January 2018

Can minimalist shoes protect against injury by increasing foot-muscle thickness?

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Improvement in abductor hallucis muscle size associated with a gradual transition to minimalist running shoes suggests that this type of strength-based approach can help improve foot structure and stability, which may help runners resist development of overuse injury or relieve existing foot pain.

By Nicholas A. Campitelli, DPM, FACFAS; and Scott A. Spencer, DPM

The approach in our society to selecting running shoes has, typically, been to find a shoe that is supportive and has plenty of cushion. Running shoe manufacturers have reinforced this notion by developing features—such as increased cushioning, elevated heel, and various motion control and stability technologies—designed to protect the body from mechanical stresses caused by running.1 Despite these efforts, running injuries reportedly affect 19.4% to 79.3% of runners every year.2 In fact, some studies suggest that cushioning mechanisms used in running shoes can alter running biomechanics by modifying kinematics and muscle-activation properties.8-14

Another common approach has been to match runners’ foot type (pes planus, neutral, pes cavus) with a shoe designed accordingly, in the hope of providing protection. However, the literature suggests that such a prescription-based strategy for recommending running shoes has also failed to decrease the rate of running-related injuries.3-7

If running shoes are not the solution to running-related injuries, what is? Mercer Rang, MD, FRCS(C), a pediatric orthopedic surgeon, once made the strong statement that “Shoes do no more for the foot than a hat does for the brain.”15 One could extrapolate from this that shoes do not make a difference in reducing the rate of running-related injuries.

The appeal of minimalism

After minimalist toe shoes for running were introduced in 2005, we (both coauthors) observed anecdotally that runners were reporting resolution of their injuries while wearing these shoes. This caught my (coauthor NAC) attention because I had still been prescribing orthotic devices to my patients who were runners and suffered various injuries.

Strengthening foot musculature, as minimalist shoes appear to do, can help improve foot structure and stability, which in turn may help to resist development of overuse injury or improve existing foot pain.

In reviewing the literature on running shoes and orthoses, what I found seemed at odds with what we, as clinicians, along with running shoe stores and manufacturers, were preaching to our patients and customers in regard to running shoe selection.7,16-25 This type of discrepancy was nothing new; there is even evidence that excessive pronation has little to do with running-related injuries, as many who recommend running shoes think.26-33

Why, then, were so many people, especially runners, taking to minimalist shoes? Given that the literature revealed little real support for modern conventional running shoe designs or explanations for why those designs weren’t significantly reducing running-related injury risk, I considered several theories. One was that the foot could become stronger by not relying on a shoe for support. Physical therapy, after all, is an entire profession based on the philosophy of strengthening muscles and improving range of motion to help prevent or rehabilitate injury. Yet, when we focus on the foot, many consider it “normal” or “protective” to reduce motion and use muscles less by wearing shoes and orthoses.

The popularity of minimalist running shoes was based in part on the belief that wearing such shoes would strengthen the foot and allow the body to find its normal anatomic alignment. Intuitively, this made sense but, at the time, had yet to be quantified or discussed in the peer-reviewed literature. In 2012, we (the coauthors and additional colleagues from the Kent State University College of Podiatric Medicine, Independence, Ohio) undertook a study to determine whether wearing minimalist toe shoes had any impact on foot strength. Our results were published in 2016.34

Our study of shoe choice and foot-muscle thickness

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Our initial goal was to obtain a sample of individuals and perform various pr-intervention and postintervention measurements that could determine if shoe wear was associated with an increase in foot-muscle thickness. In particular, we wanted to measure the abductor hallucis muscle, which has been described as being an important supporter of the arch and is frequently measured in testing overall foot strength.33,35-38 The abductor hallucis has one of the largest muscle bellies in the foot; for that reason, its volume is relatively easy to measure.

We assigned 41 subjects to four different groups:

  • Unrestricted daily walking in minimalist shoes
  • Restricted walking in minimalist shoes
  • Running in minimalist shoes (gradually increasing mileage by 10% per week to six miles.
  • Wearing traditional shoe gear.

Over 48 weeks, we collected data related to injury, arch height (assessed by radiography), muscle strength (estimated using force-plate analysis), and muscle girth of the abductor hallucis (assessed by ultrasonography). Our study population included various foot types; this variable was not recorded or measured, although we did photograph all feet prior to starting the study. Poststudy photographs were not obtained.

Yet, when we focus on the foot, many consider it “normal” or “protective” to reduce motion and use muscles less by wearing shoes and orthoses.

We were unable to publish the results of the force-plate analysis and muscle-strength calculations, as a consequence of technical problems that could have led to reporting of unreliable results. Although we observed an increase in force associated with minimalist shoe wear, this could also be explained by alterations to intrinsic foot muscles other than the abductor hallucis or by the contribution of lower-extremity muscles. We then made a decision to focus the study on the abductor hallucis.

Regarding radiographs, an institutional change during the study period resulted in the loss of digital files for a number of participants. Those files could not be recovered, which limited our analysis.

Change in muscle size

The significant results of our study involved cross-sectional measurement of the abductor hallucis by ultrasonography. The size of this arch-supporting muscle increased in all three study groups who wore minimalist shoes. Increases in muscle size suggest that strength also increased, although we did not measure strength directly.

Other, similar studies have looked at the abductor hallucis in relation to foot strength and arch height.35-37 These findings may cause us to rethink the etiology of foot pathology and injury. The traditional emphasis on strengthening the tibialis posterior muscle may not be as crucial as once believed.39,40 One hypothesis is that the posterior tibial tendon becomes damaged secondary to a breakdown in the arch, resulting from attenuation of ligaments that support the arch. Foot-muscle weakness may lead to increased stress on these ligaments, which requires the tibialis posterior muscle to work harder to prevent arch collapse; this, in turn, leads to overuse and damage to that muscle.41-44 Focusing on strengthening the posterior tibial muscle, therefore, ignores the true root of the problem: weakness of the foot musculature.

The abductor hallucis is not the only muscle crucial to foot strength. The numerous intrinsic muscles of the foot also play a crucial role in foot strength and stability. However, many of these small muscles are more difficult to isolate and measure than the abductor hallucis.

Our study was not the only study to support the idea of strength gains from wearing minimalist shoes.45-47 Chen and colleagues45 demonstrated that habitual shod runners who transitioned to minimalist shoes developed a significant increase in leg and foot-muscle volume. Johnson and co-workers46 also reported that safely transitioning to minimalist toe shoes was associated with a greater increase in intrinsic foot-muscle size compared with a control group wearing traditional running shoes.

The improvement in muscle size associated with minimalist shoes that we, and others, have demonstrated may be instrumental in treating foot pathology, such as plantar fasciitis. Strengthening the musculature of the foot can help improve foot structure and stability, which may help to resist development of overuse injury or improve existing foot pain. Given that this approach contrasts with previous thinking—that stability shoes and orthotic devices are needed to optimize foot function—more studies are needed to further explore the validity of this alternative approach.

Clinical implications and recommendations looking ahead

I (coauthor NAC) introduced this philosophy—ie, strengthening foot musculature can improve foot structure and stability—into practice approximately 4 years ago. Anecdotally, I observed improved outcomes in patients who suffer chronic foot pain that had not responded to traditional treatment with an orthosis and motion-control shoes. These improvements are, in my opinion, the result of strength gains, which are no different than the strength gains that can help resolve other muscle and tendon ailments through physical therapy. I propose that increased clinical emphasis needs to be placed on guiding patients toward strengthening their feet as opposed to splinting them or reducing motion.

Concerns exist that wearing minimalist shoes might increase the risk of certain injuries (while decreasing the risk of others).48 Transitioning too quickly to a minimalist shoe will increase the risk of overuse injuries, such as tendinitis and metatarsal stress fracture.48-50 This is why we, in our study, made certain that participants transitioned gradually to running in minimalist shoes, hypothesizing that doing so would help them avoid injury. Runners were instructed to follow a training regimen that allowed only a 10% increase in distance run each week wearing minimalist shoes, with any additional running performed while wearing the participant’s traditional running shoes.

Although we did not discuss injury data in our published study, we found that, during the 6-month data collection period, not one participant—not even in the running group—sustained an injury that was serious enough to remove them from the study. We believe that this was due to the slow transition period we implemented, which allowed runners ample time to recover and adapt before increasing their minimalist-running mileage.

This philosophy of strengthening, and the contribution of minimalist shoes to that end, still needs to be tested in future studies, with a better control group and a tighter focus. We also stress that this is a philosophy—one that coauthor SAS does not entirely adhere to. Although our opinions differ, we were still able to come together, share ideas, and complete the 2016 study. Not everyone is open to the idea of functioning with less support to prevent injury, but we—both of us—believe that it is healthy and beneficial for the field of biomechanics to explore new avenues of treatment and functioning.

Nicholas A. Campitelli, DPM, FACFAS, is a podiatrist in practice in the Akron, Ohio, area. Scott A. Spencer, DPM, is Associate professor in the Division of Surgery and Biomechanics, Kent State University College of Podiatric Medicine, Independence, Ohio.

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