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New research finds why humans walk with a 'double bounce'

AYESHA RASCOE, HOST:

Walking upright is central to human evolution and sets humanity apart. But there's one element of the human gait that's been puzzling researchers for a long time. It's our double bounce. And a group of scientists at the Technical University of Munich have been looking into it. Daniel Renjewski is a mechanical engineer and lecturer there. Thank you for being with us.

DANIEL RENJEWSKI: My pleasure, and nice to have you.

RASCOE: So tell us about this double bounce that humans have.

RENJEWSKI: So I think the easiest way to think about that is to think about our leg as a spring. And so initially, when we hit the ground with our foot, this spring compresses and so the force increases quite a bit, and we have the first force hump. And then we have a rebound. So the spring relaxes again, our whole body moves up and we fall back into the spring. And this gives us the second force hump. And we reached a peak of this hump just before the next touchdown occurs. And so this double-hump force pattern is well known for decades now, but we don't really know why it developed in this way...

RASCOE: OK.

RENJEWSKI: ...And what the function behind it is. And what we found is that it seems to be really crucial to bipedal upright gait.

RASCOE: And that is just - bipedal - that's walking on two legs...

RENJEWSKI: Right.

RASCOE: ...Upright.

RENJEWSKI: Absolutely right.

RASCOE: OK.

RENJEWSKI: Yes. And so there are a couple of factors, if we look at the human gait, that are really special. One is that we are having fairly heavy legs in comparison to other animals and even, like, other bipeds. So if you think the largest population of bipeds on Earth are actually birds...

RASCOE: Oh, my goodness. And their - whoo (ph).

RENJEWSKI: Yeah.

RASCOE: I'm sorry. I don't like birds.

(LAUGHTER)

RASCOE: No, but they do have very skinny legs - very skinny legs.

RENJEWSKI: They have very skinny legs. Yes. And so we have developed a quite special kind of locomotion with walking.

RASCOE: OK. And so what did your study find about why we walk in this unique way, specifically with that double bounce?

RENJEWSKI: So what we found is that it is really crucial in order to walk economically, to load your ankle joint, to then release this torque really explosively like a catapult to shoot your leg forward. And in order to facilitate this ankle loading, we think this is the reason why the second force hump developed. We looked a little bit into the evolution of human gait. And what we found is that our ancestors did something that is called endurance hunting. And endurance hunting is something where animals essentially just run away from you and you don't run after them, but you just walk after them. So you essentially walk more efficient - like, running is a little bit more inefficient, so you spend more energy on running. And if I chase you, but I don't run after you, but just walk after you, you will essentially spend more energy, and I give you less time to recover.

RASCOE: Wow.

RENJEWSKI: And so I can do that...

RASCOE: That's like Michael Myers.

RENJEWSKI: ...For a very long time. Yeah. Yes. Yeah.

RASCOE: That's like Michael Myers. He walks down his prey.

RENJEWSKI: Right.

RASCOE: I get it. I get it. OK.

RENJEWSKI: And so that might be a reason why we developed this kind of gait.

RASCOE: Wow. I done got - I done learned some things just in this conversation, but what is the significance of this discovery?

RENJEWSKI: Yeah.

RASCOE: Like, how can you apply this information?

RENJEWSKI: So when it comes to remobilization of humans - like building prostheses, building exoskeletons, also thinking about gait rehabilitation - you want to know what the mechanics of walking are, and you really want to implement them in these devices. And if we look at prosthetics today, they definitely expend more energy than if we walk on our natural two legs. And so I think this is really the important application here - a different way to build prosthetics, to build gait rehabilitation devices, and to control also exoskeletons.

RASCOE: That's Daniel Renjewski of the Technical University of Munich. Thank you so much for joining us.

RENJEWSKI: My pleasure. Have a great day. Transcript provided by NPR, Copyright NPR.

Ayesha Rascoe is a White House correspondent for NPR. She is currently covering her third presidential administration. Rascoe's White House coverage has included a number of high profile foreign trips, including President Trump's 2019 summit with North Korean leader Kim Jong Un in Hanoi, Vietnam, and President Obama's final NATO summit in Warsaw, Poland in 2016. As a part of the White House team, she's also a regular on the NPR Politics Podcast.