If you live on the treadmill but never lift a pound, then 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 trainers.
Image Source: POPSUGAR Photography / Rima Brindamour