When Lucy Charles-Barclay withdrew from the Ironman World Championship last year, the coverage focused on the drama of a top contender's race unravelling. From where I sit, it looked like a data problem that hadn't been solved in time.
Her core body temperature almost certainly exceeded the threshold at which performance stops being a matter of fitness and becomes a matter of physiology—heat is a blunt and indiscriminate force, and it overwhelmed her regardless of everything else she had done right.
