Dolity Office Silicone Hand Strengthening Grip Forearm Finger Exerciser Ball 40LB 600LB - 00LB

These ring are really hard and stiff when you first get them. After a little use they soften up and really work the grip. 100lbs is for advanced users. 60 is good for beginners and 80 is great for intermediate. I use 60 just for thumbs.