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 . 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 co-authors 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
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), increased rearfoot eversion magnitude and velocity [3-5], a decrease in step time / shorter stride length / increased cadence , reduced plantar loading [2, 6], reduced