Fig. 15.1
Barefoot on pavement
Foot Strike Pattern
Rear Foot or Forefoot Strike : Demographics
In the initial wave of interest in BFR/Minimalist, the dogma, especially in the lay public was that those wearing shoes were rear foot strikers (RFS ) and those BFR/minimalist were forefoot strikers. A number of observations report this is not necessarily the case. Hatala et al. observed habitually BF subjects and found 72% of runners were found to be RFS [34]. Hasegawa et al. observed elite competitors participating in a half marathon and report 75% RFS , 23.7% midfoot strikers (MFS), and 1.4% FFS [35].
A more recent look at the foot strike pattern in BFR provides a greater analysis of other variables taking place during gait such as running speed, distance, heel height (drop), training level, and running frequency rather than the use of cushioned running shoes [24, 34, 36]. Gruber et al. in 2012 determined that running surface hardness was a significant factor in foot strike pattern when it was shown that habitual shod RFS BFRs tended to RFS on softer surfaces and MFS/FFS on hard surfaces. This gives credence to the idea that the change to FFS while running BF is protective [37].
One of the themes of more recent studies is the query as to the above changes being the result of the footwear or lack thereof, the foot strike position or a combination of both [38]. As will be noted later, those believing it is not the BF condition but where the runner strikes on the foot will try and make the case for one not needing to be BF to garner the above changes. Giandolini et al. concluded in their study that a midfoot strike pattern was most effective intervention in reducing the loading rate [39].
Joint Torques
In an effort to investigate a more direct relationship between the observable changes in BFR versus shod and running injury etiology, some studies have evaluated torque forces in lower extremity joints. Kerrigan et al. describe significantly increased joint torques at the hip, knee, and ankle with running shoes compared with running barefoot. Torques appear to be due to heel elevation and medial support commonly seen in modern running shoes. How these findings may impact injuries will be discussed below [26].
In summary, although not necessarily an advantage or explanation for how BFR could lead to reduced injuries, there are significant changes that are indisputable in a comparative examination of a shod runner versus a barefoot runner. These modifications will, in part, be responsible for many of the touted advantages that are described below.
Reduction of Impact Forces
Shoes and Impact Forces
In the earlier nonacademic dogma, it was claimed shoes really were not effective in reducing impact forces. In fact, Robbins and Gouw complained of the “false sense of security” they believed shoe companies promoted with a combination of increased cushioning, comfort, and slick marketing. Runners are left with no sensory incentive to modify their foot strike [40].
Lieberman adds that those that utilize a FFS do not require shoe cushioning as they do not sustain an impact peak [41].
Fong et al. performed a systematic review that compared shod to BF as per impact attenuation and found insufficient evidence that demonstrates the ability of cushioned shoes to reduce vertical ground reaction force or loading rate during walking or running [42]. Fong claims that loading rate and tibial acceleration should be the key parameters utilized in future research because of their association with running-related injuries.
Impact Forces and Injury
Many in the running community claim most of the blame for injuries related to running is due to training errors and therefore may very well be unrelated to a runners’ footwear or lack thereof [11, 43]. Nonetheless, many investigators generally concur that impact forces especially vertical loading rate (VLR) associated with running are substantial and may be a significant contributor to running-related injuries seen by clinicians [12, 20, 26, 34, 42, 44–49]. Of note, is that VLR is reportedly lower in shod FFS versus RFS running [18, 39, 50].
Absence of the Impact Transient
As noted earlier, much of the current interest in BFR came about as a result of the Lieberman et al. paper that introduced many to the vertical ground reaction force chart that pictured the impact differences between a rear foot striker and a forefoot striker—the definitive peak is absent in forefoot and midfoot strikers [12]. The impact peak is seen below in Fig. 15.2.
Fig. 15.2
Ground reactive force graph
As noted above, it is hard to separate the kinematic changes with a BF pattern and reduced impact. Lieberman has cited the principle that most people land on the ball of the foot when they jump as the most basic example of the natural kinematic change resulting in reduced shock [41]. The following sections will look at specific kinematic findings and how the research interrelates them to impact reduction.
Decreased Stride Length/Increased Cadence
Plantarflexed Position of Ankle at Foot Strike: Forces Redirected
An anteriorly placed foot strike results in a more plantarflexed ankle and therefore will result in greater forces imparted to the posterior leg musculature [24, 56]. Although a significant mechanism for shock attenuation, the impact forces therefore do not vanish but are manifested differently. The implications of this force conversion as it relates to injuries will be discussed later.
Knee and Hip Position and Motion
Balance of Forces over Foot
As recently as 2015, Cooper et al. confirmed that most barefoot runners adopt a FFS with reduced forces and a more balanced force distribution across the plantar surface [57].
Greater Arch Deformation and Shock Attenuation
Perl et al. discuss below the economic benefits of an arch allowed to deform, which it does to a greater extent in the BF condition. This finding may also contribute to better shock absorption [22].
Increased Economy of Running
Some of the earliest studies on the possible benefits of BFR focused on one of the most obvious characteristics of being unshod, the absence of shoe mass and how this reduces energy utilization and improves running economy [58, 59]. Since that time, many more studies have taken place that evaluate this parameter.
Improved Economy : Shoe Mass, Kinematic Effects, or Both?
Most investigators assessing the changes in BFR early on believed removal of a shoe that a runner must repeatedly accelerate and decelerate over the course of a run is what led to a more efficient use of energy in a BF condition. Others believe an economy boost is appreciated irrespective of shoe mass effect.
Shoe Mass Only
Reeves et al. looked at running economy at varied intensities and found significant reductions in energy use while shod but attribute it to the reduction in mass in the BF condition [60].
Altered Kinematics May Result in Improved Economy
Divert et al., in describing impact reduction via preactivation of the triceps surae, suggest that those changes seen in a BF condition such as a FFS reduced contact time, and muscle length changes could manifest improved storage and restitution of elastic energy via augmentation of the stretch-shortening cycle behavior. Although concluding the mass of shoe was responsible for an increased oxygen consumption in shod running, they and Squadrone and Gallozzi believed shoes also caused a dampening of this energy storage and restitution, thereby adding to the reduced efficiency in the shod condition [16, 29, 61].
Nigg notes that besides the mass of the shoe that must be accelerated and decelerated, it takes work to deform and rotate the shoe sole and energy is lost to the midsole and metatarsal-phalangeal joint [62]. Besides a loss of energy through dampening, Webb et al. considered the energy required to repeatedly deform a shoe during a run reduced efficiency [63].
What a number of investigators are now looking at is an apparent improved energy use and efficiency after correcting for the mass of shoes and how this may be taking place. Specifically, they are considering if the profound kinematic changes noted in the BF condition directly improve energy utilization, thereby defining the BF condition more efficient [22, 64].
Unleash the Natural Shock Absorption as a Means to Increase Economy
Perl et al. observed that those running in minimalist shoes appear to have a greater deformation of the arch height during contact which they surmise may unleash the elastic energy storage and recoil capacity of the windlass mechanism of the arch [22]. The FFS pattern allowed a recovery of the elastic energy of the Achilles and plantar arch which would not be possible with a RFS . This study controlled for shoe mass by adding weight to the BF subjects’ feet and also controlled cadence. The improved economy was present regardless of foot strike style. Their study also reported on the finding of reduced knee joint flexion in those running barefoot or in minimalist shoe gear. That knee flexion and adduction as well as external rotation of the hip are reduced in the BF condition had been reported previously [12, 15, 26, 29, 61]. This reduced knee flexion could be another reason BFR/minimalist running is more energy efficient. The lower flexion of the knees reduces the eccentric load on the quadriceps and keeps the Achilles tendon in a more ideal state of tautness. This study also lent some support to the idea that supportive shoes and/or arch supports/orthotics may inhibit the natural function of the arch with regard to energy production. In summary, the type of foot strike, FF or RF did not seem to be as important in improving energy economy as did the nature of shoe gear did [22].
Familiarity with BFR Technique : A Factor in Energy Efficiency
Vincent et al. tried to standardize the subjects with regard to experience and foot strike pattern and found no difference in energy use between shod and BF conditions. They attribute this to the experienced status of subjects as they inherently modulate their kinematics to standardize energy use without regard to being shod or BF [28].
Warne and Warrington evaluated running economy via oxygen uptake in runners wearing minimalist footwear; Vibram Five Fingers (V5F ) but not familiar with BFR and put them on a 4 week familiarization program and noted a near 9% improvement in running economy after the familiarization program. Interestingly, there was no significant difference in running economy at the outset between shod and unshod. After familiarization, BF was 6.9% more economical than shod [65].
No Economic Benefit
Franz et al. reported that there was no significant difference in running economy as per energy utilization between shod and BFR. Indeed the reduced weight was found to impact energy use but this downside seemed negated when running in lightweight minimalist shoes. In fact they found a slight advantage to being shod that may reflect an increased VO2 due to higher cadence when BF [64].
Systemic Review of Economy Effects of BFR/Minimalist
Fuller et al. performed a systematic review and determined that although BF and minimalist shoes had a measurable effect on running economy compared to traditional running shoes, there was not a significant difference between BF and minimalist. In fact, they determined that only when the footwear is greater than 440 g, does the additional weight become a hindrance to running economy. Interestingly, traditional running shoes with characteristics of increased longitudinal stiffness, cushioning, and reported comfort had improved economy as well. They found no studies that assessed performance as it relates to shoe gear [66].
Cheung and Ngai performed a systematic review and meta-analysis on the effects of footwear on economy and concluded that barefoot running or running in minimalist footwear is potentially more economic compared to running in traditional running shoes. They present the caveat that many of the subjects were experienced in BFR and thus, those new to BFR or in transition, may not experience these benefits. The idea that reduced oxygen cost may improve performance is speculative [67].
It appears the debate as to the metabolic benefit of BFR is far from over. Kram and Franz take to task findings reported by Hanson et al. and point out only two out of seven studies found a statistical difference in oxygen consumption between BF and shod [61, 68]. Their concerns about systematic errors in methodology are at the root of disagreement [64, 69, 70].
Increased Proprioceptive Ability/Balance
Some of the earliest studies that evaluated the BF condition and led to the initial hype involved proprioception and balance. These studies assessed the performance of BF subjects, but in static circumstances. Findings stated subjects had better proprioceptive ability while BF as the shoe was said to create a barrier between the ground and plantar mechanoreceptors. These conclusions led to the extrapolation that, therefore, those running BF must likewise have better sensory input/proprioception and thus better performance and reduced injuries. In effect, besides better balance, the runner would be better able to make the kinesthetic adjustments to reduce impact via improved neurosensory feedback [4, 71–74]. Squadrone et al. found that even a minimalist shoe provided improved static and dynamic ankle position sense over that of a conventional running shoe noting that cushioning in shoes impairs the position awareness [75].
Sensory Feedback Interrelated to Kinematics
Fleming et al. in their study that demonstrated an immediate ability of habitually shod runners to convert to a BF style speculate that this is accomplished in part to the “acute awareness of altered impact forces by tactile receptors in the foot and proprioceptive organs in the shank” [17].
Ferris et al. observed the changes in leg stiffness as their BF subjects ran on surfaces of varied hardness. They point out the impressive ability of a runner maintaining the center of mass while traveling over an ever-changing surface as it pertains to hardness. They explain that this capacity is the result of an instantaneous ability to adjust leg stiffness. The authors admit that the neurological mechanism for how this takes place is unknown but suspect that the mechanism must include plantar sensory feedback that would most assuredly be greater unshod [33]. Along this thinking, Kurz et al. who described the so-called coordinative strategies evoked by BF running (see above) also noted BF runners’ gait manifested a much higher variability. That is, compared to a shod individual, the foot is in a continual state of adjusting to the running surface. They believe this variability is the result of a vastly improved sensory feedback/proprioceptive state whereby mechanoreceptor feedback modify joint kinematics in the favor of reducing impact forces a process that may be hindered by wearing shoes [31].
Improved Balance Barefoot ?
Several more recent works appear to confirm the idea that those barefoot enjoy improved proprioception and balance [75, 76]. It seems this improved balance has only been observed in either static studies or in the case of Rose et al., (dynamic) during the performance of a single leg jump landing. The improved balance was said to be the result of a reduced filtering of sensory feedback while in the BF condition [77].
Better Balance with Shoes
Horgan et al. and Koepsell et al. report that contrary to many prior studies, they found that, at least in elderly population that wearing shoes provided better balance and reduced fall risk than did going BF [80, 81].
As part of the debate it is only fair to note some propose an unprotected foot (BF) may subject the pedal mechanoreceptors to greater direct impacts and damage thus reducing their neurosensory feedback [82]. While others suspect mechanoreceptors plantarly may play a less significant role in proprioception that such things as muscle spindle afferents, golgi tendon organs and visual clues [83–85].
Increased Strength of the Musculature and Arch Structure
Shoe-Related Disuse
A number of BFR proponents have proposed that shoe gear, especially today’s running shoes, are stiff, confining and overly supportive and cushioned. The result is a foot that becomes weak and dependent. The planter intrinsic musculature atrophies and puts the shod person at risk for arch collapse and even such maladies as plantar fasciitis. One running BF would thus develop stronger plantar intrinsics and even overcome plantar fasciitis. Of the major claims about BFR discussed in this paper, changes as they relate to the plantar musculature and arch structure appear to be the least investigated.
Kinematics Change and Arch Utilization
Considering the alteration from a RFS to a more forwardly placed one, it would make sense that the arch and thus the plantar intrinsics would be more greatly utilized and become stronger with a resulting higher arch that is better at shock absorption. Indeed, this is what was suggested by Robbins and Hanna in 1987 [4].
Plantar Musculature Can Be Strengthened
Plantar musculature can be strengthened and for some athletic activities this can be an augmenting factor. Goldmann et al. concluded that toe flexor musculature was strengthened via training, and although this did not assist with basic running or walking, it did so with such activities as sprint starts, horizontal jumping, and side cutting. Although they did not look at BFR, they opined that BFR may indeed provide a similar training effect and boost performance [86].
Bruggemann et al. believed plantar intrinsic muscle strength was compromised by stiff-soled shoes with arch supports and in a project funded by Nike did find better developed and stronger plantar intrinsics in participants using Nike Free footwear during warm-ups. As may be expected, concerns regarding methodology and bias surround this report [87].
Intrinsics Recruited While Barefoot
Although not necessarily related to better performance or reducing injuries, the finding that those BF demonstrate a higher recruitment of the plantar intrinsics for balance suggests a possible advantage of the BF condition [88].
Compared to other of the claims, studies evaluating the strengthening benefits of BFR are nil. Despite some evidence that BFR/minimalist strengthens plantar foot musculature, no studies have yet associated those changes to improved performance or reduced injuries.
Reduction in Running-Related Injuries
Injury Reduction Key Motivation to BFR
It goes without saying that to serious runners, preventing and reducing injuries is paramount. It is not surprising, therefore, that the premise BFR could prevent or reduce running-related injuries is the primary motivating factor runners report for why they would consider trying BFR. Likewise, the chief reason those queried would not try BFR is an apprehension that BFR would lead to an injury [89]!
Injury Rate Unchanged Despite Changes in Shoe Gear
In an attempt to consolidate prior studies on prevalence of running injuries, van Gent et al. completed a systematic review in which they reported the rate of running-related injuries to be between 19 and 79% within a given year [90].
Some investigators conclude that yes, the rate of injury has not seemed to improve through the years but if you factor in the demographics of who is now running currently versus the runners of the 1970s, the fact that injuries have not soared is remarkable given the far greater numbers and comparatively less conditioned runners of today [91]. Indeed, maybe the significant changes in shoe gear have been of benefit?
Running Shoe Design and Dogma
Despite the continued injury rates, proponents of modern running shoe design claim that cushioning and motion control features are required in order to protect runners from injury. Dogmatically, clinicians continue to prescribe more cushioned and controlling shoe gear even though there is no evidence to support the effectiveness of high quality running shoes in preventing injury and may indeed have the potential to cause harm [92]. In fact, Clinghan et al. concluded that expensive running shoes were no better at reducing impact forces than low cost shoes were [71]. Another aspect of the traditional running shoe, the increased heel height, is thought to encourage a RFS and thus increased impact forces. Without the extra heel height, might a runner instead utilize a MFS/FFS [20]?
Along similar lines, Richards et al. did an extensive review on the available literature and concluded there have been no studies that assessed the advantages (injury rates, performance or global health and well-being) of prescribing running shoes that incorporate a design of heel cushioning, elevation, and pronation control. Indeed in their conclusion they consider the possibility that the current design can cause harm to runners [92].
More recently Knapik et al. compiled extensive prospective studies of military recruits in which attempts to match a type of running shoe to a given foot type (plantar shape) had no effect on the rate of injury during boot camp activities, therefore calling to question the dogma of providing a neutral shoe to a high arch foot, a motion control shoe to a flat foot and so on [93].
Reduced Injuries and Better Performance Extrapolated from Research Findings
As noted earlier, proponents of BFR deduced through logical assumptions that kinematic and other changes seen with BFR could pan out as a means to prevent or limit running-related maladies. It would seem that documented improved lateral stability seen in the BF condition would lessen ankle sprains in that group. Surely reducing impact forces could lessen the prevalence of conditions thought due to high impact forces seen with running. Stronger plantar musculature would lead to a cure for plantar fasciitis!
Genesis of Shoes and Injury
Well before McDougall’s Born to Run and Lieberman et al.’s Foot Strike Patterns and Collision Forces in Habitually Barefoot Versus Shod Runners, Robbins and Hanna were the first to associate a lower rate of injury with a BF condition. Their article’s conclusion “The solution to the problem of running-related injuries could be as simple as promoting barefoot activity” was a major catalyst in the association of one’s shoe gear status or lack thereof in running-related injuries [4].
Five years ago the authors’ review found that most studies focused on the observable changes seen in the BF condition [1]. More recently, there appears to be more of a focus on connecting those changes with possible alterations in genesis of, prevention of, or treatment of injuries. Another direction that research has taken is can these changes and their potential benefits be appreciated by incorporating BF kinematics but while remaining shod.
Barefoot Running and Injury Rate
It goes without saying that definitive studies associating the BF condition and injury rates be undertaken. Altman and Davis completed a prospective review of 201 runners and found the BF subjects experienced fewer overall musculoskeletal injuries but had similar injury rate to their shod counterparts. Although BF runners had a greater number of injuries to the plantar surface and calf, they had fewer involving the knee, hip, and plantar fascia [94].
Running Injury Conditions
The following sections will look at specific conditions (some unrelated to running) and related studies and how observed changes seen with BFR might be utilized to prevent or manage injuries.
Chronic Exertional Compartment Syndrome (CECS )
Although a more defined discussion of running shod but with a “barefoot style” is presented later, a study by Diebal et al. that directly associates a BFR gait modification to an injury is discussed here. They report that runners diagnosed with chronic exertional compartment syndrome (CECS) enrolled in a running program utilizing a forefoot strike significantly reduced the presenting symptoms of pain as well as the measureable pressures themselves. In this instance the authors themselves are unclear as to how a FFS causes these changes but the measureable reduction in compartment pressures and lower eccentric stress on anterior compartment seen with FFS are suggested etiologies [50].
Osteoarthritis of the Knee Joint
To set the stage for many studies described below, Braunstein et al. report significantly greater mechanical stress (as reflected by the gear ratio) placed on the knee joint while wearing shoes compared to BF [95].
Kerrigan et al. reported that wearing running shoes led to an increased torque across the knee with resultant increased pressures at anatomical locales commonly at risk for knee osteoarthritis, the medial and patellofemoral compartments. Still suggestive of a correlation only, the findings are encouraging [26].
Shakoor and Block observed gait changes as they affect the knee joint afflicted with medial joint osteoarthritis (OA) and found that ambulating barefoot resulted in reduced peak loads in the knees (and hips) as well as a near 12% lower knee adduction moment [96].
Radzimski et al. completed a systematic review on the effects of varied shoe gear conditions on the external knee adduction moment (EKAM ), a high degree of which has been associated with such pathology as medial knee compartment osteoarthritis. The evidence supports the notion that the EKAM is lessened in the BF condition with the proposed mechanism being the heel elevation in traditional shoes. A recommendation is made for developing shoe gear that can mimic the BF condition [97].
It would be presumptuous to associate DJD or other maladies of the hip and knee with wearing shoes but studies like this are seriously evaluating this connection. As a result, some investigators suggest modifications in shoe gear as a means to treatment and or prevention on OA in the hips and knees [26, 96].
Patellofemoral Joint Syndrome (PFJS )
PFJS and Impact Forces
Available literature associates an increased impact peak and increased eccentric load on the knee with the prevalence of PFJS [26, 98]. Given the observed changes regarding a reduced impact peak with BFR, a number of investigators have surmised BFR may be a preventative or therapeutic solution to PFJS in runners.
Bonacci et al. report a 12% reduction in peak patellofemoral joint stress in those subjects unshod versus a shod group and suggest running barefoot may prevent or treat patellofemoral joint syndrome. The reduced ground reaction force was attributed to the decreased stride length that resulted in a smaller knee flexion angle and extension moment [98]. Likewise, Lenhart et al. reported a 10.4% reduction of mean contact pressures of the patellofemoral joint simply by increasing the cadence by 10% in runner subjects. This change likewise reduced the overall contact area by 7.4% [54]. Of course this modification need not require a BF condition but lends evidence to the premise BF running may be of value to PFJS .
Sinclair likewise noted a significant reduction in the patellofemoral contact force and loading rate in the BF condition when compared to minimalist and traditionally shod conditions [99].
Kumala et al. compared forefoot strikers to rear foot strikers and similarly found that RF strikers had a significant increase in the patellofemoral contact force and stress. They venture a FFS pattern may reduce the risk of running-related knee injuries [100].
PFJS and Hip Mechanics
Noehren et al. note many studies that have found PFJS is more commonly seen in runners who are found to have an increased hip adduction a well as a greater peak hip rotation, two characteristics found to be reduced in a BFR pattern [101].
Iliotibial Band Friction Syndrome
Some anecdotal reports and a number of risk factors that have been associated with ITBFS that are said to be reduced in the BF condition lend to notion that BFR may be helpful for this condition. Noehren et al. and McCarthy et al. describe the association of ITBFS and PFJS with an increased adduction and internal rotation of the hip. After determining that BF running led to a significant reduction in the degree of these parameters, it could be concluded that BF running may play a role in preventing or treating ITBFS [102, 103].
Stress Fractures
Of the injuries affecting runners, one of the most attributable to vertical impact peaks and loading rates during initial contact are tibial stress fractures [45, 47]. Therefore, lessening these forces has been a major topic of investigation.
Zadpoor et al. performed a systematic review on the association of lower extremity stress fractures (the majority, tibial stress fractures) and ground reaction force . They discovered no correlation with the amount of GRF and development of a stress fracture but did find a significant association with stress fractures and the vertical loading rate of GRF [49]. This is relevant as the major distinction regarding impact forces between RFS and FFS is that although GRF may be similar the VLR is significantly reduced with a FFS [12].
Although a number of studies and reports discussed later will document clinicians and researcher’s concern with BFR as an etiology of stress fractures, Edwards et al. found that runners could significantly reduce their probability of sustaining a tibial stress fracture by simply shortening their stride length 10% which in effect, reduces the strain magnitude, a variable they believe is more contributory to bone fatigue and damage than is the number of loading cycles. Given BFR has an associated shortened stride length, one could hypothesize BFR may have a preventative benefit for tibial stress fractures as could techniques such as utilization of a BF style while shod that encourage a shortened stride length [52].
Hobara et al. also associate high loads with tibial stress fractures and suggest that increasing cadence may reduce the prevalence of developing a tibial stress fracture. They pose the caveat that as one increases the cadence the increased steps result in a higher loading cycle with a resulting muscle fatigue which could in turn increase the chance for a stress fracture [53].
Plantar Fasciitis
Early on, proponents of BFR believed participation would strengthen the plantar intrinsics and therefore prevent and/or cure plantar fasciitis [4]. No evidence to this effect has been found but interestingly, BFR may have a beneficial effect on plantar fasciitis but for a different reason. Bowser et al. determined in a prospective study a strong correlation between those runners with high vertical loading rate , impact peak, and peak positive acceleration of the tibia and a predisposition to plantar fasciitis [44]. Pohl et al. also found an association of greater vertical ground reaction force load rate and subjects with a history of plantar fasciitis [48]. If indeed, BFR results in a reduced vertical loading rate, impact peak, and peak positive acceleration, then it could be deduced that BFR could be a way to prevent or treat plantar fasciitis. At this time, studies that directly assess this are not available.
Likewise in their review, Tam et al. list four reported findings thought to cause/contribute to plantar fasciitis and how BFR has been shown to negate three of the four and thus “theoretically implicate” a reduced risk [91].
Disadvantages of Barefoot Running
During the early hype on BFR, the majority of information, especially from the nonclinical perspective, related to the potential benefits with the clinical perspective one of skepticism and concern over potential injuries. The so-called downsides of BFR are also discussed in the literature but not with the same fervor as linking the changes with BFR to possible advantages.
The following section will review studies that evaluate some of the purported drawbacks of BFR.
Stress Fracture Propensity
Shift of Impact Location to Forefoot
One of the most commonly related concerns among clinicians as it relates to BFR/minimalist is that the shift to a forefoot impact will result in a propensity to metatarsal stress fractures. It was noted earlier that Cooper et al. described a balanced force distribution across the plantar surface in most of their subjects running BF but raised a concern that this change may place metatarsals at a greater risk for injury [57].
Bergstra et al. evaluated plantar pressures encountered by experienced female runners both in traditional and minimalist shoes and found significantly greater forces in the plantar forefoot in those using minimalist shoe gear and reason a transition to minimalist shoes may put one at greater risk for metatarsal stress fractures [104].
Increased Bone Marrow Edema in Transition
At the most basic investigative level, Ridge et al. evaluated two groups of runners via MRI exam after a 10 week running program. One group ran in traditional shoes and the other in Vibram Five Fingers (V5F ) . Even with a gradual transition period, the V5F group demonstrated significantly more bone marrow edema [105].
BFR Leg Stiffness Increase: Joints Versus Bones
As noted above, Sinclair et al. reported increased stiffness in limb and knee in the BF and minimalist condition. In their discussion they suggest these findings may potentially reduce the risk of knee pathology (soft tissue injury) but suggest bone injury may be increased [25].
Major Issue May Be Transition Period/Adaptation
In their report of case studies describing runners that encountered metatarsal stress fractures, Giuliani et al. conclude that although the stress fractures appeared to be related to the use of minimalist shoe gear (V5F ), the stress fractures resulted not from BFR/minimalist per se but from an inadequate as well as an inappropriate transition to a BFR style gait. Therefore, running either BF or in minimalist footwear but maintaining a RF strike style can be fraught with increased shock and possible injury [106].
Likewise, Cauthon et al. in describing stress fractures related to minimalist shoe gear concluded they were probably the result of an absence of an appropriate transition period [107].
Salzler et al. reported on nine stress fractures (eight metatarsal, one calcaneus) that they attributed to the utilization of minimalist shoe gear but included the transition/adaptation time their runners reported. Interestingly, half of the injured did no transition period and the rest took between 2 and 8 weeks to adapt. The conclusion here is that even completing the recommended transition period was not necessarily protective from stress injury and that even though the major impact seen with heel striking may be reduced, the impact has to go somewhere, in this case the metatarsals. They also considered the concept that there is an individuality as to the ability of runners to modify their vertical leg compliance (create a better shock absorber) and those who became injured had less change in this parameter during the transition [108].
Achilles Tendinopathy
Transfer of Forces
One of the significant gait modifications seen with barefoot running is a significantly more plantarflexed ankle joint at foot strike. Intuitively, a major concern of this change is that the resulting eccentric stress on the posterior calf musculature via a far greater dorsiflexory moment would presumably set the stage for Achilles tendinopathy [24].
Almonroeder et al. report that BF peak Tendo Achilles (TA) force takes place earlier in the stance phase and results in a 15% increase in the TA loading rate which they calculated to be an additional 48 body weights encountered over the course of a mile run. The implication being BFR technique may result in higher risk of Achilles tendinopathy [109].
Sinclair likewise noted a significant increase in the Achilles tendon force in the BF condition when being compared to minimalist and traditionally shod conditions [99].
Kumala et al. compared forefoot strikers to rear foot strikers and notes that FF strikers had an increase in the plantarflexory moment and Achilles tendon force and loading. They venture a FFS pattern may increase the risk of running-related Achilles tendon injuries [100].
Effects on the Triceps Surae
Rao et al. compared the maximal muscle forces in the triceps surae during BF, minimalist, and traditional shod conditions and found no difference but note that due to the greater ankle range of motion seen in BF condition creates a more biomechanically efficient use of the triceps surae as it relates to force-length dynamics [110].
Stresses on Achilles May Be Reduced with Foot Position
Interestingly, some researchers [13] consider that stress loading on the posterior calf musculature/TA as well as the metatarsal heads can be reduced with the BFR style if the ankle is not too PF at contact, i.e., more of a midfoot parallel to ground positioning.
Increased Achilles Tendon Load Shod?
Wearing et al. describe findings that determine tensile load on the TA while walking in shoes with a 10 mm drop versus BF and found peak acoustic velocity and hence, tensile load in the Achilles tendon was actually increased while shod, a finding totally in contrast to what they expected. This would seem to confuse the premise that the BF condition or a zero drop shoe increases the load on the TA not to mention the dogma of heel lift therapy for Achilles tendinopathy [111].
Injury from Running Surface, Debris, or Soil Contaminants
Intuitively, it would seem that one running BF would encounter increased loads and thus a higher rate of injury by running on surfaces such as the pavement on a sidewalk. Lack of studies for this idea suggests this is not the case. As noted in the section on impact attenuation and proprioception improvements seen with BFR, it seems a hard surface encountered by the runner will institute mechanical changes resulting in changes in overall leg stiffness and foot position [33]. These automatic adjustments to surface impact are why Robbins and Gouw noted the human foot does not require any additional cushioning to manage the impact of running (Fig. 15.3) [112].
Fig. 15.3
Barefoot in the desert
Although no studies have assessed the risk or related injuries to BF runners [113], surface hazards such as insects, glass, rocks, and thorns are said to be risks that are overstated and as simple to deal with as taking care as to where you place your feet [2].
Runners That Require Mechanical Control for Existing Conditions
It makes no clinical sense that an existing patient successfully treated for a running-related injury that was found to be the result of a biomechanical dysfunction suddenly toss away their orthotics or motion control shoes and take up BF or minimalist running. But these patients may be hearing from friends or other nonclinical sources that all their running-related problems will magically disappear if they run BF. Foot care specialists may be called upon to advise these patient/runners and thus far, the prudent thing to do in these cases is stay the course with therapeutic biomechanical care. Despite the enormous amount of research taking place regarding BFR and injuries, no studies currently support BFR as a preventative/treatment intervention for a given running-related problem.
Diabetics and Others with Loss of Protective Sensation
Indeed, the risk of ulceration and injury for persons with diabetes is greatly increased in those that go barefoot [116]. Given the potential, it would seem quite inappropriate for one with lack of protective sensation to participate in BFR.
Shoes That Mimic Barefoot Running (Minimalist Shoes )
In response to the hype regarding BFR and its possible benefits, there has also been a major effort by most shoe companies to develop and market minimalist (barefoot simulating) shoes. What follows will be an attempt to look at the most current evidence that relates to minimalist footwear with special attention to comparing it to the BF or traditionally shod condition. Do these shoes actually recreate the kinematics seen with BFR but also provide some physical protection of the foot true BFR does not? What aspects of a minimalist shoe are most desirable to best simulate the BF condition?
History
As convinced as one might be on the merits of a BF style promoted by aficionados’ web sites and by reputable researchers from Harvard, many would-be BF runners had reservations with a truly BF status. Feet coddled and protected over lifetime were just not ready for the rocks and pebbles, temperature extremes and the weeks of “toughening up” and adaptation the true believers claimed was required. Even the Taraumara Indians in the Copper Canyons of Mexico whom Christopher McDougall wrote about in Born to Run used protective albeit, minimal foot coverings.
Nike, based on reports that elite track runners at Stanford University, incorporated BFR into their overall training program with the blessings of their coach who commented that BFR made them faster and less prone to injury, decided to develop a shoe that could simulate BFR [117]. The goal was to create a shoe that had no arch support, a very flexible structure, reduced or absent cushioning and reduced heel height. In 2004, the Free 5.0 was purported to have 50% the support/stability of a traditional running shoe. A tweak was made to the Free 5.0 creating an even less stable version as well as one more stable, the Free 3.0 and 7.0, respectively.
Enter Vibram FiveFingers (V5F ) in 2006. Time Magazine’s invention of the year in 2007 was essentially a “fingered” wet suit bootie with a Vibram sole that allowed runners to incorporate a BF style but still enjoy a degree of protection from the elements. V5F and numerous other minimalist shoes enjoyed remarkable success with huge increases in sales revenue only to have dramatic reductions in interest and sales in the last couple of years [108, 118]. Despite this waning demand for minimalist foot wear, there continues large volume of studies coming out re: minimalist shoe wear.
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