Build bigger quads and make your knees injury-resistant with sissy squats. If you're feeling masochistic enough, try using them as a drop set. Just lightly hold onto a dowel or bar for balance.
The Sissy Squat (Drop Set)
Sissy squats are one of the best, but most neglected, muscle-building exercises. While lifts like squats and leg presses do a great job at hitting three of your four quadriceps muscles, they leave your rectus femoris untouched. Even the leg extension machine does a greater job than squats at activating this area of your quadriceps (1).
The sissy squat has a unique ability to target your rectus femoris in its lengthened state, making it a useful addition to your routine for both healthy knees and bigger quads.
And no, pushing your knees over your toes isn't inherently "bad." If this were the case, walking up stairs and riding a bike would also be bad.
Your knees should be healthy enough to travel over your toes, and resilient enough to handle the extra torque in exercises like sissy squats. That's not to say you should "drop and sissy" right now, but it should be a goal to demonstrate a handful of pain-free reps at some point in the future.
Start with assisted and partial range reps. Always work within a pain-free range of motion and only increase ROM if you're still feeling good.
Related: The Forgotten Exercise for Big Quads
- Ebben, W., Feldmann, C., Dayne, A., Mitsche, D., Alexander, P., & Knetzger, K. (2009). Muscle activation during lower body resistance training. International Journal of Sports Medicine, 30(1): 1-8.