Irene S.Davis*,Hannah M.Rie,Sott C.Wearing

aSpaulding National Running Center,Department of Physical Medicine and Rehabilitation,Harvard Medical School,Boston,MA 02115,USA

bSport and Health Sciences,University of Exeter,Exeter,EX4 4PY,UK

cInstitute of Health and Biomedical Innovation,Queensland University of Technology,Brisbane,QLD 4059,Australia

Why forefoot striking in minimal shoes might positively change the course of running injuries

Irene S.Davisa,*,Hannah M.Riceb,Scott C.Wearingc

aSpaulding National Running Center,Department of Physical Medicine and Rehabilitation,Harvard Medical School,Boston,MA 02115,USA

bSport and Health Sciences,University of Exeter,Exeter,EX4 4PY,UK

cInstitute of Health and Biomedical Innovation,Queensland University of Technology,Brisbane,QLD 4059,Australia

Itisbelieved thathuman ancestorsevolved the ability to run bipedally approximately 2 million yearsago.Thisform oflocomotion may have been importantto oursurvivaland likely hasin fluenced the evolution ofourbody form.As ourbodieshave adapted to run,itseemsunusualthatup to 79% ofmodern day runners are injured annually.The etiology ofthese injuriesis clearly multifactorial.However,1 aspectofrunning thathassigni ficantly changed overthe past50 yearsisthe footwearwe use.Modern running shoeshave become increasingly cushioned and supportive,and have changed the way we run.In particular,they have altered ourfootstrike pattern from a predominantly forefootstrike(FFS)landing to a predominantly rearfoot strike(RFS)landing.This change alters the way in which the body is loaded and may be contributing to the high rate of injuries runners experience while engaged in an activity for which they were adapted.In this paper,we will examine the bene fits of barefoot running(typically an FFS pattern), and compare the lower extremity mechanics between FFS and RFS.The implications of these mechanical differences,in terms of injury,will be discussed.We willthen provide evidence to supportourcontention thatFFS providesan optimalmechanicalenvironmentforspeci fic footand ankle structures,such as the heel pad,the plantar fascia,and the Achilles tendon.The importance of footwear will then be addressed,highlighting its interaction with strike pattern on mechanics.This analysis will underscore why footwear matters when assessing mechanics.Finally,proper preparation and safe transition to an FFS pattern in minimal shoes will be emphasized.Through the discussion of the current literature,we will develop a justi fication for returning to running in the way for which we were adapted to reduce running-related injuries. ?2017 Production and hosting by Elsevier B.V.on behalf of Shanghai University of Sport.This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Footstrike pattern;Minimal footwear;Running;Running injury;Running mechanics;Tissue mechanics

1.Introduction

Some evolutionary biologistssuggestthatthe modern human form re flects numerous adaptations that facilitate bipedal running.1To the best of our knowledge,and based on anthropological evidence,it has been suggested that humans began running approximately 2 million years ago.1Human ancestors were and modern humans are relatively slow runners compared to other scavengers.However,it is posited that our human ancestorsevolved into effective endurancerunners.Thisallowed them to run theirprey into exhaustion,enabling them to getclose enough to club them to death.Indeed,humans are the only primatecapable ofendurancerunning.Despitethederived capa-bilities of the modern human to engage regularly in running,up to 79%ofmodern endurance runnersare injured in a given year, with 46%of injuries being recurrences.2These injury statistics seem inconsistent with the idea that humans have numerous morphologic features that are speci fic to running.

One explanation for this high injury rate in runners may be based in the mismatch theory of evolution.This theory generally suggests that many of the health problems in society today are the result of the rapid change in environment and diet relative to the rate at which the human body has adapted.3,4This includes the processed food we eat,the polluted air we breathe, and the relative lack of activity we now engage in.Whereas in the past,we often died of communicable diseases,we are now dying of preventable,non-communicable diseases such as those associated with obesity and cardiovascular conditions.The high rate of running injuries today may be another example of this mismatch theory.Runners may be adapting their mechanics tothe modern environment in a way that is mismatched to the mechanics with which we evolved.

There has been an ongoing debate about whether the way a runner strikes the ground plays a role in running injuries today. Up to 95%of traditionally shod runners land on their heel (rearfoot strike—RFS)when they run5–7on modern hard surfaces.According to De Almeida et al.5approximately 5%land with a flatfoot(midfootstrike—MFS)and 1%land on the ballof their foot(forefoot strike—FFS).Conversely,the majority of habitual barefoot runners land with an FFS,in slight plantar flexion.8,9Given that humans evolved the ability to run without the assistance of footwear,strike patterns during barefoot running likely represent our most natural form.Whereas primitive shoes have existed for some 10,000 years,cushioned running shoes have only existed forthe past50 years.As a softer surface encourages more of a heel strike landing,10the cushioning in modern running shoes is likely responsible for the predominantRFS pattern in runnerstoday.Therefore,itisplausible that footwear has changed the way the modern humans run, which is mismatched from the running style we evolved to use.

The purpose of this paper is to examine whether changes in strike pattern and footwear have contributed to the high rate of injury associated with running.We will provide evidence to support the argument that the strike pattern of our most natural state,in footwear that does not interfere with one’s natural mechanics,may be the optimal way to reduce injury risk in runners.We will do this by examining the mechanics of barefoot running and reviewing the differences in lower extremity mechanics between RFS and FFS patterns and how these differences are related to injury.We will then examine the effect of strike pattern on mechanics at the tissue level including the heel pad,plantar fascia,and Achilles tendon.Finally,we will elucidate the complex interactions between footwear,footstrike pattern,and mechanics.These interactions will,in turn,lend credence to the idea that running with an FFS in minimal shoes might positively change the trajectory of running injuries today. For the purpose of this paper,we will focus on the mechanics of running on relatively hard surfaces(i.e.,not sand,grass,trails) as this is where the majority of modern running occurs and where the majority of studies are conducted.We will also focus on habitual running mechanics as opposed to novice,unpracticed mechanics that may be temporary in nature.

2.What can we learn from barefoot running?

It has been suggested that our human ancestors began running over 2 million years ago,11yet the earliest example of footwear is dated back over 10,000 years.Thus,modern humans and our ancestors ran barefoot for the vast majority of our evolutionary history.As humans evolved the ability to run in the absence of shoes,we consider barefoot running to be the baseline condition that is re flected in human morphology.

The most ecologically valid means of assessing barefoot running is to examine those who habitually run this way.Most studies have revealed thathabitualbarefoot runners do not typically land on their heels,unlike their shod counterparts.12,13These studies have been limited to running on hard surfaces, which are where the majority of runners do their training volume.The primary reason for this is that loads associated with landing on the heel without cushioning during running exceeds those associated with the pain pressure threshold that occurs at fast walking.14It is logical that humans would run in a way that is least painful.It has been reported that habitual barefoot runners will use an RFS pattern when running on soft surfaces.13However,landing with an FFS is our most typical running style when running on hard surfaces.8,12,13One study has observed that habitually barefoot people from the Daasanach tribe in northern Kenya mostly run with an RFS. However,it has been noted that these individuals,who live in a hot sandy desert,are traditional pastoralists who walk long distances for herding purposes and do not run much.8,9,15

Barefootrunning hasa numberofdocumented bene fits.Ithas been shown that removing support(as provided by modern footwear)from the arch of the foot during running strengthens the foot.This is evidenced by an increase in the cross-sectional areas of both intrinsic and extrinsic foot muscles following a period of running in minimal shoes that mimic barefoot running.16It has been reported that Indian children who live in communities where they are habitually barefoot have signi ficantly higher arches than their counterparts from communities where either open toed sandals or closed toed shoes are worn.17Being barefoot also allows the maximal sensory input to the lowerextremity.This sensory inputhas been shown to be important for both static and dynamic stability.18,19Sensory input is also importantin modulating the appropriate leg stiffnessforthe surface being encountered.20–22High leg stiffness is associated with greaterloading ratesand shock,which may increasetherisk of injury to bone tissue.23–25On the other hand,excessively low stiffness has been associated with soft tissue injuries.26,27Furthermore,it has been shown that stiffness differs between RFS and FFS running patterns.28Achieving optimal stiffness is important as it is in fluences running economy and performance as well as shock attenuation and injury risk.29The heightened sensory inputavailable when running barefootmay facilitate the optimization of lower limb stiffness.30

3.Mechanics of rearfootvs.forefoot striking

Foot strike pattern,which is de fined by the part of the foot which first strikes the ground during running,plays a signi ficant role in the lower extremity mechanics during early stance.31–33During an RFS,the ankle is dorsi flexed and the rearfoot is inverted at landing.The foot lands out in front of the center of mass with the knee slightly flexed(Fig.1A).From this position, the foot dorsi flexes and everts and the knee continues to flex.At mid stance,these motions reverse until toe-off.During an FFS, the ankle is plantar flexed at initial contact with greater rearfoot inversion than in an RFS.The knee lands in more flexion with the foot placed more directly below the center of mass (Fig.1B).Due to the increased plantar flexion and inversion,the foot goes through greater dorsi flexion and eversion range of motion during stance in FFS running.This greater excursion occurs over a similar time frame as RFS,resulting in greater dorsi flexion velocities.This pattern is associated with greater plantar flexion muscle moments,as well as greater negative work required of the plantar flexors(Fig.2).In contrast,theknee goes through a greater flexion excursion during RFS but over a similar time frame to FFS,resulting in higher knee flexion velocity.Greater demands are placed on the knee extensors as evidenced by the higher knee extension muscle moments and negative work.Therefore,an FFS pattern is associated with greater demands on the foot and ankle,and an RFS pattern is associated with greater demands on the knee.

Fig.1.Lower extremity alignment at footstrike of:(A)Rearfoot strike.Note the ankle dorsi flexion,angulated tibia,and extended knee.(B)Forefoot strike. Note the ankle plantar flexion,knee flexion,and vertical tibia.

Clear differences in ground reaction force(GRF)time histories can also be seen between an RFS and an FFS pattern.13,34,35An RFS pattern often displays a distinct impact transient early in stance that is associated with high vertical loading rates(Fig.2).An FFS pattern typically has no impact transient and is associated with vertical rates of loading that are approximately half those of an RFS.However the active peak vertical force that occurs near mid stance is generally similar or slightly increased in an FFS pattern.Therefore,the majority of differences between an RFS and an FFS pattern occur in the early part of stance and are directly related to the manner in which the foot contacts the ground.

Fig.2.Vertical GRF of an RFS and an FFS runner.Note the vertical impact peak of the RFS that is not present in the FFS pattern.BW=body weight;FFS =forefoot strike;GRF=ground reaction force;RFS=rearfoot strike.

4.Strike pattern and injury

The high vertical load rates associated with an RFS pattern have been linked both prospectively and retrospectively with injury.36–38Musculoskeletal structures are viscoelastic in nature and vulnerable to injury at high rates of loading.This has been underscored by animal studies demonstrating injuries to both bone and cartilage when imposing impulsive loads rather than gradual ones.39,40This relationship has also been demonstrated in human studies.A recent meta-analysis reported a signi ficant relationship between vertical load rates and tibial stress fractures in RFS runners.41Interestingly,knee osteoarthritis with associated cartilage degradation has been linked with higher than normal vertical rates of loading during walking.42High vertical load rates may translate to abnormal loads in ligamentous structures as well.This was evidenced in a study demonstrating higher load rates in RFS runners with a history of plantar fasciitis compared to an uninjured group.38The majority of these studies have been retrospective in nature making inferences regarding cause and effect dif ficult.However,a recent prospective investigation revealed that runners who go on to sustain a medically diagnosed injury had signi ficantly higher load rates at baseline than their never-injured counterparts.36These prospective,along with the retrospective,data provide compelling support for an association between GRF load rates and musculoskeletal injuries in runners.

If humans are best adapted for FFS landings,then it follows that it should be associated with the lowest injury risk.Clearly, FFS running is associated with lower vertical load rates compared with RFS.Unfortunately,there are only a few studies to date that have examined the relationship between strike pattern and injury.Warretal.43found no difference in injury historiesof runners with differing strike patterns.However,these authors compared RFS runners to the combined group of MFS and FFS runners.Additionally,running injuries in this study were selfreported and relied on recall.Arecentreportsuggested thatMFS and FFS runners should not be combined due to the statistically higherload ratesduring MFSlandings.44In anotherretrospective investigation of a collegiate cross country team,Daoud et al.45reported thatRFS runners sustained medically diagnosed repetitive stressinjuriestwiceasoften asFFS runners.Future prospective studies examining footstrike patterns and injury are needed to further determine these relationships.

Transitioning from an RFS to an FFS pattern has been shown to have a bene ficial effect on common running injuries.One study involved a group of U.S.military(West Point)cadets presenting with anterior compartment syndrome and high intracompartment pressures.46These cadets were scheduled for, but had not undergone,a surgical release of the fascia surrounding the anterior compartment.After completing a gradual 6-week transition to FFS running,all 10 subjects demonstrated signi ficant reductions in their intracompartmental pressures(to within normal limits).Additionally,subjects reported large improvements in outcome questionnaires,and were able tocomplete a 5 km run without pain.All the outcome variables were further signi ficantly improved at the 1-year follow-up. Mostimportantly,surgicalintervention wasavoided in allcases. In a recent case series report,3 runners with a longstanding history of patellofemoral pain(mean=40 months)underwent a transition to an FFS pattern.47All had failed conventional physical therapy,which had focused on hip and knee strengthening, along with electricalstimulation forthe quadriceps.Participants underwent 8 sessions of landing pattern modi fication from an RFS to an FFS over 2 weeks,in which they used real-time audio feedback from a force sensor placed within the shoe.Feedback was gradually faded as run time was increased to 30 min by the last session.All 3 runners were able to successfully transition to an FFS pattern and reduce their vertical average and instantaneous load rates by 19%and 24%,respectively.Additionally, pain was markedly reduced.All improvements in outcome variables persisted at the 3-month follow-up.These results are supported by a modeling study by Bonacci et al.48who demonstrated that patellofemoral contact stresses are reduced when running barefootwith an FFS pattern.These studiescollectively underscore the ef ficacy of transitioning to an FFS pattern in treating runners with these common running injuries.

5.Strike pattern and tissue mechanics

In this section,we will consider how strike pattern in fluences key anatomic features of the foot including the heel pad,the plantar fascia,and the Achilles tendon.

5.1.Heel pad

The heel pad is thought to provide 3 useful functions during gait,namely:shock reduction,energy dissipation,and protection against excessive plantar pressure.49During the initial contact phase of heel–toe walking(10–20 ms after heel strike), deformation of the heel fat pad has been suggested to lower either the peak force,or the rate of loading of the lower limb,or both.50The fat pad has been noted to undergo considerable vertical deformation,about 9 to 11 mm(～45%–60%strain), during barefoot walking.14,51However,the initial loading rate of the heel pad is extremely high(～1.2 MPa/s).Additionally,the energy required to compress the heel pad(1.5 J)is relatively low51compared to the total energy exchange during walking (～21 J in a 70 kg adult walking at 4.5 km/h).52Hence,the heel pad offers minimal resistance to deformation during initial contact suggesting it has only a minor shock reduction capacity during walking,let alone running.

With every step,a proportion of the strain energy stored within the heel pad during loading is lost with unloading.This energy loss is believed to play an important role in damping high-frequency vibration within tissue.53Although the ratio of energy lost verses energy stored in the heel pad is in the order of 55%to 70%,only about 1.0 J is dissipated by the heel pad in absolute termsduring heel–toe walking.49,51Thisisconsiderably less than that of the Achilles tendon(～2.5 J)54and the ligamentousstructures(～3.1–4.5 J)ofthemediallongitudinalarch ofthe foot,55,56which have“spring like”properties and are important for energy return.The overall energy dissipated by the heel pad, therefore,is relatively low and unlikely to substantially increase with speed,making it a less than ideal structure for dissipating the impacts associated with running(Fig.3).14

Fig.3.Typical force deformation curve for the human Achilles tendon(left axis)and heel fat pad(right axis).Arrows indicate direction of loading and unloading.While the deformation lag of the heel pad on unloading(i.e., hysteresis)suggests there is a substantial loss of energy within the tissue,only 1.0 J is dissipated by the heel pad during walking,which is considerably less than that for the Achilles tendon(～2.5 J),as peak physiological loads in the Achilles tendon are around 10 times higher.

The heel pad does serve to reduce excessive pressures,and therefore pain,during ambulation.57The limit of pain tolerance for impacts involving the heel pad corresponds to a predicted heel pad deformation of 10.7 mm,which is marginally greater than that observed during walking(10.3±1.9 mm).Thus,even at preferred walking speeds,deformation of the heel fat pad approaches the limits of pain tolerance(Fig.4).Therefore,an FFS pattern adopted during barefoot running may re flect a pain-avoidance strategy.10Interestingly,cadaveric studies have shown the fibroadipose tissues of the forefoot have a higher material stiffness and higher energy dissipation than the heel pad.58This suggests that the forefoot may be more suited to attenuate the loads experienced during early stance in running.

5.2.Plantar fascia

The longitudinal arch provides signi ficant passive elastic storage and return.With de flection of the longitudinal arch,the plantar fascia and associated deep ligaments are strained and store energy and then subsequently return around 6%to 17%of the total mechanical work of running.55,59As with tendon, however,the elastic-return mechanism of the passive components of the longitudinal arch is largely strain-dependent.55An FFS pattern has been shown to induce greater de flection of the arch than RFS.60As such,an FFS pattern has greater potential to store and return elastic strain energy and contribute to overall metabolic energy savings compared to an RFS pattern.FFS runners have also been shown to have a greater volume and strength of the intrinsic foot muscles,which assist in the func-tion of the plantar fascia,when compared to habitual RFS runners.61,62In addition,the plantar fascia is also well innervated with both free nerve endings and mechanoreceptors.63These mechanoreceptors contribute signi ficantly to proprioception in the arch.63The greater plantar fascial elongation of an FFS pattern60may facilitate these mechanoreceptors,and thus proprioception,to a greater degree than in an RFS pattern.

Fig.4.Maximum deformation of the heel pad as a function of the work required to deform the heel fat padin vivo.Note the limit of pain tolerance for impacts of the heel pad(dotted line)corresponds to a predicted heel pad deformation of 10.7 mm,which is similar to the average deformation during walking at preferred speed(10.3±1.9 mm)indicated by filled circle.Adapted with permission.14

5.3.Achilles tendon

The Achillestendon isthe largestand the mostelastic tendon in the human body,reportedly returning around 95%of the elasticstrain energy stored with the loads typically encountered during running(Fig.5).64During RFS running,Achillestendon loading is typically characterized by 2 maxima and minima.Peak loadscoincide with peak eccentric muscle action during late mid stance propulsion and terminal swing,and minimum loads occur with concentric muscle action during early stance and pre-swing.65There isarapid reduction inAchillestendon force thatoccursduring initial contact in an RFS pattern,that is absent in FFS running.65This results in greater activation of the triceps surae66along with an earlier67,68and higher rate65,67,68and magnitude(8%–24%)67,68of Achilles tendon loading during FFS running(Fig.5).

Fig.5.(A)In vivoAchilles tendon force and(B)vertical ground reaction force during barefoot running with an FFS and an RFS pattern at approximately 14 kph.Achilles tendon force was measured directly via a surgically implanted buckle transducer.Note the greater Achilles force in the FFS pattern. FFS=forefoot strike;RFS=rearfoot strike.Adapted with permission.65

Greater triceps surae activation in the eccentric phase of movement,67combined with high stretch velocity65induces greater stiffness within the muscle–tendon unit.This mechanism is known to be bene ficial to storage of elastic strain energy.69Based on cadaveric studies,a 24%increase in Achilles tendon load with an FFS pattern would result in an additional 6 J energy returned by the tendon.54,55This favors the FFS pattern when it comes to leveraging the Achilles tendon for energy return.Moreover,such high-magnitude strains,often thought detrimental to tendon health,have also been shown to be critical forAchilles tendon adaptation and homoeostasis.70In support of this,a recent study investigated the Achilles tendons of jumping athletes that are chronically exposed to elevated mechanical loading.71The authors noted that the Achilles tendons of the jump leg in these athletes exhibited greater mechanical(stiffness)and material(Young’s modulus)properties.These findings suggest a clear bene fit from the stimulus of jumping.Therefore,running with an FFS pattern which increases the loading of the Achilles tendon,is likely bene ficial to the mechanical and material properties of the tendon.

Overthe lastdecade,ultrasonography hasbeen used to investigate the effects of loading on the elastic properties of human tendonsin vivo.High peak loads have been found to be most bene ficial for homeostasis and improvement of human tendon properties.70The Achilles tendon and triceps surae muscles experience higher loads in an FFS as they assist in dissipating much ofthe impactenergy associated with eccentrically controlling the ankle dorsi flexion moment.72Indeed,habitual FFS runners exhibit greater ankle plantar flexion strength than habitual RFS runners,73exposing the Achilles tendon to higher stress stimulus in FFS running.Both sprinting and minimalist footwear are known to promote an FFS pattern.15,74Sprinters have been reported to have stifferAchilles tendons than distance runners.75Additionally,it has been recently reported that minimalist footwear runners exhibit greater stiffness and crosssectional area of the Achilles tendon compared with their traditionally shod counterparts.74These studies collectively suggest that ahabituatedFFS pattern may invoke the necessary stimulus required for tendon adaptation and homeostasis,which leads to stronger calf muscles and Achilles tendons.There is a52%lifetime incidence of Achilles tendinopathy in runners76with over 90%of runners being RFS.5Additional studies are needed to determine if adaptations associated with an FFS pattern will result in fewer injuries to these structures.

6.Interaction of footwear and footstrike

There is clearly an interaction of footwear and footstrike on running mechanics.This is most evident when assessing the strike patterns and resultant GRFs.Most studies investigating the impact of footstrike pattern on GRFs have focused on the vertical component only.Speci fically,they have examined the average and instantaneous load rates associated with early stance because of their reported links with injury.These studies have all reported lower vertical load rates when running with an FFS compared with those with an RFS.13,34However,during running,the body actually experiences a resultant force comprised of the vertical,anteroposterior,and mediolateral forces. In a recent study,Boyer et al.34compared the vertical as well as the resultant load rates between habitual RFS and habitual MFS or FFS runners.In support of previous studies,they found that the FFS group had signi ficantly lower peak vertical load rates compared to their RFS counterparts.However,when assessing the peak resultant load rate,there was no difference between groups.This result was due to the higher load rates in the anteroposterior and mediolateral directions in the FFS group. However,their runners wore neutral cushioned shoes.

Preliminary data in our lab have suggested that forefoot striking in neutral cushioned shoes results in greater plantar flexion and inversion at footstrike than when barefoot.This may be due to the elevated heel and lateral flare that is characteristic of a modern running shoe,which may alter the footstrike position. Greater plantar flexion and inversion may result in the greater anteroposterior and mediolateral forces that were noted in the Boyer et al.34study.Therefore,we conducted a similar study,but with the addition of a minimal footwear group.77Minimal footwear was de fined as having little to no cushioning.This resulted in 3 groups:RFS who habitually run in neutral cushioned shoes, FFS who habitually run in neutral cushioned shoes,and FFS who habitually run in minimal shoes.In support of Boyer et al.,34we found that those who FFS in neutral cushioned shoes exhibited similar resultant load rates compared to those who RFS in these same shoes.However,we found that those runners who FFS in minimal shoes exhibited signi ficantly lower load rates than either of the traditionally shod groups(Fig.6A).This was due to lower load rates in all components of the GRF in the minimally shod group.Interestingly,the minimally shod group was made up of some runners who were habituated to full minimal shoes(no midsole,simply an outersole)and othersto partialminimalshoes (minimal midsole).A subanalysis of this group revealed that those FFS runners who habitually wearfullminimalshoes exhibited resultant load rates that were approximately 17%lower than those FFS runners who habitually wear partial minimal shoes. These results highlight an important interaction between footwear and footstrike and suggest that any cushioning in footwear in fluences mechanics.It appears that running with an FFS in full minimal shoes without cushioning results in the lowest vertical load rates at landing(Fig.6B).Future studies investigating the relationship between strike pattern and injuries should therefore include runners habituated to minimal footwear as well.

Fig.6.(A)Load rates between habitual RFS in standard shoes(SRFS),FFS in standard shoes(SFFS),and FFS in minimal shoes(MFFS).Note the statistically signi ficant reduction(***p＜0.001)in load rates in the MFFS compared with the SRFS and SFFS.(B)Subanalysis of load rates between partial minimal shoes FFS(MFFSpartial)and full minimal shoes FFS(MFFSfull). Note that load rates are the lowest when running with an FFS pattern in full minimal shoes.FFS=forefoot strike;ILR=instantaneous load rate;RFS= rearfoot strike.Adapted with permission.77

7.A word about transitioning

There have been reports of injuries associated with abrupt transitions to minimal footwear.78,79This is not surprising as the musculoskeletalsystemneedstime to adaptto changesin load so that injury does not occur.If humans began running barefoot or in full minimal shoes at an early age,there would be no need for transitioning as the body would naturally adapt to the associated loads.However,when we have habituated to heel striking in supportive,cushioned shoes,transitioning to an FFS pattern,in minimalshoeswithoutproperpreparation,involvesrisk.78,80FFS pattern increases the load on the plantar flexors as they control the heeldescentin early stance.An FFSpattern also increasesthe load to the plantar foot musculature,which is important for controlling thedeformation ofthe arch with each step.When this motion is not well controlled,additional strain to the plantar fascia or metatarsals may result.Therefore,a strengthening program that includes exercises to address the calf muscles,as well as intrinsic and extrinsic foot muscles,should precede anFFS transition.Studiesthathave incorporated footand lowerleg strengthening,along with a slow increase in training volume, have demonstrated thatan instructed transition to an FFS pattern in minimal footwear can be made safely without injury.81,82

8.Summary

In summary,barefoot running,our most natural state,ismost often associated with an FFS pattern.However,most runners today wear footwear to protect their feet.It is well-recognized that modern footwear changes our natural pattern to a predominantRFS landing thatresultsin signi ficantly differentmechanics from an FFS pattern.Some of these RFS mechanics,such as increased load to the knee and increased vertical loading rates, have been signi ficantly associated with running injuries. Running with an FFSisassociated with a loading stimulusofthe plantar fascia and Achilles tendon that bene fits their“springlike”function and may stimulate their adaptation or maintain their homeostasis.Running in full minimal footwear is associated with increases in both intrinsic and extrinsic foot muscular strength,as wellas being associated with the mostsoftlandings. Converting to an FFS pattern in minimal shoes should be done slowly and be accompanied by foot and lower leg strengthening to minimizeinjuriesduring thetransition.With propertransition, an FFS pattern in true minimal footwear that most closely mimics our natural,barefoot state,may positively change the trajectory of running injuries in the modern-day runner.

Authors’contributions

ISD and SCW both drafted and critically reviewed the manuscript;HMR helped with the literature review and critically reviewed the manuscript.All authors have read and approved the final version of the manuscript,and agree with the order of presentation of the authors.

Competing interests

The authors declare that they have no competing interests.

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17 October 2016;revised 26 December 2016;accepted 2 February 2017

Available online 31 March 2017

Peer review under responsibility of Shanghai University of Sport.

*Corresponding author.

E-mail address:isdavis@mgh.harvard.edu(I.S.Davis)

http://dx.doi.org/10.1016/j.jshs.2017.03.013

2095-2546/?2017 Production and hosting by Elsevier B.V.on behalf of Shanghai University of Sport.This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).