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If you’ve watched gymnastics in Rio, you’ve probably noticed that the gymnasts are pretty small, and that Simone Biles, the greatest gymnast probably ever, is small even compared to her American teammates. But at 4-foot-8, Biles is actually only slightly smaller than her peers. In fact, over the past 30 years, the average elite female gymnast has shrunk from about 5-foot-3 on average to about 4-foot-9.
Why are elite female gymnasts getting smaller? Because the more demanding gymnastics routines have become, the bigger an advantage it is to be small. A smaller gymnast not only has a better power-to-weight ratio. She also has a lower moment of inertia .
You can think of moment of inertia as essentially a measure of a body’s resistance to rotating. The higher the moment of inertia, the harder it is to rotate the object. And larger bodies with more weight far away from the axis of rotation have a higher moment of inertia.
Think about figure skaters. You’ve probably noticed that when figure skaters spin, they start rotating much more rapidly when they bring their arms close in to their chest. By moving their arms in, they’ve decreased the amount of weight that’s far away from the axis of rotation and they’ve decreased their moment of inertia, making it easier for them to spin at high speed.
The smaller a gymnast is, the easier it is for her to rotate in the air. You may even notice that during some movements, gymnasts tuck their legs when they start a spin or a turn in the air. That lowers their moment of inertia.
It’s not really clear if elite female gymnasts are still continuing to get smaller. At some point, you might expect that being too small would have disadvantages, like making it more difficult to sprint quickly enough to build up speed for the vault or to execute complicated floor routines. For now, it seems like 4-foot-8 is working out just fine for Simone Biles.
David Epstein is a writer for ProPublica. He is the author of The Sports Gene: Inside the Science of Extraordinary Athletic Performance .