Athletic Footwear

Athletic Footwear: A Synergy of Performance Dr Chris Bishop Director of Biomechanics—The Biomechanics Lab & Independent consultant for ASICS Australasia.

A lot of people don't give footwear enough credit.

Not only is footwear a powerful manipulator of human movement, but it can also have a drastic effect on human performance.

So what goes into the design of a shoe that can have such an effect? A lot of hard work, innovative thinking, creative design and calculated execution.

Every two years, the footwear biomechanics symposium brings together the best footwear scientists around the world, from both industry and academia, to share a mutual love of footwear for 3 days. What I often find frustrating about scientific meetings is when companies or individuals get on stage, share 5% of the story and leave you hanging. But sometimes I am left amazed, inspired and proud of the innovative work being performed to positively influence human performance.

So I want to share with you my experience from FBG, and how some of the research that was presented (namely Prof Roger Kram’s keyone address) can be translated directly into practice. And to do that, I am going to use a framework that was presented called the synergy of performance (Credit to Emma Farina from NIKE for the term).

There are three ways in which footwear can influence human performance (through a surrogate measure of running economy):

Shoe mass Longitudinal bending stiffness

Cushioning Maximizing all three creates what a 'Synergy of Performance."

So is light mass all the rage?

Firstly, we know light shoes influence performance. This was originally proposed by Ned Frederick back in the 1980s, with the finding of increasing shoe mass by 100g had a negative effect on running economy in the order of magnitude of 1%. Roger Kram and his lab have validated this finding (see Franz et al. 2012), which makes Ned's experimental findings even more impressive given the lack of equipment and knowledge 30 years ago compared to now.

But is there a point (of explicit mass) by which just putting a shoe on will result in improvements in Running economy? A colleague of mine (Dr Joel Fuller who is now at Macquarie Uni) wrote a great review that identified a US size 9 shoe that weighed < 220g would instantaneously result in an improvement in running economy.

Yet there was one comment from Kram’s keynote that really resonated with me…

“The design of competition shoes for road racing on paved surfaces should not overemphasize weight minimization at the expense of cushioning… (Tung et al. 2014)

So is light weight really better…In pursuit of light weight, what do we compromise? Durability? Energy return? Im not sure im willing to sacrifice both…

So moving on…is a stiff forefoot a good idea?

We know from the early work out of Calgary by Roy and Stefanyshyn that longitudinal bending stiffness has a benefit to the physiological cost of running, as long as it is tuned to the individual . To stiff (or not stiff enough) and the runner will receive a negative benefit. This was recently put into context by Oh and Park whom identified increasing longitudinal bending stiffness was ok as long as it does not interfere with natural 1st MTP joint motion.

Figure extracted form from ROY & STEFANYSHYN, D. J. (2006).

Enter the team at NIKE. And their innovative and game changing concept to a plate through the shoe will be discussed later as another highlight of the conference.

Emma Farina leads the running team at NIKE. And an impressive team it is. And with the budget NIKE has, I would expect nothing less! There is no doubt that the Vaporfly 4% has changed the landscape of running. I can't, in my time in footwear, remember a shoe that has had a bigger impact on the footwear industry as this shoe. And haven't they copped a fair bit of negative press about the shoe. Media outlets are calling it to be banned as it’s a spring. But when you look at it closely, and understand the mechanics, it isn't a spring. It is simply a lever to optimise the mechanical advantage of the foot.

The vaporfly works in two ways: The proprietary cushioning (see section on cushioning) The carbon fibre plate.

With the respect to the latter, Farina took us through the design and development of the plate, and how NIKE developed the unique design. It started with four prototypes:

A baseline shoe, same cu the restored after a loading shioning and no plate Prototype 1 which have a stiff plate with no curvature (same cushioning as baseline shoe) Prototype 2 which have a stiff plate with mild curvature through the forefoot (same cushioning as baseline shoe) Prototype 3 which have a stiff plate with extreme curvature through the forefoot (same cushioning as baseline shoe) So what did they find? CONTINUED ON NEXT PAGE >>

The curvature of the plate had a large effect, with the more severe curvature prototype having the greatest effect on reducing the negative effect of increasing bending stiffness (see Figure 1 below from Abstract). What one would normally expect is that a result of increasing forefoot longitudinal bending stiffness you would get net energy loss at the MTP and a net external increase in the ankle plantarflexor moment but this wasn’t the case. It’s like the runner receives the best of both worlds; maximise power around the MTP whilst not making the calf work harder.

Extracted from Farina et al. (2019). Footwear Science.

And how is this done?

Through the shoe of course! Roger Kram, who oversaw the external validation work of the shoe at University of Colorado Boulder, presented data to support the fact that the shoe itself was doing some of the work. This is likely the holy grail, reducing the work at the MTP without increasing it at the ankle, knee or hip.

But remember this is fast running. Elite runners (sub 30 min 10 km runners), running at 18km/h. Whether this data stacks up to recreational runners at paces more attributable to say a 4 hr marathon remains to be seen.

So what about cushioning?

In short, cushioning has a big impact on both the perception of comfort, as well as the vibrations applied to the body. However, what a lot of people fail to understand about cushioning is what influences it?

There are two distinct factors Compliance Resilience

The compliance of a shoe has formed the traditional view of cushioning. Soft bouncy midsoles like BOOST, Everrun etc have entered the market with large applause. The immediate perception of a comfortable ride has sold many a millions of pairs of shoes (irrespective of durability). Interestingly, research has suggested that regardless of whether you run on hard or soft materials, the magnitude of load applied to the body is the same. However the perception of the load can be different, and likely explains why some people prefer different materials to others.

So if it isn’t compliance that makes cushioning so good, then what is it?

The magic in these new smart materials we are seeing (i.e. Zoom X in the Vaporfly 4%) seems to be the resilience of the material. Resilience of foam is defined as the energy restored from the material after a load has been applied. This is commonly referred to energy return. In presenting loading data of the 4%, Kram eloquently demonstrated that just because you have a compliant midsole, does not mean that has a negative effect on energy return. Infact, high compliance, couple with high resilience, results in a material that loses minimal energy through the loading cycle. Where the Zoom Streak returned 65.5 % energy and the Adidas adios boost 2 (the shoe Denis Kimetto broke the world record in) returned 75.9%, Zoom X was recorded a staggering

87.0% of energy. According to Martyn Shorten who tests shoes for Runners World, this is the highest recording of energy return ever recorded by a shoe. What ultimately results is less energy lost in the midsole, and therefore more returned to the runner to assist with propelling forward momentum.

Impressive huh…YEP

Despite these three factors combining together to create a synergy of performance, I am going to take the opportunity to discuss some of the findings we presented at FBG on midsole density. Yes I will at this point declare a conflict of interest given my consultancy role with ASICS, but I do feel our results suggest the dual density midsole design can also contribute to this synergy of performance.

DuoMAX - ASICS dual density midsole system. What does it do?

There has been a staple in the ASICS footwear range since Kayano 1 (yes that it 26 years!). And that is duomax. ASICS' patented dual density midsole system.

Now I want to address the elephant in the room of the role of dual density midsoles. I honestly feel they have been harshly dealt with on the online stratosphere. There are many individuals calling for the extinction of this technology. I routinely read 'they are heavy', 'they don't control pronation' so why include them in the shoe. Yet I am yet to ready a really good study showing these shoes don't work (and yes tehre are multiple ways to define 'work'). In fact, there is a range of really good studies that show benefits of medial posted shoes. This makes it all the more surprising that companies are deviating away from this technology (despite anecdotal feedback from consumer that some find these type of shoes more comfortable) given the lack of conclusive evidence to support their lack of effect.

I also want to highlight that not all dual density systems are the same

So what did we do…

We built two versions of the Kayano 23, with the only difference between the shoes being the presence (or lack of) DUOMAX TM design. We then got people to run both fresh and in the presence of fatigue (with fatigue created throughr unning 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 this design feature. And something that should be considered in a lot of recreational runners…fatigue influences performance!

So to summarise, of course there is a desire for every runner to find the perfect shoe. And at the moment, we don't know what that is. But we do know that when we get footwear right, different components combine to create a synergy of performance.

So what contributes to this synergy?

A shoe that is durable. If it can be light, that is even better! A forefoot stiffness that is tuned to the individual needs, one that doesn't restrict MTP ROM and one that doesn’t increase the moments acting around the ankle A shoe that is resilient and returns energy stride after stride after repeated loading

And if running in the presence of fatigue, perhaps considering a shoe with DuoMax might help too!

References

• Bishop, Chris (2019) Dual density midsole design improves running economy and performance when fatigued, Footwear Science, 11:sup1, S136-S137, • Farina, Emily M, Haigh, Derek & Luo, Geng 2019, ‘Creating footwear for performance running’, Footwear Science., vol. 11, no. sup1, pp. S134–S135. • FRANZ, J. R. , WIERZBINSKI, C. M. & KRAM, R. (2012). Metabolic Cost of Running Barefoot versus Shod. Medicine & Science in Sports & Exercise, 44(8), 1519–1525. doi: 10.1249/MSS.0b013e3182514a88. • Roy J and Stefanyshyn DJ. (2006) Shoe midsole longitudinal bending stiffness and running economy, joint energy, and EMG. Med Sci Sports Exerc. 38(3):562-9. • TUNG, K. D. , FRANZ, J. R. & KRAM, R. (2014). A Test of the Metabolic Cost of Cushioning Hypothesis during Unshod and Shod Running. Medicine & Science in Sports & Exercise, 46(2), 324–329. doi: 10.1249/MSS.0b013e3182a63b81.