Exercise balls are useful, in a gym or therapeutic setting, for strengthening your core muscles. The inherent instability of the round, prone-to-rolling exercise ball forces core muscles to constantly stabilize your body while you're on the ball. Even though big-name trainers like Bob Harper from "The Biggest Loser" endorse the use of exercise balls in place of desk chairs, some still oppose the idea.
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Exercise balls are sized by their diameter in centimeters. Your height determines what size ball you need. For general exercise applications, a 45 cm ball is appropriate for anyone up to 5 feet tall. A 55 cm ball suits those 5 feet to 5 feet, 5 inches; a 65 cm ball suits individuals who are 5 feet, 5 inches up to 6 feet; a 75 cm ball is appropriate for those over 6 feet. Hard-to-find 85 cm exercise balls are appropriate for individuals over 6 feet, 8 inches.
If you're using the exercise ball as a chair, however, the University of Minnesota Extension recommends using a ball one size larger than you would for typical exercise. So, for example, an individual up to 5 feet tall would bump up to a 55 cm exercise ball.
One of the biggest arguments against using exercise balls as desk chairs is that balls lack the adjustability and support that a proper ergonomic--by its very definition, "stable"--chair provides. Features that an ergonomic chair may have that a ball cannot provide include adjustable seat depth and angle, lumbar support and armrests.
Exercise balls don't offer the same support that an ergonomic office chair would. After all, forcing your body to stabilize itself is part of the appeal if using the ball instead of a chair. But an exercise ball should still conform to some of the same principles of ergonomics that govern office chairs.
To test whether your exercise ball has you properly aligned, sit squarely on the ball in front of your desk, close your eyes, and adjust your head angle, as necessary, so that it's facing straight forward. Open your eyes. You should be looking right at the center of your computer screen. If you're not, adjust either the screen or where your ball chair is situated. Next, sit up straight on the ball with your upper arms straight down by your sides. Bend your elbows and place both hands on the computer keyboard or, if you have no keyboard, the desktop. Your elbows should be bent at a 90-degree angle; if they're not, you need to adjust the size of your exercise ball or your desk accordingly.
Arguments against using exercise balls as desk chairs include the risk of falling off the ball; excessive strain from continued muscular exertion; the fact that one can slump just as easily on an exercise ball as in a chair; the lack of support for those with back injuries; and the potential risk of a sudden deflation.
The chief touted benefit of using an exercise ball as a desk chair is that it will give you a constant core workout as your muscles struggle to stabilize your core, so that even while working at your desk, you'll still be getting some exercise.
Simply rolling your exercise ball over a fallen thumbtack might cause it to suddenly deflate. If you're sitting on it when this happens, a serious injury might result. Help cut the risk of this happening by selecting an exercise ball that is clearly labeled as either puncture- or burst-resistant, or with the letters SDS (slow deflate system).