I am a paramedic, I ride fixed, I blew a ligament in one knee riding a couple decades ago, I have cartilage damage in both knees from skiing over the last two decades. Do NOT rehab a knee injury on a fixed gear if you are new to cycling. Anyone who tells you to is a moron. No single sports physiologist or orthopedist will disagree with me, not one. Don't even consider it. Unless you live in a very flat area with no hills and never plan to ride more than a few miles near home, don't rehab that knee on a single speed either. Get a nice multispeed bicycle and use it wisely with input from your doctor and/or physical therapist. Better yet, invest in a winter pool membership. Spend the winter in the pool slowly building up those muscles in a lower impact sport. Then switch to cycling in the spring. With a multispeed bicycle for at least your first year of riding. As I said, there isn't a single sports medicine professional who will disagree with this sane advice. You need that knee, don't f*ck it up by blowing it out during rehab.