I have very bad knees and need to lose about 100 lbs. I've started swimming twice a week in a saltwater pool nearby and I'd like to start riding my bike again. My dr. says I can do low impact aerobics but to use an eliptical trainer (walker?) instead of a tread mill. I've checked into them but I am afraid I will lose my balance with the foot plates swinging back and forth. Next knee injury I receive could mean some serious surgery. How can I avoid this prospect? Is there a safer alternative?