Saturday, July 23, 2011

Shoes and inserts: how the broken model works

One of the first things we blame whenever we get injured is our shoes.  Most runners are keenly aware of all the minute details of their footwear, and they are easy to pin the blame on.  Whether it's the shoddy upper of the updated model, a too-stiff midsole, shoes that are too run-down, or an abrupt change in brands, we are quick to link our shoes with our maladies.  And for good reason, many podiatrists and doctors would state--shoes can be the cause of a new injury, especially if it occurs within a week or two after a change in footwear.  Problem is, no one can agree on exactly what about running shoes makes them good or bad.

If you try to buy shoes at a chain store, you're probably on your own, unless the salespeople are trying to earn a few extra bucks.  At a specialty running store, you'll probably be exposed to some arcane witchcraft to divine the perfect shoe--the wet foot test, meant to measure arch height, and the standing pronation test, intended to measure rearfoot motion, are the two most common.  Inspecting wear patterns on an old shoe and a pronation examination while running are sometimes also done.  The general idea is as follows: people who underpronate/supinate (the terms are used interchangably) and have high arches can't absorb shock effectively, so they need a "cushioning" shoe.  People who have mild pronation and medium arches need support from a "supportive" shoe.  Finally, people with low arches who pronate severely need very supportive "motion control" shoes.  The driving principle is that pronation is bad and arches need support.

There are several problems with this approach.  The biggest one is that it doesn't work.  As we'll soon see, pronation does not reliably predict injuries, and attempting to correct it does not avoid them either.  Furthermore, the standard tools for correcting pronation, namely, shoe changes and custom orthotics, don't reliably reduce pronation! But perplexingly, switching to a more supportive shoe or getting a custom orthotic from a podiatrist works a good amount of the time! 

Before we tackle that, I'll briefly go over how pronation works, since there's a lot of misinformation about exactly what it is.  Biomechanicists (or is it "biomechanics"?) and podiatrists scoff at the word "pronation"--they prefer "rearfoot eversion," since this more accurately describes what is going on.  Here is an excerpt from a RunnersWorld video that illustrates a runner pronating:

I've skipped ahead to the interesting part. Also, stop watching after about 1:00, since pretty much nothing the narrator says is true.

In principle, pronation during running is fairly simple: after the foot strikes the ground and flattens (whether it is flattening from a forefoot strike or a heelstrike), the foot rotates inward about the subtalar joint.  In my previous post, I mentioned that studies have shown that no matter the stiffness of the surface, peak impact forces are the same.  In a 1998 paper, Wright et al. showed that this phenomena can be explained by a passive mechanism.  That is, the muscles do not interfere in a dynamic way to absorb impact shock.  Rather, the muscles, bones, and joints act like a system of springs, rods, and hinges during the impact phase. 
Wright's model of the leg was significantly more sophisticated than the simple "three hinge leg" model used in some other studies.

The role of pronation in this process is to translate vertical impact force into rotational force, which attenuates the impact by spreading it out over a longer period of time.  Everybody pronates; some do it more than others. Pronating was probably originally suspected of causing injury because it looks odd and out of place.

Running shoes and inserts attempt to correct pronation by supporting the medial arch of the foot.  Remember, pronation is when the foot rolls inwards, so putting a wedge on the inside of the foot should (theoretically) prevent that inward rolling.  This correction is easily visible in a static position; images like this one are common on shoe and insert websites:

 The difference is fairly obvious.  However standing is not running, and as we'll soon see, the difference in degree of pronation while actually running is not nearly as drastic or straightforward as the image above.

So why doesn't the "pronation paradigm" work? First, there is little evidence that the degree of rearfoot eversion/pronation is related to injury.  The biggest study I'm aware of that has found excessive pronation to predict injury is this one by Williams et al.  The study examined 400 subjects over three years.  Some 46 developed "exercise related lower leg pain" (shin splints), 29 of which developed it in both legs.  This gave the researchers 75 injured legs, which they compared with 167 uninjured legs from the students who stayed healthy.  Why they chose 167 is beyond me, but their statistics showed a significant difference in several parameters related to pronation.  The injured group displayed pronation that was faster, greater in magnitude, and occurred later in the stance phase than the uninjured group.  Unfortunately, some experimental oversights make these results untrustworthy.  First, the subjects were recreational athletes who competed in a variety of sports; the study did not use runners exclusively, even though the biomechanical evaluation was a running test.  Second, this biomechanical evaluation was done barefoot.  Running barefoot has a variable effect on rearfoot kinematics (including pronation), so there is no way to know whether someone who pronates barefoot pronates to the same degree in a shoe!

To be sure, there are some other studies that have found a weak relationship between pronation and injury.  But there have been just as many that have found no relationship (like this one, which concluded that "lower-extremity alignment is not a major risk factor for running injuries in [a] relatively low mileage cohort").

Additionally, the two tools used to alter pronation--shoe support and orthotics--don't reliably alter pronation! Benno Nigg, whose work I've cited several times in this post and the previous one, concisely summarizes in a seminal 2000 paper:

"Results from studies with bone pins in the calcaneus, the tibia, and the femur showed only small, nonsystematic effects of shoes or inserts on the kinematics of these bones during running. Even more surprising, the differences in the skeletal movement between barefoot, shoes, and shoes with inserts were small and nonsystematic.The results of this study suggest that the locomotor system does not react to interventions with shoes, inserts, or orthotics by changing the skeletal movement pattern. These experimental results do not provide any evidence for the claim that shoes, inserts, or orthotics align the skeleton."

If shoes don't have the ability to reliably alter pronation, we would expect large-scale studies using familiar tests like the pronator/supinator or wet foot tests to show little or no difference between people assigned the "right" shoe for their foot type and people assigned the "wrong" one.  In fact, this is exactly what we see.  This study by Ryan et al. divided up 81 women training for a half marathon into three groups: neutral, pronators, and severe pronators.  Each group was divided into three subgroups which were each assigned either a cushioning, stability, or motion control shoe to wear for all of their training.  So, for the 30 women who were deemed to be "pronators," 10 received a stability shoe (the "right" one), 10 received a cushioning shoe, and 10 received a motion control shoe. Surprisingly, runners who got the "wrong" shoe got injured slightly less than runners who got the "right" shoe, and everyone who got a motion control shoe was more likely to become injured than those who did not.  A similar study by Knapik et al. with over 1300 Air Force recruits which assigned shoes based on the "arch height" strategy saw no difference between groups.

All I've told you up until now is somewhat old-hat.  The failure of pronation control in preventing injuries has been covered in more detail by others, and the results of the aforementioned studies have been declared as proof of an enormous conspiracy wrought by an unholy alliance of money-hungry shoe companies and doctors.  The problem is that the conclusion is not as simple as it seems, and few people seem to bother with exploring why.  The evidence in the Ryan and Knapik studies, combined with Benno Nigg's review article, seems to lead to a simple conclusion: running shoes and running inserts can't prevent injuries because they don't change pronation, and pronation doesn't cause injuries (which is quickly misconstrued into "running shoes cause injury").  However, this is not the conclusion the data support!  As usual, the details are more nuanced. 

Why?  Because shoe inserts (and probably the right shoe) can prevent injury.  Several studies have found that inserts can relieve or prevent injury; here is one example by M√ľndermann et al. So, even though the inserts are not significantly altering pronation, they are preventing injury.  This points to a different mechanism for injury, which brings us back to Benno Nigg's article. Nigg spends the first half describing why impact forces and pronation have little or no relation to injury risk.  The second half proposes a new model for skeletal movement and its relation to injury.

According to Nigg, the body has a "preferred movement path."  Regardless of the footwear condition or presence or absence of inserts, the body activates the muscles to stay as close as possible to the preferred path.  This is why adding a thick medial wedge on the inside of the foot to oppose pronation does not significantly change the amount of pronation while running.  To overcome a medial wedge attempting to divert the foot from its preferred path, the body simply activates the eversion muscles of the leg more strongly.  Apparently, the muscles controlling the foot are easily strong enough to overcome a small foam wedge.  In contrast, if a shoe or shoe insert encourages the foot to move along its preferred path, the foot control muscles will not have to be as highly activated.  In his own words:

" If an intervention counteracts the preferred movement path, muscle activity must be increased. An optimal shoe, insert, or orthotic reduces muscle activity. Thus, shoes, inserts, and orthotics affect general muscle activity and, therefore, fatigue, comfort, work, and performance."

There is concern that "interventions" (shoes and inserts) that increase muscle activity may cause injury, but no evidence of this yet.  There is evidence that shoes which decrease muscle activity decrease the energetic cost of running, increasing efficiency.  Muscle activity is connected very closely with muscle vibration on impact--muscles act like springs on impact, and just like a real spring, they vibrate.  Much of Nigg's current work is looking at the effects of these muscular vibrations and whether they are related to injuries. 

Benno Nigg uses electromyography machines to prove the changes in muscle activity.  Fortunately, you don't need an EMG machine to tell whether shoes or inserts are encouraging or discouraging your preferred path of motion.  Your body communicates this in a fairly clear way: comfort. Nigg (and other researchers) have published studies indicating that the shoe and insert choices which prevent injury and reduce muscle activity are also the ones which subjects report to be the most comfortable.  In the M√ľndermann study above, the military recruits in the experimental group (which recieved shoe inserts and experienced a lower injury rate) rated their inserts as more comfortable than the control group, which had a simple flat insole.

The factors that affect comfort are not the same from person to person; this is why stability shoes are the right choice for some, and the wrong one for others (and this seems to have nothing to do with degree of pronation).  So, in a long and circuitous way, we have shown that in this case, common sense is right: wear the shoe that feels most comfortable!

Things are, from a scientific perspective, a bit more complicated than that, and I've been itching to do some more reading on how changes in muscle activity and vibration affect injury, as well as how various other changes (texturing for example) affects the way the foot interacts with the shoe and the ground.  Further, there are several factors that aren't taken into account in many of these studies that do come into play in real life.  Most studies use small variations of the same shoe, testing different arch heights, medial wedges, midsole hardness, etc.  But in the real world, much more changes from shoe to shoe.  Flexibility, the fit of the upper, the elevation of the heel, and the lacing system are just a few examples.  So even though one of the new "minimalist" shoes with a low-profile, flexible midsole may feel very comfortable, you may not be ready to run in it.  For exaomple, abruptly switching to a shoe with a lower heel height could induce Achilles tendonitis, or wearing a more or less-flexible shoe could cause foot problems.  Of course, it could prevent them as well! These are all topics for another time, though.  In the next week or two, look for another post on the anatomy of a running shoe and some analysis on the usefulness of some of these things.

Tuesday, July 19, 2011

New York Times article on running surface stiffness

The New York Times has a nasty habit of writing poorly-researched exercise science articles. They go something like this: A new study by professor so-and-so at such-and-such university upends some widely-accepted fact about exercise, and we're darn luck to have these scientists (and the clever journalist) telling us that common sense is wrong.  You have to realize, of course, that the New York Times is a business, and no one's interested in an article titled "Exercise is still the best treatment for a variety of health conditions."  That's old news.  What does sell is controversy: you should run when you are sick, cool-downs are completely unnecessary, exercise is bad for you, abs are bad for you.  I don't disagree with the message in all of these articles; what I disagree with is the methodology.  The articles never put the new findings into context.  It seems like they seek out an exercise-related muck-raker, then report on his or her controversial idea, then include some vague comments like "the full impact of this study remains to be seen."  This most recent New York Times article is a perfect example: posted yesterday, it claims that soft surfaces don't prevent injury, and moreover, that soft surfaces can themselves be injurious.  This happens to be an area of research that I have done quite a bit of reading on, so I'll go through this article piece-by-piece.

The very first picture is anathema to the trained eye: some overstriding, uncouth runner romping his way though the countryside, slamming his heels into the ground, straight-kneed as his legs stretch several feet in front of him.  Now, this is probably just a stock photo from the archives, but it raises a good point: regardless of what we talk about regarding surfaces, running form is a big factor in injury prevention, and ignoring that is unwise.  No matter what surface or shoe you wear, if you run like the man in the stock photo, you're going to have problems.

Moving on, the facts in the first part are more or less correct: there have not been any controlled studies linking soft surfaces to lower injury rates.  This is a bit misleading because there have not been any controlled studies that link much of anything to lower injury rates.  In survey-studies, such as this famous one, no link has been found between injury and shoe type, foot type, running surface, overall fitness, or weight.  The only reliable predictors of  injury are mileage and previous injuries.  Some factors (like hip muscle imbalances) are related to injury risk, but have not been proven (yet) to have a causal relationship.  Still other factors have evidence linking them to injury risk, but have not (yet) been shown to be related.  This is where running surfaces belongs.

The NYT article moves on, touching on the surface-stiffness paradox: surprisingly, the impact force does not differ from surface to surface.  For a given runner at a given speed, the foot will hit the ground with the same force no matter the hardness of the surface.  Therefore, the article concludes, soft surfaces are no better than hard ones since the impact force is the same.  Furthermore, the author concludes that soft surfaces are worse because they tend to be uneven, risking turned ankles and the like.

However, the surface-stiffness paradox is not the end of the story.  Concluding that the injury risk is no different because the impact risk is no different presumes that injury is related to impact, which is not at all clear.  Understanding how the body modulates impact on different surfaces is essential to understand the implications on injury risk, and this is not covered in the NYT article.

During impact with the ground, the leg muscles act like springs.  Before impact, the muscles tense up to absorb the shock.  Unlike mechanical springs, however, the body's muscles can have their stiffness altered.  When you are running, the brain does this automatically.  In order to optimize performance, the brain tenses the muscles to minimize the vertical motion of your center of gravity upon impact.  Using feedback from the skin and muscles, the brain  tenses the muscles so that the stiffness of the overall system remains the same.  What is the overall system? It's the combined stiffness of the surface you are running on, the stiffness of the shoes you are wearing (if any), and the stiffness of your leg muscles.  If the overall system stiffness is to remain constant, the body must modify the stiffness of the leg muscles in order to change it, since surface stiffness and shoe stiffness are out of your body's immediate control.  So, when running on a soft surface, the leg muscles are tighter, and when running on a hard surface, the leg muscles are looser. And unlike the New York Times article suggests, the adjustment is not slow or gradual--it is instant.  In a brilliant 1999 study, Daniel Ferris showed that runners adjust muscle stiffness before the first step onto a different surface.  This is pretty cool.  When you move from pavement to grass, your eyes see that you will land on the grass, your brain interprets that information, and relays it to the legs in the form of increased muscle stiffness.  All this happens on auto-pilot. 

 The muscles and bones of the leg can be modeled as a system of levers and springs.  A realistic model is illustrated on the left, and a more simple one is illustrated on the right.  Unlike real springs, the muscles can be changed dynamically to have a different stiffness--imagine the effect of swapping out a loose spring in the model on the right for a stiffer one.  This is the effect that different muscle pre-tension has on impact.

But impact force is not the whole story when it comes to collision with the ground.  Daniel Lieberman, an anthropologist at Harvard University, has come into the spotlight recently for his work on barefoot running and footstrike style.  More relevant to the current topic, however, is this important illustration of the effect of cushioning: in the graph below, the force at various times in a footstrike are plotted.  The black line represents a barefoot heelstrike, while the red line represents a shod heelstrike.  This is effectively a test of the effect of surface stiffness on the impact parameters (the surface just happens to be attached to the foot).
It's easy to see that a less-cushioned impact (barefoot, black line) has the same impact force (about 2.4x body weight) as a more-cushioned impact (shod, red line).  However, it is also easy to see that the total duration of the less-cushioned impact is shorter.  This means that the loading rate, or the change in force over time, was higher.  This is mostly because the foam in the shoe takes time to deform, spreading out the load both in time and area.  Whether the loading rate affects injury risk is highly controversial: Irene Davis, a highly respected researcher, claims they do (and has data to back it up).  But Benno Nigg (who is also highly respected) claims they don't, and also has data to back it up.

Impact loading rate is the most obvious difference between hard and soft surfaces, and I'll probably deal with the nuances of it in a separate post.  But there are other differences too--the pressure distribution on the foot is one, and the evenness of the surface is another.

Plantar pressure distribution has been overlooked for a long time, since biomechanics researchers assumed impact control was passive for a long time.  We now know that it is active--it involves dynamic feedback between your legs and your brain.  The body senses what type of surface you are in contact with, and modifies muscle activity accordingly.  There are some very interesting results from this: for example, it is easier to balance on a textured surface than on a smooth one. In running we know that the body attempts to minimize pressure on the sole of the foot.  Sensory organs called mechanoreceptors detect the local pressure on the sole of the foot.  If it is too high, the body modifies gait in an attempt to reduce it.  When running barefoot, especially on a hard surface, your body forces you to take choppy, quick steps.  A quicker stride frequency means less impact force per footstrike, so the pressures drop.

In a study published in 2008, Vitor Tessutti and coworkers measured the pressure at different points on the sole of the foot during running on natural grass and asphalt.  Predictably, running on grass resulted in longer contact time with the ground and lower peak pressure.

So if the body can adjust to any surface/shoe stiffness by modifying muscle stiffness, and high plantar pressures should be avoided, why not run with pillows strapped to your feet? Well it turns out that the body can't adapt to any surface/shoe stiffness, and that some plantar pressure is a good thing.

One emerging idea regarding muscle and surface stiffness holds that the body has a Zone of Optimal Leg Stiffness.  Impacts that occur when the muscles are forced to operate outside this zone are definitely uncomfortable and probably injurious.  The zone of optimal leg stiffness is easy to demonstrate using extremes: First, imagine sprinting as fast as you can, barefoot on asphalt.  Of course, this would be a painful and regrettable experiment.  Second, imagine trying to sprint in a high-jump pit.  Not as painful, but nearly as uncomfortable.  In the first case, the leg muscles can't be loose enough, so the legs are forced to absorb too much impact too fast.  In the second case, the leg muscles can't be tight enough and cannot act as effective springs to return the energy from the impact.

I suspect that the zone of optimal leg stiffness changes as muscles become more fatigued--so at the end of a 90 minute run, your body is less tolerant of various surface stiffnesses than it was at the beginning.  This is purely speculative, but from good old-fashioned experience, I (and most other serious runners) can tell you that 13 miles on pavement feels less comfortable than 13 miles on dirt and grass.  I also suspect that most runners encounter the lower end of their zone of optimal leg stiffness more often than the upper end--that is, I suspect that the surface/shoe combination is too hard more often than too soft.  Again, only a suspicion supported by experience. But if I'm correct, a soft surface is a much safer bet, as you will be in less danger of being outside that zone of optimal stiffness.  Fortunately, there are some very smart folks over at the podiatry arena who are on the same page as me.  Too bad the New York Times didn't interview them.

With regard to plantar pressure, emerging research is showing that, although the body tries to avoid high peak pressures, it is also used for feedback.  "Proprioception" is the $5 word for the dynamic feedback between the plantar mechanoreceptor cells and the brain.  As I mentioned earlier about the ease of balance on a textured surface, better feedback about the surface from the soles of your feet is a good thing.  There is some evidence (though not by any means GOOD evidence) that part of the benefit in taping sprained ankles is not from the support, but from the increased proprioceptive feedback.  A 2003 study found that soccer players with a textured insole had much better sensory feedback versus a smooth insole. Unfortunately, thicker cushioning means less proprioceptive feedback.  Alas, this is a topic for another day--shoes, insoles, bare feet, etc.

Getting back to the topic at hand, one final advantage soft surfaces have is their unevenness--this is why I brought up proprioception in the first place.  An uneven surface puts different stresses on the body, and the proprioceptive feedback from the varied surface will result in muscles being tensed slightly differently.  Uphill, downhill, bumpy, and smooth surfaces all stress the body slightly differently.  One step might stress the medial side of the foot more, while the next stresses the lateral side more.  Accordingly, the muscles of the leg are stressed slightly differently.  Sports orthopedists often blame hip and knee injuries on running on the same side of a cambered road, inferring that the same stresses over and over are a bad thing.  So why not switch up the stresses on your body? Repetitive stress is a bad thing in many fields--if I keep typing up this article much longer, I'll get a repetitive stress injury.  To my knowledge there are no studies looking at how slightly uneven surfaces affect injury or running gait.  Most biomechanical studies are done in a lab on a smooth concrete surface, so the dearth of studies is completely understandable. It's hard enough in a lab, imagine doing it on a trail.  This last paragraph is by far the least supported in this whole article, so toss it out if you must--but plantar pressure, impact loading rates, and leg stiffness are the more meaty arguments anyways. 

So in closing: why is the New York Times article wrong? Because all surfaces are NOT the same.  Hard surfaces may can push the body outside of its zone of optimal leg stiffness.  They also increase the peak pressure on the sole of the foot, which we know is a bad thing because the body does all it can to avoid high peak plantar pressures.  Finally, uneven surfaces stress the body in a more varied manner, lessening the risk of repetitive stress/overuse injuries.  Saying there is "no evidence" that soft surfaces are better is a gross mischaracterization.

Monday, July 18, 2011

Something New in Training: The Methods of Renato Canova

This is a piece I finished a few months ago after spending considerable time going over Renato Canova's training methods.  Renato Canova is a world-famous coach who instructs many of the best athletes in the world.  He has worked with the Italian national team in the past, but today, he works mainly with athletes in Kenya.  His athletes have won Olympic and World Championship medals, as well as setting national and world records.  As of 2015, Renato Canova's athletes have set 6 world records, won 42 medals in the World Championships, and 8 medals in the Olympic Games.  He has coached 9 athletes under 2:05:04 in the marathon, and 9 men athletes under 26:55 in the 10,000m.

More importantly, his training philosophy is significantly different than that of any other coach I am familiar with.  I wrote the following article in an attempt to understand the mechanics of his philosophy so it could be applied to any training program, not just one for an Olympian.

>> Click here to download the PDF <<

I recommend actually downloading the PDF from Google Drive, since the in-browser viewer does not always render text in an easily-readable way.

Hello world

Glad you're reading my blog.  I'm a recent Carleton College graduate with a penchant for running and an inclination towards writing.  My high school coach encouraged his athletes to become "students of the sport," and I quickly took to it.  I've spent a lot of time reading and studying running-related topics in the past six years or so, and this blog is a way for me to start synthesizing thoughts and making comments about different aspects of running.  My main area of focus is high-level training and racing: training methodologies, injury treatment and prevention, and the like.  I'm not sure what this blog will eventually turn into; for now, you can expect a mixture of science, common sense, and coaching know-how that has rubbed off on me from many of the brilliant individuals I've met in the running world.  While my training right now isn't going to impress even the lowliest of hobby-joggers, I've trained at a fairly high level in the past.  Even if nobody reads this, it'll be a way to sharpen and develop my writing skills and possibly pass on some knowledge about the sport and lifestyle I love.  Hope you enjoy it.