Bad knees are a bad excuse for not exercising -- there are too many types of workout equipment that don't stress your knees. Some even help you build the muscles in your calves and thighs that support your knees, making that next flight of stairs less intimidating -- and those short skirts more attractive on you. Bad knees might keep you away from the long morning runs of the past, but picking the right cardio exercise equipment can be soft on your knees and hard on calories.
Running and jumping can stress your joints and can be a bad idea if you have bad knees. That doesn't mean you should avoid the treadmill -- just keep to a brisk walk instead of running. For a no-impact option, hop on the elliptical machine. It lets you work all sides of your legs as you rotate the pedals forward or backward. Vary the intensity on a recumbent bike so you push yourself for a minute and pedal a bit slower for about two minutes to add some interval training to your workout.
A leg press machine typically has you lying on your back with both feet on a plate situated above your hips. The beauty of this machine is that you are in control of the weight you're lifting and the range of motion; if deep bends hurt your knees, keep the move more shallow. Ideally, bend your knees to a 90-degree angle. A rowing machine moves your knees in a similar fashion as the leg press, spreading the weight across both legs while keeping your feet stationary.
With the variety of leg curl machines available, you're sure to find a few that fit in with your workout routine. These allow you to work the hamstrings and quadriceps in your thighs without putting stress on your knees. Instead of lifting your body weight, like you do in a squat, you control the weight level you lift.
Cables offer a portable way to work out your legs without stressing bad knees. Hook the cable to something stable -- such as a closed door, not your significant other -- and connect the other end to your ankle. You can lie on the floor and pull your knees in toward your chest to work your quadriceps and abdominals, or stand up and lift one leg at a time forward, backward and side to side. Try standing leg curls using cables as well, bending your knee to lift your foot toward your butt.