About this item Quality MaterialThe exercise balls are made of high-quality thermoplastic rubber (TPR) and food-grade silicone, which keep it's original shape after repeated squeeze and stretch Portable kitThis kit is suitable for anyone to exercise the hands at anywhere no matter you are sitting at home watching TV, walking on the way, or in a lunch break in the office Effective FunctionsBy using the hand grip balls and finger stretchers to improve hand strength, finger dexterity, joint flexibility. These also can be used to assist injury recovery, ease muscle pain, and relieve stress Progressive ResistanceThe grip training ball set includes four balls, with four resistance levels: 15kg (yellow), 18kg (orange), 25kg (red), and 30kg (blue), provide different resistance levels for beginners and advanced users. It's great for progressive training Wide ApplicationsThe hand exerciser kit is ideal for athletes who play baseball, tennis, golf, boxing, and fitness, for outdoor adventurers who love rock climbing and bodybuilding, for musicians playing piano, guitar, bass, and violin, and for people with daily stress