What About Energy Return In Your Running Shoes?

How many of you recognize this shoe? Reebok ERS 5000- from 1988

Runners, don't you want running shoes that not only protect your feet but give you energy as you go?
It seems that many shoe companies are joining the energy return movement again. Why did I say again?

Well, looking at the picture above, how many of you will know what I'm talking about if I mention "ERS"? To be specific, the Reebok ERS or energy return system technology Reebok came up with in 1988 to compete with the Nike Air technology. That was way back in 1988!

Let me sidetrack a little. The ERS comprises of a series of cylinders made from Dupont Hytrel placed in the midsole of the shoe to act as springs. This was meant to help propel the runner forward after foot strike or so Reebok claims. (Note - ERS was phased out when Reebok invented Hexalite their next technology). Come talk to me if you wanna discuss shoe technology from the 80's and 90's.

The Adidas Boost

In the present day, there's Adidas claiming its Boost midsole material will "keep every step charged with an endless supply of light, fast energy." Puma's IGNITE foam says "Energy in. More energy out." Saucony tells you its new Everun foam will give you "increased energy return" and a "lively underfoot sensation."

When a running shoe cushions well, it also lowers the responsive response of that shoe (since it has to absorb the force). A responsive shoe means firmness in that shoe and that firmness allows you to transfer the force from your stride into running faster.

And cushioning and responsiveness are mutually exclusive. Either you have a shoe that is soft and cushions your landing by dissipating that energy or a shoe that allows your to put your energy directly into propulsion.

These new energy return shoes try to combine the two properties. They absorb more than the previous traditional foam used in running shoes and then store that energy that was absorbed to return the foam to original shape quickly producing a responsive feel as they push back on the bottom of your foot during push off.

Despite what the ads say, no material can actually produce energy that can propel your next running step. No foam can actually do that. The basic law of physics states that no system creates or destroys energy. Energy can only be transformed. If anything, what energy that is returned tends to be generated by your own stride in the first place.

Even if the foam did bounce back harder than it was compressed, it is unlikely it can propel you in any meaningful way. This is because running involves forces generated by your muscles, joints, tendons and bones along with gravity and friction. In order to get that "energy return", the energy has to be returned at the right time, frequency and right location (Nigg, 2010). It just doesn't happen so easily.

The running shoe industry has yet to get all these variables working together, regardless whether the rebound is from foam, tubes, springs or mechanical trampolines used by Newton, Spira or any other brand currently.

Moreover, the current energy return foam is fairly heavy. Weight has a large impact on running shoes. Previous studies show that for every 100 grams (or 3.5 ounces) added to your foot, energy cost is increased by one percent. This is also why you want to use racing flats when you race as a lighter shoe allows your foot to turnover faster and thus leading to faster race times.

If you have tried some of these newer "energy return" shoes though, you just might be sold on them. They actually feel fantastic when you first try them on. One of my patients who's tried it said his feet felt like a million dollars. But like I always say, it's the legs (and running technique) that makes you fast not the shoes.

Previous studies have shown that your own muscles and tendons and a good running technique will reduce overall impact forces better than the midsole of your running shoe can. What is important in the cushioning of the shoe is spreading the load across your foot. This may explain why those who switched to minimalist/ barefoot running type shoes had no more knee or hip pain but ended up with stress fractures on their metatarsals (or foot bones) instead. This of course lead to a huge outcry against such minimalist/ barefoot running type shoes.

So, now you know that getting energy return or propulsion is not so straight forward and not something you should expect from a shoe. Not yet anyway.

References

Liberman DE, Venkadesan et al (2010). Foot Strike Patterns And Collision Forces In Habitually Barefoot Versus Shod Runners. Nature. Jan 463(7280): 531-535.

Nigg BM (2010). Biomechanics Of Sports Shoes. Calgary, Alta. : University of Calgary, c2010.