For years its has been known that in running, different distances favor different body types. Sprinters are generally more powerfully built and thus have a higher BMI, while distance runners are generally thinner and smaller and have a lower BMI. This is due to the tradeoff between weight and power in land athletes. Stronger athletes generally weigh more due their larger muscle masses, which in turn, requires more strength to move quickly. Larger muscles also require more oxygen transport.
Whether a similar tradeoff exists in swimming has long been unclear. Drag, not weight, is the primary limitation on the speed of swimmers. According to a new study (paywall) published by Christian M. Gagnon, Michael E. Steiper, and Herman Pontzer of CUNY, swimmers face no such tradeoff between body mass index, or BMI, and endurance. The study, published in Proceeding of the Royal Society B, examined swimmers who completed in the London Olympics in 2012. They showed that swimmers competing there did not have statistically different BMIs across a wide range of events, from the 50m to the 10,000m open water competition. The BMIs of most swimmers completing in London fell within a fairly narrow range – 22-24 for the men and 20-22 for the women. The study also looked at the height and weight of swimmers across various distances and found insignificant differences in each for both genders. Runners, in comparison, get significantly smaller and lighter the longer the distance they compete in.
Only freestyle events were considered here, so whether there may be differences in ideal body types for different stokes was not looked at. Also unknown is whether elite swimmers have these body types naturally or whether intense training causes swimmers to naturally converge to a single body type.
BMI is just a ratio of a person’s weight to the square of their height, so does not take into account other body factors such as body fat percentage. It remains to be seen whether sprinters exhibit any other body types differences compared to endurance swimmers. Another recent study (paywall) in the Scandinavian Journal of Medicine & Science in Sports by Bex et. al. showed that endurance swimmers tend to have a higher concentration of slow twitch muscle fibers as compared to sprinters. Unlike BMI, slow-twitch muscle fiber composition is something which is measurable without special equipment and techniques, proton magnetic resonance spectroscopy in this case.
Based off of the study, it certainly isn’t advisable for coaches to pigeonhole swimmers into certain events or distances based on body type alone. Certainly however, much work remains to be done to find what sort of biomechanical or body-type tradeoffs exists for swimming speed vs. endurance.