Selecting a ball is simple. The ball size that will allow the most amount of versatility in Balance Ball exercises will allow you to sit on it with your knees and hips at 90 degrees.

The size of exercise balls varies. Physical therapists and exercise trainers suggest using an exercise ball that fits each persons height and leg length, where the hips and knees are at or slightly greater than a 90 degree angle (thighs parallel to ground or point down slightly) with feet flat on the floor.

Sit on a fitness ball so that your knees are bent at a 90 degree angle and your feet are flat on the floor. Roll down into a lying position and see if its still comfortable. The following guidelines give you a general idea of what size ball to buy but keep in mind that its your comfort that matters most. You can be 54 tall and find that a 65 cm ball works better for you.

42 cm. ball (16 in.) 4 ft. 10in. tall
53 or 55 cm. ball (22 in.) 4 ft. 11 in. to 5 ft. 6 in. tall
65 cm. ball (25 in.) 5 ft. 7 in. to 6 ft. 2 in. tall
75 cm. ball (29 in.) 6 ft. 0 in. tall

Using a different size balls will allow you more flexibility and variation with your Balance Ball training. The different ball sizes allow body weight to be shifted in the appropriate direction as to remove or add weight to any exercise.

The size of the ball can also take into account for flexibility differences amongst users. One of the big advantages of working with a fitness ball is that it targets your core muscles (back and abs) improving your posture, balance, and core strength.

According to Michele Olson, focusing on core strength improves functional strength, making you more able to handle every day tasks such as carrying groceries and picking up your kids. A strong core also enhances performance of all fitness activities and decreases low back pain and injuries. The big bonus is that working your core makes you appear taller and whittles your waste a lot faster than other ab exercises!