[Mountain_Skies]

Another contributing factor that has been theorized is that women are somewhat more buoyant than men due to a different distribution of bodyfat. As a result, they expend a bit less energy keeping themselves afloat and can instead use that energy for moving forward. It's probably no single factor but rather several working together to give women an advantage in long distance swimming.

[thrower123]

At the very tippy ends of the spectrum of humanity that these crazy ultra-endurance athletes are on, some freak genetic mutations can account for significant differences. There have been a bevy of stories about Michael Phelps having the perfect body for swimming. Likewise I recall a nordic skier that dominated because he had a gene that boosted naturally his amount of red-blood cells, as if he were doping. I won't even touch what is going on with intersex runners in track and field...

[Mountain_Skies]

It's interesting to ponder how many people there are who have these far end of the spectrum genetic traits that could give them an edge in some sport but have no idea and may have never even tried out the sport. How many other Michael Phelps are there in the world who never had access to a swimming pool, much less a social environment that would have encouraged taking up competitive swimming?

[obpacheco]

A common misconception about Michael Phelps is that he has an abnormally long wingspan, while it is about average for his height. Studies show that a wingspan is on average 2.1 inches longer than your height and Phelps is 6'4 and has a 6'7 wingspan. To compare some other althetes with actually abnormal wingspans

Draymond Green height: 6'7 wingspan 7'1

Kawhi Leonard height: 6'7 wingspan 7'3