Exercise balls can be used to treat back pain and are effective in preventing, reducing and rehabilitating lower back pain in particular. Exercise balls are a wonderful addition to the fitness arsenal, but to get the most out of using one you must choose the right size ball for you. An excellent way to ensure you get the right ball is to schedule a personal consultation with a fitness and rehab professional. However, since that is not always possible, the following are some general guidelines these professionals have developed to help you choose a ball.
There are typically five different diameters of exercise balls to choose from, in centimeters: 45, 55, 65, 75 and 85 cm. They are each designed to fit a certain body type. Your height is not the only determining factor in terms of ball size; your weight and body composition are also considerations.
Be sure the various balls are inflated and available for you to test before purchase. Sit upright on the ball, ensure that your feet are flat on the floor and that your weight is evenly distributed. Your knees should be level with your pelvis, or just slightly lower. This position is key because you want an angle of 90 or greater at the knees and hips, to avoid stressing either of these pivot points. Your ears, shoulders and pelvis should form a vertical line, without your body leaning in any direction to maintain stability. You can check this alignment by gently bouncing up and down.
In general, people 5' and shorter do best with the 45 cm balls, and people 6'8" and taller do best with the 85 cm balls. If you are heavier than average, you will compress the ball further when sitting on it. Therefore you should consider buying the next ball size up in order to ensure you have the 90-degree angle or greater for your knees and hips.
Also remember that you can adjust the ball somewhat yourself. For example, if the angle of your hips and knees is much greater than 90 degrees, you can deflate the ball a little. This can be useful if you are out of shape and/or new to using an exercise ball, as the flatter ball will be more stable and easier to use. However if you are in better shape or are more experienced with exercise balls, a flatter ball will be less effective at training your back muscles to improve their balancing and stabilizing abilities. If you initially buy a ball and deflate it, you can always add more air later to increase the difficulty level and train your muscles further.