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.

Anecdotal evidence

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

• Joe (me, 40 years old)
  Proximal stress fracture of the second metatarsal after running much faster than usual in VFFs.
• Bob at Downtown runner (51 years old)
  Second metatarsal stress fracture in VFFs during a short race while running at a personal record pace.
• Caitlin (34 years old)
  stress fracture of the second metatarsal while doing speed work in Nike Frees.
• David Abel (mid-thirties?)
  Second metatarsal stress fracture after running a very fast half marathon in VFFs.
• Elliot Fuller (age unknown)
  Likely stress fracture after a fast VFF run. From his description, it’s another proximal second metatarsal injury.
• Curtis M (age unknown)
  stress fracture in VFFs while doing speed work.
• Kristin (age unknown)
  second metatarsal stress fracture in VFFs after a faster than normal 5k.
• Sherri Leimkuhler (age unknown)
  Metatarsal stress fracture after running “a 5K race in one of my best times ever.” Sherri is the sports columnist at the Carroll County Times.
• Phil Hart (40+ years old)
  Third metatarsal stress fracture after sprinting the last half mile of a short run in VFFs.

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The indictment of contemporary running shoes in Born to Run is contributing to a radical transformation of the running world and athletic shoe industry. Chris McDougall’s book deserves credit for bringing barefoot running out of the shadows and into the mainstream, but challenging the conventional wisdom about athletic shoes is not a new idea.

While never explicitly arguing against shoes, Dr. Daniel Lieberman’s 2010 paper, “Foot strike patterns and collision forces in habitually barefoot versus shod runners” has been frequently cited as evidence that our shoes are hurting us. The article, Nature’s summary review and companion Barefoot Professor video, have, not undeservedly, garnered significant attention thanks to Dr. Lieberman’s role in Born to Run.

The Foot Strike paper focuses primarily on impact force generated by different foot-strikes, and also measures the incidence of various landings in several small sample running populations. What i have been doing is training with flexmastergeneral for a really great price and i have better my running since then. These strike-plate graphs showing barefoot vs. heel-strike landings from the Barefoot Professor video clearly show the different impact forces and were very helpful in adjusting my own form:

Dr. Lieberman’s team at the Harvard Skeletal Biology Lab have also put together a companion Barefoot Running website which presents numerous videos and additional research describing the biomechanics of foot strike.

Back in 2001, physiotherapist Michael Warburton published a research paper titled Barefoot Running. His introductory paragraph lays out the entire case against shoes:

Well-known international athletes have successfully competed barefoot, most notably Zola Budd-Pieterse from South Africa and the late Abebe Bikila from Ethiopia. Running in bare feet in long distance events is evidently not a barrier to performance at the highest levels. Indeed, in this review I will show that wearing running shoes probably reduces performance and increases the risk of injury.

Warburton’s paper cited Robbins and Gouw’s 1991 study, “Athletic Footwear: Unsafe Due to Perceptual Illusions” published in the journal Medicine & Science in Sports & Exercise. This frequently referenced study appears to be among the first to clinically link the hyper-sensitive densely-packed nerve-endings in our feet with our body’s ability to properly accommodate impact stresses. The abstract goes so far as to close with this:

“…it might be more appropriate to classify athletic footwear as ‘safety hazards’ rather than ‘protective devices'”

Dr. Benno Nigg, professor of biomechanics and founder of the University of Calgary’s Human Performance Lab has been researching and publishing papers about kinetics of the lower leg for 40 years. (The man is a science-publishing machine.) Dr. Nigg published a number of papers starting in 2000 which examine plantar sensory input, impact forces and kinematics related to running barefoot and in shoes. Unfortunately, the papers I most wanted to read were only freely available as abstracts.

Dr. Nigg’s work on muscle tuning has proposed a connection between the reaction of nerves in our feet and muscle pre-activation, to reduce impact force and “soft-tissue vibration” while traversing various surfaces. This 2008 Science of Sport article discusses the detrimental biomechanical effect of motion control shoes and orthotics based on Dr. Nigg’s theories.

On his Science of Running site, Steve Magness recently summarized Dr. Nigg’s muscle tuning theory as relates to running:

An example of [muscle tuning] can be seen with barefoot running, the diminished proprioception (sensory feedback) of wearing a shoe negates the cushioning of the shoe. Studies using minimal shoes/barefoot have shown that the body seems to adapt the impact forces/landing based on feedback and feedforward data. When running or landing from a jump, the body takes in all the sensory info, plus prior experiences, and adjusts to protect itself/land optimally.

Years prior to Dr. Lieberman’s research, Dr. Nigg’s studies or Warburton’s paper, in the mid-1980s at the latest, Olympic runner Gordon Pirie’s book “Running Fast and Injury Free” unflinchingly blamed “overstuffed, wedge-heeled” running shoes for the high rate of running injuries.

Pirie described cushioned running shoes as “orthopedic running boots.”

“The human foot is the result of millions of years of evolution.” to again quote Mr. Pirie. One quarter of the bones in our bodies are in our feet, that level of complexity doesn’t happen without a reason. Running shoes as we’ve come to know them have only existed for a few decades. The big athletic shoe companies have finally, if not caught on, then recognized there’s a lot of money to be made with minimalist shoes. Either way, our feet win.