Al Pratt was the guy in the muscle ads that got sand kicked on him-he was 5'1" and 98 lbs. He was trained by Joe Morgan, who also trained Wildcat and the Guardian. Originally, he was a tough scrapper, later on due to exposure to radiation in a battle with Terry Curtis, also known as Cyclotron, he developed super-strength, speed, reflexes, durability and was immune to radiations. He could also charge his super-strength and release it in a powerful blow that was called the Atomic Punch.
Atom is the first guy into a fight and the last guy out. He's the classic Irish scrapper guy, even though he was also a scientist. He was loosely based on real-life strongman Joe Greenstein who was known as "the Mighty Atom", and in fact the Atom was sometimes billed as that.
One of the interesting things about the JSA is how the team was split into the blue bloods and the blue collar guys. You had guys like Carter Hall, Charles McNider, Bruce Wayne, Kent Nelson, Terry Sloane, Ted Knight, etc who were wealthy aristocrats born into money, then you had more average joe guys like Jay Garrick, Al Pratt, Clark Kent, Jim Corrigan, Ted Grant and Johnny Thunder. And Alan Scott was a working guy who got rich.
Class doesn't come up in comics as much as it should, except with Spider-Man being broke and Batman being insanely rich. I think it should.