If the sit-stands hurt your knees and lower back, you may want to just do regular squats here, or another lower body move that doesn't bother the knees or back. When you do these sit-stands, be sure your knees don't come out beyond your toes and hold your abs in tight to give your lower back support. Try that and if you still feel pain, you need to try another alternative. Maybe try not doing as many reps. Like Cathe always says, listen to your body.
Sharon