I herniated 3 disk over ten years ago. Broke my left knee in 2 places 3 years ago. Plus I am 45 years old. You can build great leg's from leg pressing. Muscle only responds to stress created. Muscle does not know what exercise your doing. It takes me 1 hour to an 1.5 hours to train quads. 20-30 minutes warm up (leg press). then 5-8 working sets leg pressing once a week. My stats. 6'0", 220lb, body fat, well really low. An example of my workout for quad's would be, warm up 1 plate per side 20 rep per set increasing 1 plate each set. 8 plates 30-40 reps, 9 plates 20-30, 10 plates 17-25, 11 pltes sets of 12-15. Back down to 8 plate 2 sets of 20. There are different variance's to my workout this is just one example were I choose to use 8 plates a side for my first set. A may choose 9 plates a side for my first set, in this case 8 would be in the warm up range and my rep range at 9 plus plates would be extended probably in the 4-8 range per set. I may choose 7 plates as my first set and try to push for 50 reps! then 8 for 20-30 and so forth on. Either way the intensity level of this training forces quad development. Oh and I'm a classic ectomorph. My nutrition and supplementation are spot on.