After you have a knee replaced many people get concerned about what they can and cannot do physically. The orthopedic surgeon also in most cases will try and scare you to death concerning the replacement and re-injuring the knee if you are not careful.
No doubt there will be limitations after your knee is replaced after all, it was a major surgery and, your knee is no longer the original. After having my knee replaced in 1999 and being told that I had to settle for swimming and biking only I set out to discover what else could be done to get my legs stronger again.
Having worked out in the gym for years prior to the surgery, I knew with some temperance that I could go back to several exercises for the legs, and the one that I feel builds the most strength and gets some mass and size back in your leg at any age is the leg press.
The leg press while being seated protects your low back providing you use it properly and does all the balance work for your upper body so that you can concentrate on the movement and, the development of the quadriceps. The important piece of advice I can give you is that you want to use a weight that is light enough that you can get a full range of motion from so, weight is not as important as is technique.
With the leg press also, you can either work both legs together or, work them unilaterally. The leg press itself is a safe alternative to the squat not only for your back but will not put as much stress on the knee prosthesis itself.
Be sure you are medically cleared by your orthopedic surgeon prior to starting or continuing your exercise program. Also be prepared for your surgeon to advise against most exercises in the gym. Most are very conservative and, unless they workout themselves or have had a knee replaced they will advise you against weight training.
You have to be the judge yourself. Use light weight and go for higher repetitions instead of heavy weight and low repetitions. I use anywhere from 15-25 repetitions and stay with a weight from 185-235 pounds. Settling for a world of limitations should not be for you.