Lots of Japanese guys who do OK on the show are "trampoline enthusiasts." I don't know if it's a sport in Japan or what. But just bouncing on a trapoline is kind of basic, but super dynamic and fast. It's like plyomeyrics, only with way more force and rebound, and less impact.
Dudes I know who are good at wake boarding have mentioned practicing on trampolines.
For grip I would try tons of non-maximal holds, going for time. Pinch grip plates. Put plates in a pillow case, and grab it like a sack. Fat Dumbbell Handles. Something round, kinda like a door nob or something. Hanging off a chinup bar, from all different kinds of handles and grips. Use workable weights, and shoot to hold them for a long time.