As a science fiction writer, I really wish I knew what it’s like to walk on the Moon or Mars or any other low gravity world. It would help a lot with that whole “writing from lived experience” thing. Of course there are ways I could experience hypogravity for myself, but I don’t have that kind of money. So instead, I’ve turned to medical research papers like this one from Frontiers in Physiology.
First off, let me just say this: I’ve read some really complicated stuff over the years, but I don’t think I’ve ever read anything as complicated as a scientific paper trying to describe how we humans walk.
But if we want to understand what it would really be like to walk on another planet, we have to start by understanding—in meticulous mathematical detail, apparently—how we do this walking thing here on Earth.
Gravity Makes Walking So Much Easier
The mathematical relationship between walking speed, leg length, and gravity was determined back in the 1870’s. It was later used in what sounds like a rather whimsical research paper about the walking pace of the Lilliputians from Gulliver’s Travels. And then it was used for more pragmatic purposes to estimate the running speeds of dinosaurs.
For those sorts of calculations, the force of gravity would have been treated as a constant, but gravity can easily be treated like a variable, and that’s when things get interesting. You see, when you walk, your body uses energy to complete the full arc of a footstep, especially at the beginning when you’re lifting your foot off the ground. But gravity helps you (perhaps more than you realize) when your foot comes back down to the ground.
So if you reduce the force of gravity, gravity provides you with less assistance, and you end up having to expend more energy to complete each step in your walk cycle.
Walking-Mode vs. Running-Mode
The muscle actions involved in walking and running are different enough that there’s no real grey area between “walking-mode” and “running-mode,” as that paper from Frontiers in Physiology calls them. These two “modes of locomotion” take advantage of gravity in distinctly different ways. Walking-mode ends up being more metabolically efficient at slower speeds, and running-mode is the more metabolically efficient way to travel at higher speeds.
So what happens when you alter the force of gravity? The transition point where running-mode becomes more efficient than walking-more changes too. Lower gravity means your body will naturally want to switch modes at a lower speed.
On the Moon, for example, walking-mode only works well when you’re moving very slowly. To achieve what we might consider a normal walking pace, you’ll have to switch to running-mode. And if you want to reach Earth-like running speed, you’ll probably have to try hopping-mode or jumping-mode—modes of locomotion that we don’t use often here on Earth except under certain specialized circumstances. Skipping-mode also seems to be more metabolically efficient on the Moon than it is on Earth.
Moon-Walking or Mars-Walking in Science Fiction
I’ve read plenty of Sci-Fi stories set on the Moon or Mars. For the most part, I feel like science fiction writers just mention the reduced gravity thing in passing and then move on with the story as quickly as possibly. I don’t blame them. It’s really hard to imagine what hypogravity must feel like, and even harder to communicate that feeling to readers.
But one of my highest ambitions as a writer is to write something that makes you feel like you’re there on the surface of a hypogravity planet like Mars. I want to capture that experience of “running in order to walk” and “hopping in order to run.” Hopefully this line of research will someday help me pull that off.