With the exercise printed directly onto the ball, the Stability Ball is a fun, yet challenging way to stretch, tone and tighten your body. What makes the Stability 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. Ball Height Recommendations: 55cm (blue) – recommended for persons 5′ to 5’5″ 65cm (red) – recommended for persons 5’5″ to 6′ 75cm (silver) – recommended for persons 6′ and over.