The GoFit Ultimate Round Wobble Board is a great tool for balance, stability, rehabilitation & strength training. With the new, adjustable height feature you can now increase the difficulty of your workout. Mark is the author of the best-selling books titled "Core Performance" and "Core Performance Essentials". Mark"s training methods are used by world-class athletes including Mia Hamm, Phil Mickelson, Nomar Garciaparra, Brett Favre and many others. The board features a molded plastic base that is screwed into the board for added safety and durability.