Fitness fads come and go -- remember the thighmaster? -- but if you want a well-rounded workout, then embrace the fitness ball. It's no secret that exercise balls are a great way to add variety to your workout and improve strength in your core and lower body. But working out on a large, bouncy object might be intimidating for beginning exercisers. The key is to find the right size for your height. A ball that's 55 cm in diameter works best for those who are 5'4" or shorter. If you're 5'5"-5'11", try a 65-cm ball, and if you're taller than that, then a 75-cm ball will work. Exercise balls put any workout you do in an unstable environment, forcing your bottom half to contract to keep you stable. So if you're doing a chest press or curls, then your lower body is working as well, thereby increasing the work you're doing.