When you run, the arches in your feet really do put an extra spring in your step. According to a new study published in Scientific Reports, each time your shoe hits the pavement, the arch acts like a spring that supplies "free" energy. In other words, you get an additional lift without your muscles having to work for it. But if you restrict the arch's function by wearing orthotics, you can lose up to 6 percent of your body's total energy efficiency, which can slow down your pace.
Surprisingly, the arch's role in running hasn't been well studied. The fact that it has a springlike function was discovered in the 1980s, but nobody had looked at how it impacts runners when that free energy is taken away. Researchers at the University of Western Australia wanted to find out, so they created two types of custom orthotic inserts. One restricted the arch's spring by 50 percent, while the other squelched it entirely. Next, they recruited 17 runners who didn't normally wear orthotics and had them run on the treadmill three separate times, all while having their energy expenditures tracked. For their first run, they wore shoes without orthotics to provide a basis for comparison. The second and third times they sported each type of insert.
Sure enough, wearing either orthotic hindered the volunteers' running efficiency. The insert that suppressed the arch halfway resulted in an average energy cost of 4 percent. The orthotic that blocked it out entirely translated to a 6 percent energy cost. What that means, essentially, is the runners' muscles had to work 4 and 6 percent harder to make up for the free energy no longer available from the arch.
"When your foot strikes the ground, those springs are stretched and they store elastic energy," says study co-author Jonas Rubenson, now a kinesiology and physiology professor at Penn State. "Then the spring recoils to help propel your body forward, just like how when you snap a rubber band, it releases energy and flies across the room." But when you block that spring from storing up energy, such as by wearing orthotics, you'll also miss out on the energy it would otherwise release.
Another interesting finding was that the arch's spring is nonlinear. Many springs, such as the metal coils you'll find at a hardware store, are linear, which means they store and release energy uniformly at every stage of compression and extension. But the foot's arch doesn't work like that, says Rubenson. Instead, more energy is stored up during the end range of compression than the beginning. "That last 20 percent of arch compression gives you the biggest energy bang for your buck," he explains. Therefore, if you restrict arch compression even a small amount, it'll have a disproportionately big impact on energy cost.
The study also confirmed that suppressing the arch while walking doesn't impact energy expenditure much at all. That's because the arch has a different function for walking than it does for running. As Rubenson explains, running requires a spring-like gait while walking does not. Therefore, you don't load up your arch with nearly as much energy when you walk. "So when that energy is removed, it really doesn't matter," Rubenson says. "The stored-up energy was negligible to begin with."
Even though orthotics may cause you to lose some energy when you run, don't go ditching your inserts just yet. The point of this study was not to blast orthotics, Rubenson says, but rather to gain a better understanding of the arch's function. Crafting and testing out different orthotics was simply the best way to do that, and this research underscores just how important the spring action is. Even so, Rubenson insists orthotics are still crucial for many runners because they help correct biomechanical issues and prevent injuries. That benefit likely makes losing a little bit of free energy worth it.
- A new study shows that the arches of your feet act as natural springs when running.
- Insoles that suppress the arch can cost you up to 4 percent of your natural energy.
- Suppressing the arch while walking, on the other hand, doesn't impact that energy.
- Don't ditch your orthotics just yet. The connection between your arch's spring action and injuries — rather than performance gains — isn't well-studied.