Why do people move their arms when they walk?

Let’s say you’re watching a bunch of people run a marathon. The runners are pumping their legs, sure. But they’re also swinging their arms. Why is that? And why do we swing them when walking too?  We asked Rachel Adenekan of Stanford University to help explain why that swinging motion is so universal. Got a Moment of Um question that’s keeping you at arm’s length? Send it to us at BrainsOn.org/contact, and we’ll point a finger at the answer.