Human strength is amazing. I'm fairly strong for my size and have done physical work a good part of my life. Still, I'm a city boy.
Back in 1999, I was working on a couple of films as art Dept., which is basically a furniture mover and cordless drill operator. We needed to film a barn scene with lots of haystacks, sort of a Hollywood art director's idea of how the inside of a barn should look. We were shooting in a farm town in MN. They contracted w some local guys for a large load of hay bales to be delivered to the barn we were using for the shot.
They showed up in the morning w a PU truck pulling a huge flat trailer, low to the ground but really big dimensions. It was loaded 10' high at least w hay bales. The plan was to have them back the trailer into the barn, which was quite large, and then we would unload the hay on the far end where the director wanted it. Of course, the trailer got hung up on the threshold of the barn entrance because it was so loaded down w hay. What would have been a fairly easy day became a relay line of guys tossing the bales from one guy to the next and people on the far end stacking it. Did I mention that it was a 100 degree day w 98% humidity? Must have been 120 easily inside the barn.
The farmer who delivered it was in his 40s and his helper was his 17 y.o. nephew. When I say that these guys were the picture of health, I'm not exaggerating. Strong as freaking oxes. I was in the first position, catching bales as they tossed them to me from the trailer. The first couple bales didn't feel too heavy, by number 8 or 10, they seemed to be gaining weight and it was getting hard to toss them to the next guy down the line. By 15 or 20, I thought I was going to pass out from heat stroke. There were about 100 of them. The farm guys did not seem to barely be breaking a sweat. I made some remark about them not having to join a gym and the older guy said, "no gym."
Like others have said, it's a lifestyle thing.