Chapter Seven
Shoe Gear Unlike the majority of mammals possessing insensitive hooves or thick pads, humans are forced to traverse the terrain on our comparably soft and extremely sensitive feet. Because they are so easily injured, some argue that the human foot was poorly designed to manage the stresses associated with wandering across our oftentreacherous landscapes, since feet were originally intended to help us get around the thick branches present in cool dense forests, where arboreal climbing skills were key to our early survival. The difficulties associated with prolonged bipedality presented significant challenges to our feet, as ground-reactive forces were drastically amplified (increasing the risk of puncture wounds) and the higher ground temperatures outside the forest increased the potential for thermal injury and/or surface heat loss. Given the potential for lacerations, abrasions, and/or thermal injury, it seems odd that for almost all of our 7 million year history as bipeds, we walked around the planet barefoot. Although we perceive our feet as being delicate structures in need of protection, when barefoot from birth, the human foot is remarkably resilient. As demonstrated by Robbins et al. (1), the skin beneath the heel and hallux is designed with a tight trabecular tethering of the epithelial membrane that resists perforation. The authors tested the integrity of this membrane by measuring skin resiliency and perceived pain as barefoot subjects were exposed to a “penetrometer� (a pointed device that applied 9 kg of pressure through a 10 mm spherical ball). Surprisingly, when the heel was compressed with the device, only 6% of the subjects reported pain; when the penetrometer compressed the distal hallux, just 32% reported pain. This contrasts to compression of the plantar first metatarsophalangeal joint, in which 66% of the subjects reported significant discomfort. The authors state that painful plantar stimulation beneath the metatarsal heads is necessary to initiate a protective muscular response in the digital flexors, which has been proven to distribute pressure over a broader area. In contrast, the heel, which is the initial point of contact while walking, is relatively insensitive to small objects applied with light to moderate force. The plantar heel is also relatively impervious to thermal injury, since the fat pads located beneath the heel and forefoot possess 4-times the polyunsaturated fat of regular adipose tissue, which, due to its lower freezing point and viscosity, prevents against heat loss to the environment and dissipates shock even at subzero temperatures (2).
In a study comparing lifelong shod feet with the feet of people who have never worn shoes, D’Aout et al. (3) confirm that the unshod forefoot is 16% wider than the shod forefoot. The increased width allows for improved distribution of ground-reactive forces during the propulsive period of barefoot walking. In their analysis of plantar pressures centered beneath the forefoot in lifelong shod versus unshod individuals, the authors confirm that regular shoe use is associated with significantly higher peak pressures beneath the second and third metatarsal heads. This is consistent with an analysis of skeletal remains dating back 100,000 years, confirming metatarsal pathology is more severe in shod populations (4). To enhance protection against perforation, the skin of an unshod foot becomes extremely tough and is remarkably similar to leather. These features allowed the feet of our earliest ancestors to easily manage the stresses associated with moving around sub-Saharan Africa. Surprisingly, our unshod feet could even handle the extremely cold temperatures and jagged mountainous terrain associated with traversing Eurasia, as evidence suggests that we did not begin routinely using protective footwear until 30,000 years ago. This means that for 80,000 years following our exodus from Africa, we crossed the Swiss and Italian Alps and quickly spread through the harsh climates of Europe and Asia without protective shoe wear. Determining the exact date that we began routinely using shoes has been difficult, as the early shoes were made of leather, grass and other biodegradable materials that left no fossil evidence. Although Neanderthals and Homo Erectus were suspected of occasionally using insulated foot coverings, the first direct evidence of shoe use dates back only 3,500 years, to a leather shoe found in an Armenian cave (Fig. 7.1). While primitive sandals and moccasins discovered in Oregon and Missouri have been carbon-dated to 10,000 years ago, the actual time period that our ancestors first introduced protective shoe wear remains a mystery. To get around the fact that ancient shoes rapidly decayed leaving no evidence of use, Trinkaus and Shang (5) decided to date the initiation of shoe wear by searching for changes in the diaphyseal diameter of the second through fourth proximal pedal phalanges in our early ancestors. Because shoe use lessens strain on the digital flexors, the authors theorized that habitual shoe use would 333
Human Locomotion: The Conservative Management of Gait-Related Disorders The Greeks also prized their shoe wear. Although the first Olympic athletes competed barefoot, the average individual routinely wore ornamental sandals. Analysis of Greek art reveals that shoes and sandals were used as status symbols to identify the social status of the wearer (this was also true of the Egyptians). While the Egyptians were credited with adding the first heel to a sandal, it was the Greeks that developed the first high heel shoe, and it is suggested that Greek prostitutes wore the elevated heels because the heavy heel produced a clicking sound that announced their presence to potential clients. The trend for wearing high heels was only temporary and did not come back into fashion until the reign of Queen Elizabeth, when women wore platform heels as high as 24 inches. Because of the frequency of serious falls while wearing the excessively elevated heel, they quickly fell out of favor. Surprisingly, high heels became popular among French men in the 1700s, as Louis XIV was shorter than average and favored wearing 3-inch heels that flared out at the bottom. This type of heel is still around today and is known as the Louis heel. Despite evidence that regular use leads to the development of forefoot arthritis (6), high heel shoes remain popular. Although the affluent Greeks and Egyptians had separate shoes/sandals made for their right and left feet, the practice of wearing different shoes on each foot was short-lived, and throughout the Dark and Middle Ages, shoes were made to be worn on either foot. Improvements in manufacturing techniques before the American Civil War changed that. By modifying a duplicating lathe used to mass produce wooden gunstocks, a Philadelphia shoemaker was able to manufacture mirror-image lasts for
Figure 7.1. The earliest shoes resembled stitched leather bags. Drawn from a photograph in Pinhasi et al. (36).
be associated with the sudden appearance of a thinning of the proximal phalanges. By precisely measuring all aspects of phalangeal shape and composition, the authors discovered a marked decrease in the robusticity of these bones during the late Pleistocene era, approximately 30,000 years ago (Fig. 7.2). Because there was no change in overall limb robusticity, the anatomical inference is that shoe gear resulted in reduced strain on the long and short digital flexors, eventually resulting in the development of narrower proximal phalanges. The authors state that because there is no evidence of a meaningful reduction in biomechanical loads placed on human lower limbs during the late Pleistocene era (e.g., reduced foraging distances), the only logical conclusion is that the slender phalanges could only have resulted from the use of shoes. The authors evaluated numerous skeletal remains from different periods and concluded that based on the sudden reduction in diaphyseal diameter of the second through fourth proximal phalanges, the use of footwear was habitual sometime between 28,000 and 32,000 years ago. The first shoes were most likely similar to the shoes discovered in the Armenian cave, in that they were simple leather bags partially filled with grass to insulate the foot from cold surfaces. Because shoe gear varied depending on the region, the earliest shoes worn in tropical environments were most likely similar to the 3,000-year-old sandals recently found in Israel. Once discovered, use of protective shoe wear quickly spread. The early Egyptians were believed to be the first civilization to create a rigid sandal, which was originally made from woven papyrus leaves molded in wet sand. Affluent citizens even decorated their sandals with expensive jewels.
Figure 7.2. Dorsal view of the proximal phalanges from the early (bottom row) and late (top row) Pleistocene era. Trinkaus and Shang (5) claim that the decreased strain on the toes associated with regular shoe use produced bony remodeling with a gradual narrowing of the proximal phalanges (compare A and B).
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Chapter Seven Shoe Gear upper comparable to the Converse All-Star. The first orthopedic sneaker was developed by New Balance shortly before the Great Depression. This company continues to be the world’s largest manufacturer of sneakers made with different widths. The German shoemaker Adi Dassler formed Adidas in the 1930’s, while his brother Rudi formed Puma in the 1940’s. Adidas was the more popular company and was the dominant manufacturer of sneakers until the 1960’s, when Phil Knight and Bill Bowerman created Blue Ribbon Sports. Renamed Nike Inc. in 1978, after the Greek goddess of victory, this company has remained the world’s largest producer of sneakers and sporting apparel for more than 40 years, with 2009 revenues exceeding $19 billion (8). The design of the earliest sneaker was simple: a thin rubber sole was covered with a canvas upper, providing nominal cushioning and protection. In contrast, modern sneakers are made with synthetic leather or mesh uppers, foam midsoles and synthetic rubber outsoles to resist abrasion and improve traction (Fig. 7.4). Because runners usually make initial ground contact with the lateral heel, this area is often reinforced with a durable synthetic carbon rubber. The upper, in addition to providing space for the toes, also possesses an elaborate lacing system that has the ability to modify motion (Fig. 7.5). In their detailed analysis of foot motion and pressure distribution in runners wearing the same type of sneaker tightened with different lacing techniques, Hagen and Hennig (9) demonstrate that the high 7-eyelet lacing pattern secured with moderate
the production of separate shoes for each foot. Using this new technology, the Union Army supplied over 500,000 soldiers with matching pairs of right and left leather shoes. The basic components of a leather shoe are illustrated in figure 7.3. Leather continued to be the most popular material used for making shoe gear until the 1890s, when Charles Goodyear accidentally dropped rubber into heated sulfur creating vulcanized rubber. Prior to his serendipitous discovery, rubber was a relatively useless material. The newfound resiliency of this material would have numerous applications, including the production of the first sneaker. Although alternate names for the new foot wear include tennis shoes, trainers and runners, the term sneaker remains the most popular, and its origin can be traced back to an 1887 quote from The Boston Journal of Education (7): “It is only the harassed schoolmaster who can fully appreciate the pertinency of the name boys give to tennis shoes-sneakers.” Apparently, the soft rubber soles allowed schoolchildren to quietly sneak up on unsuspecting teachers. Spalding manufactured one of the earliest sneakers: the Converse All-Star. Used by athletes at Springfield College to play the newly invented game of basketball, the sneaker was immediately popular. Since their introduction in 1908, more that 70 million pairs of Converse sneakers have been sold worldwide. In 1916, the U.S. Rubber Company (currently named Uniroyal) introduced Keds, a sneaker made with a flexible rubber bottom and canvas
Figure 7.3. Components of a well-made leather shoe. The heel counter should fit securely and the bisection of the shoe should be vertical to the supporting surface. Poor quality control often allows for an asymmetrical heel counter that is either inverted or everted relative to the table top; see A. In addition, the shank should be able to resist forceful compression without deforming (B), and it should be angled in such a way that when the heel seat is compressed (C), the plantar forefoot lifts no more than a few millimeters (D). The toe box should provide ample space so as not to compress a dorsomedial or lateral bunion. Because it allows for greater separation of the upper, Blucher lacing (E) may be necessary to accommodate the midfoot in individual’s with high arches.
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Human Locomotion: The Conservative Management of Gait-Related Disorders by its weight (it is the heaviest midsole material), and by its tendency to turn yellow over time. Ethylene vinyl acetate (EVA) is another common material. Despite its tendency to rapidly deform with repeated impacts, EVA is often used in entry level sneakers because it is inexpensive to produce. Other hybrid materials have recently been incorporated into midsoles, such as Phylon, which is made from EVA pellets heated and cooled in a mold, and Phylite, a combination of Phylon and rubber. Both of these materials can be injection-molded and easily shaped. Because Phylite is durable enough to be used without an outsole, it makes for an extremely light and flexible sneaker. As demonstrated by Frederick (10), the firmness or durometer of a midsole affects its ability to alter motion since soft midsoles allow for excessive pronation while firm midsoles discourage pronation. Because softer midsoles improve shock absorption while the firmer midsoles more effectively control motion, it has become standard practice for manufacturers to combine a softer lateral midsole with a firmer medial midsole. Referred to as a duodensity midsole, the softer material on the lateral side softens impact forces and decreases the initial velocity of pronation, while the firmer material on the medial side provides protection against excessive pronation. The duodensity midsole essentially creates a functional rearfoot varus post that lessens the amount of rearfoot pronation following heel strike. Because early research on sneaker design focused on static models, the first midsoles were designed with excessive medial and lateral flaring. Although these flares provided stability during static stance, the lateral flare was troublesome during the gait cycle since it struck the ground prematurely, supplying ground-reactive force with a longer lever arm for producing subtalar joint pronation (11) (Fig. 7.6). Conversely, a negative lateral flare effectively shortens the length of the lever arm (X’ in Fig. 7.6B), lessening the range and speed of initial pronation (11). The same biomechanical principles associated with lateral flares also applies to the negative posterior flare, since it shortens the lever arm between the ankle joint axis and the ground, lessening the velocity and range of initial ankle plantarflexion following heel strike (Fig. 7.7). This modification should be considered in all recreational and competitive walkers as a method of reducing strain on the anterior compartment musculature. In contrast to large lateral flares, medial flares effectively reduce the range of pronation by acting as a physical barrier that blocks excessive motion (12). This is also true of sneakers that have extra midsole material placed directly beneath the medial longitudinal arch (13). While the rearfoot of the midsole may be modified with various positive and negative flares, the forefoot of the midsole may also be modified by adding different degrees of toe-spring (Fig. 7.8). This modification, which represents a superior angulation of the distal aspect of the midsole,
Figure 7.4. The modern sneaker. Although every manufacturer has proprietary differences in construction, the typical sneaker is manufactured with a carbon rubber outsole (A), a foam midsole (B) and a nylon mesh upper (C).
Figure 7.5. Variation in lacing patterns. (A) Standard 6-eyelet lacing, which may be tightened various degrees;(B) low lacing, in which only the first and second eyelets are tightened; (C) alternate lacing of the first, third and fifth eyelets; (D) high lacing with all seven eyelets used. In this lacing pattern, the laces are pulled from outside the sixth to the seventh eyelet on the same side, and then to the resulting loop formed between the sixth and seventh eyelet on the opposite side. Redrawn from Hagen and Hennig (9).
tension produced significant reductions in peak pressure beneath the heel and lateral forefoot, along with reduced loading rates and pronation velocities. In contrast to the very low lacing and the tightly laced 6-eyelet technique, the participants in the study found the moderate tension 7-eyelet lacing technique to be very comfortable. The authors claim that because this technique creates a firm foot-to-shoe coupling that lessens loading rates and rearfoot pronation velocity, the firm 7-eyelet lacing pattern may play an important role in reducing the risk of lower extremity injury. Although lacing may favorably modify impact forces and pronation velocity, the most functional portion of a sneaker is the midsole, which is made from a variety of foams to enhance shock absorption and durability. Polyurethane (PU) is the most resilient of these materials, because it provides maximum resistance against compression without breaking down. It can be identified 336
Chapter Seven Shoe Gear
Figure 7.8. Many running shoes have a superior angulation of the distal midsole known as a toe-spring (arrow) that lessens the degree of digital dorsiflexion necessary for the propulsive period.
Figure 7.6. Although the overall range of pronation will remain unchanged, a large lateral flare (A) provides ground-reactive forces with a longer lever arm (X) for pronating the subtalar joint at heel strike. This feature produces significant increases in the initial range and velocity of pronation. Note that a midsole with a negative flare (B) provides ground-reactive forces with a shorter lever arm (X’) for pronating the subtalar joint.
effectively shortens the functional length of the shoe while also allowing the metatarsophaÂlangeal joint to move through a lessened range of motion during propulsion. This midsole design is invaluable in the treatment of Achilles tenÂdinitis, plantar fasciitis, metatarsal stress syndrome, and/ or hallux limitus/rigidus. When treating Achilles tendinitis, the toe-spring should be coupled with a flexible midsole, while hallux limitus responds best to a toe-spring with a stiff midsole; i.e., depending on the midsole material, a sneaker will bend with anywhere from 5 to 50 lbs of force (Fig. 7.9). To allow for the biomechanical requirements associated with different foot types, running sneakers are divided into cushion, stability, and motion control models that are specifically designed for high, neutral and low arched individuals, respectively (Fig. 7.10). As a rule, cushion models are made for people with high arches who require a curve-lasted shape to fit the cavus foot, along with softer, flexible midsole materials to improve shock absorption. In contrast, motion control models possess stiffer, bulkier midsoles that are extended beneath the medial arch. The midsoles in motion control shoes are
Figure 7.9. Evaluating midsole stiffness by grasping the proximal and distal aspects of the sneaker and twisting. Although a stiff midsole may lessen pain in an individual presenting with hallux limitus, it could aggravate an Achilles tendon injury. Figure 7.7. The negative posterior heel flare (star).
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Human Locomotion: The Conservative Management of Gait-Related Disorders
Figure 7.10. Bottom view of the 3 basic types of sneakers. Cushion sneakers (A) are made for individuals with high arches, as they are slightly curved to match the shape of the typical cavus foot and possess flexible midsoles with significantly less bulk in the midfoot region (x). The reduced midsole material in the midfoot gives the shoe an hourglass appearance when viewed from below. Stability sneakers (B) are made for individuals with neutral foot types, as they are straighter and have slightly more midsole material reinforced beneath the arch. In contrast, motion control sneakers (C) are very straight and are strongly reinforced throughout the midfoot with extra-thick midsole material. Because of the additional midsole material, motion control sneakers are extremely stiff.
hypothesis that excessive midsole thickness may dampen sensory input, amplifying the potential for injury (15). In another study evaluating the value of prescribing sneakers according to arch height, Knapik et al. (16) divided 1,400 male and female Marine Corps recruits into two groups: an experimental group in which sneaker recommendation was based on a visual assessment of arch height, and a control group that wore neutral stability sneakers regardless of arch height. After completing an intensive 12-week training regimen, the authors concluded that prescribing sneakers according to arch height was not necessary, since there was no difference in injury rates between the two groups. The preliminary research by Ryan et al. (14) and Knapik et al. (16) should be viewed with caution, because these studies only evaluated pain and lost days from running, which may not be an appropriate method to assess sneaker efficacy. Because so many factors may be responsible for injury (e.g., deconditioning, prior injury, decreased neuromotor coordination, even inappropriate sneaker lacing patterns), a more accurate method to evaluate the functionality of a sneaker is to evaluate kinetics and kinematics to determine if the different types of sneakers actually do what they are supposed to: motion control
usually duodensity and are manufactured with straight lasts to match the shape of the typical pronated foot. Stability sneakers are a blend of the two extremes and are made with semi-curved lasts for people with neutral foot types. Despite the billions of dollars spent annually on sneakers worldwide, there has been little research attempting to determine whether the prescription of sneakers based on foot structure is clinically justified. To determine if the widespread practice of prescribing sneakers according to arch height will lessen the potential for injury, Ryan et al. (14) categorized 81 female runners as supinators, neutral or pronators and then randomly assigned them to wear neutral, stability or motion control sneakers. Although it was assumed that wearing the appropriate sneaker would reduce injury rates, the authors concluded that there was no correlation between foot type, sneaker use, and the frequency of reported pain. In fact, the individuals classified as pronators reported greater levels of pain when wearing the motion control shoes. Overall, regardless of foot type, use of the stability shoe was associated with the fewest missed days from running (51) while use of the motion control running shoe was associated with the greatest number of missed running days (79). This is consistent with Robbins and Hanna’s 338
Chapter Seven Shoe Gear height may be responsible for lessening the potential for injury, individuals should be encouraged to try a range of models to evaluate comfort. Despite their ability to lessen leg fatigue (18), and decrease calcaneal eversion (20), bulky motion control sneakers should not be worn by running athletes, since they may impair proprioception thereby increasing the potential for injury. Obviously, because there is so much variation in foot movement and shape, the most important factor when prescribing shoe wear is that the shoe/sneaker actually fit the person’s foot. Because toe lengths vary, shoe length is determined by matching the widest part of the forefoot to the widest part of the toe box. The last shape should match the person’s foot shape (Fig. 7.11) and, in addition to selecting the appropriate forefoot/rearfoot width, the upper should be roomy enough to accommodate the toes. This is especially true in the presence of claw and hammer toe deformities. One of the most important qualities to look for in a sneaker is that the heel counter securely stabilizes the rearfoot. Besides maintaining the fat pad (which protects the plantar calcaneus from injury), a well-formed heel counter has the ability to lessen musculoskeletal transients, decrease activity in the quadriceps and triceps surae musculature, and reduce VO2 (21). It is also possible to accommodate injured tissues and/or unusual foot shapes by altering a standard shoe/sneaker with various balances and/or modifications (Fig. 7.12). In part due to the failure of mainstream sneaker manufacturers to deviate from their standard models, a growing industry of alternative shoe gear has appeared. The original alternative shoe was the Earth Shoe. Based on the observation that footprints left in the sand are formed with the heel being lower than the forefoot, Earth Shoes were designed to accommodate this pattern by making the
sneakers should limit motion and cushion trainers should attenuate shock. To test the biomechanical attributes of different sneakers, Butler et al. (17) quantified arch height (using the arch height index) and evaluated instantaneous loading rates, tibial accelerations, peak eversion range and peak eversion velocities as high and low-arched runners were randomly assigned to wear cushioned trainers and motion control shoes. Their detailed kinetic and kinematic analysis confirmed that motion control sneakers do in fact control rearfoot motion better than cushioned trainers, and cushioned trainers attenuate shock better than motion control sneakers. In a study evaluating the effect of motion control versus neutral shoes on overpronators, Cheung and Ng (18) strapped surface electromyography sensors to the anterior and lateral compartment muscles as subjects ran 10 km. The authors noted that when wearing motion control shoes, pronated runners had reduced levels of fatigue in tibialis anterior and peroneus longus. They state that the motion control shoe “may facilitate a more stable activation pattern and higher fatigue resistance” in overpronators, which the authors claim may reduce the risk of overuse injuries. In a separate study of excessive supinators, Wegener et al. (19) evaluated plantar pressures and comfort when high arched individuals wore either cushioned sneakers or a control shoe. The authors confirm the cushioned running shoes more effectively distribute pressure and were perceived as being more comfortable than the control sneaker. In fact, the cushioned running shoes reduced peak pressure by 17% and pressure at the forefoot was reduced by 6%. The results of the previously listed studies suggest that the practice of prescribing sneakers based on arch height has merit, particularly for people on the far ends of the arch height spectrum. Because factors other than arch
Figure 7.11. Straight and curve-lasted sneakers. The last refers to the foot-shaped high-density polyethylene mold that a sneaker or shoe is constructed around. A straight-lasted shoe is well-aligned in the forefoot and rearfoot and is recommended for individuals with rectus foot types (A). On the contrary, curve-lasted shoes are angled medially at the forefoot and should be used only for individuals with metatarsus adductus (B). The inadvertent use of a curve-lasted shoe by an individual with a metatarsus rectus most often results in a painful adventitious bursa forming over the dorsolateral fifth metatarsal head.
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Figure 7.12. Shoe/sneaker modifications. (A) By supporting the metatarsal necks, a Thomas bar may decrease pressure beneath the metatarsal heads. (B) A Schuster heel wedge is useful when heel lifts greater than 7 mm are required. (C-E) Decompression pads may be used to distribute pressure away from a variety of bony prominences (including dorsomedial and lateral bunions and Haglund’s deformity). (F) A wing-heel may be added to reinforce the medial heel while a varus wedge may also be incorporated into the heel itself (G). (Note that external modifications of shoes are not as effective at controlling motion as in-shoe orthoses [37].) (H) An overly flexible shank may be reinforced with a filler material while the addition of a rocker-bottom (I) allows the patient to proceed through the propulsive period without bending the metatarsophalangeal joints. (This modification is often essential when treating hallux rigidus deformities.) The final modification requires making cuts in the sole of the shoe in order to encourage a high or low gear push-off (J) A high gear push-off should be encouraged in an individual with a rigid forefoot valgus and recalcitrant interdigital neuritis while a low gear push-off should be encouraged in an individual with a hallux abductovalgus deformity associated with excessive propulsive period pronation.
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Chapter Seven Shoe Gear ankle and arch while walking and running by designing sneakers with various energy storage devices built into the midsoles. The early models tended to place springs beneath the heel with the belief that energy absorbed during early stance could be returned during propulsion. Unfortunately, the heel springs rapidly compressed and returned to their original shape during early stance, making them useless for returning energy during the propulsive period. To return energy during the latter half of stance phase, newer energy return sneakers have added bendable materials to the midsole beneath the forefoot, which theoretically allow the stiffer midsole to store and return energy during the propulsive period. Although early research with carbon/graphite insoles suggests it is feasible to store and return energy (22), the stiffness of the graphite material may cause problems in other areas, because it may limit dorsiflexion of the toes, reducing strain on the flexor digitorum brevis muscle. This could have consequences as it might lessen the ability of this muscle to distribute pressure and/or protect the metatarsal shafts against bending forces. Other models have added materials to store energy beneath the midfoot, which when worn by a midfoot striker, allows for the storage and return of energy. The only difficulty with this method is that in order to take advantage of the stored energy, the individual must stiffen their lower extremity during early stance, and pushoff is achieved with a bouncing action through the knee (comparable to walking on a trampoline). The obvious downside to this approach is that it may lessen the natural ability to store energy in the Achilles tendon, as the ankle range of motion is significantly reduced. Of all the new alternative sneaker models, the ones designed to mimic barefoot activity have received the most attention, e.g., the Vibram 5-finger, (Fig. 7.14) and the Newton. The theory behind these shoes is that by sensing surface irregularities, cutaneous receptors in the plantar feet initiate reflexive contraction of the long and short digital flexors, effectively distributing pressure over a broader area. The extreme flexibility of the midsoles also encourages powerful contraction of the digital flexors to decelerate dorsiflexion of the toes during the propulsive period. In addition to strengthening the arch, regular use of a barefoot running shoe will lessen vertical forces traveling through the lower extremity because the person usually shifts to a mid or forefoot strike pattern, allowing the gastrocnemius and soleus muscles to dampen impact forces. Although proponents of barefoot running often take on evangelical overtones (23), the concept of reducing midsole thickness has merit. As demonstrated by Lieberman et al. (24), the switch to a more forward contact point while running barefoot increases ankle compliance during impact, essentially “decreasing the effective mass of the body as it collides with the ground.� This is consistent with several studies (25-27) confirming that forefoot strike
sole beneath the forefoot thicker, creating a negative heel. Possibly because this shoe produced so many Achilles tendon injuries, it was quickly taken off the market. The logic of accommodating a footprint left in sand is questionable since the resultant footprint has a lower heel because the calcaneus is rounder and narrower than the forefoot, causing it to sink deeper into the sand. Allowing the heel to drop below the level of the forefoot greatly increases strain on the plantar fascia and Achilles tendon. Additional alternative shoes/sneakers include models such as MBT (an acronym for Masai Barefoot Technology) and Skechers. These sneakers incorporate negative posterior heel flares and modified rocker bottoms, which lessen strain on the anterior compartment muscles during early stance and reduce the range of digital dorsiflexion during late stance (Fig. 7.13). The resultant gait pattern is often reported to be subjectively more comfortable, causing manufacturers to claim that these shoes are helpful in the management of a range of lower extremity and low back disorders. To evaluate these claims, Buchecker et al. (33) compared muscle activity and lower extremity joint loading as 10 overweight males walked at a self-selected pace wearing either MBT sneakers or conventional shoes. Compared to the conventional shoe gear, the MBT sneakers reduced the peak tibiofemoral adduction moment during the early stance phase, appreciably diminishing medial compartment loads at the knee without overloading the hip or ankle joints. The only potential drawback to the MBT sneaker was that it increased vastus lateralis/gluteus medius coactivation during midstance and propulsion, as these muscles fired with greater intensities during the latter half of stance phase. Despite the increase in muscle activity (which may lessen once the individual becomes accustomed to wearing the sneaker), the reduced adduction moment at the knee may be helpful when managing medial knee joint osteoarthritis, especially in obese individuals with greater than 5° genu varum. To improve metabolic efficiency, a few sneaker manufacturers have attempted to take advantage of the natural storage and return of energy that occurs in the
Figure 7.13. The MBT sneaker.
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Human Locomotion: The Conservative Management of Gait-Related Disorders of the calcaneal fat pad for absorbing shock (30), every effort should be made to protect this important structure, including not running barefoot on asphalt. A final factor associated with barefoot running is that the early hominids, like all lifelong barefoot populations, had different foot architecture than the average shod modern human. Most notably, lifelong barefoot populations have wider forefeet (3) and higher arches (31) that allow them to dissipate force more evenly through a stable bony architecture. If an individual with a narrow forefoot were to attempt running barefoot on asphalt, the increased groundreactive forces would be distributed over a smaller area, possibly resulting in the development of metatarsal stress fractures and/or interdigital neuritis. Furthermore, the low arches often present in lifelong shod individuals are poorly designed to manage the valgus torques associated with barefoot running, since low arches place more strain on the muscular restraining mechanisms on the medial side of the leg (34,35). This explains why low arched people transitioning into barefoot running frequently complain of abductor hallucis myositis and/or tibialis posterior tenosynovitis. Despite the potential for medial ankle and/or fat pad injury, barefoot running is an effective way to strengthen the digital flexors and serves as an excellent gait retraining tool because it encourages the individual to switch to a more forward contact point while simultaneously shortening their stride length. Although almost always associated with a reduced running speed, these simple gait alterations markedly lessen impact forces in the proximal structures, making them invaluable when treating recurrent tibial stress fractures and various knee, hip and low back disorders. When worn recreationally while walking or slow jogging, these sneakers favorably stimulate the digital flexors without overloading them, possibly resulting in an increased arch height when worn regularly. After an appropriate break-in period, running athletes should consider doing speed workouts on grass or soft dirt as a way to increase tone in the digital flexors. Despite their questionable value for improving performance, these sneakers have revived interest in the theories initially presented by Robbins et al. (1,15,32), who in the 1980s cautioned that excessive midsole cushioning was unnecessary and potentially hazardous. Whether or not the complete removal of sneaker midsoles prevents or produces lower extremity injuries remains to be seen. Because the modern foot has had little time to adapt to the stresses associated with impulsive running on asphalt, and because excessive support may result in weakness and impaired proprioception, the ideal shoe, in addition to fitting all aspects of the individual’s foot, will most likely provide just enough support to reduce the exaggerated impact forces associated with modern surfaces without limiting the degree of sensory stimulation necessary to elicit an appropriate motor response.
Figure 7.14. The Vibram 5-finger sneaker is designed to mimic barefoot activity.
patterns reduce retropatellar pressure by as much as 50%, which can be invaluable when treating a wide range of the lower extremity injuries. The problem with barefoot running is that the reduced impact loading of the proximal structures comes at a price, since the resultant forefoot/midfoot strike pattern increases ankle dorsiflexion velocity, rearfoot eversion (excursion and velocity) and increases Achilles tendon stress during the contact period when running (26,27). This greatly strains the medial and posterior compartment musculature, potentially increasing the risk of injury. The increased flexibility of the midsole may also play a role in the development of plantar fasciitis, as the flexible midsole may allow for a more rapid dorsiflexion of the digits during propulsion, which is a proven predictor of this injury (28). If flexor digitorum brevis lacks the strength to decelerate digital dorsiflexion, tensile strain is transferred from this important muscle into the plantar fascia. Another concern with barefoot running is that when it is done for years on modern materials such as asphalt, it could damage the plantar fat pads. This is especially true at higher speeds of impulsive running, as vertical forces may reach 5-times body weight. Because barefoot running results in a 60% deformation of the calcaneal heel pad (29), compared to the 35% deformation when running with conventional sneakers, lifelong barefoot running on hard surfaces may result in plastic deformity of the walls of the fat pad, limiting their ability to protect the calcaneus from trauma. Because almost all lifelong barefoot individuals walk/run on natural surfaces such as dirt and grass, it is unknown whether barefoot running on asphalt will permanently damage the fat pads (both the heel and forefoot fat pads may be traumatized). Since atrophy of the fat pad may result in chronic pain and because no synthetic material comes close to matching the ability 342
Chapter Seven Shoe Gear 19.
References: 1. 2. 3.
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