I'm sure the higher-end treadmill models have better interfaces, but this one in particular uses a standard audio jack which uses encoded sounds to control the various speed and incline settings found on the treadmill. For example, playing the sound encoded for speed-3, incline-7 will set the treadmill accordingly. As you can imagine there are quite a few of these sounds. I was able to acquire these sounds after a little prying into the JavaScript for the treadmill application provided by the vendor.

Sound controlled interface?! Don't they realize that is how the Decepticons stole all our government secrets?

Controlling your Treadmill from Silverlight