If you live on the treadmill but never lift a pound, you're missing out on one of the most important pieces of the fitness puzzle. Not only does weight training prevent injury by strengthening the joints, but it also builds muscle mass and increases metabolic rate. Bonus: thanks to a revved-up metabolism, you'll keep burning calories long after you've slipped off your sneakers.
New to lifting weights? These tips for starting strength training will help you out.