It’s hard to find a cardio machine that works your body harder than an indoor rower. It requires equal effort from your upper and lower body, extreme cardiovascular fitness, and ridiculous muscular endurance to blast out rep after rep. And it’s great for fat loss, too. A 185-pound guy can burn 377 calories by rowing vigorously for only 30 minutes, report researchers at Harvard University. Want to try it for yourself? Check out these four fat-blasting, muscle-pumping rowing workouts.