Here's one more isolation exercise that's especially good for rehab of hand injuries & can be done by folks who may not be in any condition to do pull-ups or hefty weight-training exercises that contribute to the forearms:
* Newspaper Wadding
-- (1) place 1-sheet of newspaper on a flat surface (i.e. a table-top); (2) place one hand (with fingers extended & spread-out) on top of the center of the newspaper sheet; (3) bring fingers together, grabbing sheet; (4) wad paper into a very-small ball using the fingers of the one hand only (see below for progression)
---> 1st-Phase: Once newspaper sheet is grabbed, keep working-hand next to same-side rib-cage (where grip-strength is strongest)
---> 2nd-Phase: Once newspaper sheet is grabbed, keep working-hand next to opposite-side pec-muscles (where grip-strength is weker, but still fairly strong)
---> 3rd-Phase: Once newspaper sheet is grabbed, keep working-hand stretched-out forward (where grip-strength is weakest)
This exercise was being done with great-effect in the 1920's & is still a very effective exercise to this day
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One more great (old, circa 1930's, advanced) finger exercise was done originally with a magnum champagne bottle, but I did a variation with a baseball bat that worked quite well (although I forgot the exact/given name of it)...
* Necking-Up (?)
-- (1) grab a baseball bat with one hand by the small-end, thumb-up, large-end downward; (2) with fingers of one hand only, "walk the fingers" & raise the bat until fingers are at the bottom (that is the thick end of the bat); (3) add an ankle-weight to thick-end of bat as strength & speed improve (this exercise is a killer, folks)