Joe Maller.com

Barefoot running stress fractures: A theory

Stress fractures are ridiculously common among runners, but this post will specifically address the apparent rash of metatarsal stress fractures afflicting minimalist and VFF runners, including myself.

First off, the term “barefoot” needs to be clarified. In every case I’ve found the runner wasn’t actually barefoot. We were all running in minimal shoes: Vibram FiveFingers, Nike Frees or something similar.

I noticed several consistencies among runners’ accounts of their injuries:

  1. Running in Vibram FiveFingers or other minimalist shoes, not barefoot.
  2. Running faster than normal (races, speed work, or just having too much fun)
  3. Mostly older than 35 years old (past the age of peak bone mass)
  4. Injury occurs months after transitioning to minimalist or barefoot running

So here’s my theory:

The majority of stress fractures affecting minimalist runners are not impact-related, but rather result from overloading weak metatarsals with increased toe push-off. These injuries arise several weeks or months after switching to natural running because the runner’s muscles strengthened faster than their bones.

Metatarsals are most susceptible to injury because they’ve been immobilized and weakened by conventional shoes. The second metatarsal, being the longest, endures the most stress.

Skin protection offered by VFFs and minimal shoes likely increases the risk of injury, friction on bare toes would have limited activity before the bones could have been hurt.

Barefoot & Minimalist stress fractures: A theory Pushing off with the toes places an enormous amount of stress across the midfoot. With the metarsals acting as a lever, the forefoot is pulled down and back against the ground with enough force to throw our full body weight forward. This motion also creates substantial sliding friction under the toes. Bare toes, no matter how calloused, would not be able to withstand this friction and would limit activity before bones could be overstressed.

Anecdotal evidence

As I’ve been researching this, I’ve found a lot of stories similar to my own:

If you’re a near-barefoot, minimalist or VFF runner and have suffered a stress fracture, please email me or leave a note in the comments.


Stress fracture blues

So I got my foot x-rayed and my recent suspicions proved true. Apparently in mid-December I suffered a stress fracture of the second metatarsal on my right foot. It was healing well, but needed more time and was re-aggravated during the Manhattan Half Marathon on January 22nd.

Doubtlessly, I’m going to be dealing with a lot of “this is why you shouldn’t run barefoot” comments, so this will be something of a pre-emptive refutation.

Is barefoot running a failure? No.

As described in Essentials of Surgical Specialties, “Stress fractures are the classic overuse injury.” Stress fractures were originally identified in the feet of army recruits resulting from long marches (while wearing boots). Wikipedia’s March Fracture entry precisely described my injury: “mostly occurs in the second metatarsal of the right foot.” Yep. My exact fracture is on the bone’s shaft, towards the base (proximal, meaning towards the body).

The American Academy of Family Physicians Common Stress Fractures page describes this injury as “among the most common sports injuries” with “track and field sports” acounting for 50% of stress fractures in men and 64% in women. One quarter of all stress fractures affect the metatarsal bones of the feet. Runners are exceptionally well represented among stress fracture sufferers.

Yes, there are apparently a number of Vibram FiveFingers runners who’ve suffered stress fractures. But clearly, shod runners get them too. The clinical literature notes that changing equipment can lead to stress fractures, but much more blame is assigned to increased amount and intensity of excercise.

Barefoot and minimalist runners are running more because it just feels great. When I switched to minimal shoes my running distances more than tripled, I was having a blast and my knees, hips, feet and back didn’t hurt. I know I’m not alone in blowing past my previous physiological limitations.

Am I just weak? Probably.

Our bodies stop stop automatically adding bone mass at around 35 years old. That’s not to say we can’t strengthen our bones through exercise, but the process is slower than it was–and bone growth is already much slower than muscle growth.

My skeleton seems to be in pretty good shape, with the distinct exception of my feet. Having spent most of my life in stiff, over-cushioned and inflexible shoes, the bones in my feet are doubtlessly weaker than they might have been.

This leads to what is probably the most difficult question:

Am I simply too old to recover from a lifetime of shoe-weakened feet?

I’m inclined to say no.

Ten months might have been long enough to build up the muscles and tendons in my feet and legs, but seemingly wasn’t nearly long enough to make up for 39 years of under-use and weakness in my foot bones.

Much more on this idea: Barefoot running stress fractures: A theory

So is the VFF Bikila blameless? Maybe, maybe not.

My fracture is basically directly aligned over the unnatural flex-point of the Bikila’s forefoot. That doesn’t mean anything, but the correlation is troubling.

Consistent with my previous observation about limited toe dorsiflexion in the VFF Bikilas, a suspected cause of stress fractures is muscle fatigue. Tired muscles fail to absorb shock and transfer mechanical stresses direclty to the bones. Here’s how Sports Injury Bulletin describes the causes of a stress fracture:

The ‘fatigue theory’ suggests that during repeated efforts (as in running), the muscles become unable to support the skeleton during impact as the foot strikes the ground. Instead of the muscles absorbing the shock, the load is transferred to the bone. As the loading surpasses the capacity of the bone to adapt, a fracture develops.

If my extensor muscles were exhausted by trying to flex against the restrictive toes in the Bikila, then it’s completely possible that fatigue could have contributed to this injury.

Additionally, the VFF Bikila muffles ground feel much more than thinner-soled VFFs or Soft Star RunAmocs. Reducing feedback from the ground can result in harder foot-strikes.

Ultimately though, this happened after my third (non-consecutive) 100+ mile month–which is a lot for me. That last month also coincided with colder weather and increased use of the Bikilas–often with socks, further reducing ground feedback.

What’s next?

I’m off the roads for a few weeks but I’m not planning on sitting still. First off, I’m going to learn to swim better.

This is also a great opportunity to fulfill the volunteer requirement for NYRR’s 9+1 guaranteed entry to the 2012 NY Marathon. I can’t run, but I can still help.

Accepting my potentially not-yet-sufficient bones, it’s possible that the reduced ground feedback of cold weather running presents a hazard for me. I may just need to accept that when I can’t feel the ground my feet still need a bit of padding to make up for weakness in my bones. Whether that will prove a tradeoff for pain everywhere else remains to be seen.

I’m not sure what to do about races. They’re ridiculously fun and while I’m not “racing” racing, I do tend to run much faster during races than I would normally. Maybe I need to wear a more forgiving shoe, or just train better.

I’m not about to stop running or give up running barefoot. The first thing I did after the doctor confirmed the white fuzzy spot in the x-ray (bone callus, a sign of healing) was to mark a date in my calendar. Unless there’s some clinically compelling reason not to, on the first Saturday in March, I’m going running.