’t entirely bad. Too much tension is bad because it will force some compromises, but a gentle stretch is great. The best scenario is to find just the right height that allows you to hit all of the points of performance (full depth in the working knee, upright torso, neutral spine) while just finding a moderate stretch in the back leg at the bottommost position.