Dual Density MidsolesASICS ARTICLE

Dual Density Midsoles

Dr. Chris Bishop PhD

Is there a role for dual density midsoles in modern athletic shoes?

Running is a popular activity. And despite a small population that continue to support the barefoot and minimalist movement, most runners are wearing shoes. Each year, recreational runners purchase up to three pairs of running shoes, with the purchases being in part motivated by the expectation of improved running performance. But how do we identify the right shoe for an individual? Well despite the large number of running shoes sold each year, there are currently no criteria to help runners decide which is the best running shoe for them. This results in shoe selection based on subjective assessments of shoe characteristics and anecdotally-based marketing literature and product information from shoe manufacturers, or video gait analysis results from technical retail stores.

Traditionally, running shoes have been categorised based on levels of support ranging from structured cushioning to motion control. Underpinning the concept of support is the presence of different midsole designs. Dual density footwear (characterised by two differing density layers of a midsole) has been a keepsake of athletic footwear design for the best part of 30 years. The long held belief was that dual density midsoles stopped the foot from pronating through loading response and midstance, as well as providing medial stability to the shoe. Despite this anecdotal categorisation of running shoes, evidence from the literature does not support the premise the footwear can control motion [1]. However, runners continue to purchase this shoe on the basis that it feels more comfortable and stable for them when they run.

There has no doubt been a lot of pressure with regards to the effect of this type of footwear and whether it should still exist of recent times. Largely anecdotal and pseudo expert opinion without a huge amount of published evidence to support such strong statements. There are certainly brands removing dual density systems from their range, despite evidence presented by Malisoux and coauthors that some dual density designs are associated with lower injury risk. So is there a role for dual density midsoles in 2019? I want to delve into the best part of four years research to share our investigation findings and the two big lessons we learnt along the way.

Lesson 1: Not all dual density systems are the same.

I think its unfair to label dual density systems don’t work. Because the Brooks Adrenaline was a different design to the ASICS Kayano, which was then different to the NIKE Structure Triax and New Balance 860. Some posts were hard density. Some posts were different in geometric design. If we consider ASICS DUOMAX system, it is simply a thin (say 10mm thick) wall in the medial midsole in the midfoot that was never purported to act on the subtalar joint, where as the former Adrenaline was a wall of stiffer midsole material in the posterior heel of the shoe. Similar concepts yet differing positions equals different purported function. Heck you could argue the Nimbus is a dual density system in the past with fluid ride midsole system actually being composed of two differing midsole materials which will each respond in different ways.

If we are going to make conclusive statements about something working/not working, you have to be comparing apples with apples.

Lesson 2: The literature hasn’t actually properly investigated the effects of these designs on running biomechanics.

When you look at the design of most studies investigating dual density footwear, the outcomes used are a result of the instantaneous effects of the shoe, and it is therefore unknown what the acquired effect of the shoe is overtime, whether that be after defined periods of running or accumulative periods of wear.

This design error becomes really important in terms of clinical translation.

Runners often don’t report issues straight away, but more so 5-10kms into a run. The question has to be asked what is happening at this point when someone starts to experience symptoms? Perhaps they are getting tired and the function of the body (not just foot and ankle) is changing. Not ignoring the role of age of shoes in performance, but fatigue has been shown to reduce performance. Research suggests that stride patterns deteriorate in the presence of fatigue ( or running near exhaustion)[2], increased rearfoot eversion magnitude and velocity [3-5], a decrease in step time / shorter stride length / increased cadence [6], reduced plantar loading [2, 6], reduced peak vertical GRF [7, 8] and reduced leg-spring behaviour [2, 7]. This is of interest as the role of footwear as a ‘protective’ mechanism to changes in postural demand (e.g. fatigue) rather than a ‘motion control’ system to change foot function has not been evaluated. Given our internal wear test data suggests that shoes with varying midsole densities are more stable, it is plausible to hypothesise that the effect of such shoes may not be one of motion control, but of improved stability in the presence of changes to postural demand (e.g. exerted state or fatigue).

This last point has driven me to search for answers on what the DUOMAX system in ASICS footwear actually does. It has been in Kayano for 26 years. It’s sat within most stability models in the range. And yet we didn’t (nor did the industry) have a really good understanding of what it actually did.

So what did we do to investigate this?

I presented our work on this recently at the Footwear Bio

mechanics Congress in Kannaskis, Canada. For those interested, the abstract is published in Footwear Science

from the meetings proceedings. But we built two versions

of the Kayano 23 in the factory, with the only difference

between the shoes being the presence (or lack of) DUOMAX TM design. For those interested in the difference in shoe mass, it was between 2 and 7 grams.

We then got people to run both fresh and in the presence of fatigue (with fatigue created in a protocol where participants were asked to run back-to-back 3km time trials as quick as they could). What we found was instantaneously, there was a physiological benefit (2%) running in the DuoMAX shoe regardless of foot posture, with the benefit increasing to 3% when running in the presence of fatigue. This also translated into a performance benefit, with the dual density shoe assisting in maintaining performance (i.e. faster time trial performance in the dual density shoe) in the presence of fatigue. This was measured as a 6.8s improvement in 3km time trial performance, equating to an improvement of some 24 m. This is a clear and substantial benefit of the DUOMAX design feature. And something that should be considered in a lot of recreational runners…fatigue influences performance!

So to sum up…is there a role for dual density systems in modern athletic shoes?

Based on our data, the answer is YES. And that is why it is exciting to see DUOMAX continue in the latest evolution of GT 2000-8.

But we can’t say the results we found are applicable to all dual density systems. Different systems will work differently, and individuals will respond differently to each system. And yes I am sure there are other design concepts that may be able to achieve similar effects. Let me be clear that the results we found were specific to ASICS DUOMAX midsole system. The concept of shoe design is about providing a depth of choice for runners. Where we now need to head and more our attention towards is how best to identify runners in need of these type of features in their shoes.

That’s another discussion all together…

References—on request