SMARTFLY 1 Piece(Six Colors Optional) Silicon Ring Hand Grip Trainer for Forearm Hand and Finger Strength - Extension, Crushing & Pinch Grip Training Solution SMARTFLY Hand Grip Trainer is a finger-hand-forearm strength and endurance exerciser Material--The grippers are made from premium silicone, the high quality item can strenghten your muscle. Great for all three major grip trainings types. These rings can be used for Crushing, pinching and even extension. Very portable and easy to use, almost anywhere/anytime - at your desk, stuck in traffic, walking, on the treadmill, etc. Sized especially for adult hands, to maximize the benefits of the exercise and ensure a challenging, yet safe experience. External external : 88mm,Inside diameter : 44mm.