Wednesday, July 20, 2011

Mick Wilkinson Part II: The Sensitive Sole of Barefoot Running

For Dr. Mick Wilkinson, barefoot running has got to have sole. Exposed soles. In part I of this interview with Dr. Wilkinson, we looked at the rationale for going truly barefoot from a largely musculo-tendon and propriocpetive perspective. In part III, we look at minimalist vs barefoot running, and related social constraints, and in part IV, we'll do a wee clinic on technique and adaptation.

Now, in part II, we're gonna focus a bit more on some other mechnoreceptive action and skin types.
In other words, barefooting is really great to let a foot be a foot in terms of getting joints to move that are designed to move, and muscles to work that may otherwise get no work in a shoe, since that's a really good idea to give the brain a better idea of where we are in space.

But more than just moving joints cuz their designed to move is to consider the surface of the foot, and what it's designed to do, and how that actually also needs to inform movement - and how movement is changed because of this feedback.
So we pick up again with Dr. Wilkinson

o Gotta Have Sole
My fave stat about our feet is that 24 percent of the body's joints are in our feet - that's a whole lot of proprioception going on.

But you brought up this other incredible observation i've simply never heard before that's right there in front of us in the brain's representation of the body in the cortex: that the feet are HUGE like the hands and lips. And your observation is that this is mapping the sensitivity - the nerual activity - in the skin of the foot.

Yes indeed. The density of nerve endings in the sole of the foot - called the plantar surface - is analogous to that of the lips, finger tips and genitals, all renowned for their sensitivity.

Mapping of homunculus to sensory/motor cortex;
foot and toes take up 10% of the total sensory cortex;
in the motor cortex get this: the foot is 8%
and the ANKLE is 17% (lips are 14%). These
stats suggest those little clay models with small
feet are off in terms of representation. These Illustrations
seem to be more accurate.

Moreover, the type of nerves supplying these areas are known as type IV afferents - the fastest conducting nerves fibres in the body. Your feet are incredibly sensitive and they are so for very good reason.

I so did not know that about nerve types in the foot.
You have some really cool ideas about this sensitivity's development that
shoes obviously compromise, running from evolutionary survival mechanism to
running economy. Please, share:

I'll try to do this by combining what findings of studies have shown with what anthropologists tell us about human evolution. Hopefully that way, you can see how I have arrived at my hypothesis on the matter.

Gait Alternations with Bare Feet.
We know that gait is altered when barefoot, reduced rates of loading and in some studies reduced impact forces are associated with these gait alterations, economy is also improved (i.e. energy cost to run at a given speed is reduced) and could be linked to the altered gait pattern as well as simply removing the mass of the shoe.

Previous studies have shown a relationship between plantar sensation, impact avoidance behaviour and impact force in contrived, static activities (Robbins 89).

There is anthropological and fossil evidence that endurance running was a key survival-related behaviour for humans and that the this behaviour shaped the anatomy and biology of homo sapiens (LIEBENBERG 08). Early humans would have and indeed many existing clusters of humans still do run barefoot. Incidence of lower limb injury is lower in these barefoot populations (Carrier 84, Leibermen 10)).

hypothesis: The nerve supply is very dense in the plantar surface of the foot and those nerves are type IV afferents, very fast conducting nerves that provide great sensitivity of feedback.

Sensitive Soles
ya heel strike only once
in bare soles.
that's all it takes to
suggest a different approach
My hypothesis is that we have highly sensitive feet because the feedback received from them when walking and running barefoot results in continuous regulation and alteration of gait to minimise perceived plantar discomfort. This moderation of perceived plantar discomfort comes about by adopting a gait pattern that minimises rates of loading and peak force. It just so happens that the gait patterns that do this also seem to make better use of elastic structures in the legs and feet and also result in more economical locomotion.

Rate of loading and peak force are two known causative factors in injury, and economy is a known determinant of endurance-running performance. It would make good sense from an evolutionary perspective that highly sensitive feet, by virtue of the injury reducing and economy enhancing benefits, would be a trait that conferred a survival advantage in humans for who running was a key survival behaviour, and would thus be perpetuated by evolution through natural selection, hence we still have them. If sensitive feet were a limitation to early human runners, they would not have survived to pass on this trait.

So there you have it, we have sensitive feet so that we moderate every moment of every step to make it comfortable and we make it comfortable by altering loading rate, friction  and peak force - the bonus is that these attempts to make landing comfortable also might cause reduced energy cost of movement and be related to injury prevention.

How can a running shoe top that? Two million years of evolution cannot be surpassed by an inch or two of cushioning foam rubber that in fact interfere with natural locomotion.  

Awesomely sensible. You're looking at researching this sensitivity effect right now. Can you
tell us a little bit about how you're going about doing that and what the
hypothesis around running economy is?

Yes. Some fairly dated but excellent studies by Steve Robbins and co-workers in the mid-to-late eighties got there before me and really were ahead of their time. They proposed a plantar sensory feedback loop in which increased plantar sensation (imposed by pressing the barefoot down from above on to a force plate topped by surfaces of varying roughness) resulted in automatic impact avoidance reflexes the same as the withdrawal reflex when you touch something sharp or hot and a resulting lower plantar force.

bad bad bad sensation blockers
They also published some results collected from people dropping down onto a force plate from an elevated box. They covered the force plate in varying thicknesses of cushioning material used in the manufacture of running shoes or nothing at all. Impact force and subjective rating of discomfort were recorded. The findings showed that the thicker the cushioning the more comfortable the landing was perceived but the higher the actual impact force was!

So, shoes block the sensation of impact NOT the impact
which is actually worse than when barefoot. Robbins and colleagues [in the late 80s] actually warned against the deceptive advertising of running shoes saying that you buy them believing they would protect you when in fact the opposite is the case. Many more studies from this group reinforced these findings.

Revisiting Robbins on Barefoot Force/Sensitivity Adaptation
The limitation of the these excellent studies was actually also one of their strengths - they were very tightly controlled in a laboratory setting with contrived loading patterns. All I am working on at the moment is an extension of this early work in actual running.

I also want to quantify the link between plantar sensation and impact avoidance as it manifests itself in whole-body gait alteration. Briefly, I plan to have willing volunteers run at a self-selected speed both shod and barefoot over surfaces of varying roughness outdoors.Leg-muscle activation (via EMG), gait (via motion analysis), impact force (estimated from a mathematical model) and subjectively rated plantar sensation will be recorded.

I hypothesise that as plantar discomfort increases, the difference in gait characteristics and impact between the shod and barefoot run will also increase (i.e. the moderation of gait will become more extreme as the plantar discomfort increases). Though this design cannot show causality, it might provide some evidence for the link between plantar-sensory input and gait moderation that could open the door to other studies. 

Running economy and Barefoot Running.
Current literature (and my own work that is currently under review) is fairly consistent in that barefoot running is less costly in energy terms, but also suggests that the advantage is most likely explained by simple removal of the extra mass of the shoe (Divert 08). There is, however, a strong theoretical basis to suggest that the gait patterns characteristic of  experienced barefoot runners could also be related to energy saving. Specifically, the greater knee, hip and ankle angles could all place greater stretch on elastic elements in the legs and feet improving energy storage and the shorter ground-contact times mean that more of the energy is returned (the longer you stay in contact with the ground the more energy is lost as heat).

An interesting finding from our own lab that could also relate to energy saving is that the horizontal distance between point of initial ground contact and the body's centre of mass is much less in barefoot than shod running.

In other words, the foot land more underneath you than in front of you. This is important as the further you land out in front (shod), the bigger the braking impulse and the more you slow down with each foot strike. That speed loss has to be actively overcome to maintain a constant speed. In contrast, the reduced braking impulse of the barefoot strike means less energy to maintain speed - corresponding to my personal experience of effortlessly falling forward over the foot underneath me.

The work needed here is with experienced barefoot runners as the current literature all uses runners for who barefooting is novel. It is entirely plausible that a skilled barefoot runner will have economy advantages over shod runners that exceed that explained by the mass of the shoe - if you know any experienced barefoot runners who would be interested in this study, let me know as I haven't found many (actually any) up here!

The call goes out, and thank you for the references to related work.

And with these insights into the effect that and exposed plantar surface of the foot has on gait, we close today's post, with this request: if you try exploring barefooting this week, please come back and post - let me know how it goes. Especially if you try mulitple surfaces to notice the effect each may have on gait.

In Part III we look at other forms of running (like pose), the role of shoes in sports, being barefoot in public, a bit more about Dr. Wilkinson

In Part IV we have a wee clinic on barefoot running technique - best heard after giving barefoot running on mixed surfaces a go, when there will be more motivation than ever to hear these Sage Words. We'll also look a little bit at Making It Safe for Loved Ones to Accept your Peculiar Desire to Run Barefoot.

And for ref, this all started with Part I: the mechanical advantage of barefoot running. 

Also, again, if you're finding this series interesting, please do consider sponsoring Mick on his
  Great North Run in his bare feet at:

See you in Part III. Remember, please do let me know about your initial bare soled experience.

Carrier, D. (1984). The Energetic Paradox of Human Running and Hominid Evolution Current Anthropology, 25 (4) DOI: 10.1086/203165
Divert, C., Mornieux, G., Freychat, P., Baly, L., Mayer, F., & Belli, A. (2008). Barefoot-Shod Running Differences: Shoe or Mass Effect? International Journal of Sports Medicine, 29 (6), 512-518 DOI: 10.1055/s-2007-989233
LIEBENBERG, L. (2008). The relevance of persistence hunting to human evolution Journal of Human Evolution, 55 (6), 1156-1159 DOI: 10.1016/j.jhevol.2008.07.004
Lieberman, D., Venkadesan, M., Werbel, W., Daoud, A., D’Andrea, S., Davis, I., Mang’Eni, R., & Pitsiladis, Y. (2010). Foot strike patterns and collision forces in habitually barefoot versus shod runners Nature, 463 (7280), 531-535 DOI: 10.1038/nature08723

Robbins SE, Gouw GJ, & Hanna AM (1989). Running-related injury prevention through innate impact-moderating behavior. Medicine and science in sports and exercise, 21 (2), 130-9 PMID: 2709977

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1 comment:

SeKi said...

Two million years of evolution cannot be surpassed by an inch or two of cushioning foam rubber that in fact interfere with natural locomotion.

Exactly right. It is delusional arrogance on an extraordinarily grand scale to believe that we can improve on such a finely tuned, naturally evolved structure.

My particular hobby horses are the raised heel, and lack of custom fitting footwear.

We do not naturally walk on a surface that is permanently sloped in one direction (up or down). On average we walk on a surface that is flat, ie perpendicular to the line of gravity. So why does almost every shoe I have ever seen have a raised heel? On first principles alone it is utterly absurd, and it is not just the foot that pays the price, even a modestly raised heel has potentially serious long term implications for the rest of the body's biomechanical function.

The custom fit should need no explanation. Other than to concede that we possibly could get away with a much greater range of standard length/width combinations than we currently have available today, instead of fully customised footwear for each individual.

(Declaration of interest: I have feet that are disproportionally much wider than longer, and getting off the shelf footwear that both fits comfortably and functions well has been a constant nightmare for me since early childhood. I avoid all but the most minimal footwear.)

The overarching principle of footwear design should be to interfere with the natural function of the foot as little as possible. No artificial enhancement of any kind is required, except possibly in a few rare cases of deformed feet. Ideally, a shod foot should produce static and dynamic pressure patterns identical to the unshod foot. That is, the transmission of forces between the plantar surface and the locomotive surface should not be distorted in any way.

On the good side, I believe that both the scientific understanding of foot function, and the development of footwear materials and production techniques (including the economics of customised production), are very close to a major synthesis that will deliver serious benefits to us all.


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