There are lots of so-called aero socks out there. But the Castelli Fast Feet sock has the development and testing to back up the claim. The few socks that have been tested have probably been tested with a static, nonmoving test because it's easier. But a foot is always in motion, so we created a jig that replicates the movement of the lower leg. And we test on actual riders to verify the results. This sock can save 2 watts at 50 km/h. The newest version has an updated cuff fit for more comfort and aerodynamic gains.