Is there an alternative to Squats?

I have serious cartilage degeneration in my knees, so I can't squat, kneel or run (considering also a BMI of 34). But I really want to push my leg muscles and was wondering if there are effective routines to work quadriceps while sitting or lying on my back.

Thanks a lot!

I am currently doing NROL for Life and the level one squat is body weight holding onto a fixed object and just working to get down as far as you can.  I don't know if there is an alternative from the floor.  If you can "sit" and get up from a chair try just doing that.  Even if you have to hold onto another chair-

I would talk to a physical therapist to see what leg exercises you can do with your knees.

I agree with Smash.

When I was rehabbing after a knee surgery which involved removal of cartilage and ACL reconstruction, I remember doing straight leg raises while lying down... But I recommend asking a professional about this, as you don't want to end up aggravating it further.

I agree with smash, but squatting with body weight only on a chair or Swiss ball will develop leg strength. Lunges are also a great exercise for legs, there are several variations including front, side, back to keep your muscles guessing.

For low impact, you can also swim. Use a kick pad or the side of the pool if you are unable to swim distance at first until your endurance builds.

Thanks all, 

My physiotherapist had already shown me the leg raises and drawing circles with my feet while lying flat. And also I am allowed to cycle on flat routes but i live in a very hilly area. And we have 1 public pool that doesn't allow individual therapy - only fast swimming. So i'm stuck at home, which isn't too bad, but i'll try to continue the leg raises with ankle weights then.