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The study found that the central nervous system is constantly working to make sure we are moving in the most efficient ways possible.TORU HANAI/Reuters

Jessica Selinger was developing a high-tech energy-harvesting knee brace when she got sidetracked by a more fundamental question. "Why," wondered the PhD student at Simon Fraser University's Locomotion Laboratory, "do people move the way they move?"

Every move you make is actually a symphony of staggering complexity. To simply scratch your nose, your brain has to choose which of your millions of muscle fibres should contract in which order – a task that can be achieved in a near-infinite number of seemingly equivalent ways.

And when figuring out which of all possible combinations to go with, your body tends to pick the one that expends the least amount of energy.

Selinger, along with lab head Dr. Max Donelan, used the knee brace to explore the hard-wired "laziness" of the human body. Their results, published last week in the journal Current Biology, suggest that our continual and automatic adjustments of our movements are in the service of burning the least energy possible. "For 50 years at least, we've known that people prefer ways of moving that minimize their energetic cost," Donelan says. "But what's unknown is how we do this."

For example, when you walk at any given speed, you'll settle into a cadence – the number of steps you take per minute – that minimizes your energy cost at that speed. It's a remarkable feat of physiology that invites practical questions: Is there any benefit to learning a new running style, for example, if your body has already figured out its most efficient stride?

It's not clear whether this trait has developed through millennia of evolution, whether we learn these patterns individually as we grow up, or whether we're constantly learning and relearning them on the fly.

To try and find out, Selinger adapted a leg-mounted exoskeleton previously developed in Donelan's lab. (That exoskeleton harvests wasted energy much like regenerative brakes in hybrid cars, and can charge four cellphones after an hour's walking; it is being commercialized by a spinoff company called Bionic Power and has received millions of dollars of funding from the Canadian and U.S. militaries.)

Selinger used the brace to apply a slight resistance at the knee, making walking relatively harder at either faster or slower cadences. That meant the cadence that subjects instinctively chose was no longer the most efficient one. "We're dropping people into a new world with new rules," Donelan explains.

In the initial experiments, Selinger fitted subjects with the knee brace, turned on the resistance, and … nothing happened. The subjects kept walking at their initial cadence. "That was a huge surprise to me," she admits. Simply walking on a treadmill at a steady pace, the subjects had no reason – either consciously or unconsciously – to suspect that it would feel easier if they adjusted their cadence.

In the real world, of course, variations in terrain and gradient force people to change their stride frequently. So in the next stage of the experiment, the subjects were led through a "guided exploration" of their new world, following the cues of a metronome to increase and decrease their cadence to varying degrees. The resistance added by the brace was subtle enough that the subjects weren't consciously aware of how it was trying to alter their strides; some thought (incorrectly) that the treadmill speed was changing.

But when Selinger once again allowed them to freely choose their cadence, the subjects immediately settled into the new point of maximum efficiency. A few minutes of exploration had allowed their brains to absorb the rules of the new world, overruling a lifetime of experience under the old rules. This suggests that the body is indeed capable of continuously optimizing energy consumption.

The subjects saved a remarkably small amount of energy: Over the course of an hour of walking, Donelan estimates, the savings would amount to roughly the amount of energy contained in a single peanut. It's a testament to how extraordinarily energy-conscious – or, if you prefer, lazy – our bodies are.

Exactly how the body makes these adjustments remains a topic of considerable controversy, notes Dr. Samuele Marcora, a physiologist at the University of Kent in Britain (who was not involved in Selinger's study). Rather than minimizing energy cost, he said in an e-mail, the subjects could simply be choosing the stride pattern that feels easiest – a choice that sometimes, but not always, coincides with energy efficiency.

Selinger and Donelan hope to answer that question with further experiments on how the body might be able to make real-time measurements of energy cost. Among the candidates are internal sensors that detect muscle force and elongation, oxygen and carbon dioxide levels in the blood, or effort-related metabolites such as lactate.

For now, the safest conclusion is that whatever activity you undertake, your central nervous system is working hard to make sure you do it as efficiently as possible. That means that, in the short term at least, deliberately changing your running stride (by increasing cadence, for example) will almost certainly make you burn more energy.

That doesn't mean it's impossible to get better at activities such as running, but it suggests that you can't simply decide to get more efficient. You actually have to change your body – undertake the slow and painstaking process of building new muscles and ingraining new habits. Once you've done that, the good news is that your brain will automatically take care of the rest.

Alex Hutchinson blogs about exercise research at His latest book is Which Comes First, Cardio or Weights?

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