Rear-foot elevated split squats, Bulgarian squats, or whatever fancy name you have for them, are easily adjusted so they're knee friendly while simultaneously strengthening muscles around the knee joint. Just adjust the movement to maintain a vertical shin.
- Begin with your front foot more forward than usual. Place the back foot laces-down on a bench or roller. As you descend into the negative, drive your working hip down and back to maintain a vertical shin (a hip that only moves down shoots your knee forward).
- Allow your back knee to bend as you descend (many lifters instinctively lock this knee, reducing hip range of motion). Pivot your torso forward while maintaining flexed abs and neutral spine. Retract the shoulder blades to maintain a neutral spine against the load.
- Reverse direction at the bottom by pushing evenly through the entire foot. You can emphasize pushing through your heel if it tends to lift off. Finish all reps on one leg and then switch.
For heavy loads, rest between working the left leg and right leg. Start with the weaker leg or crankiest knee.