The GoFit Exercise Ball is a fun, yet challenging way to stretch, tone and tighten your body. What makes the GoFit Ball so challenging? Unlike training on a stable, flat surface, the ball is a round, unstable surface, so you not only train the primary muscle groups, but also the smaller stabilizing muscles as well. The stabilizing muscles are small, yet very important muscle groups primarily used for balance, alignment and posture. The ball total body workout is very efficient, allowing you to see the results you want in a much quicker period of time. As a general rule, the exercise ball should be large enough so that when you sit on it, your knees bend at a 90 degree angle. The 55cm ball is recommended for most people height 5ft to 5.5ft tall. Suggested weight limit of 300lbs. Not recommended for usage with weight training routines. Comes with ball pump, 2 ball plugs and an easy to follow exercise sampler. GoFit Stability balls are made of PVC plastic and contain no Latex.