http://www.nytimes.com/2014/10/19/sports/alan-webb-grinds-out-a-bumpy-transition-to-triathlon.html
He was a competitive swimmer as a child, and baseline testing by USA Triathlon revealed that his swim times over shorter distances were comparable to those of top triathletes. He lacked endurance for the Olympic distance, but Hall believed it would come. Webb has always been efficient at processing oxygen — the hematocrit levels of his blood make him an “outlier among outliers,” Hall said — and then there is his unusual pain threshold.
“Alan could always train — so hard,” Ritzenhein, his fellow Olympian, said.