Jedd over at Diesel Crew has posted an article about sledgehammer training for nail bending strength.
Isometric Hammer Work for Bending Success
This isn't completely revolutionary stuff. If you've read John Brookfield's Mastery of Hand Strength, you'll recognize most of these. But unlike that book, this is a free article. Levering a hammer - holding it by the handle and moving it around with grip and arm strength - is a great way to train, and it's tremendously difficult. Jedd's article has a lot of good information on how to do it. If you have a hammer and want to get a better grip, read it!
2 hours ago