The BOSU® Ballast® Ball stays put, so your ball does not wander during your workout. Constructed of high quality, burst-resistant material, it has a six-sided surface design for proper alignment and positioning and has 2.5 lbs. of multi-dimensional load, which provides a fun, visual effect and audible feedback during lifting, shifting, and shaking exercises. With the BOSU® Ballast® Ball, individuals can perform exercise progressions and dynamic drills that are not possible on a regular stability ball. Multiple Ballast® Balls stack easily on their own without a need for unsightly storage contraptions.