Stepping Back to Minimal Footwear

Applications Across the Lifespan

Irene S. Davis; Karsten Hollander; Daniel E. Lieberman; Sarah T. Ridge; Isabel C.N. Sacco; Scott C. Wearing

Disclosures

Exerc Sport Sci Rev. 2021;49(4):228-243. 

In This Article

Minimal Footwear in Adult Running

Up until 60 yr ago, running shoes were quite minimal, typically consisting of a thin rubber sole and a canvas or leather top.[1] These shoes were flexible and lacked any midsole cushioning, arch supports, or heel counter stiffeners. Although running injuries likely occurred, they were not reported in the literature until the 1970s, suggesting that this may have been when they began to become more prevalent. This coincided with the running boom as millions of untrained people started running. Unfortunately, we lack the data necessary to explore the causes of this apparent uptick in injuries. However, a number of sports medicine professionals at that time attributed them to untrained runners landing too hard and without adequate foot control.[1] We hypothesize that, instead of these new runners developing the ability and strength to cushion and control their landings, footwear was adapted to do this passively for them. Shoe companies began to add midsole cushioning, arch support, and heel counter supports to address these deficiencies. Elevated heels were added to reduce the load on the Achilles tendon, and toe springs were added to reduce the work of the toe flexors.[43] These changes were made with the goal of increasing comfort and reducing injury risk. However, we postulate that these changes in footwear, intended to assist the runner, may be increasing injury risk.

As we evolved to run without footwear, barefoot running provides a reference for our most natural running gait. Strike patterns of barefoot runners are noted to be variable, depending on running speed and substrate hardness.[20] On softer surfaces, there is a greater tendency to RFS. However, habitual barefoot running is mostly associated with landing on the ball of the foot (referred to as a FFS pattern).[21] Those who are habituated to cushioned running shoes tend to land consistently on their heels (referred to as an RFS). A recent study demonstrated that the more time individuals spend running in cushioned shoes, the more likely they will be a rearfoot striker (Figure 3).[20] This is because the cushioning allows landing on the heel without the pain that would be experienced if landing on it barefoot. An RFS places less demands on the calf musculature, which must contract eccentrically at the onset of stance in an FFS to control the heel descent.[44] However, there are consequences of this RFS landing style. As stated previously, it results in an abrupt, characteristic impact transient in the vertical ground reaction force time series curve that is typically missing in FFS landings (Figure 2).[21] Impactful loads have been shown to produce damage to both cartilage and bone in animal studies.[45,46] This impact transient is associated with a steep rise to its peak, leading to a significantly increased loading rate compared with an FFS pattern.[47,48] Increased loading rates have been associated with some of the most common running injuries such as tibial stress fractures, patellofemoral pain, and plantar fasciitis.[49–54]

Figure 3.

Comparison of the vertical ground reaction force during running of a rearfoot striker (RFS) and a forefoot striker (FFS). Note the abrupt impact force of the rearfoot striker that is missing in the forefoot striker. [Adapted from Samaan CD, Rainbow MJ, Davis IS. Reduction in ground reaction force variables with instructed barefoot running, J Sport Health Sci, 2014; 3(2):143–151. CC BY-NC-ND 4.0.]

In an attempt to mimic barefoot running, the first modern, widely available minimal running shoe was introduced by Nike in 2005. Like many racing flats, the Nike Free lacked arch support and heel counter stiffness, and it had a flexible sole (Figure 4A). However, it had a cushioned midsole, which permitted an RFS pattern.[55] In the same year, the Vibram FiveFingers shoe (Figure 4B) also became available. This shoe had five pockets for the toes, which allowed them to move independently from each other. The shoe was extremely flexible and had no midsole or heel counter. It was originally designed for boating but quickly was adopted by the barefoot running community who wanted a shoe that was as close to barefoot as possible. Other minimal shoes also began to emerge (Figs. 4C–E) However, traditionally shod runners who wanted the barefoot experience also began running in these shoes. Many simply replaced their cushioned shoes with minimal shoes without reducing their running mileage. These runners lacked the benefit of adaptation that the experienced barefoot runners had. Therefore, many of these runners sustained injuries to the foot and ankle due to the lack of cushioning and support that their traditional shoes offered. Reports of Achilles tendinitis, plantar fasciitis, and metatarsal stress fractures appeared in the literature.[56–58] This was unfortunate, as it may not have been the shoe but the lack of appropriate transition to it that led to the injuries. Although there continued to be steadfast believers in the minimal shoe, these injury reports led to a reduced enthusiasm for this type of footwear.

Figure 4.

Top row: Early minimal shoes. A. Nike Free*; B. Vibram FiveFingers. Middle row: Examples of current minimal footwear. C. Vivobarefoot Stealth; D. Xero Prios; E. demonstration of the flexibility of minimal shoes. Bottom row: Examples of partial minimal shoes. F. New Balance Minimus; G. Saucony Kinvara. *Now considered a partial minimal shoe.

As a result of the reported injuries, some footwear companies decided to retreat from minimal shoes. Instead, they produced a shoe that had less cushioning and support than their traditional running shoe as a compromise to runners between minimal and cushioned shoes. These are sometimes classified as a partial minimal shoe and include shoes such as the New Balance Minimus and Saucony Kinvara shoes (Figs. 4F, G). However, studies have suggested that mechanics while running in partial minimal shoes are similar to those while running in traditional shoes and statistically different than running barefoot.[59–61] Only when running in shoes with little or no cushioning are mechanics similar to barefoot running.[60,62]

Minimal footwear promotes an FFS pattern, and this pattern has been shown to actually resolve some injuries. The benefit of an FFS pattern was demonstrated in a case series of 10 West Point cadets diagnosed with anterior compartment syndrome and recommended for fasciotomy surgery.[63] The cadets underwent a 6-wk training intervention to transition from an RFS to an FFS to shift the load from the anterior lower leg musculature to the larger, posterior musculature. After the training, all compartment pressures returned to normal, with significant improvements in pain and function, as well as in their 2-mile run times. Most importantly, surgery was avoided in all cases. Another study involved 16 runners with patellofemoral pain who were randomized into a retraining group to transition to an FFS pattern or to a control group.[64] Those who transitioned to the FFS pattern had near complete resolution of their knee pain. In addition, the patellofemoral contact stresses, which have been associated with this pain,[65] were reduced by 50%. This likely is due to two factors. There is greater knee flexion at foot strike with an FFS pattern,[66] which increases the contact area between the patella and femur.[67] In addition, forces at the knee during early stance are lower due to the decreased slope of the vertical ground reaction force typically seen in an FFS pattern. Lower forces and greater contact areas lead to lower patellofemoral contact stresses and likely to reduced pain.[65]

There is an important interaction between footwear and foot strike patterns that must be considered. An FFS runner in cushioned shoes demonstrates a lower vertical ground reaction force load rate compared with an RFS runner in cushioned shoes. However, the mediolateral and anteroposterior load rates of an FFS runner in cushioned are increased above that of an RFS runner.[47,68,69] This likely is due to the elevated heel and lateral flare of a cushioned shoe. These structural features often place the foot in greater plantarflexion[70] (which is coupled with inversion) at foot strike than when running in a minimal shoe (Figure 5). This is associated with greater posterior and medial ground reaction force load rates at foot strike. These increased posterior and medial load rates coupled with the decreased vertical load rate in FFS runners habituated to conventional shoes result in similar resultant load rates between them and RFS runners habituated to conventional shoes (Figure 6).[47] However, when forefoot striking in minimal shoes, all components of the ground reaction force load rates are significantly lower than when either rearfoot or forefoot striking in traditional shoes. Thus, forefoot striking in minimal shoes results in the lowest impact loading in all directions. Reducing impacts in the vertical direction has prospectively led to a 62% reduction in running injuries over the course of a year.[53] Reducing impacts in all three directions may potentially lead to even fewer injuries, but this needs to be examined further.

Figure 5.

A. A habitual forefoot striker (FFS) runner landing in their cushioned shoe. B. The same runner in a minimal shoe. Note the reduction in plantarflexion and inversion in the minimal shoe.

Figure 6.

Comparison of load rates between habitual rearfoot striker (RFS) in standard shoes (SRFS), forefoot striker (FFS) in standard shoes (SFFS), and FFS in minimal shoes (MFFS). Note that resultant load rates are only lower in the FFS in the minimal shoe. [Adapted with permission from (47). Copyright © 2016 American College of Sports Medicine. All permission requests for this image should be made to the copyright holder.]

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