Originally designed to refine sports & musical skills, this amazing exercise ball strengthens & improves grip by building forearm, wrist, hand & finger strength. Easily activated using the starting cord (spare included), it takes your wrist through a range of dynamic exercises, building muscle along your entire arm. The faster you spin, the greater the resistance. Easy & quick to use just 3-5 minutes a day. Ideal for rehabilitation. H7cm. Dia. 5.5cm.