First off, if your back hurts, go see a Dr and make sure its not a herniated disc, or you are going to end up like me, and its no fun. Secondly, do not do lunges either. Lunges twist your back, in almost the same position as a squat, which could be worse than squats. Do seated leg presses, making sure your back is securely up against the back of the seat, then leg extensions, then hack squats with your legs together, and with a lighter weight. IN THAT ORDER. Hack squats are the same movement as a regular squat, but if you do them at the end of your training session, your legs will be tired and a lighter weight will be enough to burn them. I have been going through this for a year, and its hard to make your legs grow. But if you injure yourself further, they aren't going to grow at all whyen you are sitting on the couch at home. Later