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It’s bloody hard! I have one. Available on Amazon
Senshi Japan Forearm Wrist Curler Exerciser - Curling Trainer - Best Forearm Strengthener - Wrist Strengthener - Grip Strengthener - Grip Curler For Bigger Forearm Muscle Growth - Weight Twister - Easy Wrist Curling Exerciser Bar - Increase The Strength In Your Forearm https://www.amazon.co.uk/dp/B013GV6N3C/ref=cm_sw_r_cp_api_glt_fabc_C5JTWRK1S1NH1BR56PFN
What is it?