Meet the Aktiv Ninja Grip Kit: a solution for rock climbing, ninja warrior, and parkour training with grips that lets you suspend, hang, and swing like a pro. These are ideal for extreme grip strength training. You can use them to do pull-ups and holds.
Ninja Grips are made from polyurethane. The set includes 3″ cones, 2.5″ spheres, 8″ flares, and 3″ spheres. You can find out more here.