Effectively performing exercise ball activities requires careful selection of the right exercise ball size. Because personal consultations are not always possible, physical therapists, exercise trainers, and other professionals have constructed several guidelines to use when selecting the proper exercise ball size.

When sitting upright on an exercise ball:

  • Feet should be flat on the floor - with an even weight distribution.
  • Knees should be level or slightly lower than the pelvis - creating an angle of 90 degrees or slightly greater at the hips and knees (thighs parallel to ground or pointing down slightly).
  • Pelvis, shoulders, and ears should be in a vertical line - the body should not be leaning in any direction as a counterbalance. Bouncing up and down lightly will usually produce this alignment.

Exercise balls generally come in five different diametrical sizes. Each of these sizes is accordingly used for people of differing body compositions.

It is important to note that height alone is not the only factor in determining ball size. Because the exercise balls are flexible and offer resistance, weight is also an important factor.

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A general guideline for height correspondence to diameter of exercise ball is as follows (this is assuming average body weight is proportional to height):

Exercise ball diameter Person's height
45 cm
5' and under
55 cm 5'1"– 5'8"
65 cm 5'9"– 6'2"
75 cm 6'3"– 6'7"
85 cm 6'8" and taller

If body weight to height is larger than the average proportion, sitting on the exercise ball will compress it down more, so individuals usually should try using the next larger exercise ball size in order to maintain the 90-degree rule. Another factor to keep in mind is that most exercise ball sizes have some adjustability to them. If the angles at the hips and knees are much greater than 90 degrees, some air can be released to compensate and vice versa.

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Bear in mind, releasing air from the exercise ball will cause it to lose air pressure. As the ball flattens out, this will actually make it more stable, as it has a larger contact area with the resisting surface and the body. This means that stabilizing and balancing exercises will become easier and will lose some effectiveness.

Exercise balls also lose pressure because of stretching from regular usage. Therefore, as the ball ages, it may require further inflation. On the other hand, adding excessive air to the exercise ball will increase the difficulty of balancing and stabilizing, as the contact area decreases.