Dr Max Donelan, Simon Fraser University
It seems humans are wired for laziness; in a new study, volunteers walking on a treadmill altered the way they moved under different conditions to minimise how many calories they burned. This is likely to have been something our ancestors evolved to do to restrict energy used when food was scarce. Ginny Smith spoke with Max Donelan, who led the study at Vancouver’s Simon Fraser University.
Max - There's an infinite number of ways to get from point A to point B when you're walking. But people have a preferred way of doing it. They prefer a certain speed, they prefer a certain step length, and step frequency, they prefer a certain step width. And it turns out their preferred way of moving minimizes their energetic cost. When I say energetic cost, I mean, literally the amount of food needed to get them from point A to point B. The question was, even though they had millions of steps experience with their normal way of walking, would they be able to learn the gait that’s now cheaper to walk at, how quickly would they learn it, and for how small of savings would they do it for?
Ginny - The team led by PhD student Jessica Salinger put participants on a treadmill moving at a constant speed and measured the amount of oxygen they consumed and the carbon dioxide they breathed out to calculate their energy consumption. Once they determined each participant’s preferred step frequency and how efficient it was, they then had to do something to make that preferred way of walking more energetically demanding. To do this, they used a special kind of exoskeleton.
Max - There's one in each leg and they look like a very high-tech athletic new brace if you will. It has gears and a motor and we can use those gears and motor to apply a resistance to the knee joint when people are walking that we can control, almost in any way we like to. And so, in this case, we control the resistance as a function of people’s step frequency. That resistance makes walking harder. And so, we can make it so that the resistance is higher at high step frequencies and lower at low step frequencies so that we’re penalising high step frequencies. The idea is that we’d shift the energetic minimum to be at lower step frequencies than what people normally prefer. But we can also give the control function in the opposite direction and shift in the other direction. So the idea allows us to give people new energetic landscapes that render previous predictions about what the right thing to do is, the energetically often thinks to do is obsolete. And then study how people navigate these new energetic landscapes and whether or not they're able to converge on a minimum in these new landscapes, all accomplished with these exoskeletons.
Ginny - The results show that people did indeed adjust their way of walking to expend the least energy possible for the conditions they were under. When food was scarce, it made sense that our ancestors would’ve evolved to preserve as much energy as they could. I wanted to know if Max thought that that was what's going on here.
Max - We don’t know for sure that that’s how we evolve the system and why we evolve the system. But it does make some rational sense that, if you move as cheaply as possible when you're hunting, then you could spend less time hunting, less time exposing yourself to predators, more time reproducing, all the things that increase your evolutionary fitness. So, it does make some sense that you like your nervous system to be helping you in the background to make sure your movements are cheap. But I should add, it does it for a surprisingly small savings. So, that was one of the remarkable things about the finding is that the nervous system was fine-tuning to gain as small as a 5 per cent change in energetic cost. And so, if you're walking for an hour, that savings adds up to be about a peanut worth of calories or kilojoules. That’s a remarkably small savings, literally, peanuts of savings.
Ginny - While a peanut-worth of saved calories hardly seems worth worrying about, this study shows that without us even realising it, our nervous systems are constantly monitoring our way of moving and adjusting it subconsciously. This may have been great for our ancestors who were trying to save energy, but for most of us in the modern world, we’d prefer to burn more calories. Meaning, we’re actually fighting against ourselves.
Max - If you'd try to change your way of moving in some way during this to make it even more expensive for you, what you'd find is that you'd be working against your nervous system in doing so. Let’s say, you put heavyweights on your feet or something like that to make it more costly for your exercise, but what your nervous system is going to do is try and figure out while you're doing the activity, how to adapt your way of moving in order to make that cost as small as possible. So, it’s not that you can't add a cost to it, but your nervous system is going to try and reduce it by as much as it can.