Midsole Foams - History, Properties, and Differences Between EVA, TPU, and PEBA

DOR Podcast #84: Midsole Foams - History, Properties, and Differences Between EVA, TPU, and PEBA | Part 2

In the second of a two part podcast, Nate & Matt discuss the makeup of shoe foams with physiologist and researcher Dr. Geoff Burns. Dr. Burns takes us on a deep dive into the history, properties, and durability of common running shoe midole foams. Ever wondered what the difference between EVA, TPU, and PEBA is? Dr. Burns has the answers.

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About Geoff Burns

Dr. Geoff Burns is a physiologist and engineer with an expertise in running. He works for the US Olympic and Paralympic Committee as a Sport Physiologist, and he conducts academic research with the University of Michigan. His work has focused on running biomechanics and performance. Geoff is a runner himself and competes internationally in ultramarathons. He was the 2016 USA national champion in the 100km and has finished 5th at the 100km World Championships, twice, in 2016 and 2018.

Check out Geoff's website: https://www.geoffreyburns.com/stream
Geoff's Twitter

Chapters
0:00 - Intro
1:18 - The history of EVA and its properties
6:21 - TPU: a new foam on the scene
13:00 - PEBA: a revolutionary foam
20:24 - The confusing state of foams & marketing
24:14 - Supercritical foams
31:24 - Defining foam resilience & longevity
35:40 - Lab testing midsole foams
45:16 - Assessing foam durability
53:46 - Foam thermosensitivity
1:00:31 - Wrap-up

Science Blog:

What Sole Stiffness/Rigidity Delivers
By Chief Editor Matt Klein

We often talk about sole stiffness or rigidity in reference to plates, but a midsole can be stiff without an internal device. Longitudinal bending stiffness specifically refers to how easily a shoe bends in the sagittal (front to back) plane of motion. This can be influenced by many factors, including the density of the sole, plates, stack height, interactions between different components in the sole and more. Whether this is good or bad depends on the person. We know that there are variations in how much people benefit or worsen from increased bending stiffness, with some improving by up to 3% and others losing out by 3% in regards to running economy. The average amount people may improve their running economy by with increased stiffness is around 1%.

However, it is well known that each person's body responds best to a different amount of stiffness. Some people will do well in stiff shoes while others will do well in flexible shoes. That is the nature of the human body: it always depends on the person. Stiffness in the sole has the most significant impact on the toe joints and the ankle joints. A shoe that has just the right amount of stiffness, particularly at the forefoot, which often facilitates forward movement as the body transitions over the sole and it snaps back during toe off. If the sole is too flexible, the person will be responsible for all forward momentum and propulsion. If the sole is too stiff, the individual will have to work harder at those areas (and potentially elsewhere) to transition over the stiffness. Alternatively, with a sole that is too stiff, the individual may choose to alter their movement pathway to avoid the sole. The body will generally move toward the path of least resistance, which may be trying to move around the stiff segment. Used correctly this can actually create stability, whereas when used incorrectly can cause some intense movement and musculoskeletal compensations

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