I have to admit that regular squats always seem to hit my hams and glutes moreso than my quads. I use the hack squat and front squat as they both seem to hit my quads a little better. Also, squats with a smith machine with legs in front do a better job at isolating the quad muscle.... for me at least. I have done this exercise before with grabbing a bb behind your legs. It's a tough one and you cannot use a lot of weight, but the isolation is great.
Oh I should also mention seeing a girl do this exercise once. She ended up falling flat on her back because she used too heavy a weight, got up, looked around to see if anyone saw her, then quickly walked away lol. But I have to give her credit, she working hard.