You can get an effective aerobic workout with both an elliptical machine and a treadmill. In general, you can let your fitness goals and personal preferences determine whether you choose an elliptical machine, a treadmill or a different piece of exercise equipment.

However, elliptical machines might offer some advantages over treadmills. For example:

• Using an elliptical machine can be less stressful on your knees, hips and back than is running on a treadmill. Walking on a treadmill, however, exerts about the same force as using an elliptical machine.
• Unlike treadmills, some elliptical machines are equipped with movable upper body handles or poles, similar to ski poles. These allow you to exercise both your arms and your legs.
• Most elliptical machines can be pedaled in reverse, which allows you to work your calf and hamstring muscles a bit more than does forward motion.

Using an elliptical machine is generally considered a low-impact activity, and it shouldn't cause knee pain if you are using it correctly. Since elliptical machines provide low-impact aerobic activity, they can be a good alternative to running or jogging for someone who has joint pain due to arthritis. Talk to your doctor about what exercise is right for you if you have any injuries or health concerns.

And what if you're training for a 5K run or other road race? A treadmill is probably a better tool to prepare you for running events. But even if running is your main aerobic fitness activity, cross-training with an elliptical machine or other low-impact exercise equipment can help keep you fresh and prevent overload injuries, including stress fractures.

If you use an elliptical machine, remember to maintain good posture to help ensure the most effective workout. Keep your shoulders back, your head up and your abdominal muscles tight. Look forward, not down at your feet. And don't lean on the handles — let your lower body support your weight.