They’re so bouncy and enormous that,when you sit on one, your core works overtime just to keep you upright[br]What is it? A large inflatable ball used for exercise. You may know it as a balance ball, or birth ball,body ball, fitness ball, or gym ball,stability ball, therapy ball or yoga ball.
How much does it cost? Shop around and you can find them for approximately a fiver. Alternatively, or your gym will be packed to the gills with them.
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Source: theguardian.com