The Running Shoe Industry Part 2: Where do we go from here and the problems with Running Form research.
It’s easy to point out problems. It’s harder to come up with solutions. In this post, I’ll give my take on what to do with all of the information discussed in the last post and then end with a topic that needs to be covered: understanding the limitations of running form and shoe research.
It’s obvious that form and shoes are connected. One of the problems in the running shoe research is that it’s based on heel striking, which we now know is partially caused by the raised heels of many of today’s modern shoes. So we get results that say cushioning improves economy and retards impact forces. That sounds great, but it only does that when heel striking. If you strike midfoot, then all those things improve even more. This is but one example. Here are the pertinent questions that need to be asked and answered:
1. How do we classify shoes?
It’s obvious that the old motion control/stability/cushioning model holds little weight scientifically or practical, so how do we classify shoes for selection? Contrary to the belief of some hard core minimalist, there are differences in the structure and function of the foot that need to be addressed. Just like not every runner can or needs to run 100mpw, not every runner can survive wearing racing flats.
My preliminary suggestion is that classification should be done based on barefoot foot strike. If you heel strike barefoot and plan to keep doing that, then you need a shoe with cushioning in the heel. If you midfoot strike barefoot, then give you a shoe that still allows you to midfoot strike but offers a varying level of cushioning and protection.
2. What do impact forces actually mean?
We used to think the big impact number meant something in regards to injury. Well that hasn’t turned out to be true. Now we think the rate of that vertical impact, called the impact transient is important. In heel striking that transient is higher, and some good data shows a correlation with greater impact transient and injuries. But the bottom line is we know little of what those numbers actually mean. We need to make some sense out of the measurements we take and see if they are actually useful and provide meaning.
3. What makes people respond differently to shoes?
In a study by Nigg (2001), he found that when subjects were tested for economy while running in a shoe that had an elastic heel or a visco elastic heel the results were rather interesting. Some subjects had better economy in a elastic heel, while others had better economy in a visco-elastic heel. The question that needs to be asked is why? Why do runner’s respond differently to the property of the cushioning?
4. Why do we tend to heel strike when fatigued?
Various studies show that a lot of runners convert from a mid/fore foot strike to a heel strike when fatigued at distances ranging from 1 mile up to the marathon. The question is why?
One possibility is that stride length tends to decrease with fatigue and runners are trying to compensate by lengthening their stride, but instead of doing it by pushing off and covering more distance, they simply let their lower leg reach out. Another possibility is that fatigue may impact fine control of the lower leg. As fatigue builds up, the body tends to ignore less critical functions, so the sensory feedback that tells the higher motor centers that we are heel striking are filtered out and ignored. While early on, the brain paid attention to the feedback and didn’t want us to heel strike so that injury was avoided; under heavy fatigue the body is more worried about protecting critical functions so impact forces due to heel strike are seen as unimportant. Lastly, in a surprising twist, our body could make us heel strike subsconciously as a way to slow us down. While this seems contradictory, on a subconscious level it knows that if we slow down fatigue will start to dissipate, therefore heel striking might be a sort of protective mechanism when we are venturing too far away from homeostasis. This question needs to be resolved. What’s interesting is that the top runners don’t seem to alter their mechanics to the heel strike. They tend to maintain mechanics much better.
5. What actually causes injuries?
This is a loaded question so I’m not going to be able to do it justice, but we need to figure out what causes most injuries. As pointed out in Nigg (2001), the total impact force idea which states that impact forces are major causes, has led to little improvement in injury prevention from a shoe standpoint. So a better model needs to be developed. Common sense says impact has to play some sort of a role, but perhaps it’s not total impact or the rate of the forces, but instead what happens during fatigue? Our body might be fine with dealing with those impact forces during normal running, but be ill equipped under fatigue. Another potential mechanism is mechanics. A lot of good recent research has looked at the relationship between certain types of mechanics and injury and I think this a good thing to explore.
Where I think the shoe industry should go:
Many minimalist believers would have us get rid of all cushioning and have all shoes be a zero drop shoe. This is a mistake. It’s the same mistake that was made in going all heavy cushioned shoes. There is likely a balance point in between the all or none offerings. In an email with former Boston marathon champ Amby Burfoot, he pointed out that the shoe industry is like a pendulum. For far too long we were too far in the heavy cushioned side, and we don’t want to make the mistake of swinging too far into the complete opposite side. I agree completely and there is likely a balance, or a happy medium, in between. As I’m fonding of saying, whenever a new trend occurs, we tend to go too far in that direction, before it settles down into its rightful place. Let’s not make that mistake in running shoes.
Why do we need some cushioning? Possibly for fatigue resistance. Our body has a natural built in cushioning system in that we alter muscle activity to adjust the impact based on feedback. However, as we fatigue, it’s likely that these natural cushioning systems don’t work as well. It could be due to the lower leg muscles themselves being tired or because the body stops paying attention to the feedback from the lower leg under heavy stress because it is concerned with what is going on elsewhere. It’s possible that having some cushioning to supplement, but not overwhelm, the bodies natural cushioning system early on could aid in delaying fatigue. Or, having some sort of cushioning simply could prevent injuries when we continue to press on during a run or race when our natural cushioning is gone (and we start heel striking for example).
The questions that need to be answered are how much is enough, how much is too much, and where should it go? I’m afraid I don’t know the answers. But if I were a shoe company I’d take a bunch of runners who land midfoot barefoot, then have them put on shoes with ever increasing levels of cushioning and heel to toe rise. Monitor the change in lower leg mechanics. There is likely to be a point where the shoe starts to change lower leg mechanics and when you get to this point, you went one step too far in the amount of cushioning/heel-toe drop. As far as where it should go, I’ll cover that shortly, but one question that needs to be addressed is does cushioning in the fore/midfoot area reduce the impact transient? Research is mixed on this subject, but it could have more to do with where the cushioning is traditionally placed in shoes or the types of materials used. One study I looked at found that 1 out of 3 material reduced the impact transient when midfoot striking. That shows that we might need a different material, than is traditionally used in heel cushioning systems,in the midfoot/forefoot area to provide cushioning.
I also think shoe companies should pay more attention to how the foot function changes during the stride. More research should be done based on how a shoe alters mechanics. Right now shoe companies and research looks at how shoe types change GRF or center of pressure data. Recent studies have shown that how the foot lands or the mechanics of the lower leg change both of these data sets to a much larger degree than any cushioning or shoe manipulation. What that tells me is that foot mechanics should be most important, with shoe manipulations of secondary importance. Companies should look at how there shoes change joint angles, velocities, etc.
This is one reason why I think classifying based on footstrike is needed. I understand and believe that heel striking is not desirable, but let’s be realistic in that some runners aren’t going to take the time to change. Look at the center of pressure and anterior/posterior GRF data of the three foot strike types and you see a completely different picture. A heavy heel striker needs some cushioning in the heel that is soft enough to absorb the impact. A light heel striker wouldn’t need such soft cushioning as someone who lightly heel strike generally hits the heel but is quickly flat foot. For both of these types of runners, the foot has a more distinct back to front absorption and propulsion phase, so some of those nifty propulsion systems companies have designed might make sense. But for midfoot and forefoot runners, they make zero sense.
A midfoot striker on the other hand usually lands on the outside of the foot under the arch and then moves medially. So you’d have to have some sort of durable material on the outside that might provide a slight amount of cushioning. The medial and front side of the shoe probably needs to be a harder material with good grip because the majority of the propulsion will take place in this portion. For forefoot strikers, the heel is almost useless. Little cushioning needs to be there, though it should have just enough of a heel so that the runner puts the heel down instead of holding it off the ground. The shoe should probably be zero drop and have some sort of minimal cushioning in place along the outside metatarsals that spread the impact over the foot, instead of localizing it here. Lastly, all shoes should be as light as possible given the above requirements.
One of my favorite spikes was the Nike Kennedy’s. Take a look at these spikes and you’ll note that they only have spikes on the inside. Why? Because that’s where a distance runner pushes off, and when you need the most grip because the posterior forces are the highest! More ideas like this looking at the function of the foot would be good.
The bottom line is that shoe companies should design shoes that let the foot function. It should work in concert with it, not in opposition. And the shoe should allow for individual biomechanical differences..
Lastly, I’d like to end with the problem with evaluating running form in research studies. This is originally part of an email I sent to Pete Larson and he suggested I post it so I am. It fits well in this post, because it once again demonstrates that we have to be careful with how we evaluate research and be smart about so that we don’t end up in a situation like we have with running form or running shoes. The email below was originally in response to a Matt Fitzgerald article that can be found below:
The problem with scientific research on running form changes are:
1. The studies are short term. Most are a few weeks to max around 10wks at length.
So, we are going to go dramatically change someone’s stride from what they’ve been doing for the past 15-20years+ in most cases in a matter of weeks and we expect that the body will rapidly adjust, resync all the neuromuscular firing patterns and be magically more efficient? Of course this doesn’t happen. Most of the time when you make a form change the acute effect is that efficiency gets worse! Why? Because, it’s awkward and new. Your body has to go through a whole period of adjusting everything from the neural signalling, to motor unit recruitment, to who knows what. We’ve all experienced this. When I was a younger soccer player, I had to learn to kick with my left leg. At first I felt totally uncoordinated, but I kept at it and by the end of my soccer career I could kick with my left leg pretty much as good as my right. Do your own experiment and go try and write with your left hand. It’s hard, but if you do it for a while you’ll get a lot better. My contention is that in an acute setting, efficiency will decrease or stay the same, but in the long term, the ceiling for efficiency is higher.
2. The measurements used.
There are so many different ways to measure or express running economy, and I think most are flawed. First, we are using oxygen consumption to represent energy expenditure, which might not be the best case. For instance look at the graph below from Kyrolaine (2001) that compares Oxygen consumption, calculated energy that takes into account anaerobic sources, and energy expenditure. As you can see, even at low speeds, there’s already a gap between oxygen consumption and estimated energetics. At faster speeds, it’s almost useless if you don’t correct.
Second off, economy is either expressed as VO2/distance, VO2 at an absolute speed, or in terms of caloric cost. If you look at the literature, all give vastly different results. For example, the traditional VO2 measurement usually shows fast runners (who generally have a high VO2max) with worse economies than slightly slower runners. Just from a practical standpoint that doesn’t make a whole lot of sense. While caloric cost shows that better runners have better economy. Finally, the name of the game is to cover a given distance as fast as we can, so a lot of misinterpretation happens when we look solely at economy measurements.
3. Efficiency really is three things rolled into one. Biomechanical, Neural, and Metabolic efficiency. When we talk about running economy, we are talking about a gross measure that kind of represents how all three work together. It’s one reason why people who look bad running, can still have pretty good efficiency. If they’ve optimized two out of three that contribute to total efficiency, they’re going to be pretty dang efficient.
All that being said, in Fitzgerald’s article most of the study results make sense if you recognize the above two things. For example, of course economy would get worse if we consciously tried to think about it. What happens when you do that? You start forcing things to happen and it’s not natural. You are forcing your body to think about running in a slightly different way. That’s why you do that in practice to ingrain it so you don’t have to think in a race! Look at other sports. They consciously think about the action when working on it in practice, then instinctively do it in a game. For example, in tennis, you work on your serve in practice by tweaking the mechanics of it, but in the game you are just doing it, relying on what you learned in practice to take over.
Lastly, as for the findings by Stephen Mcgregor. It just confirms what we already know. You get more efficient at running by….running. If you go run 100 miles a week, your body will figure out how to maximize it’s efficiency of whatever stride you are using. My contention is that if we have a biomechanically correct stride, the ceiling is higher. Why? Because we know certain actions have to be mechanically more efficient. If we can maximize biomechanical efficiency, then put a lot of training under our belts while running that way, we’ll reach a higher level of efficiency then if we just ran with bad biomechanics but maximized our efficiency at running with bad biomechanics.
The problem isn’t the research; it’s the human element, the interpretation.