Today finally I did the barbell squats, I called a trainer to check out my form, but turned out he himself didn't know the correct form: his knees were going way forward his toes and he was holding the barbell almost on this shoulders, I told him that and he started 'you do squats on the smith machine'. After telling him it is bad for you, the position of legs and straight back can be dangerous etc, he bluntly disagreed and started telling me 'these machines are made for a reason, they can't be wrong, and if you don't want to exercise on machines then why are you using leg press, extension etc'. I shooed him away.
While I was doing squats, I kept looking straight, chest up, neutral spine, tight core. Starting with hips going back, I was able to do almost 10 perfect repetitions, but on the 13-15th ones my toes were coming up, and I was sort of falling backwards.
My Question: how do I stabilize my feet so the toes don't go up and at the same time knees don't pass the toes?
I felt some tightness in my lower back and I was slightly bent forward, is that normal?