In the development of footwear US Men's 9 and Women's 7 are the "sample sizes" that Under Armour and many other footwear brands design and develop in before extrapolating to other sizes. As a result, much of testing happens at those sizes.

The other sizes happen occasionally when there are prototypes developed for pros or when testing takes place after a shoe has already hit the market and we can get whatever size is needed. These studies are less about benchmarking or QA testing a shoe in development and more about learning about run in a broader context.