It's more about the size of the ball and angle of you knees when sitting.
Online - Ideally, your knees should be about 10cm (4in) lower than your hips when you sit on it. As a general rule: If you're up to 1.73m (5ft 8in) in height, it's best to get a 65cm ball. If you're 1.75m (5ft 9in) or taller then it's best to get a75cm ball
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