Thursday, February 9, 2012

Running Toward Injury?

There seems to be a spate of articles in the Health section of the NY Times lately about running and the most recent is no exception.  In the article, the author reports on a recent study published in "Medicine & Science in Sports and Exercise" by a group at Harvard that investigated the relationship between foot strike and injury rates in endurance runners.  Although the piece in the NY Times is informative enough, I wanted to read the actual article and judge their findings and conclusions for myself.  As most runners are well aware, the debate between heel-striking and forefoot-striking is thick with passionate opinion on both sides and with no real resolution expected anytime soon.  Like many runners, my interest in minimalist running was piqued by Christopher McDougall's "Born to Run" -- I was clearly enamored by it in my blog post -- and decided to follow up my enthusiasm with a slow transition to minimalist shoes.  Unfortunately, I think my enthusiasm may have been either premature or misplaced altogether since, insofar as I can tell, the onset of Achilles tendonitis in June 2011 may have followed, in part, from nine months of semi-regular minimalist running.  I'm obviously not 100% sure of the cause, nevertheless, my enthusiasm for minimalist shoes has tempered -- my Vibrams are languishing in the corner of my bedroom -- and I'm running all my road & trail miles (as modest as they are these days) in a pair of trail shoes, the Montrail Mountain Masochists.  Commentary aside, I downloaded the article from the journal (you are also welcome to access it by clicking here) to read what these researchers actually did and how they (statistically) did it. 

First, the authors cite the three primary strike patters among distance runners:  rear-foot strike (RFS), forefoot strike (FFS), and mid-foot strike (MFS).  With RFS, the heel contacts the ground first whereas with FFS, the ball of the foot strikes the ground first, and with MFS, the heel and ball of the foot contact the ground simultaneously.  The context in which their research question and hypothesis are introduced makes clear that their study was motivated by the increasing number of runners adopting FFS or MFS patterns and jettisoning their cushioned shoes because of claims that are often, at best, anecdotal and, at worse, unsubstantiated.  These authors aimed to add some rigor to the debate by way of investigating the differences in injury rates/types in a retrospective cohort of 52 Harvard cross country runners.  The authors hypothesized that certain injuries would associate with different strike types:  Achilles tendinopathies, foot pain, and stress fractures of metatarsals with FFS and hip pain, knee pain, lower back pain, tibial stress injuries, plantar fasciitis, and stress fractures of lower limbs (excluding metatarsals) with RFS.  The statistical analyses seem reasonable (t-tests for two-sample comparisons and a General Linear Model, GLM, for the multivariable model) although the numbers of tests run, sans a multiple test correction, gives me pause since they are analyzing four outcome variables -- repetitive injury rate, traumatic injury rate, FFS injury rate, and RFS injury rate -- and are comparing FFS vs RFS both by sex and overall.  This results in a lot of p-values, some of which may reach significance at an alpha level of 0.05 just by sheer chance.  That said, however, the overall rate of injury across the study was quite high:  nearly 75% of subjects suffered at least one moderate or severe repetitive stress injury per year.  When only repetitive injuries are only considered, the rates appear to be much higher for RFS than for FFS:  2.5 times higher when only mild & moderate injuries are considered and 1.7 times higher when moderate & severe combined.  There was no statistical difference between RFS and FFS for traumatic injury rates.  In short, the authors conclude that habitual RFS experience rates of repetitive injury are nearly twice that of habitual FFS (see table below, copy-and-pasted from the article).  Compelling evidence?  Sure, but not completely impenetrable.  


First, the sample size, although modest (N=52; n=16 FFS and n=36 RFS), should be larger (especially given the number of statistical comparisons made in the paper).  Second, the sample isn't generalizable:  a cohort of highly competitive, highly trained Ivy League runners isn't representative of the at large running population.  Third, it is unknown how and why the subjects adopted the strike pattern they did -- the lack of randomization makes it difficult to assert that the only discernible difference between the two groups is their foot strike.  Fourth, the senior author (Daniel E. Lieberman) acknowledges receiving funding from VibramUSA.  This doesn't automatically undermine his findings -- a fair amount of respectable medical research is underwritten by pharmaceutical companies -- although there is evidence in the literature to suggest that negative or null research is either left unpublished or is presented such that it better reflects the product of the funder.  (I'm in no way suggesting the senior author acted in such a way, just that it could.) 

All things considered, this paper is a solid contribution to the RFS vs. FFS debate.  Unlike a lot of retrospective studies that rely on subject recall -- a major limitation -- this one analyzed data that was collected contemporaneously (data collection and occurrence of event coincided) so report of mileage and injury are likely to be quite accurate.  

Lastly, the authors' comment about transitioning from RFS (cushioned shoes?) to FFS (minimalist shoes?) rang true (reproduced below).  If only I'd been this deliberate after "Born to Run". 

"Another point to consider is that this study did not test  for the effect of transitioning from RFS to FFS running, and it is unclear and unknown if runners who switch from RFS to FFS strikes will have lower injury rates. FFS running requires stronger calf muscles because eccentric or isometric contractions of the triceps surae are necessary to control ankle dorsiflexion at the beginning of stance, and shod FFS runners also generate higher joint moments in the ankle. Runners who transition to FFS running may be more likely to suffer from Achilles tendinopathies and calf muscle strains. FFS running also requires stronger foot muscles, so even though impact forces generated by FFS landings are low, runners who transition are perhaps more likely to experience forefoot pain or stress fractures. They may also experience plantar fasciitis if their foot muscles are weak. However, these injuries are treatable, and they may be preventable if runners transition, slowly, gradually, and with good overall form."

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