This is the ultimate deep squat--one you should NOT do with a lot of weight.
Many people worry that this exercise will be hard on your knees, and doing it with a lot of weight can definitely increase your risk of injury as you squat. However, the fact that you're moving your hips back as you squat helps to focus the workout on your glutes rather than on your quads, and the strain is taken off your knees.
Butt to Floor Squats
Adopt your classic squat stance: head up, back straight, knees spread slightly beyond shoulder width, toes pointed outward. If you are going to use weight, use very little.
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Slowly drop into the squat, lowering your butt toward the floor. You don't actually touch the floor, but you want to bring your butt as close to the ground as possible. Make sure that your knees don't pass over your toes, but thrust your hips backward to maintain proper posture.
Repeat as desired.