The GoFit Professional Grade Core Stability Ball is a fun, yet challenging way to stretch, tone and tighten your body. What makes the GoFit Core 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. Combine the ball workout with the GoFit cardio vascular jump rope program and resistance tube training and you have a complete workout that can be done anywhere, anytime, and at a very affordable price.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 Professional Ball has a 1200lb rating and is ideal for core training routines. Each ball comes with a Core Performance Stability Ball Training DVD, Training Manual, ball plugs and a hand pump to inflate the ball.GoFit Stability balls are made of PVC plastic and contain no Latex.GOFGF55PRO. GF55PRO. Gofit GOFIT GF-55PRO PROFESSIONAL STABILITY BALL & CORE PERFORMANCE TRAINING DVD (55 CM DARK RED).