The sissy squat isolates your quads better than any other barbell or dumbbell exercise. It does this by eliminating most of the involvement of your glutes and hamstrings. When you do it right, your heels will be above your toes, and your body, angled backward instead of forward, will form a straight line from neck to knees throughout the movement. That reduces the posterior-chain muscles to a supporting role, making them work with the rest of your core muscles to keep your body in that straight line. All the action is in your knee joints.