Not that it matters but I thought it was his abnormally large heart or lungs that made him a monster athlete?
I thought it was his big feet made him better at the kick.
I thought it was his enormous teeth and dorsal fin that- no wait I’m thinking of someone else.
It’s a lot of things. You’d be hard pressed to engineer a better swimming body, and he has the mental drive to take full advantage of it.
Why Michael Phelps Has the Perfect Body for Swimming

With his extra-long torso and flipper-like feet, it’s no wonder Olympian Michael Phelps earned the nicknamed the “Flying Fish.”

Biography
I knew a guy who swam at the same college under a scholarship, and this guy was only like 5’9” but worked his ass off. Swam with Phelps a couple of times and then realized he might not be destined for swimming greatness.