My grand-daddy worked in a blacksmith shop when he was a young fella, and he used to tell me, when I was a little nipper, how he had toughened himself up so he could stand the hard work of blacksmithing.
One story was how he had developed his arm and shoulder muscles. As he told it, he would stand outside behind the wood shed, with a 5 pound potato sack in each hand, extend his arms straight out from his sides and hold them there as long as he could.
After awhile he tried 10 pound potato sacks, then 50 pound potato sacks and finally he got to where he could lift a 100 pound potato sack in each hand and hold his arms straight out for more than two full minutes. . . .
Then, he started putting potatoes in the sacks…DOH!