Cross trainers provide a full body workout, working your upper and lower body simultaneously. And if you just use your legs to power the machine (no arms), you can engage your core for some tummy toning too. Because you're using so many muscles at once, they're great calorie-burners. Ramp up the intensity enough and you may even burn more calories than you would running or cycling. Result! And last but not least, they're low impact (if any impact at all) so they don't put any stress on your joints making them particularly good if you have knee-troubles.