After my lower back injury at the age of 18, I've used a lot of one-leg hack squats to develop my quadriceps. These put very little stress on the lower back, but put very high intensity on the quadriceps. By doing one leg at a time, I could put almost twice as much energy in each of my legs. After 8 sets of one-leg hack squats (4 sets of 10 reps for each leg) my quads had an incredible pump and were almost paralyzed from exhaustion. After the one-leg hack squats, I added an additional 4 sets of leg extensions or one-leg leg presses to squeeze out the last remaining drop of glycogen. This usually was enough for several days of deep muscle soreness.