Minimal shoes are defined as those that have less cushioning

IRENE S. DAVIS, PT, PhD 1,2 The Re-emergence of the Minimal Running Shoe Minimal shoes are defined as those that have less cushioning and stability than traditional running shoes. While the minimal shoe is often considered a new trend in running footwear, it has actually been around for a very long time. It has only been in the past 50 years that footwear has progressively evolved into the modern-day, elevated cushioned-heel, motion-control shoe. While such changes were purported to assist in reducing running injuries, these injuries have persisted at a high rate. It has been suggested that this modern-day footwear has significantly changed the way we run and may contribute, in part, to the high rate of running injuries. This clinical commentary describes the various aspects of the minimal shoe, including the history of running footwear leading to its re-emergence and how it differs from the traditional running shoe in regard to biomechanics and injury potential. Although this clinical commentary is not intended to be a comprehensive review of shod-running mechanics, it TTSYNOPSIS: The running shoe has gone through significant changes since its inception. The purpose of this paper is to review these changes, the majority of which have occurred over the past 50 years. Running footwear began as very minimal, then evolved to become highly cushioned and supportive. However, over the past 5 years, there has been a reversal of this trend, with runners seeking more minimal shoes that allow their feet more natural motion. This abrupt shift toward footwear without cushioning and support has led to reports of injuries associated with minimal footwear. In response to this, the running footwear industry shifted again toward the development of will review the biomechanical studies that have compared minimal shoes to barefoot and elevated cushioned-heel shoes. In addition, it will discuss the progressive increase in running injuries since the 1970s, as well as review the injuries associated with different types of minimal shoes. The importance of proper transitioning into minimal shoes will be underscored, and studies relating to transitioning to minimal shoes will be presented. Finally, the application of minimal footwear beyond running will be discussed, and areas of future research will be suggested. lightweight, partial minimal shoes that offer some support and cushioning. In this paper, studies comparing the mechanics between running in minimal, partial minimal, and traditional shoes are reviewed. The implications for injuries in all 3 conditions are examined. The use of minimal footwear in other populations besides runners is discussed. Finally, areas for future research into minimal footwear are suggested. J Orthop Sports Phys Ther 2014;44(10):775-784. Epub 11 September 2014. doi:10.2519/jospt.2014.5521 TTKEY WORDS: biomechanics, footwear, running injuries, transitioning A BRIEF HISTORY OF FOOTWEAR Humans have been running essentially barefoot for the majority of their evolutionary history. Homo erectus, emerging nearly 2 million years ago, is credited with being the first endurance runner. 4 Homo erectus may have initially covered his feet for protection, using materials found in the environment. The oldest shoes recovered were found under layers of volcanic ash in Fort Rock, OR and radiocarbon dated to 10 000 years old. 6 These sandals were fashioned from sagebrush bark woven into a flat surface, and had rope ties to secure them to the sole of the foot (FIGURE 1). It is clear that the purpose of this shoe was simply to protect the plantar surface of the foot from the elements. Shoes have undergone many changes over the past 10 000 years. However, athletic shoes emerged in the early 1800s, and their evolution was described in The Sneaker Book by Vanderbilt. 35 In 1832, rubber soles were added to the bottom of the leather shoes for greater durability. The next evolution in athletic footwear was the development of vulcanized rubber, perfected in 1868, which allowed for the formation of a flexible sole with a tread. This sole was attached to a canvas upper, and Keds (mixture of kids and peds ) were introduced in 1916 by the manufacturer Goodyear (FIGURE 2A). They were advertised as sneakers, as 1 Department of Physical Medicine and Rehabilitation, Harvard Medical School, Boston, MA. 2 Spaulding National Running Center, Cambridge, MA. The author certifies that she has no affiliations with or financial involvement in any organization or entity with a direct financial interest in the subject matter or materials discussed in the article. Address correspondence to Dr Irene S. Davis, Spaulding National Running Center, 1575 Cambridge Street, Cambridge, MA 02138. E-mail: isdavis@partners.org t Copyright 2014 Journal of Orthopaedic & Sports Physical Therapy journal of orthopaedic & sports physical therapy volume 44 number 10 october 2014 775

FIGURE 1. First documented footwear, made of sagebrush bark, found in the Fort Rock caves in Oregon. These shoes date back 10 000 years. Copyright University of Oregon Museum of Natural and Cultural History, photograph by Jack Liu. they were quiet and allowed one to sneak up on someone. In 1895, J.W. Foster and Sons (now Reebok) added spikes under the forefoot of the athletic shoe and developed the first running shoes (FIGURE 2B). The father of the modern running shoe is Adolf Dassler, who in 1926 introduced the first customized running shoe. This shoe had customized spike patterns and lengths, depending on whether it was to be used for sprinting or distance running. Dassler s shoes were worn by runners such as Jesse Owens in 1936, and their success later led to the formation of Adidas. In 1964, Mr Onitsuka of Japan produced the first pair of cushioned running shoes, the Onitsuka Tiger (FIGURE 3A), which eventually led to the creation of ASICS (Kobe, Japan). 35 These shoes were also marketed in the United States by Bill Bowerman, a University of Oregon track coach, and Phil Knight, one of his disciples. However, Bowerman and Knight soon decided to form their own company, which eventually became Nike. In 1972, they developed their own version of the Tiger that they named the Nike Cortez (Nike, Inc, Beaverton, OR) (FIGURE 3B), which was an instant success. If we fast-forward 40 years, we find that numerous athletic-footwear companies have emerged, including Brooks, New Balance, Reebok, and Saucony, to name a few. 35 Throughout these years, shoes have become increasingly more cushioned and supportive. Jeff Johnson, who worked for Nike between 1965 and 1983 and created the Nike name, was FIGURE 2. Early sports shoes. (A) Keds sneakers that were fashioned from vulcanized rubber and a canvas upper (courtesy of Keds). (B) First running shoes developed by J.W. Foster and Sons (now Reebok) in 1895 (courtesy of Reebok). Adi Dassler (now adidas) later customized the spikes according to the distances to be run in them. FIGURE 3. First cushioned running shoes. (A) Onitsuka Tiger (courtesy of Asics). (B) Nike Cortez. able to shed some light on why this progression occurred (personal communication, March 12, 2014). Jeff began running at 9 years of age in a pair of sneakers and continued to run in high school. He remembers running in thin, solid-rubbersoled shoes with canvas uppers, recalling that his training was of higher intensity and much lower mileage than what is typical today. He and his teammates developed what he referred to as iron legs from running on hard surfaces with these flexible, thin-soled shoes. What we have since learned is that we have an incredible ability to modulate the stiffness of our leg spring according to the surfaces with which we come into contact. 2,8,9 When training on hard surfaces, the leg adapts by increasing its compliance, requiring increased demands on the muscular system. This may be what Jeff refers to as iron legs. One point that Jeff was very clear about was that running injuries were rare at that time. Running Injuries: The Impetus for New Shoe Design There were no reports of injury incidence found in the literature in the 1960s, which may, in fact, be indicative of a low occurrence of running injuries at that time. However, reports of running injuries did begin to emerge in the 1970s, when the running explosion began (FIGURE 4). In fact, a relatively newly minted magazine, titled Runner s World, conducted the first survey of running injuries in 1971 (800 runners) and repeated it again in 1973 (1680 runners). 6 This was followed by an unpublished study conducted by Lloyd Smith and colleagues in 1979 (974 runners) of injuries at a runner s clinic at St Elizabeth s hospital in Massachusetts. 6 These reports demonstrated an increasing percentage of runners sustaining injuries. As is true today, knee injuries were the most common injury reported in these reports. However, the percentage of knee injuries in all runners increased from 17.9% in 1971 to 22.5% in 1973. Knee injuries as a percentage of all injuries have also increased from 30.5% reported in 1979 6 to 42% in 2002, 33 one of the most recent reports. Jeff Johnson recounted that the first reports of injuries in the Runner s World 776 october 2014 volume 44 number 10 journal of orthopaedic & sports physical therapy

Introduction of the cushioned-heel running shoe (Onitsuka Tiger) First runninginjury survey by Runner s World Addition of motion control and more cushioning into shoe Introduction of the Nike Free Born to Run published Emergence of the first published transition programs 1963 1971 1977 2005 2009 2013 1970 1973 1979 2006 2011 Formation of Nike; beginning of the running boom Second runninginjury survey by Runner s World Third running injury survey by Dr Lloyd Smith Introduction of the Vibram Five Fingers shoes; other minimalfootwear companies begin to proliferate First published case report of stress fracture in 2 minimalfootwear runners FIGURE 4. Fifty-year timeline from the beginning of the cushioned-heel shoe to the development of transition programs to minimize injury during transition to minimal shoes. study prompted Nike to collaborate with 3 pioneering sports podiatrists, Dr Steven Subotnick, Dr Harry Hlavick, and Dr Dennis Vixie, who, seeing these injuries, could provide some input into shoe design (personal communication, March 12, 2014). Though there were no empirical data, these 3 prominent clinicians felt that the injuries they were seeing were related to excessive impact and excessive foot motion, in particular, foot pronation. Jeff reports that this was the clear catalyst for the addition of cushioning and motion control in the running shoe. This increasing trend has continued for over 4 decades. With time, running shoes also became differentiated into 3 classifications: cushioning, stability, and motion control (Jeff Johnson, personal communication, March 12, 2014). Cushioning shoes were developed for runners with high-arched, rigid feet that tended to excessively supinate or exhibit reduced pronation. These shoes were relatively flexible, with midsoles of lower durometer, a measure of hardness. Stability shoes were designed with some cushioning and some control and were intended for runners with a neutral foot and normal mechanics. Motion-control shoes were intended for runners with flat feet and were designed to control rearfoot and midfoot pronation. They were constructed with higher-density midsoles and additional stiffening of the heel counter. Throughout these years, there have been countless running-footwear developments, FIGURE 5. (A) Nike Free, the first modern minimal shoe. (B) Flex grooves in the sole allow for maximal flexibility (www.nikeinc.com). such as air soles, torsional springs, kinetic wedges, etc. There has even been a shoe touted as altering its stiffness to a female s menstrual cycle. Despite all of these technological advancements in the running shoe, it has been reported that up to 79% of modern runners are injured in a given year. 36 The purpose of matching footwear to foot type was to assist in reducing injury. The validity of these claims was tested by Knapik and colleagues 13-16 in 3 large, randomized controlled trials conducted in differing branches of the military. In all 3 studies, these authors reported that matching footwear to foot type had no influence on injury. Therefore, one is left to question whether any of the technological developments in running footwear designed for motion control and cushioning have been helpful in reducing injury. The Rebirth of the Minimal Shoe: The Nike Free In the midst of the increasing technology, Nike unveiled the first modern minimal shoe in 2004. The shoe was developed in response to feedback from Nike representatives, who in 2001 observed the Nike-sponsored Stanford University track team doing some of their training barefoot. In fact, the legendary coach Vin Lananna was quoted saying, I believe that athletes that have been training barefoot run faster and have fewer injuries. 21 The goal of the Nike Free was to mimic barefoot running as closely as possible. Jeff Pisciotta and a team from the Nike Sports Research Lab embarked on 3 years of research to better understand the mechanics of barefoot running (Jeff Pisciotta, personal communication, March 3, 2014). This team noted that barefoot runners tended to land with a flatter foot orientation and different pressure distribution under their feet, and exhibited greater hallux dorsiflexion at push-off. They therefore sought to design a shoe that mimicked these characteristics. The Nike Free running shoe represented a revolutionary change and journal of orthopaedic & sports physical therapy volume 44 number 10 october 2014 777

FIGURE 6. Some of the first modern, true minimal shoes with no cushioning and no motion control. (A) Huarache sandals of the Tarahumara indians in the Copper Canyon of Mexico, fashioned from discarded tires and leather laces for tying them onto the foot. (B) Vibram FiveFingers shoes (courtesy of Vibram). (C) LUNA Sandals (Seattle, WA). Courtesy of Ted McDonald. a large departure from traditional running footwear. It was constructed with an extremely flexible upper, including a fully flexible heel counter. Additionally, there was no arch support. The midsole thickness and the heel-to-toe drop were both reduced. Finally, the sole of the shoe had so many deep flex grooves that it allowed the shoe to be rolled up into a ball (FIGURES 5A and 5B). The shoe was designed on a numbering system, with 0 being referenced to completely barefoot and 10 being their most supportive, cushioned shoe, the Nike Air Max 2000. The first shoe they produced was the Nike Free 5.0, which was halfway between these conditions. Pisciotta and colleagues fully realized that this shoe would place greater demands on the foot and lower leg. Therefore, each box of shoes came with instructions on outlining a progressive transition to minimize injury risk. The shoe was initially released only to Nike Town stores and specialty running stores, and only after staff attended educational seminars at Nike headquarters. The shoe received a lot of publicity initially, but quietly fell into the background of Nike s extensive line of running shoes. It eventually became more widely available, and a 3.0 version (less support) and a 7.0 version (more support) were added to the inventory. The Impact of Born to Run 21 While Nike Frees were fading from the limelight, a new revolution in running began. The catalyst for this movement was the 2009 release of the bestselling, epic nonfiction story Born to Run. 21 Author Chris McDougall introduced the world to the Tarahumara (meaning running people ) indians who live in the Copper Canyon of Mexico. He noted that this reclusive group of ultrarunners did not appear to be plagued with the injuries of most modern-day runners. More surprising is that they were able to run high mileage throughout most of their lifetime and without the aid of modern running-shoe technology. Their running shoes were huarache sandals, consisting of a rubber sole fashioned from discarded auto tires, held on with leather laces (FIGURE 6A). They are similar in design to the first sagebrush bark shoes dating back 10 000 years, whose purpose was simply to protect the sole of the foot from the environmental elements. Before the Born to Run 21 explosion, longtime barefoot runner Ted McDonald, aka Barefoot Ted, resided in Burbank, CA and did much of his training in the steep, rocky, and hot foothills of Los Angeles (Ted McDonald, personal communication, February 3, 2014). As a result, he had been experimenting with all types of minimal footwear to protect his feet. These included rope, yucca, and rice-straw sandals. Unfortunately, none of these were durable enough for his running. However, in 2005, Vibram introduced the FiveFingers shoes (Vibram SpA, Albizzate, Italy) (FIGURE 6B). They were invented by Robert Fliri, a design student from Vinschgau, who wanted to figure out a way to move around in nature better. They were originally marketed for yacht racers to maintain grip on slippery decks while maintaining the barefoot feel. Ted contacted the CEO of Vibram to suggest that these shoes might be the ideal minimal running shoe. The rest, as they say, is history. In 2006, Ted headed into the Copper Canyon of Mexico in his Vibram FiveFingers to further his study of the natural selection of footwear in human societies. While he was there, a local Tarahumara indian, Manuel Luna, fashioned him a pair of the huarache sandals out of discarded automobile tires and leather strapping. Little did Manuel know that this would be the impetus for the launch of Ted s company, LUNA, and a line of premium huarache sandals (FIGURE 6C). The Minimal-Shoe Explosion Although there was no empirical evidence, many of those who read Born to Run 21 were convinced that running barefoot or in minimal shoes would be the answer to their running injuries. Research was revealing that most traditionally shod runners land on their heels due to the additional cushioning that cushioned shoes offered. 17,18 There were increasingly more studies suggesting that the repetitive vertical impact loads associated with a heel-strike landing might be detrimental to the musculoskeletal system. 7,23,26,40 Habitually running barefoot, or in shoes that lack cushioning in the heel, promoted more of a forefoot strike pattern, which typically eliminated the vertical impact transient. 16 It appeared that a paradigm shift in running footwear and foot strikes was occurring. This led to the emergence of a number of new footwear companies focusing on minimal 778 october 2014 volume 44 number 10 journal of orthopaedic & sports physical therapy

running shoes, such as Inov-8, VIVO- BAREFOOT (FIGURES 7A and 7B), and Altra. There is also a new line of huarache sandals called the Invisible Shoe, which consists of a flat bed and straps to hold it onto the foot. Additionally, some of the traditional companies began to develop their own lines of minimal shoes, such as the New Balance Minimus and the Merrell foot gloves (Merrell Footwear, Rockford, MI) (FIGURE 7C). Runners have also begun to run in low-cost water shoes such as the Teva Nilch (Deckers Outdoor Corporation, Goleta, CA) (FIGURE 7D). The common characteristics of all of these shoes are no drop between the heel and forefoot, no arch support, no midsole, and either no heel counter or a flexible heel counter. Basically, the shoe provides protection for the sole of the foot and a means for attaching it to the foot. While many consider minimal footwear a new trend, the shoes of Ron Hill, who won the Boston Marathon in 1970 (FIGURE 7E), tell a different story. Hill's shoes were custom made by Reebok and named the World 10 shoe because Ron had broken two 10- mile world records in them. These shoes are more minimal than the majority of the most minimal shoes of today, suggesting that we may be returning to an old trend. In fact, when the minimal shoes recently resurfaced, Ron s response was, Hello, we ve been here before years ago! (personal communication, July 24, 2014). BIOMECHANICS ASSOCIATED WITH MINIMAL SHOES Minimal shoes have been designed to mimic barefoot running as closely as possible. Most of the studies that have incorporated true minimal shoes have found this to be true. For example, Squadrone and Gallozzi 32 compared the biomechanics between barefoot, minimal-footwear (Vibram Classic FiveFingers), and traditional neutralshoe conditions. All participants were experienced barefoot runners. These authors reported that the minimal-footwear condition was similar to barefoot in strike indices, vertical impact peaks, sagittal plane foot and knee angle at touch-down, as well as foot and knee excursions during the support phase. Oxygen consumption was also similar between these conditions. The only difference noted was that stride length was longer when running in the minimal shoe compared to barefoot. However, in the traditional shoe, strike index indicated a more posterior strike pattern, the vertical impact peak was significantly higher, and the foot was in greater dorsiflexion and the knee in less flexion at foot strike compared with both barefoot and minimal-footwear conditions. Oxygen consumption was also significantly higher in the traditional shoe. These results suggest that the minimal shoe used in this study closely mimicked the mechanics of barefoot running and was clearly different from the traditional, more cushioned shoe. It is important to differentiate studies using habitual barefoot or minimalfootwear users compared to novice ones. The majority of habitual barefoot/minimal runners do not land on their heels, 15,16 most likely because it hurts to do so if maintained over a prolonged period. This typically results in lower vertical impact forces and lower rates of loading than when landing in traditional shoes, which are associated with a heel strike in up to 89% of runners (FIGURE 8). 17,18 However, in a study of novice barefoot/minimal-footwear runners, vertical impact peaks and rates of loading were significantly higher in the barefoot and minimal-footwear conditions compared with traditional shoes. 25 These novice runners likely did not, on average, alter their mechanics significantly during the single testing session. What is still unknown is just how long it takes for someone to adapt to the minimal-footwear condition, resulting in a change of mechanics. It has been shown that habitually shod, rearfoot-striking runners who undergo a progressive, 12-week program of running in minimal shoes (Vibram FiveFingers) transition to a pattern that FIGURE 7. The next wave of minimal running shoes. (A) Inov-8 (Inoveight Ltd, Crook, UK) Bare 200x (with permission from Inoveight Ltd); (B) VIVOBAREFOOT (London, UK) Aqua Lites (http://www.vivobarefoot. com); (C) Merrell foot gloves (courtesy of Merrell); (D) Teva Nilch (water shoes that can be used for running) (with permission from Teva). (E) Shoes worn by Ron Hill, who won the Boston Marathon in 1970 (courtesy of Amby Burfoot). Note that these shoes are more minimal than many of the minimal shoes of today. journal of orthopaedic & sports physical therapy volume 44 number 10 october 2014 779

FIGURE 8. The vertical ground reaction force during running in traditional running shoes (left), minimal shoes (middle), and barefoot (right). Note the similarity between the minimal-shoe and barefoot conditions and their difference (additional impact peak) from the traditional-shoe condition. Abbreviation: BW, body weight. is similar to barefoot running. 20 This includes a plantar-flexed ankle at foot strike, shorter ground contact, and reduced knee flexion excursion. A related study demonstrated a significant reduction in heel pressures following a 4-week training program in Vibram FiveFingers. 38 Both of these programs incorporated a foot and ankle strengthening program as well. These studies indicate that, with time, individuals do adopt a more anterior strike pattern. More studies are needed to determine the exact time of this transition and the permanence of these changes. CAUTION: INJURIES ASSOCIATED WITH MINIMAL FOOTWEAR Along with the new movement in minimal shoes have come reports of injuries. The first report in the literature was by Giuliani et al, 11 which described 2 runners with second metatarsal stress fractures following transition to Vibram FiveFingers minimal shoes. However, both of these runners incorporated this minimal footwear into their running programs without any alteration in their running volume or any preparation of their foot and ankle musculature. Salzler et al 29 reported a case series of 10 experienced runners who had been injured during their transition to minimal footwear. All runners had been injury free for 1 year prior to the transition. Nine of the 10 injuries were stress fractures and 8 of these were to the metatarsals. The majority of these runners transitioned very quickly (average, 0.7 months) and most injuries occurred within the first few months of minimal-footwear running. This indicates that these injuries might be predominantly from poor transitioning, as opposed to the minimal footwear itself. Ridge et al 27 randomized 17 runners into a traditional shoe group and 19 runners into a minimal-footwear group (Vibram FiveFingers). The traditional-shoe runners continued with their typical training mileage. The minimal-footwear group was provided very liberal guidelines for transitioning into the minimal shoes, and was not given any exercises for foot strengthening. Magnetic resonance images were taken at baseline and following a 10-week transitioning period. Ten of the 19 minimal-footwear runners exhibited bony edema in their feet consistent with a stress injury (marrow edema score between 2 and 4, with 4 being a stress fracture). Interestingly, the majority of these cases were subclinical (ie, the subject had no symptoms). Most of these were, again, to the metatarsals. Two were diagnosed stress fractures, with the remaining diagnosed as stress reactions. It is unclear whether those classified with a marrow edema score of 2 (out of 4) would have progressed to stress fracture or were indicative of remodeling. However, this transition program was admittedly loosely structured and might have allowed runners to transition too quickly. Collectively, these injury reports underscore the need for a well-controlled, slow transition to minimal footwear. Unsupervised transition programs should be conservatively designed to increase minimal-footwear running very slowly. The transition program proposed by Warden et al 37 for returning to run from a stress fracture is a very reasonable one that can be easily adapted for transitioning to minimal footwear (FIGURE 9). To follow this program, one must be able to work up to walking briskly for 30 minutes in minimal footwear. The 30 minutes are then divided into three 10-minute intervals. In each of these 10-minute intervals, walking is slowly replaced with running. Rest days are provided and runners are guided to progress only if they are pain free at the current level. Therefore, the transition is customized to the runner. Running with less shoe support places additional load on the foot. If progressed gradually, this has been shown to increase the strength of some of the foot and ankle musculature. 5 However, if the muscles are not adequately conditioned, they are likely to fatigue quickly, and muscle fatigue has been shown to increase strain of the bones that the muscles support. 22 Therefore, to further protect against injury, a foot-strengthening program should accompany any transition to minimal footwear. This should include calf stretching and strengthening (FIGURES 10A and 10B), as minimal-footwear running is associated with more of a forefoot strike pattern, which loads the calf musculature. To strengthen the intrinsic muscles of the foot, towel curls and toe abduction 780 october 2014 volume 44 number 10 journal of orthopaedic & sports physical therapy

(spreading) and adduction (squeezing) exercises should be included (FIGURE 10C). To address the muscles that support the arch, foot doming should be performed by pressing the toes down into the ground and raising the arch (FIGURE 10D). The importance of foot doming has been highlighted in the study by Jung et al, 12 demonstrating the significant increase in activity of the abductor hallucis when performing foot-doming exercises. The abductor hallucis has been shown to play an important role in the maintenance of the medial longitudinal arch during static stance. 10 Others 24 have shown that training with foot-doming exercises results in a significant reduction in navicular drop, an indication of less arch collapse. REGRESSION TO CUSHIONING AND SUPPORT: THE PARTIAL MINIMAL SHOE Graduated Loading Program Stage 0 Stage 1 Stage 2 Stage 3 Stage 4 Stage 5 Pre-entry to graduated loading program Pain during walking in normal activities of daily living Walk and jog stage (2 days loading, 1 day rest) Level A Walk 30 min B Walk 9 min and jog 1 min ( 3) C Rest D Walk 8 min and jog 2 min ( 3) E Walk 7 min and jog 3 min ( 3) F Rest G Walk 6 min and jog 4 min ( 3) H Walk 5 min and jog 5 min ( 3) I Rest J Walk 4 min and jog 6 min ( 3) K Walk 3 min and jog 7 min ( 3) L Rest M Walk 2 min and jog 8 min ( 3) N Walk 1 min and jog 9 min ( 3) O Rest Jogging every second day Level A Jog 12 min B Rest C Jog 15 min D Rest E Jog 15 min F Rest G Jog 17 min H Rest I Jog 17 min J Rest K Jog 20 min L Rest M Jog 20 min Jogging (4 loading days in 1 week) Level A Jog 25 min B Rest C Jog 25 min D Rest E Jog 30 min F Jog 30 min G Rest Individualized running (5 loading days in 1 week) Level A Running B Running C Rest D Running E Running F Rest G Running Return to activity (individualized) FIGURE 9. Graduated loading program for transitioning from walking 30 minutes in minimal footwear to running normal mileage. Reproduced with permission from Warden et al. 37 Copyright 2009 Elsevier. It is likely in response to these transition injury reports that shoe companies have begun to produce partial minimal shoes. These are shoes that may have a midsole, some heel counter, some arch support, and a mild heel-totoe drop, but all to a lesser degree than traditional running shoes. A comparison of these footwear characteristics between traditional, partial minimal, and minimal shoes is provided in the TABLE. Partial minimal shoes are touted as a safer compromise to the true minimal shoes. The problem is that, despite having less midsole, these shoes have enough cushioning that runners still tend to land on their heels and experience impacts. For example, Bonacci and colleagues 3 reported that running mechanics in partial minimal shoes (Nike Lunar Racer, Nike Free) were similar to running in traditional running shoes (Nike Air Pegasus), but statistically different from running barefoot. For example, stride length as well as ankle and knee kinematics and kinetics were generally similar between all shod conditions, but different from barefoot. This suggests that running in the partial minimal shoes produced a similar pattern to the traditional shoe, but a different one from the barefoot condition. In another study, Willy and Davis 39 reported that runners landed harder and with more dorsiflexion when running in a partial minimal shoe (Nike Free) compared with a traditional shoe. Further highlighting the potential danger of partial minimal shoes, a recent prospective study reported that runners in partial minimal shoes sustained more injuries than those running in true minimal or traditional shoes. 28 These studies together should raise caution regarding the use of par- journal of orthopaedic & sports physical therapy volume 44 number 10 october 2014 781

tial minimal shoes. Runners appear to continue to heel strike in these shoes, yet there is less cushioning to protect them. The culmination of the research to date also suggests that if a runner s goal is to adopt barefoot-like running mechanics, he or she should transition slowly, and do so with true minimal shoes. Alternately, if runners are going to maintain a rearfoot strike pattern, they need to have adequate cushioning under their heel, and they need to replace their shoes when the cushioning becomes compromised. THE APPLICATION OF MINIMAL SHOES BEYOND RUNNING There have been a number of studies recently suggesting that shoes for the elderly should also be more minimal. A study of women between the ages of 55 and 75 years determined that performance on the forward reach test, the timed up-and-go test, and the 10-meter walk test was better when walking barefoot or in flexible, flat, thinsoled shoes compared to standard heeled shoes. 1 These minimal-type shoes appear to be better for individuals with knee osteoarthritis (OA) as well. Shakoor et al 30 reported that the knee loads associated with the development and progression of knee OA are reduced when walking in thin-soled, flexible shoes compared to either a control stability walking shoe or a self-chosen walking shoe in individuals with knee OA. This same research group 31 has reported similar results when walking barefoot, in flip-flops, or in thinsoled shoes as compared to clogs and standard heeled shoes. Finally, there is a prospective randomized controlled trial in 56 elderly women with knee OA that is currently under way. 34 These authors have provided thin-soled, flexible, minimal shoes to half of the women, whereas the remainder will continue to wear their own shoes. Measures of pain, function, and mechanics will be recorded at baseline and at 6 months. Based on their prior work, these authors hypothesize FIGURE 10. Recommended foot and ankle exercises to prepare for minimal-footwear running. (A) Calf stretches off a step; (B) calf raises off a step; (C) foot intrinsic exercises, including towel curls and toe spreading and toe squeezing; and (D) foot doming, pressing down with the toes while raising the arch. significant improvements in all variables in the intervention group only. These types of studies are needed to determine if the concept of minimal footwear can be extended to populations other than runners. Limitations This clinical commentary presents a historical perspective on how running footwear has evolved from very minimal to very cushioned and supportive, then reverted to minimal again. As some of this 782 october 2014 volume 44 number 10 journal of orthopaedic & sports physical therapy

TABLE history has not been documented in the literature, personal accounts were solicited from key individuals in the running community over the past 50 years. These accounts are subject to bias and therefore limit the strength of this evidence. Additionally, a focus of the commentary was the relationship between footwear and injury as it relates to biomechanics. However, it must be acknowledged that other factors not examined in this article, such as fitness level, running experience, age, culture, and environmental changes, might also have played a role in injuries. SUMMARY It appears that the footwear pendulum has swung from the far left (our barefoot/minimal footwear beginnings) to the far right (heavily cushioned and supportive shoes) and back to the far left (barefoot/minimal footwear), and is now trying to find a settling point. Current running research suggests that the mechanics of running in partial minimal shoes are similar to running in traditional shoes, but with less cushioning. Available studies to date suggest that true minimal shoes are needed to promote our natural barefoot running style. However, this style of running places greater demands on the foot and ankle, and transition injuries of the foot and lower leg have been reported. Therefore, a transition must be undertaken slowly and carefully to allow proper adaptation. This is no different from taking on any new physical activity that loads the body in a new way. Characteristics of Traditional, Partial Minimal, and Minimal Running Footwear Characteristics Traditional Partial Minimal Minimal Arch support Moderate Minimal None Midsole thickness, mm Heel 24-32 12-24 0 Toe 14-22 10-17 0 Heel-to-toe drop, mm >12 4-12 0 Heel counter Stiff Semi-flexible Flexible/none Sole Stiff Semi-flexible Flexible Unfortunately, simply putting a different type of shoe on does not automatically trigger a need to reduce training and has resulted in injuries. Research has also suggested that running in true minimal shoes results in landing with a flat or slightly plantar-flexed foot, with less impact. As these impacts have been associated with common running-related injuries, running in this manner with minimal shoes may reduce injury risk. However, prospective randomized controlled studies are needed to determine if injuries are actually reduced with this footwear. Additionally, research is needed to determine the best way to transition safely to reduce the incidence of injuries during the transition period. Until then, runners should err on the side of caution by strengthening their foot and lower leg and progressing their mileage slowly as they transition to minimal footwear. t ACKNOWLEDGEMENTS: The author would like to acknowledge Erin Futrell, PT for her assistance with the literature review for this article. REFERENCES 1. Bhatia A, Kalra S. Footwear effects on balance and gait in elderly women of Indian population between the ages 55 and 75 years. Indian J Physiother Occup Ther. 2011;5:1-4. 2. Bishop M, Fiolkowski P, Conrad B, Brunt D, Horodyski M. Athletic footwear, leg stiffness, and running kinematics. J Athl Train. 2006;41:387-392. 3. Bonacci J, Saunders PU, Hicks A, Rantalainen T, Vicenzino BG, Spratford W. Running in a minimalist and lightweight shoe is not the same as running barefoot: a biomechanical study. 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The JOSPT Reference Library can be found at: http://www.jospt.org/page/authors/author_reviewer_tools 784 october 2014 volume 44 number 10 journal of orthopaedic & sports physical therapy