See these guys? They’re *racewalking*. It’s like running in that you do it as fast as possible, but you’re not allowed to have both feet off the ground at any time. One foot has to be planted on the track at all times. The guy to the far right is slightly cheating – both his feet are off the ground.

But take a closer look at the foreground guy in the red, white, and blue. He illustrates the proper form very well. Think a little abstractly and imagine how the leg moves. The foot is planted and so the leg forms the radius of a circle. The hip therefore traces out part of that circular arc until at its forward point that foot comes off the ground and the same happens on the other leg.

And that’s uniform circular motion, which we know how to do. We’ll put the acceleration we need for circular motion on the right, and we’ll put the acceleration we actually get from gravity on the left:

Here g is not always pointed toward the center of the circle, so this equation is not quite right, but it’s pretty close. Solve for v:

Now let’s say the average person’s leg is a meter long. Plug in the figures, and I get a speed of 3.13 meters per second. You can’t walk any faster than that. Gravity won’t keep you held to the ground if you try to go faster than that.

But that’s only an 8.5 minute mile. Racewalkers can go faster than that. What gives? The answer is that they use a funky walk where they swivel their hips, flattening out the circle. That makes the effective r larger and lets the fastest racewalkers reach paces of a little over a six minute mile, or about 4.3 meters per second.