Stepping Back to Minimal Footwear: Applications Across the Lifespan

Stepping Back to Minimal Footwear: Applications Across the Lifespan.

Exercise Sport Sci R. 2021;49:228–43.

doi: 10.1249/JES.0000000000000263

Abstract

keywords

Keypoints

There is an evolutionary mismatch between modern foot-wear and the way our feet were adapted to function.

Modern footwear is associated with the development of footdeformities such as hallux valgus and pes planus.

Minimal footwear promotes strengthening of both the in-trinsic and extrinsic foot muscles.

Minimal footwear promotes walking and running gait pat-terns more similar to our natural barefoot gait.

Minimal footwear is beneficial t o healthy older adultsand t hose with some pathologic conditions such as knee osteo arthritis.

現代の履物と私たちの足の機能が適応された方法との間には、進化上のミスマッチがある。

現代の履物は、外反母趾や扁平足などの足の変形を引き起こす原因となっている。

裸足感覚シューズは、内在性と外在性の両方の足の筋肉の強化を促す。

裸足感覚シューズは、人間の自然な裸足に近い歩き方や走り方を促進する。

裸足感覚シューズは、健康な高齢者や、膝OAなどの病的な状態にある高齢者にも有効である。

INTRODUCTION

The barefoot condition is our most natural state, and the foot is well adapted for walking and running gaits without footwear. Although footwear originally was developed more than 10,000 yr ago to protect the sole of the foot, footwear over the past 50–60 yr has become both more cushioned and supportive (1). This type of footwear often is recommended for athletes, as well as elderly with musculoskeletal dysfunction. However, we will review how these shoes have been shown to interfere with natural foot and lower extremity mechanics in ways that may increase the risk for injury. The purpose of this article is to present the novel hypothesis that minimal footwear may lead to improved musculoskeletal health across the lifespan. To evaluate this hypothesis, we will review evidence regarding the relation between minimal footwear, foot strength, mechanics, and injuries in both athletic and nonathletic populations. Here, we define minimal shoes as those lacking any support or cushioning. We begin with an evolutionary perspective on foot development and footwear. We then address studies of minimal footwear in children. This is followed by a review of minimal footwear and lower extremity mechanics in adult runners. Next, we address minimal footwear and the foot musculoskeletal system. The relation between foot strike pattern and tissue properties (tendons and fat pads) is then reviewed. Finally, the use of minimal footwear for healthy older individuals, as well as those with pathology such as knee osteoarthritis (OA) and diabetes, is discussed. We conclude with a summary of recommendations for future studies needed to address current research gaps. The topic of minimal footwear is one that is hotly debated in both clinical and scientific arenas. We hope this perspective article will begin to create a paradigm shift in the way we think about footwear, spark debate, and be a catalyst for additional research.

IN THE BEGINNING

Most mammals walk and run on their toes, but humans evolved from African great apes that have plantigrade feet that are well adapted for climbing trees. African great apes have long, curved toes, an abducted hallux, a relatively short and flexible midfoot that lacks an arch, and a less developed calcaneus with a mobile ankle joint. However, over the 7 million years since the human and chimpanzee lineages diverged, hominin feet evolved substantially. This first occurred to facilitate bipedal walking and then later running over longer distances than apes (2). For example, human feet have adapted to include an enlarged calcaneus. This helped stabilize the rearfoot and bear repeated, higher stresses during the impact phase of walking considerably longer distances on two versus four legs (3). In addition, they developed an elongated midfoot that is stiffened by transverse and medial longitudinal arches (4,5) and a thicker plantar fascia (6). The hallux became elongated, and the toes became shorter and straighter with dorsally oriented metatarsophalangeal joints (7) (Fig. 1). Together, these adaptations compromise our ability to climb trees. However, they optimized the human foot for both walking and running (2).

Figure 1

Figure 1:

For most of the last 7 million years, humans also walked and ran barefoot. They did this over a variety of surfaces from soft grasslands to hardpacked savannah. As such, humans have evolved with the ability to adjust their overall leg stiffness to the hardness of the substrate they negotiate to maintain a constant stiffness of the system. For example, they increase their leg stiffness when encountering soft surfaces and reduce their leg stiffness when encountering hard surfaces. This has been demonstrated in a number of modern studies (8–10) and underscores that humans are equipped to walk and run on a wide variety of surface stiffnesses.

Until about 600 generations ago, all humans were hunter-gatherers who walked on average 9–15 km·d−1, approximately 10–15,0000 steps, either barefoot or in minimal footwear (11). The oldest preserved sandals are about 10,000 yr old (12), and the oldest shoes are from about 6000 yr ago from Armenia (13). However, it is reasonable to hypothesize that footwear was available by at least 40,000 yr ago when needles and other tailoring technologies first appear in the archeological record (14). Around this time, there also is some evidence for a reduction in metatarsal robusticity (cross-sectional thickness relative to bone length). This indirectly suggests that the use of footwear such as sandals decreases bending forces on the midfoot during propulsion (15).

For most of human evolutionary history, shoes probably were used only occasionally. In addition, until recently, almost all footwear were minimal such as sandals or moccasin-like shoes. Features common in modern shoes such as toe springs, stiffened midsoles, elevated heels, and arch supports are generally quite recent. Ethylene-vinyl acetate-cushioned shoes have been available since only the 1970s (16). Features in these shoes provide some benefits, notably protection and comfort. However, these structured shoes potentially contribute to several hypothesized evolutionary mismatches. Evolutionary mismatches are conditions that are more prevalent or severe because bodies are inadequately or imperfectly adapted for novel environmental conditions (17). Put differently, although shoes provide some advantages, they may also have some drawbacks for which we are not well adapted.

Evolutionary mismatches related to footwear fall into three categories. First, cushioned (elastic) soles slow the rate of loading at impact and decrease sensory perception (18,19). As a result, people who habitually wear cushioned shoes experience higher ground reaction force impulses when walking (19), and they are more likely to rearfoot strike (RFS) when running (20). This results in an abrupt impact transient of the vertical ground reaction force not seen with forefoot striking (21) (Fig. 2). Second, these supportive shoes reduce the demand on the foot muscles, which can result in weaker feet, as evidenced from smaller muscle cross-sectional areas (22). Finally, structured shoes alter our foot mechanics. The added sole flares increase the external torques to the foot, creating abnormal loading to the foot and lower extremity (23). In addition, the arch support in structured shoes reduces the longitudinal and transverse arch compression during midstance (24), which is important for elastic energy storage. All of these changes potentially lead to mismatches between the way the foot was adapted to function and how it functions in structured shoes, which can lead to dysfunction and injury.

Figure 2

Figure 2:

Relation between footwear history and footstrike angle in Kenyan runners. Note that as footwear usage decreased, runners adopted a more FFS pattern. (Reprinted from (20). CC BY 4.0.)

MINIMAL FOOTWEAR IN CHILDREN

The foot undergoes crucial developmental changes during childhood and adolescence. Although several bones ossify prenatally, the main ossification period extends over the first 10 yr of life (25). Epiphyseal union of all long bones in the foot, as well as the talus and calcaneus, occurs throughout late adolescence or early adulthood, representing the end of foot bone growth (26). The foot typically achieves its final length at an age of about 13 yr in girls and 15 yr in boys (27). The arch of the foot also develops during childhood. Although infants have no arch, it begins to develop once toddlers begin to walk. The shape of the arch is, thus, determined, not just by the shapes of the bones, but also by the muscles and ligaments.

Childhood and adolescence are critical periods in which the developing foot is more prone to external influences. One of these influences is incorrectly fitted footwear. The prevalence of incorrectly fitted footwear has been estimated to be up to 66% (too narrow) and 72% (too short) in school children (28). Improperly fitted footwear has been shown to increase the risk of foot deformities such as pes planus or hallux valgus in children and adolescents (29–31). In contrast, children and adolescents who grow up barefoot have been shown to have significantly higher arches than those who have grown up shod (32–35). One large-scale study reported on 2300 children in India between the ages of 3 and 15 yr (25). In one community, children were barefoot, in another they wore sandals, and in a third they wore closed-toe shoes. Flat footedness was most prevalent in the group that wore shoes and least prevalent in those who were barefoot.

Low arches are associated with pathologies such as pes planus deformity (29), which can lead to altered function. For example, children with low arches have been shown to walk with greater foot progression angle and greater external rotation of the lower extremity (30). Along with higher arches, children who are habitually barefoot also demonstrate improved jumping and balance skills (36). A recent, large-scale study compared Japanese children from two schools in the same city that incorporate a running program, with one being barefoot and the other shod (37). Children in the barefoot program exhibited significantly greater performance in jumping and sprinting, and a greater proportion ran with a midfoot or forefoot strike (FFS) pattern compared with the shod group.

Minimal footwear has been associated with the ability to mimic some of the barefoot walking characteristics in children (38,39). For example, Hillstrom et al. (38) compared the gait of toddlers with only a few months of walking experience as they walked barefoot, and in minimal and structured shoes. Similar plantar pressure distributions between the barefoot and minimal footwear were noted compared with more structured footwear. The authors conclude that this similarity may enhance proprioception, which they suggested was important for developing gait in young children. In addition, Wolf et al. (39) studied a cohort of 6- to 10-yr-old children. They noted that walking kinematics in minimal footwear were closer to the barefoot condition than in structured footwear in 12 of 15 parameters tested. They also noted that minimal footwear allowed the medial longitudinal arch to deform more naturally than in traditional, stiff footwear (39).

Running mechanics in children also are influenced by minimal footwear. Hollander et al. (40) conducted a comparison of minimal and cushioned footwear with barefoot running in 6- 9-yr-old children. The greatest differences in mechanics were found between the cushioned and barefoot conditions, and the most similarities were noted between the minimal and barefoot conditions. For example, the rate of RFSs was highest for the cushioned shod running and lower but similar for the barefoot and minimally shod running. This pattern was also true for other variables. The impact force and step length were higher, and cadence was lowest in the cushioned shoe, but similar between the barefoot and minimal shoe conditions.

Based on these collective studies, minimal shoes may be optimal for the developing feet of children. These shoes are designed to better match the natural shape of the foot with additional width in the forefoot. This helps overcome the issue with improper fit of shoes, which is especially important for the developing foot. They seem to replicate many aspects of both walking and running mechanics as being barefoot while protecting the sole of the foot. Despite these potential benefits, recommendations for youth footwear, to date, still do not address minimal shoes (41,42).

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 (Fig. 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 (Fig. 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

Figure 3:

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 (Fig. 4A). However, it had a cushioned midsole, which permitted an RFS pattern (55). In the same year, the Vibram FiveFingers shoe (Fig. 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

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 (Fig. 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 (Fig. 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

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

Figure 6:

MINIMAL FOOTWEAR AND THE MUSCULOSKELETAL SYSTEM

The human musculoskeletal system normally adapts to the mechanical loads it experiences. As proposed by Frost's (71,72) mechanostat model, tissues responds to the mechanical demands placed on them by altering their mechanical properties to better meet the new demands. Although criteria governing this response are not well understood, there is emerging evidence that footwear may influence the adaptation of the musculoskeletal system including fibroadipose and dense connective tissue structures. The majority of this research has been conducted on adults.

Influence on Bone

There is a dearth of articles examining the effect of minimal footwear on bone health, and these articles focus on running. One study measured bone mineral density in runners before and after a structured, 26-week transition to running in minimal footwear (73). The authors reported no significant changes in the apparent density of the measured bones, including the tibia, calcaneus, and metatarsals. However, other more detailed measures of bone quality, such as cortical thickness and trabecular bone density, may be more indicative of strength. These measures are acquired using high-resolution, peripheral, quantitative computed tomography, which is not currently widely available. As its use becomes more prevalent, we will be able to better study the effect of progressive load on bone strength. The effect of minimal footwear on bone injury has been addressed in a few more studies. Bone marrow edema (BME) is used as an indication of both bone turnover and bone injury. Ridge et al. (74) reported on the Bone Marrow Edema Score after a largely unstructured, 10-wk transition to running their habitual mileage in minimal footwear. Increased edema was noted in 10 of the 19 runners. However, not all had pain that would indicate an injury. It was noted that those with the greatest amount of edema were the ones who reported pain. It is possible that some with lower levels of edema were cases of bone remodeling that would be expected with an increased load, as opposed to an injury. However, there have been other case reports of individuals with bony (primarily metatarsal) injuries associated with minimal footwear. In these cases, it was pain that sent them to seek medical attention (56,75), and definitive stress fractures were diagnosed. In both these reports, runners transitioned rapidly to their full mileage rather than progressing slowly. These studies indicate the need for engaging in a slow transition to minimal footwear for running to provide the time for adaptation. This gradual addition of loading may not only reduce injury risk but may also potentially result in some bone strengthening, according to Wolf’s law.

Influence on Muscle

There have been a number of investigations of the effect of minimal footwear on muscle size and strength. A study by Holowka et al. (22) reported that habitual daily users of minimal footwear had larger abductor hallucis and abductor digiti minimi (ADM) muscles compared with a supportive shod population. This likely is due to the greater demand placed on these muscles when walking in unsupportive footwear. Other studies have shown foot muscles hypertrophy when transitioning to minimal footwear for walking (74). A recent study reported that an 8-wk, progressive walking program in minimal shoes increased intrinsic and extrinsic muscle size and strength (74). In fact, walking in these shoes produced similar increases in the size and strength of foot muscles as the strengthening program completed by the foot strengthening group (Fig. 7). As the loads of running are higher than those of walking, the potential for strengthening in minimal footwear is greater (76). Studies of minimal footwear use during running and athletic activities (76–79) have shown increases in the size and strength of a number of intrinsic and extrinsic foot muscles (EFM). In fact, every study that has examined the effect of minimal shoes on foot intrinsic and extrinsic foot muscle size or strength has reported increases (Table). The benefits of stronger intrinsic foot muscles include improved propulsion during walking and running (80,81) and control of midfoot deformation (81–84). In addition, simulated contraction of cadaveric foot intrinsic muscles during loading has been shown to reduce the bending strain in the metatarsals (85). Finally, a recent study has demonstrated that runners who completed an 8-wk foot exercise program were 2.4 times less likely than the control group to develop a running-related injury (86). Therefore, we postulate that a gradual transition to minimal shoes, which promote foot strengthening, may also reduce the risk for injury in runners.

Figure 7

Figure 7:

TABLE - Effect of physical activity (walking, running, and exercising) in minimal footwear on muscle size and strength

EDB, extensor digitorum brevis; FDL, flexor digitorum longus; FHL, flexor hallicus longus; GT, great toe; LT, lateral toe; MF, minimal footwear; MPJ, metatarsal-phalangeal joint; PER, peroneus; R, running; RW, restricted walking; TA, tibialis anterior; TP, tibialis posterior; UW, unlimited walking.

Although habituating to minimal footwear results in foot muscle strengthening, muscle injuries can occur if transitioning is done too quickly. In 1 study, 7 of 14 runners reported pain in the gastrocnemius/soleus/Achilles tendon complex during a 12-wk transition to running in minimal footwear (57). Similar to bone injuries, it is possible that many of the muscle strains or soreness injuries could be prevented by a slow increase in activity in minimal footwear. A foot core program (87) has been shown to significantly increase the size and strength of the intrinsic and extrinsic foot muscles (74,88). The addition of such a program can also help prepare the foot for the transition and reduce injury risk during this period.

Foot orthotic devices provide support to the foot. They are often prescribed for long-term use, which may negatively affect the foot. As these devices support the arch, the demand on the foot intrinsic muscles is reduced. In fact, an article by Protopapas and Perry (89) reported a 10%–17% reduction in the foot intrinsic muscle sizes as a result of 12 wk of orthotic use. Therefore, just as minimal footwear that removes support from the foot has been shown to strengthen muscles, adding chronic support to the arch likely will weaken them. Therefore, if a foot injury requires additional temporary support of a foot orthosis, it should be gradually removed once the injury has healed to help strengthen the foot once it has recovered.

Influence on Tendon and Aponeurosis

The Achilles tendon is an important component of the stretch-shorten cycle of the triceps (TS) surae muscle-tendon unit. The tendon's mechanical properties, and particularly its material stiffness, affect force production and the performance of complex movement. There is some controversy regarding the capacity of mature tendon to adapt to loading. However, animal studies suggest that, like bone, the material properties of tendons can be dramatically increased with loading during growth and development (90). High peak loads have been shown to be most beneficial for homeostasis and adaptation of human tendon properties (91). An FFS strike pattern during running results in greater activation of the triceps surae and a higher rate and magnitude (8%–24%) of Achilles tendon loading than heel strike running (92–94). Hence, minimalist footwear is associated with a loading stimulus that is more likely to induce Achilles tendon adaptation. Indeed, runners who wear minimalist footwear have been shown to have greater stiffness and cross-sectional area of the Achilles tendon than traditionally shod runners (95,96). Moreover, the Achilles tendon of habitual FFS runners has been shown to be functionally stiffer during both walking and running, thereby aiding its “spring-like” function (97). Therefore, the additional loading of the Achilles tendon associated with minimalist footwear likely results in a stiffer tendon. This may be beneficial for activities requiring rapid force development and protective against strain-induced injury for a given load.

The plantar fascia effectively connects the expanse of the medial longitudinal arch and has been regarded as the primary structure stabilizing the arch during weight-bearing. Along with the ligaments of the medial longitudinal arch, the plantar fascia also may contribute to the elastic behavior of the foot and to improved locomotor efficiency (98). However, the medial longitudinal arch also is traversed by the intrinsic muscles of the foot and the long tendons of extrinsic foot muscles. These muscles are well positioned to reduce the load borne by the plantar fascia (99–102). Running in minimal footwear is associated with intrinsic muscle hypertrophy (68–71), a thinner plantar fascia, and a less compliant medial longitudinal arch (103). These findings are consistent with a shared load bearing role between these structures. With stronger intrinsic muscles, the plantar fascia can be thinner and more compliant. When the muscles are weaker, the plantar fascia adapts to become thicker to control the arch deformation during loading. Indeed, a thickened and stiff investing muscle fascia has been implicated in the development of other pathologies, such as chronic compartment syndromes of the lower leg (104,105).

Fibroadipose Tissue

The calcaneal heel pad is a specialized fibroadipose tissue that is thought to play a number of mechanical roles during gait (106,107). The first is shock reduction. During walking, the heel pad undergoes approximately 9–11 mm of vertical deformation, which is thought to lower the peak impact force (108). However, the heel pad offers minimal resistance to the rapid deformation induced by initial heel strike, suggesting it has only a minor role in shock reduction during walking and running (109). The second role of the heel pad is energy dissipation. However, only about 1.0 J of the strain energy stored in the heel pad during walking is dissipated with unloading (110). This only equates to about 20% of the impact energy of the foot and approximately 1% of the total energy exchanged during a single gait cycle (~100 J for a 70-kg human) (111). This is less than that of the Achilles tendon (~2.5 J) and ligaments of the medial longitudinal arch (~3.1–4.5 J) (112–114). These structures reportedly behave as “springs” and are key structures associated with energy storage rather than energy dissipation. The energy dissipating properties of the heel pad are relatively insensitive to strain rate (111). Thus, the relatively low level of energy dissipation provided by the heel pad is unlikely to change substantially with increases in gait speed. This results in the heel pad being a less than ideal structure for dissipating the impacts associated with running (109). The third role of the heel pad is the protection against excessive plantar pressure. The mechanoreceptive and nociceptive nerve endings of fibroadipose tissues are localized between fat cells (115), and their sensitivity is related to the degree and rate of deformation of the tissue (116,117). This endows the fat pad with a proprioceptive role for monitoring mechanical vibrations associated with heel strike, as well as for detecting pain (118–120). Deformation of the heel pad during barefoot walking (approximately 60% or 10 mm) approaches that associated with the limits of pain tolerance. Hence, the FFS pattern adopted during barefoot running likely reflects a pain-avoidance strategy (121).

In conventional shoes, the heel pad is constrained and deforms only about 35% during walking and running (106,122). Therefore, conventional footwear likely lowers the potential for strain-related injury of the heel pad. However, by inducing a slower loading rate and a lower final strain in the tissue, it also has the potential to lower the sensitivity of the heel pad to detect pain and potentially harmful vibrations (108). Minimalist shoes, in contrast, tend to promote a forefoot foot strike gait pattern during running. Cadaveric studies have shown that the fibroadipose tissues of the forefoot have both a higher material stiffness and greater capacity to dissipate energy than the heel pad (123). These tissues also have a higher density of vibration-sensitive mechanoreceptors (124,125). Hence, fibroadipose tissues of the forefoot may better damp the impact vibrations associated with running than those of the heel and tend to be preferentially loaded in minimalist footwear.

MINIMAL FOOTWEAR IN OLDER ADULTS

Minimal Footwear in Healthy Older Adults

Minimal shoes have been shown to be beneficial for older adults. Shoes with cushioning are likely to filter out important sensory information (126,127), which is important for balance and stability, especially in aging populations. It has been shown that when sensory input is lost, such as through anesthetization of plantar afferent nerves, stability during quiet stance becomes impaired (128). This may explain why balance during standing and walking in an elderly population is improved in minimal shoes compared with cushioned ones (129). Second, although older adults tend to experience general lower extremity muscle weakening, there is a shift in joint power during walking gait from distal to proximal (130). This suggests that foot and ankle function degrades with age, which may increase the risk for falls in this population (131). Falls have been related to foot weakness (132,133), and unfortunately, the intrinsic foot muscles (IFM) have been noted to become weaker with age (134). Nearly one in four older adults experiences falls (131), which are the leading cause of injury-related deaths in older adults (135). Along with weakness and loss of function that accompany aging, chronic support of the foot can lead to further foot muscle weakening (6,89).

It has been reported that standard features of conventional shoes can be detrimental to the elderly. For example, these shoes often have constrictive toe boxes, which has been associated with hallux valgus (136) (Fig. 8). Other features, such as elevated heels, stiff uppers, and flared outer soles have been shown to negatively impact gait mechanics. For example, Aboutorabi et al. (137) conducted a systematic review of the effect of footwear on static and dynamic balance in elderly individuals. They reported that balance during standing posture and functional activities (i.e., the timed get up and go and functional reach tests) was improved when soles were thin and hard. This recommended footwear shares some characteristics with minimal shoes, such as thin soles, low heel-to-toe drop, low weight, and lacking sole flares (138). Studies about the effect of minimal shoes on gait in this older population are still scarce. In one relevant study, Cudejko et al. (139) compared the center of pressure trajectory while older adults stood and walked in several footwear conditions. This included 11 variations of minimal shoes, a barefoot, and a conventional shoe condition. The older adults performed better on the timed up and go test with the minimal shoes compared with the conventional shoes The center of pressure excursion and velocity in the anteroposterior and mediolateral directions were also reduced during standing and walking in minimal shoes, indicating greater stability. Results between the minimal shoes and barefoot were similar. These collective results suggest that minimal shoes may offer a more stable alternative for healthy older adults.

Figure 8

Figure 8:

Minimal Footwear in Older Adults With Knee OA

Knee OA is one of the most common musculoskeletal conditions of older adults. Although the etiology of knee OA is multifactorial (140), mechanical aspects such as the intra-articular loads are the primary risk factors for its development and progression (141–143). The external knee adduction moment (EKAM) is often used as a surrogate measure for these internal loads. Increases in the EKAM have been reported to increase the risk for the severity (144) and progression (145,146) of knee OA. Specialized footwear is an emergent conservative strategy to reduce EKAM. This footwear has included variable-stiffness soles (147–150), rocker soles (151), and laterally wedged insoles (152–154). Although footwear with laterally wedged or arch support insoles results in a small reduction in the EKAM, this footwear type increases the frontal plane torques at the ankle. Preservation of normal ankle torques and kinematics is recommended to prevent adverse effects at the foot-ankle complex (155).

Among footwear interventions to reduce knee joint loads, minimal shoes have been one of the most promising both in the short term (156–159) and long term (160). Shakoor et al. (157) studied the acute effects of minimal footwear in patients with knee OA. The minimal shoe was custom engineered to mimic barefoot walking and was composed of a flexible poly carbon sole with flex grooves and a mesh top. They reported an 8% decrease in EKAM in a minimal shoe compared with self-chosen walking shoes and a 12% reduction compared with control (wearing a cushioned sports shoe). Others have compared the effect of a commercially available shoe, called the Moleca (Calçados Beira Rio S.A., Novo Hamburgo, RS, Brazil) shoe, to a modern heeled shoe on the gait of women with knee OA. The Moleca shoe (Fig. 9) is a low-cost women's canvas flat walking shoe. It has a flexible 5-mm antislip rubber sole, and its mean weight is 0.172 ± 0.019 kg. These features qualify it as a minimal shoe (109). In two cross-sectional studies, the Moleca shoe demonstrated reductions in EKAM of approximately 12% during walking and 15.5% during stair descent (158,159). Trombini-Souza et al. (160) then conducted a clinical trial with older women with knee OA randomized into the Moleca shoe or a neutral athletic shoe and followed them for 6 months. They reported a 22% reduction in EKAM in the minimal shoe group: nearly double that of the study of the acute effects (159). In addition, they experienced a 66% reduction in the Western Ontario and McMaster Universities Arthritis Index (WOMAC) pain domain, a 62% reduction in WOMAC stiffness domain, and a 63% improvement in WOMAC function domain. Pain medication also remained low and unchanged after 6 months. In stark contrast, the women in the control group experienced significant increases in EKAM (15.4%), WOMAC pain, stiffness, and a decrease in function. In addition, there was a 36% increase in the pain medication. Therefore, the minimal footwear resulted in less pain and better function than conventional footwear in patients with knee OA.

Figure 9

Figure 9:

Results of a minimal shoe (Moleca shoe, center) intervention in women with knee osteoarthritis (OA). Note improvements in the Western Ontario and McMaster Universities Arthritis Index (WOMAC), pain medication intake, external knee adduction moment (EKAM), stiffness, and pain compared with the standard shoe intervention.

Minimal Footwear for Individuals With Early Stages of Diabetes

Diabetes mellitus is a metabolic disorder that results in high glucose levels in the blood that eventually can damage both motor and sensory nerves. With time, diabetic peripheral neuropathy can ensue and cause major problems with the foot. Approximately 50% of diabetics experience neuropathy between 25 and 30 yr after the diagnosis of diabetes (161). When it does develop, muscles become weakened and foot deformities, such as claw toes, can develop (162). This can result in an anterior displacement of the already thinning fat pad, increasing the exposure to the metatarsal heads (163). This, combined with the sensory loss, leads to ulcerations, most commonly at the metatarsal heads (164,165). Most individuals with diabetic neuropathy are prescribed full contact soft foot orthosis and structured, cushioned shoes, often with a rocker bottom (166). This intervention is aimed at redistributing the plantar load and reducing the load on the metatarsal heads. However, this intervention strategy is typically used for all patients regardless of their risk for ulceration or musculoskeletal status. Unfortunately, if used before the neuropathy progression, this passive approach might result in foot muscle atrophy (6,89), leading to impairments in muscle strength and foot function. This can accelerate the degenerative changes that may occur.

Studies suggest that motor loss may occur before the sensory loss in the neuropathic process (128,167,168). Diabetic neuropathy has been shown to affect both the intrinsic and extrinsic muscles of the foot (169–174). The intrinsic muscles are small with short moment arms and primarily provide foot stability (74). The larger extrinsic muscles can generate more force and with larger moment arms produce joint rotations. These muscles serve as prime movers of the foot (74). Strength loss of both intrinsic and extrinsic foot muscles increases the risk for the development of foot deformities, which leads to an increased risk for ulceration (164,175). Therefore, foot strengthening should be part of a diabetic treatment approach long before the neuropathy progresses. Studies of weight-bearing exercises that address the foot and ankle have shown improvements in range of motion, plantar pressures, and overall gait mechanics (176–179). As a result, foot exercise prescription is now part of the International Working Group on the Diabetic Foot Guidelines 2019 (170).

Another way to encourage foot strengthening is through the use of minimal footwear in walking (74). Minimal shoes are not currently recommended for those with diabetes due to the lack of support and cushion that a neuropathic foot requires. However, those diagnosed with diabetes typically have many years before a neuropathy progresses. This could provide a fairly extensive time for individuals to address the strength of their feet. Usage of minimal footwear during this early stage of diabetes may enable patients to maintain their foot motion and muscle strength for a longer period. This may help delay the development of foot deformities that can result in pressure ulcerations. Minimal footwear coupled with a foot strengthening program may provide a way for individuals to maintain foot strength and function before development of neuropathy. These feet might be more resistant to dysfunction if the neuropathy develops. This type of a program would need to be monitored by a medical professional who could routinely assess the sensory and motor status of the individual.

SUMMARY

私たちは、生涯にわたって最小限のフットウェアを使用することで得られる多くのメリットを明らかにしましたが、まだまだ多くの研究が必要とされています。

現在の文献のギャップに基づいて、私たちは以下のことを必要としています。

従来の靴とミニマムシューズが子供と大人の足の発達、バイオメカニクス、筋骨格の健康に与える影響を比較する大規模なプロスペクティブ研究

歩行時のミニマムシューズの使用が筋骨格系に与える影響について、他の活動と同様に、より多くの研究が必要である。

異なる足型（ハイアーチ、ローアーチなど）の組織の適応反応を考慮した、ミニマルフットウェアへの最適な移行方法の理解を深める。

ミニマルフットウェアへの移行を容易にするための、内在性筋および外在性筋の最適な強化介入の研究

足の健康に対する内在性および外在性筋の強化の効果に関する前向きな研究

足の複雑な動きに対するミニマムフットウェアの効果を調べるために、バイプラナービデオラジオグラフィなどの高度な方法を用いた研究

高齢者の足の強さ、バランス、および転倒に対するミニマムフットウェアの効果に関する研究

ミニマルフットウェアがどのような病的集団に最も効果的であるかを調べる研究

以上のことから、現代のフットウェアは進化上のミスマッチであり、傷害のリスクを高める可能性がある。そこで、我々は以下のことを提案する。人間は裸足でいることに適応していることを考えると，裸足の状態に近いミニマルフットウェアで習慣的に運動することは，筋骨格系の傷害に対する脆弱性を軽減する。近年、ミニマルシューズが再登場したのは、より裸足に近い状態のロコモーションに戻すことを目的としたもので、これにより傷害リスクが減少するという仮説が立てられている。本邦では、自然な足の仕組みへの干渉を最小限に抑えた靴を履くことの利点をいくつかまとめた。私たちは、年齢を問わず、また病的な集団においてもミニマルフットウェアの利点を示す証拠を提示した。その中には、足の筋肉が強くなる、アキレス腱が強くなる、着地が柔らかくなる、バランスが良くなる、足部機能が向上する、痛みが少なくなる、膝の負担が減る、下肢のトルクが減る、などが含まれている（Fig.10）。ミニマルフットウェアにまつわる主な懸念事項の1つは、移行に伴う怪我のリスクである。従来のフットウェアに慣れている人は、特にランニングなどの高いレベルの活動をする際には、ゆっくりとミニマルフットウェアに移行する必要があると推奨されている。理想的には、体が自然に順応するように、若いうちからミニマルフットウェアを履き始め、移行に伴うリスクを排除することである。

Figure 10

https://gyazo.com/024a8d115383ca0c869ad6f40f74a6d4

Schematic of the novel hypothesis that suggests that the use of minimal footwear across the lifespan will result in reduced injuries and a healthier musculoskeletal system. FFS, forefoot strike.