Whenever a new running shoe or new design feature is proposed or marketed, I tend to take notice, but not often write about it. I decided to write about this one. A Swedish company, Airia have started marketing their Airia One running shoe with the claims that it is the biomechanically ideal or perfect running shoe and that it can improve performance. If you make a claim like that, you going to get my critical attention!
I was first alerted to this shoe via Pete Larsen and then a thread on Podiatry Arena (that the Airia CEO contributed to). My initial kneejerk reaction as along the lines of OMG and roll eyes! How can there be one shoe that is ideal or perfect when I had just last week wrote a post with one possible explanation as to why one size does not fit all when it comes to running?
To get a better understanding of what this shoe is, here is the marketing video from Airia:
And here are two graphics on the shoe from Airia:
Probably the key design feature is the elevation under the lateral metatarsal heads that tapers, giving a 6mm difference between the first and fifth metatarsal heads. I never sure what to call this type of design feature: Pete Larsen called it a ‘slant’. I usually call this a ‘lateral column elevation’. Others might call it a ‘cant’, ‘forefoot valgus post’ or ‘lateral forefoot wedging’. Other design features in this shoe is the zero drop between heel and forefoot and the 35 degree toe spring. I not going to focus on those design features and address my comments at the slant/cant/lateral column elevation/forefoot valgus post/lateral forefoot wedge in the context of the performance claims made by the company and the potential injury risks from this. The comments are also made based on the publicly available information and not seeing or trying the shoes.
As with most other shoe brands there is a certain about of ‘marketing speak’ that goes into the promotion of the shoe. Airia claims a “as much as 7%” improvement in performance using these shoes. You can read more on what they did here in their white paper. It is preferable to see this sort of research properly published in a peer reviewed journal by independent researchers rather than something that is proprietary to the company. Without going into detail they used 10 runners doing 5km time trials in two Airia One running shoe prototypes vs five other shoes. With the exception of one shoe, they were all the same weight. The order was randomized. Its not clear from the white paper if the runners were blinded as to the condition and the purposes of the study, which could introduce bias. The results appear to show that the participants could run faster over 5km in the Airia running shoe. However, it is not as clear cut as claimed in the white paper and on the Airia website. The statistical method they used (Paried Wilcoxon signed rank test) was used to do ‘multiple comparisons’ or ‘multiple testing’. You are not supposed to do that and this is a significant flaw in the white paper, so a conclusion can not be drawn from the data until its analysed properly.
Having said that, how plausible are the claims that performance can be improved with the use of the Airia shoe? Way back in 1998 one of the students I was co-supervising for an undergraduate research project looked at how high you could jump with a lateral wedge under the forefoot (like the design in the Airia shoe) and with proper randomization, blinding, repeatability etc he showed that the participants could jump almost 1cm higher wearing the wedge compared to the other conditions. So, yes, there is possibly some biological plausibility to the performance claim from Airia. It is only possible to speculate on the mechanism for this. Back in 1998 we speculated that one mechanism could be that the lateral wedging lowers the force to get the windlass mechanism established (subsequent testing shows it does) and if the windlass is easier to establish, then it is theoretically easier to get the heel off the ground, therefore this could explain why you could jump higher and in the case of the Airai shoe, potentially run faster. See my previous post on the windlass mechanism and more discussion on it below. It is possible that issues associated with the windlass mechanism are important to the design features used by Airia (including the 35 degree toe spring). However, there are so many if and buts and its probably going to be a subject specific response, I would not want to draw any hypothetical across the board conclusions.
Naturally, I don’t agree because I believe in the subject specific responses to different design features in running shoes and that different running techniques load different tissues differently and there is no such thing as one size fits all because of natural biological variability. Airia have the one shoe, the Airia One, whereas other running shoe companies have a range of shoes to try and accommodate that biological variability and individual preferences.
Just what are the biomechanical affects of that lateral wedging in the Airia One running shoe? As alluded to above and discussed in this post, if you elevate the lateral forefoot, you lower the forces to get the windlass mechanism established. Try this: Get someone standing and then grab their big toe and dorsiflex it. Take note of how hard or easy it is to do. If it is hard to dorsiflex, then imagine how harder it is going to be to run to elevate the heel off the ground. Imagine how high that the loads going through the tissues are going to be to overcome that higher force (~a hypothetical decrease in economy and an increase in injury risk). Now find something (a 5-6cm thick notepad, or even your hand) and put it under the lateral (2-5) metatarsal heads and then grab the big toe again and see how much force is needed to dorsiflex compared to before – most of the time it is significantly easier to dorsiflex. Now, imagine how much easier it would be to run in that circumstance: hypothetically, not as much force is going to be needed to move forward during gait over that big toe on the ground and the loads going through the tissues will be lower (~a hypothetical increase in economy and a decrease in injury risk). Is this biomechanical perfection? Unfortunately probably not as there are individual variations in the response to this and there are other factors at play, but this model or thought experiment could account for the performace claims made by Airia.
A shoe like this one could be useful to lower the forces to get the windlass mechanism established, but its probably going to have no affect on those who already have a normal or low force to get it established and we have no idea if this design feature can be detrimental to these people. It could be helpful to those with a high force. This lateral column elevation (or whatever you want to call it) is something I do routinely in those with plantar fasciitis to lower the forces in the damaged tissues (see more on that here), so we could get into some interesting speculation about the use of this shoe in those with chronic issues with the plantar fascia.
A similar case could hypothetically be made for peroneal tendinopathy (more on that here), but the mechanism there would potentially be via the influence of the lateral wedging at the subtalar joint and not on the windlass mechanism ….. which brings us to the other issue with the lateral wedging and its affect on other structures in the foot and further up the kinetic chain and if it is potentially detrimental or not. The lateral wedge in these shoes has a lever arm to the subtalar joint axis, so will provide a pronatory moment at that joint and a theoretical increased risk for injury if the forces are high enough (see all the “overpronation” posts). Again, the response is likely to be subject specific and potentially one of trade-offs (eg more risk from the increased pronatory moment and less risk from the easier force to establish the windlass mechanism).
Lets look at this hypothetical situation. I used this diagram in the one size does not fit all post:
Look at the variability of the subtalar joint axis in the transverse plane (4 to 47 degrees). Look where the lateral wedge in the Airia shoe would be relative to that axis. Consider the differing lever arms that the wedge would have in feet that have an axis at 47 degrees compared to an axis at 4 degrees. In those with an axis at 47 degrees, the post tib muscle is working very hard as it has such a short lever arm to that axis to move the joint. The wedge in the Airia shoe is going to make it work much harder. Is this a disaster waiting to happen (~hypothetical decrease in economy and increase in injury risk). In those with an axis at 4 degrees, the post tib muscle is not having to work so hard as it has a larger lever arm. The wedge in the Airia shoe is not going to have much of an affect on this as it has such as short lever arm.
Can you see why I think the affect of the Aira running shoe is probably going to be subject specific. Look at the situation of posterior tibial tendinopathy. Those with a more medial subtalar joint axis are potentially going to be put at increased risk with the Aria shoe. BUT, if they have a high force to establish the windlass mechanism and the Airia shoe lowers this, then the posterior tibial muscle is not going to have to work as hard to supinate the foot as the windlass mechanism will be able to do more. In these runners this might hypothetically reduce the risk for posterior tibial tendinopathy. Its one of trade offs and subject specific responses. I have only touched on the biological variation of the force to establish the windlass mechanism and the effects of lever arms from variations of the subtalar joint axis. There is plenty more to go into this complicated mix. It is never going to be easy.
How will this shoe go? I am intrigued by it. I would like to try it in runners with plantar fasciitis. I would like to try it in those with a high force to get the windlass mechanism established. I would not be keen on trying it in those runners with a more medial subtalar joint axis.
Those with a memory will recall the WARP (windlass activation and resupination paddle) technology that Nike brought out in some shoes 10 or so years ago. It was specifically designed to facilitate the windlass mechanism. While I do not know officially why the design feature was dropped, but my understanding is it was partly due to the retailers not getting it. The earlier version of the New Balance MT110 trail shoe had a slant like this one and caused at least one person a problem with their post tib. Therein is the problem with these design features. We really do lack clear guidelines, preferably evidence based (rather than the thought experiments above) as to who should get what design feature. Even if we did have these guidelines, then who should apply them. Can the retailer do it? I can clinically determine the windlass forces and subtalar joint axis position. Can or will a retailer do that? Is there life one mars?
Despite my initial negative kneejerk reaction to these shoes, I do like the concept of the Airia One as it can be another tool in my arsenal for specific clinical scenarios (as that is what I do), as for it meeting the blanket claims for performance: Hypothetically, yes, but lets watch this space.
POSTSCRIPT: I have a follow up post on this: Another look at the performance claims by the Airia One running shoe; a theoretical context