The same is true if you increase your stride length beyond what you are forced to do by the speed of the run. Right leg is shown over a two stride range. Left leg, not shown, alternates the same way. The body always stays erect for efficiency and less back muscles stress. Standing still position requires very little muscle use.
Frame supports the body.
Quadriceps muscles lock knee as the body is shifted forward. The shifting is done by the weight being placed ahead of the grounded foot by releasing the pressure of the toe, and swinging the raised foot forward. No power to vault is exerted by gravity. Air time is the slowdown time in the stride cycle. It shouldn't be made longer than necessary to comfortably exchange the feet.