Glide. Elliptical cross-trainers are designed to be easy on the joints, but they don't work unless you use the proper form. "Moving with a short, jerky stride rather than a smooth, full-length one puts stress on joints," Harris says. "If you find yourself moving that way, it probably means your resistance is too high for your present capabilities." On the other hand, setting the resistance too low―and swinging out of control at astonishing speed isn't an effective way to exercise, either. Discovering the balance between too much and too little resistance may take some experimentation. "A good rule of thumb is that if you are using a higher resistance, your muscles should be challenged but not straining to complete each stroke," says Harris.