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 5' to 5'5" tall. Suggested weight limit of 300lbs. Not recommended for usage with weight training routines.