The advertisements are compelling. Adidas claims its Boost midsole material will “keep every step charged with an endless supply of light, fast energy.” Puma’s IGNITE foam promises, “Energy in. More energy out.” Saucony says its new Everun foam will give you “increased energy return,” and “a lively underfoot sensation.” Other companies are clamoring to join the energy-return movement.
Who wouldn’t want running shoes that not only protect your feet, but also give you energy as you go? Before you shell out the bucks for a new pair of trainers, however, take a minute to determine what is true, what is implied and what “energy return” means.
While the ads may imply it, no material will actually produce energy that propels you down the road. These foams aren’t contradicting the law of conservation of energy. One of the fundamental laws of physics states that no system creates or destroys energy—energy can only be transformed. Any energy being returned from the shoe was first put in by your stride.
Martyn Shorten, biomechanics expert and head of the Runner’s World Shoe Lab in America, uses a device that compresses the soles of shoes then measures the percentage of the input energy that is recovered during rebound. A shoe that returned all of the energy would score 100 percent. Most shoes with traditional EVA foam tend to fall in the 50 to 60th percentiles. The best new foams are returning 70+ per cent of the energy.
That still leaves 30 per cent of the energy that sinks into the shoe, being lost to heat. So don’t expect any superhuman propulsion pushing you along.
What the materials do accomplish is reduce the amount of energy being lost by the shoe. And that’s good—nobody wants to be wasting energy with each stride. But does it make you a better runner? Maybe. Two factors to consider are scale and timing.
Shorten points out that the energy and recovery forces coming from the muscles and tendons of your legs during running are more than 10 times that which occur in the shoe. Since all shoes return some energy, the difference in energy return between the best and worst shoes represents at most one percent of the energy of your stride. To a runner, one percent of time or effort is worth gaining, but you’d see these gains only if all the energy that comes back from the shoe is translated directly to making you faster.
The fact is, even if the sole did bounce back harder than it was compressed, it is unlikely it could propel you in any meaningful way. Running involves a complex interaction of forces that generates from your muscles, tendons, and bones in a context of gravity and friction. In order for that energy to be applied to your stride, it has to be returned at the right time, the right frequency, and the right location, explains biomechanical expert Benno Nigg in his book Biomechanics of Sport Shoes.
The industry has yet to perfect all of these variables, whether the rebound is from a bouncy foam or mechanical trampolines, tubes, or springs such as those used by Newton, On, and Spira.
Shorten also explains that the timing and frequency are different for every runner’s weight, stride, and speed. “Energy return is not a property of a shoe or a material,” Shorten says. “It is an outcome that depends on the forces applied.”
What feels right for one runner may swallow the stride of the next one. “It’s a matter of how your body is working, not just how the shoe works,” White says. The more fine-tuned you make energy return, the narrower the range of runners it will work well for—further reducing the chance of it matching you perfectly and reducing your energy expenditure.
Consider also that other variables can and do change your energy use as much or more than the rebound of the sole. “This [energy-return foam] material is quite heavy,” points out Simon Bartold, podiatrist, biomechanical expert, and current consultant for Salomon. “Weight has a large impact on the energetics of running. Studies have shown that for every 100 grams added at the foot, the energy cost is increased by one percent.”
Shorten, while not ruling out possible benefits from a rebounding midsole, says, “The potential and as yet unproven benefits of energy return must be weighed against the known performance and injury prevention benefits of cushioning, stability, light weight, and flexibility.”
Even if midsoles aren’t actively returning energy to your stride, don’t dismiss the new foams altogether. Something else is going on that makes you feel good when running in them. “These materials give instant gratification,” Bartold says. “There is no question they feel fantastic at first try-on.”
What you’re feeling underfoot is a combination of cushioning and responsiveness. Both of those words can and have been used to mean a variety of things.
When it comes to cushioning, we tend to think mostly of reducing the impact of landing. “We are not certain we are looking at the correct component of cushioning,” Bartold says. “If we are only talking about attenuating the initial impact, or even the initial rate of impact (called the loading rate), there is not very strong evidence to say this is important. Other forces are at play that might be far more important.”
Studies show that your muscles and tendons do a far better job at reducing overall impact forces than the sole of your shoe can. A more important function of cushioning may be spreading the load out across the foot. This would explain why those who switched to minimal shoes a few years ago often cured their hip and knee problems, but got fractures in their toes or foot bones.
When cushioning is all about impact, increasing it traditionally lowers the responsiveness of a shoe. Responsiveness denotes a shoe you can “pop” off quickly and with more force—a shoe that feels fast. “Responsiveness ended up being firmness,” says Brewer about how midsoles are usually created. “And cushioning and responsiveness were mutually exclusive.” You either had a firm shoe that let you put your energy directly into propulsion or a soft one that cushioned by absorbing and dissipating that energy.
High energy-return foams attempt to combine these two properties. They deflect more than traditional foams of similar density, letting protruding parts of your foot sink in, then molding around them to spread out forces across your sole. At the same time, they store the energy of that compression better and return to their original shape quickly, providing a responsive feel as they push back on the bottom of your foot during push off.
“It’s all about creating a platform that allows the foot to land and bounce back, reducing energy loss to the point where it feels like hyper cushion, hyper bounce back,” says Brewer.
Getting propulsion from “endless energy” is science fiction and not something you should expect from a shoe. Shoes aren’t the engine, but they are like tires and shock absorbers, tuning the connection between the drive train and the ground.
New foams provide an enjoyable, bouncy feeling that can make you feel energized if they match your stride dynamics. This “energy” comes from a unique sensation that combines the softness of a cushioned shoe with the powerful, quick turnover of a responsive one.
It’s no wonder every company is coming out with their own “energy” foam.