The same lake can chill one swimmer and overheat another. A new preprint from the University of California, San Francisco and Stanford University puts that into one simple formula for open-water swimming.
Race rules treat water temperature as the test of safety. World Aquatics allows competition between 16 and 31 degrees Celsius. The authors argue that this fixed range cuts across limits that differ from swimmer to swimmer.
A personal tipping point
A swimming body makes heat with its muscles and loses it to the water. The model finds a critical water temperature where the two balance. Below it, the swimmer cools down; above it, they warm up.
For a reference swimmer of 70 kilograms and 175 centimeters, that point sits at 26.5 degrees Celsius. A larger body moves it lower, so bigger swimmers stay warm in colder water. Swimming faster lowers it too, and so does a wetsuit.
Four sets of swims
Henry van den Bedem and Ellen Kuhl tested the model on real swims, from wetsuit swims at 10 degrees to elite races without wetsuits at 25.5 degrees. In one swim in 13.5-degree water, core temperature fell from about 37.9 degrees to below 35 within the first hour.
"No single water temperature can separate safe from unsafe swimming for every athlete," the authors write. The preprint has not yet been peer reviewed.
