Great idea from synergy-athletics.com These grips are homemade with a couple bolts, nuts, and softballs. Ball grip pull ups taxes the grip to a greater extent. Usually used for 1-2 sets of pull ups, and other grips that will allow more reps for the other 2-3 sets.
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This one is a no brainer! Drill a hole though a hard ball, pop a bolt with an eye through it, attach a nut and attach the other end to a chain or rope and fix onto your pull up bar.
Alternatively, just make a shorter version and attach weights for training the grip
