How your arches make you a faster runner

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Each time your foot strikes the ground on a run, your body claims something valuable that few species can - free energy from the arch in your foot.

A recent study published in Scientific Reports examined the role of the human arch while running. The authors wanted to see how energy is stored in the arch each time your foot hits the ground, and more specifically, how much is lost if you restrict the arch with orthotics. They discovered that blocking arch compression by even a small amount causes your energy efficiency during a run to decrease by up to six percent.

They also note that the energy you get from your arch is “free.” Instead of requiring muscles to expand and contract for forward momentum, the arch uses the ground to compress like a spring, according to study co-author Jonas Rubenson. The tension and release of the tendons provide a mini-boost to propel you forward, kind of like a pogo stick. 

Except unlike a pogo stick, “the arch is a non-linear spring,” Rubenson says. This means it becomes harder to compress the arch the more a force is applied. A pogo stick, which is a linear spring, requires the same amount of force when it starts to compress and when it is completely coiled. 

This distinction becomes important when the spring-like arch releases. The study authors found that while running, most of the arches' boosting power is stored in the final 20 percent of compression. That means if you restrict the compression of the arch by just a small amount, it could cause you to lose up to six percent of your total energy efficiency each time the arch uncoils. Put more simply, if you block the arch from doing what it wants to - compress and release - you will waste energy on your run.

It also means that muscles in other parts of your legs and around your hips have to work a bit harder to push you forward when your arch can’t function properly. 

“I think this just shows how important your arch is,” Sarah Stearne, the study’s lead researcher, said. “Six percent is actually quite a big difference.”

To make their calculations, Stearne and her colleagues fashioned two types of custom orthotics: one that completely blocked the arch from compressing, and another that allowed the arch to compress halfway. They recruited 17 runners who don’t normally use orthotics to test the insoles while on a force-plate treadmill. Sensors inside their shoes measured energy expenditure. To get a baseline measurement, the participants ran in just shoes first, then ran in the same shoes at the same speed while testing the insoles.

In the orthotics that limited compression by 50 percent, researchers measured an average energy cost of just over four percent, while the orthotics that blocked all compression caused another two percent jump in energy loss. 

Rubenson makes it clear that these findings should not make you trash your orthotics. “We don’t want to say orthotics are good or bad,” he said. That’s because for many runners custom insoles prevent injury, which is more important than the slight amount of energy lost while using them.

The more important finding, Rubenson said, is just how vital the arch is specifically when running. The study participants tested the insoles while walking and found hardly any difference in energy expenditure. It was only when they began to run that significant energy loss occurred.

So, as Rubenson and Stearne agreed, instead of worrying about whether or not you should ditch your orthotics, simply appreciate the tiny power boosters in your feet. And the best part, they don’t require much energy to activate - just the road or trail beneath you.

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