Athletic footwear: A synergy of PerformanceASICS ARTICLE

Athletic Footware: A synergy of

Dr. Chris Bishop PhD Director of Biomechanics, The Biomechanics Lab Performance

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.

As a result of this hard work and innovative thinking, World Athletics has now established rules in footwear design limiting the number of carbon fibre plates as well as the stack height of the midsole. On one hand we can thank NIKE and the controversy around AlphaFLY, yet every company was doing something similar. They were exploring the ability of footwear to influence performance.

But this was right at the top end of runners. The elite of the elite. What about our recreational runners? Is there a way we can help them improve performance too?

We know from the science that 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 is known as a 'Synergy of Performance."

So let's explore these factors.

Firstly - is light mass all the rage?

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.

Figure extracted from Franz et al. (2012)

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 at the Recent FBG symposium in Canada 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? I'm not sure I'm willing to sacrifice both…

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

and Stefanyshyn that longitudinal bending stiffness has a benefit to the physiological cost of running, as long as it is tuned to the individual. Too 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 from ROY & STEFANYSHYN, D. J. (2006).

Enter the team at NIKE. And their innovative and game changing concept to a plate through the shoe. Is this any different in function to applying a propulsion trusstic through the midsole or manipulating material density to create a similar effect (think racing flats for the last 30 years)? Or is just a sign of the current market where engineering and production advances have now made this possible and feasible?

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. 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:

1. The proprietary cushioning (see section on cushioning)

2. The carbon fibre plate.

With respect to the latter, NIKE have clearly explained the design and development of the plate. It started with four prototypes:

1. A baseline shoe

2. Prototype 1 which have a stiff plate with no curvature (same cushioning as baseline shoe)

3. Prototype 2 which have a stiff plate with mild curvature through the forefoot (same cushioning as baseline shoe)

4. Prototype 3 which have a stiff plate with extreme curvature through the forefoot (same cushioning as baseline shoe) m

So what did they find?

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.

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, has 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.

Is a plate the only way to do this? Well arguably not. Think about toe springs and midsole geometry. It is feasible to create a propulsion mechanism through the forefoot that may assist runners in a similar way. That’s the concept of the EvoRIDE from ASICS. It may not improve performance 4%, but the impact that it can have on optimising propulsive mechanics may well be similar for the recreational runner.

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

2. 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 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? 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 as 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, coupled 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 Runner's 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 recently 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 there 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 consumers that some find these types 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

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 through running 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?

1. A shoe that is durable. If it can be light, that is even better!

2. 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

3. 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.