[The best balance ball for core training] The TRNR Gym Ball (diameter 55 cm) is an essential training tool that increases core strength, balance, coordination, range of motion, joint stability and postural coordination. By adding instability to your workout, you can train your abs, back, and pelvic floor muscles. This makes it great for strengthening muscles that are often neglected and preventing muscle imbalances, back pain, and injuries. Although designed specifically for core strengthening, gym balls can also be used for a wide range of exercises targeting the arms, hips, buttocks, and legs. It comes with an innovative dual tool of manual pump and TRNR that allows you to inflate the ball to the right size (align the two holes with the marks on the ball) or remove the plug (using the fork-like ends).