In competitive running, body type is destiny: If you’re a sprinter powering yourself over a short stretch of track, you better pack a lot of muscle onto your frame. But all that bulk is too much to carry through longer-distance endurance races, which favor more of a whippet type.

Swimmers, though, somehow seem to escape that endurance/speed/size trade-off, according to a new study published in July in Proceedings of the Royal Society B, which for the first time closely examines the morphology of a large group of world-class swimmers and runners. Its findings provide evidence that while there is a body type associated with success in each sport’s various distances, in swimming that body type is consistently the same.

For the research, three anthropologists with an interest in human physical performance gathered publicly available biometric data about the male and female swimmers and runners at the 2012 Summer Olympic Games in London. The information included each athlete’s height, weight and competitive events. (The researchers focused on freestyle swimming races, for the sake of simplicity.) The scientists used that info to calculate each athlete’s body mass index; differences between the B.M.I.s presumably would indicate differences in musculature, because competitors at their level do not carry much fat.

The resulting graphs plotting B.M.I.s and events looked quite different by sport. In running, B.M.I.s dropped precipitously as event distances increased. Two-hundred-meter runners were considerably more massive than marathoners. But there was no similar drop among swimmers. Those contesting the 50-meter freestyle shared a similar body mass with those swimming the two-hour, 10,000-meter open-water marathon.

This relationship held true whether the swimmers were tall or short, male or female, although, not surprisingly, the typical B.M.I. for male and female swimmers differed. Among male Olympic swimmers, B.M.I.s hovered around 23, whatever their event length. (That B.M.I. would be considered “normal weight” for nonathletes, but those tabulations are based on people who are not mostly muscle.) Female Olympic swimmers had a B.M.I. of around 21.

What these figures suggest, most obviously, is that top swimmers, unlike runners, do not necessarily need to be slight to race long, says Michael Steiper, a professor of anthropology at Hunter College, who conducted the study with the master’s student Christian Gagnon and the evolutionary anthropologist Herman Pontzer, who are now at Boston University and Duke University, respectively. “For runners, there is a cost to carrying extra mass over long distances,” Steiper says. “For swimmers, there is less cost to having more muscle mass.”

But the study’s broader subtext suggests that a short, well-muscled fireplug could out touch a lean six-foot swimmer in any distance, if both share a similar B.M.I., which happens to be about 23 for men and 21 for women. Swim events, in other words, can welcome wildly different athletes — an inspiring possibility for many of us.


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