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Metatarsal Stress Fractures in Minimalist Runners: What Are the Causes?

Posted on July 20 2011

From Gray's Anatomy.

Image via Wikipedia

One of the big topics of discussion swirling around minimalist running shoes right now is whether they increase the likelihood that a runner might experience a stress fracture. Clearly, fractures of the 2nd metatarsal are occurring in individuals who give minimalist running a try (for example, check out this great post by Joe Maller), but it’s also important to keep in mind that this is the most common type of foot fracture in shod runners as well. The big question, for which we don’t yet have an answer, is whether incidence is in fact higher among minimalist runners.

Anecdotal reports from some medical professionals suggest that injuries resulting from minimalist running have exploded over the past year or two. To a certain extent it wouldn’t surprise me at all if this were true. Runners are notorious for jumping on new trends and doing dumb things in their training, and the exuberance with which some have approached minimalist running is likely to be a big factor in causing these injuries (as I suspect it was in the two cases reported by Giuliani et al., 2011). The body needs time to adapt to any major change in footwear or training, and someone who has been running their entire life in cushioned shoes is going to need to take it really slow at first in a shoe like the Vibram Fivefingers until the bones can strengthen (look up Wolff’s Law if you’re interested in this – I teach about it every year in my Human Anatomy and Physiology class).

As an analogy, suppose you do all of your miles in the form of long, slow distance. One day, you hear that interval training is really where it’s at if you want to improve your performance, so you start hitting the track and running hard intervals 2-3 times per week. What’s going to happen? My bet is you get injured, simply because you jumped into speedwork too quickly and didn’t let your body adapt to the new type of training.

Given the above, I think there are a number of significant questions that need to be answered regarding the metatarsal stress fracture issue. Among these are:

1. Is incidence truly higher among minimalist runners? Hard numbers are needed here, as anecdotal evidence points in both directions depending on who you ask.

2. If incidence is higher among minimalists, when do these fractures most commonly occur? In other words, are these mostly transition injuries that occur because people apply a new stress to quickly to their body without allowing time for adaptation? For more on this, read my post on repetitive overuse injuries in runners.

3. Which shoes are most frequently associated with this problem? Most commonly I see the Vibram Fivefingers referenced and not something more moderate like the Saucony Kinvara. Minimalist shoes come in a variety of types, and some may require more care than others.

4. What type of stress typically causes these injuries? Is there something specific about minimalist shoes or the type of gait that they encourage that makes a stress fracture more likely?

caseykerriganI’ve thought a lot about these questions, and have some thoughts on each, but for this post I wanted to concentrate on point number 4. To do this, I solicited the help of Dr. Casey Kerrigan (see photo to the left), who has published over 90 peer-reviewed papers on walking and running gait (with a particular interest on the causes of osteoarthritis). Casey has an amazing background and an immense amount of clinical experience – she founded the gait labs at both Harvard and the University of Virginia. Recently, Casey took the amazing and gutsy step of resigning from her tenured faculty position at the University of Virginia so that she could start a shoe company called OESH. She had come to the point in her career where she felt that she could have a more positive impact on the well-being of her patients by developing a shoe that works rather than by spending her days in the clinic or publishing papers. As a fellow tenured college professor, I can assure you that one does not give up a job like this without great risk (both professional and financial), and I’m incredibly impressed by willingness to make such a move.

I asked Casey if she would be willing to share her thoughts on what might be causing these stress fractures – here was her response:

Dr. Casey Kerrigan on Metatarsal Stress Fractures

My thoughts below are based on the biomechanical research I’ve done on walking, running and the (ill-effects) of current shoe designs. To learn more about my research and how that led me to re-think the midsole of an athletic shoe and start a shoe company, go to OESH Shoes. Don’t worry – I’m not trying to sell shoes here – what I have available now is a limited, high-demand, women’s all purpose athletic shoe that is not specific to running.

Let’s start with a post by Joe Maller and the comments that followed it suggesting that these second metatarsal stress fractures seem to be occurring with some amount of frequency in runners who are running in minimalist shoes. Pete was also kind enough to give me a heads up, sending me over this just published case report by Giuliani et al., which describes the occurrence of this fracture in two runners wearing Vibram Five Fingers. My physician colleague, Bob Wilder, who now has my old job as professor and chair of the department of physical medicine at the University of Virginia, and has a clinical practice made up almost exclusively of runners, told me he’s seen only two cases this past year but admits he hasn’t seen a lot of patients in his clinic running in minimalist shoes.

I can add my own close to home experience… one of my daughters, Kellyn, who is 13, had one. She greatly enjoys running and has been running her whole life either barefoot or in assorted racing flats that over the years I would scrupulously check to make sure they had no raised heel or arch support. There was no way, I as her mother, with all that I knew from my studies, would ever let her wear traditional running shoes. Now that she’s big enough to wear OESH, that’s what she trains in and only wears flats (track spikes – without the spikes) when racing.

Back to Joe Maller’s post. His theory and his nice cartoon video about how these fractures are happening is not that far from what my research would indicate. Joe believes, and I agree, that these fractures are occurring later in stance, not at impact as Giuliani et al. suggests. But before I delve into my analysis, I need to first review why I believe most all injuries, not just this particular one, occur when the foot is fully planted during midstance, not at impact. From there I can explain why the second metatarsal seems to be more vulnerable than other areas in runners who are running in minimalist shoes.

For years, I’d been putting together force plate data with 3-D motion analysis data in walking. Combining these data and evaluating them in light of our clinical information (all the problems that came to our general clinic or to our specific runner’s clinic), we could finally understand how and why we get the types of repetitive injuries we do. Remarkably, this had never been done before. You could say that the “a-ha” event of all my best research was synthesizing the force plate data with the 3-D motion data—this was the classic 1 + 1 = 3 breakthrough that every researcher hopes to discover. If you’re interested you can read more about all this in my post The Rest of the Story.

I first started looking at the forces associated with knee osteoarthritis. But what I found applies to nearly all of the typical injuries we see. The peak forces stressing nearly all the injury sensitive areas of our body occur when our foot is fully planted (not at impact) and fully pronated. The specific forces at the moment of impact are miniscule and sometimes in the complete opposite direction to where injuries occur. More critically, the forces across virtually every injury prone area are highest at two points in walking that occur well after impact – when loading weight onto the foot and when lifting the foot off. These injurious forces peak at one point in running – at midstance, well after the peak impact force.

Our findings are quite logical when you think about them – we’re at risk for injuries when we also have to support and transfer our full body weight. But they really do fly in the face of what everyone, (including the athletic shoe industry) has always promoted – that foot impact is what causes the really bad things that happen to bones and joints, such as stress fractures and osteoarthritis. To this, I like to say that the only way a cushioned athletic shoe can protect you from injury is if you are in a car accident and happen to have them strapped around your head.

What is the effect of a traditional running shoe at this critical moment when we’re fully supporting and transferring our body weight and are at risk for nearly every single common running injury, second metatarsal fracture included? The raised cushioned heel and arch support / motion control features that are both inherent to every traditional athletic shoe (not just motion control shoes) essentially freeze out the foot’s natural function in providing compliance that helps protect the rest of the body. Also, the cushioning, which doesn’t really give back in any physiologically meaningful way, just makes it harder to transfer the body weight, which increases peak joint torques at this critical moment.

If you are running barefoot or in minimalist shoes, your foot is going to be freed to provide compliance at this critical moment. We’ve demonstrated that the foot arch compresses and releases in perfect harmony with the rise and fall of the peak body weight force. Of course, that compliance is good – it protects your body north of the foot from all the major injuries at that time. It also healthfully exercises your plantar fascia and foot intrinsic muscles. Good, good, good! But it does put more stress through the second metatarsal than there ever was before.

Again, back to Joe’s theory. The stress is occurring around midstance or perhaps a little after, at the time when the foot, and more specifically the foot arch, is providing maximal compliance and then springing back. Joe depicts in his video cartoon the stress to be associated with quite a bit of movement but in fact, there really isn’t that much movement then. It’s really the forces, not so much the movement, that is responsible for the fractures.

One reason that it seems to be occurring more often when wearing minimalist shoes compared to running completely barefoot is that folks wearing minimalist shoes take longer strides compared to running completely barefoot. But note, I don’t think it’s the harder heel strike associated with shoes that is directly causing injury. Rather, it’s that the longer stride length causes higher forces at midstance. Again, the physics of bearing all that body weight trumps the modest forces at impact. Also, whatever cushioning there is in a minimalist shoe makes it harder, not easier, to transfer the body weight at midstance. Thus a minimalist shoe that has cushioning but lacks a heel or arch support / motion control ends up putting the greatest stress on the second metatarsal because the lack of those two things frees up the foot while the cushioning simultaneously puts greater demands on it.

Now, how do you minimize this risk?

After all that, my advice probably sounds a lot like what you’ve already heard. You should transition slowly from your current traditional shoes to running barefoot or in minimalist shoes. You need to do this very slowly (as much as I hate the thought of you continuing to place more torque than necessary at your knees with traditional or just moderately minimal shoes), probably over a few months.

A bit concerning is that it seems that these fractures can occur months after you’ve made the transition. Also, my daughter’s experience makes me especially concerned about adolescent girls and young women who are at greater risk for stress fractures in general than the rest of the population… I suspect that running in minimalist shoes, especially on non-compliant surfaces (see below), places them at greater risk than the rest of the population for a second metatarsal fracture.

So what else can you do? Besides the obvious of making sure you’re eating well and getting enough calcium and Vitamin D for good general bone health? And not increasingly mileage too quickly (a general rule is to not increase your mileage by more than 10% per week)? You might look to see if you can find a compliant but not foamy soft surface to run on (see my post on the Harvard Indoor Track to see the distinction of compliant but not soft which I think can be found in a lot of natural surfaces). Unlike foam in a shoe, a compliant surface that truly compresses and releases in tune with the rise and fall of the peak forces can reduce injury.

I would avoid a minimalist shoe that has any kind of cushioning (as I think that maximally stresses your second metatarsal) especially if you are recovering from a stress fracture or develop any “top of the foot” pain. Finally, while this isn’t a problem when running completely barefoot, as it is when running in minimalist shoes, I think you need to work extra, extra hard to take shorter strides than you normally feel comfortable doing. Of course, this advice goes for traditional shoes as well.

Runblogger’s Thoughts: For me, one of the key take-home messages here is that it’s not impact that is causing these fractures in the metatarsals, but rather stresses in the metatarsal shaft that occur during mid to late stance as body weight passes over the foot and the heel begins to come up off of the ground. In addition to shortening stride and taking a gradual approach, a third piece of advice might be to avoid pushing off too hard when running in very minimal shoes!

Casey’s anecdote about the UVA gait lab not seeing many stress fractures caused by minimal shoes is also worthy of note – you see different reports from different people. This is why this issue needs to be looked at in more detail.

An interesting personal note is that I often find that I get an ache under my second metatarsal on my right foot if I run long in my Vibram Bikilas. One of the things I have noticed is that because my toes touch the tips of the toe pockets, it can be very hard to flex them while I’m running (somewhat paradoxically, toe movement actually seems more restricted to me in these shoes – may be a fit issue), and by the end of a long run I’m desperate to take my shoes off so that I can wiggle my toes up and down. Part of me wonders if this might be a common experience, because an inability to push down with the toes may put added stress on the metatarsal heads (since the toes can’t share the burden), which in turn causes greater bending forces on the metatarsal shafts during stance. I don’t have this problem wearing other shoes without the toe pockets since my toes have greater freedom of movement in a one-fingered toebox.

That’s it for now, we’d love to hear if anyone else has thoughts on the stress fracture issue!

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