There are lots of so-called aero socks out there. But the Castelli Fast Feet 4 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 tested it 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.