Casey! She correctly answered that running is defined by never having more than one leg touching the ground at one time.
For roughly 20% of the walking cycle, both feet are on the ground–a period known as the “double-support” phase. That percentage decreases as speed increases. But when you stride in a way that you have only one leg or no leg on the ground the entire cycle, you’re officially running. In other words, walking becomes running when you no longer have any double-support period.
Congratulations, Casey! You won a free subscription to Runner’s World!