Material Matters: What's Up With Energy Return Foam?
These days energy return seems to be at the heart of every new design, but not so long ago the term was seldom heard. The goal is to produce a shoe that converts as much as possible of a runner’s momentum into energy that pushes forward, to make the most out of what they’ve got. It’s become a high-tech sole foam face-off, which has seen athletic footwear manufacturers recruit some of the world’s chemical engineering elite. The big question is, does energy return live up to all the hype?
Despite the term’s recent rise as a ubiquitous marketing cry, sneaker brands have sought to harness forward propulsion for decades. High school physics probably taught you that energy can’t be created from nothing – it has to be converted from one form to another. But for a sneaker designer looking to push a runner forward, there isn’t an abundance of energy sources to draw from. In fact, there’s only one: the runner. A massive amount of energy is lost every time a runner strikes the ground. A well-cushioned shoe aims to disperse that impact to keep you comfy, whereas an energy-return design stores and transfers it into forward momentum or lift-off springiness – it’s cushion that kicks back.
The earliest efforts to improve energy-return capabilities favoured a more structural approach. Reebok’s ERS utilised a series of tubes, Karhu’s Fulcrum provided a pivot point that rocked the foot forward, while Nike’s Shox created a series of EVA columns that acted differently to traditional foam soles. It’s thanks to advancements in chemical engineering, however, that we’re hearing so much about the issue now. Sneaker manufacturers reached out to chemical giants seeking their help to create all new forms of midsole foam that would give the properties they desired; adidas and Brooks tapped BASF for BOOST and DNA AMP, while more recently Under Armour sourced HOVR from Dow Chemical. Now, everybody’s doing it.
Traditional EVA sneaker soles have an energy-return rating of around 50 per cent, meaning that the other half of the energy expressed in a runner’s stride is lost to heat and sound. The challenge for designers can be looked at in two ways: they can either increase the amount of energy returned or decrease the amount of energy lost. But that’s the easy part. If a shoe throws back energy at a runner in the wrong way it can have adverse effects – for instance, if it pushes back too quickly it can produce a jarring effect. To make matters even more difficult, designers can only guess at the energy input of any given athlete. There are different running styles, but the biggest variable is the weight of the wearer.
Modern foam compounds boast an energy-return rating of around 70 per cent, a big improvement on older tech. Their chemical compositions are as varied as their on-foot properties. BOOST uses TPU; Brooks combine TPU and PU; Under Armour uses Dow Chemical’s proprietary blend that’s neither EVA nor TPU; New Balance’s Fresh Foam uses EVA; and ever-enigmatic Nike simply state that their React foam is non-EVA. Even with all this experimentation and variation, the simple fact is that no shoe will ever hit 100 per cent and they may never be much better than they are now.
When it comes to the question of whether these energy-return foams actually work, that’s entirely up to the runner. Companies market these foam compounds as if they’ll change the way you run. But they won’t. However, they may compliment your running style, cushion in a way that suits you, rebound at a rate that suits you too, and make you feel a bit more energised when running. The way that your joints, tendons and muscles interact with the properties of the shoe is entirely up to you to judge. At the end of the day the solution is simple: if they feel good, wear them. Every one of these new-gen foams is miles more cushy than old-world equivalents. So whether or not they help you run faster, they’re likely to make your feet that little bit happier.
Material Matters is our weekly tech section where we peek behind the mesh curtain and examine the building blocks of the industry. Recently, we’ve looked at Denim Dissected, adidas and Parley Reinvent Recycling and Nike React vs. adidas BOOST.