lundi 5 octobre 2015

You’re Not Lazy, You Just Quickly Minimize Exertion, Study Suggests

You’re Not Lazy, You Just Quickly Minimize Exertion, Study Suggests

(Peter Mooney/Flickr Creative Commons)

(A new study finds we quickly change our gait for greater efficiency. (Photo:Peter Mooney/Flickr Creative Commons)

By Josh Eibelman
CommonHealth intern

If you’re feeling guilty and blaming yourself for being lazy, take heart: We evolved to minimize how much we move, and new research suggests we adjust our bodies quickly to expend the least possible energy.

In a new study, “Humans Can Continuously Optimize Energetic Cost During Walking,” published in the journal Current Biology, researchers found that people optimize their gaits — the manner in which they walk — in real time in order to expend less energy.

Subjects in the study were fitted with exoskeletons that forced them to walk in abnormal ways. The scientists found that participants automatically fine-tuned their manners of walking to more energetically efficient ones in response to the exoskeletons.

I spoke with Jessica Selinger, lead author of the study and a doctoral candidate at Simon Fraser University, to learn more.

How would you sum up your results?

What we found was that people quite readily will tune or change really fundamental characteristics of their gait –characteristics that have been established over millions of steps over the course of their lifetime — in order to move in a way that uses the least amount of energy.

That’s probably intuitive for a lot of us. We know that we like to do things that require the least effort and do them in the least effortful way. I might prefer to take a bus to work when I could walk or I might prefer to sit when I could stand. But what’s really interesting is that even when you make a conscious choice to exercise or spend energy, what our study shows is that your nervous system is optimizing and tuning behind-the-scenes your movements so that you’re burning the fewest calories possible.

What message do you want people to take away from these findings?

For one, it’s really remarkable that the body can do this. There are countless ways that someone could walk from point A to point B. We can choose different speeds, step rates, and even muscle activity patterns, yet we have very strong preferences for particular gaits — the energetically optimal gait. It’s really amazing that our body is able to home in on what is the most energetically optimal way to move. It’s a complex problem and an impressive feat. You have to be smart to be that lazy!

And the other really interesting thing was that that people would adapt their gait even in response to very small savings in energetic cost. We’re talking about just a few percent of the body’s total energy use. It seems that the body is really sensitive to this measure. Energetic cost is not just an outcome of our movement, it is continuously shaping the way me move.

Can people do anything to counteract this laziness?

Yes, absolutely. We can make conscious choices to override this wired laziness. As I said, we can choose to get off the couch or go out for a run. But when you do, your body is still going to be tuning your movements so that it’s doing it in the most efficient way possible. That can be viewed as a good thing or a bad thing, I suppose. If you are running just to lose weight, then maybe you wish you weren’t as efficient or your nervous system weren’t as clever. But when you’re running in a race or marathon, you want your body to be quite efficient and save some of that energy for the final push.

How do you think people became so good at optimizing movement?

Well, we’ve known for a long time that people do walk in the way that’s most energetically optimal. That’s not the new finding. We know that out of a whole range of gait parameters that someone can select — their speed, step frequency, or muscle coordination — they choose those that are metabolically efficient. But what we haven’t known, and what this study has added to the body of literature, is over what timescale this preference is formed.

You can imagine that this could have occurred over evolutionary time. We might have evolved to move this way. Or it might have occurred over the course of our lifetime. Perhaps this took years and years of walking to learn. But what we’ve shown is that it’s something we can continually optimize in real time, over quite short time scales. At the moment we are conducting experiments to try to figure out exactly how the body solves this very complex optimization problem so quickly.

In other words, the findings showed it was not evolutionary or it was both evolutionary and during the course of a lifetime?

What our study showed in particular is that this is something that happens continuously, in real time. It is still likely that we have evolved the ability to do this, but it’s not the case that our chosen gait is just hard-wired. We can adapt to our surroundings remarkably well and tune our movements to new environments and constraints.

That was the motivation behind using the exo-skeletons. We wanted to drop subjects into a totally new world where suddenly there are new rules around the optimal way to move. That way any walking strategies they may have learned over evolutionary or developmental timescales are obsolete. Instead, in order to move in an optimal manner in this new world, they have be able to sense and optimize energetic cost in real time. And, we found, this was indeed what they did.

Do you think these findings have practical considerations for someone who does exercise? Say someone who is running — should they change the way they’re thinking about running?

I don’t think our results suggest that someone should change the way they run, at least not if the goal is to improve economy. In some running circles, people advocate that athletes should run at a certain cadence or change their cadence so they are no longer at their preferred. Our results suggest that you probably can’t do much better than what your nervous system is already doing. Your body tends to pick the cadence that is optimal for you.

And that brings me to my last question: I saw that the U.S Army provided a grant for the study, and I was curious whether there was a specific reason for that.

Max [Donelan, the principal investigator of the research group] was awarded a grant from the U.S Army. The money from this grant has been used to explore fundamental research questions, aimed at understanding why people move the way they do. The army is also interested in the application of these principles in the design of exoskeletons or assistance devices, potentially to restore or even enhance movement.

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