"USATF Footwear vs. Barefoot"
The following paper appears in 'Track Coach' #168, Summer 2004 Barefoot Running: A Natural Step for the Endurance Athlete Dennis G. Driscoll firstname.lastname@example.org Presented to: USA Track & Field Coaching Education August 2003 In accordance with the requirements for Level III coaching certification. "The human foot is a work of art and a masterpiece of engineering." Leonardo Da Vinci Introduction Abebe Bikila raced barefoot to a gold medal in the 1960 Olympic marathon. Herb Elliott, the 1500 m victor at the same Games, ran 17 sub-four-minute miles and was never defeated at either 1500 m or 1 mile. Photographs of Elliott during barefoot training runs twice graced the cover of Sports Illustrated. Two time world cross country champion and former 5K world record holder Zola Budd competed and trained barefoot. The first lady of American distance runners, Doris Brown Heritage, as a youngster enjoyed barefoot ten mile runs on the beaches and forest trails near her home. And the stories of barefoot Kenyans running throughout their homeland are legendary. Running barefoot does not appear to have been detrimental to the development of these elite endurance runners. In fact, it may have been beneficial to their training. Why, then, don’t we see more barefoot endurance runners? In some cases the answer has been legislated. The National Federation of State High School Associations, for example, mandates in its rule book that track & field and cross country athletes wear shoes. Another reason is certainly that runners believe they are better off donning footwear. After all, athletic footwear has been touted as reducing the number of running-related injuries as well as improving performance. It would seem, however, that footwear may interfere with some naturally selected adaptations to the human form. There are some anthropologists who believe man evolved as a diurnal endurance predator. Stories exist of Bushmen relentlessly pursuing and wearing down the much faster zebra and of Navajo Indians doing much the same with pronghorn. The evolution of an anatomy featuring long, tapered limbs with more mass concentrated near the hips, energy conserving spring-like tendons, and the cooling effects of sweat glands working with a nearly hairless body gave us a being that was an adept endurance runner.1,2 It follows that the human foot would also have evolved as a mechanism for efficient endurance propulsion. The purpose of this paper is to investigate some of the scientific studies that have been done in the area of running both barefoot and with footwear. Is there an advantage to training, or racing, barefoot? Or, is the modern running shoe a technological marvel that helps reduce injury and makes us more efficient runners? Biomechanical analysis of the stance phase An analysis of the biomechanics of the stance phase during barefoot running reveals several differences when compared to shod running. Unlike the dorsiflexed ankle of the shod runner, at heel contact the barefoot runner’s ankle is plantar flexed leading to a more horizontal position for the foot. One study described a sole angle difference of 14° for a runner at a velocity of 4.5 m/s with increasing angular differences as velocity increased. A more horizontal foot would have fewer shear forces acting on the heel. Also, maximum pressure on the heel is reduced with a more horizontal foot at touchdown. The lower leg is more vertical for the barefoot runner at this point thanks to a greater knee flexion. Both the increased plantar flexion of the ankle and increased knee flexion occur prior to touchdown suggesting an "actively induced adaptation strategy to barefoot running."3 The initial foot strike is at the posterior end of the heel. Unlike shod running where heel strike typically occurs at the lateral posterior portion of the heel, the barefoot heel strike is centered between the lateral and medial parts of the heel. As the barefoot runner moves forward, the heel smoothly rolls over the center of the calcaneus. The bottom of the calcaneus features a concave tuberosity and bursa which allow for a smooth forward roll of the heel as weight is shifted forward.4 The shod runner, whose foot strike occurs away from the center of the heel, is unable to take advantage of the naturally intended function of this anatomical design. The entire plantar surface of the barefoot heel is in contact with the ground whereas the shod foot makes contact only with the posterior lateral edge of the heel. The greater plantar area covered for the barefoot runner leads to an increased deformation of the fatty heel pad and superior shock absorption. A barefoot runner ambling at 4.5 m/s can expect a maximum heel pad deformation of 60.5% (+/- 5.5%) compared with only 35% (+/- 2.5%) for a shod runner.5 During this initial contact phase, sensory feedback from the glabrous epithelium of the bare foot brings about greater flexibility of the foot. This suppleness helps the foot adapt to irregular ground surfaces and allows it to act as a shock absorber. Much of the shock absorption during this phase occurs through natural pronation and the associated downward deflection of the medial longitudinal arch. Much of this is tempered, or even lost, in the shod foot. Sensory feedback is greatly diminished by the insulating sole of the shoe. The result is a more rigid foot which disables the deflection of the medial longitudinal arch reducing the foot’s ability to moderate impact shock. Arch supports built into most shoes further reduce the ability of the arch to deflect. 6 Additional evidence exists relating sensory feedback the foot receives during impact to intralimb coordination patterns of the lower extremity. Kinematic changes of the lower extremity diminish the quantity of impact forces. The kinematic changes related to intralimb coordination appear to depend on this sensory information. Running shoes, typically designed to decrease the forces on the body at impact, tend to reduce this important sensory feedback.7,8 The early contact phase of barefoot running is characterized by a smaller vertical deceleration distance for the ankle, likely due to the more plantar flexed position of the foot and the lack of a deformable shoe sole. Consequently, eversion of the foot during this phase is reduced in the barefoot condition resulting in less deceleration of the support leg. One study noted a compensation in the form of higher knee flexion velocity immediately after contact which reduced impact loading by lowering the effective mass of the barefoot runner’s support leg.3 At midstance the foot supports the entire weight of the body. For the barefoot runner, this support is confined to a rather small base area formed by the calcaneus, the base of the fifth ray, and the unit formed by the five metatarsal heads. These three sites alone support the body’s superstructure. With this base in its natural position, the muscles, tendons, ligaments, and fascia of the foot can work most efficiently. The elevated heel found on typical training shoes disrupts this natural position of the foot at midstance. The normal weight-bearing function of the fifth ray is unavailable as the midfoot is raised off the ground by the elevated heel. Supplementary support provided by the cuboid is also reduced. This shifts additional weight-bearing responsibilities to the calcaneus and the metatarsal heads.4 Additional effects of the raised heel on the toes, Achilles tendon and calf muscles will be discussed later. Midstance is the time of the greatest ground reaction force. One study found the impact peak in barefoot running to be 14% lower than in shoe conditions.9 This reduction may occur because the impact peak and the end of midstance happen significantly sooner for the barefoot runner. The percentage difference in the amount of time it takes to the end of midstance is directly proportional to the velocity of the runner; a study found barefoot runners moving at 3.5 m/s reached the end of midstance on average 17% sooner than the shod runner, the difference became 22% at 4.5 m/s, and it grew to 24% at 5.5 m/s.3 An additional benefit of the barefoot runner reaching the end of midstance sooner is the related rapid rate of pronation that has been associated with a decreased chance of developing overuse injuries.10 Once peak pronation is reached at a point 40% into the stance phase, the foot begins to supinate about the subtalar joint in order to reach a more neutral position. The transverse tarsal joint locks resulting in a more rigid foot. The now rigid foot is prepared to act as a lever for push-off with the metatarsal heads acting as a fulcrum. As the toes dorsiflex, the plantar fascia tightens locking the metatarsals, deepening the longitudinal medial arch and resupinating the foot. This process, known as the windlass mechanism, provides a stable and centered foot for efficient propulsion. The properly spaced toes of the bare foot grasp the ground while keeping the runner balanced and directing the foot forward. Unfortunately the tapered toe box found in most shoes constricts the toes and prevents their natural spacing. Also, the shoe’s toe spring – the upward tilt of the toe area visible in the side profile of the shoe – lifts the digits of the foot away from their natural flat position impeding their grasping responsibility. The instability caused by inadequate toe spacing and grasping leads to imbalances that generate compensations by the legs and upper body. Inefficient gait and possible injury result.11 Footwear modifies some of the characteristics of the propulsion phase in other ways. Several of these changes are related to the aforementioned elevated heel of the shoe. One byproduct of heel elevation is a shortening of the Achilles tendon and calf muscles. Three of the calf muscles – the posterior tibial, flexor hallucis longus and flexor digitorum longus – play important roles in the function of the arch. As these muscles become shorter, they fail to pull properly on the back of the heel thereby increasing the flattening of the arch. Pronation occurs at a time when the foot should be in a neutral position. The unnatural position of the elevated heel also disrupts the work of some tendons connected to the toes. These tendons, which originate in the lower leg, apply their pull around ankle bones above the heel to hold the toes against the ground while the body passes over them during propulsion. The raised heel leads to an imbalance in the tug of these tendons thereby interfering with efficient propulsion.11 Perhaps the greatest hindering effect of elevated heel is the loss of the involuntary stretch reflex of the Achilles and posterior lower leg muscles. This stretch reflex is designed to aid the forefoot with propulsion, yet it can only be activated if the heel comes close to the ground. The elevated heels of most available footwear, including athletic shoes, prevent this stretch reflex from occurring.11 The result is a loss of propulsive power. The runner’s body is forced to borrow power from other areas – knee, thigh, hips, trunk – to compensate for the sidelined Achilles tendon and calf muscles.12 According to podiatrist William A. Rossi, "ANY shoe with an elevated heel, even a one-inch heel, automatically places the foot at a functional disadvantage."4 Additional implications The barefoot runner can expect reduced knee injury frequency. The association between high- heeled shoes and knee problems has been well documented. Wearing high heels, normal ankle function during gait is disrupted forcing the knee to compensate. Abnormal forces result across the patellofemoral and medial compartments, the sites of typical degenerative joint changes. Additional loads are also placed on the quadriceps muscles and the hip.13,14 Though it is not being suggested that running shoes should be classified as "high-heeled," many training shoes exhibit a sizeable heel. (A difference of approximately 1 inch is easily observed between the posterior center heel and the point on the forefoot closest to the surface on many popular training shoes.) Even these smaller heel heights can be expected to increase knee pressures to some extent. Another reason for potentially fewer knee woes for barefoot runners can be traced to adaptations at the ankle and knee joints. Studies have shown that runners adapt to a running surface by modifying their lower leg stiffness.8,15,16 For the barefoot runner, the changes in lower extremity geometry include a decrease in knee joint stiffness and a corresponding increase in ankle joint stiffness compared to the shod runner.17 This results in the ankle becoming the site of greater impact absorption. For the shod runner, the impact absorption demand on the knee is greater. Considering that up to 30% of all running injuries are related to anterior knee pain, the adoption of barefoot running must be considered as a method for reducing knee injuries in runners.18 Barefoot runners can also expect fewer sprains of the ankle. Ankle sprains are the most common acute injury suffered by athletes. Runners who frequent trails or uneven surfaces may be especially vulnerable to this type of injury. Since nearly all ankle sprains are inversion injuries, it behooves athletes to find ways to improve their lateral stability. The best lateral stability, with mostly reduced inversion, is found in the barefoot condition. This is because a shoe’s sole increases the lever arm thus escalating torque around the subtalar joint during a stumble.19,20 Also, imperfect proprioception can cause the foot to be placed in an awkward position. Compared to being barefoot, foot position awareness has been shown to be 107.5% worse when wearing athletic footwear. The authors of this study believe, "The inescapable conclusion is that footwear use is ultimately responsible for ankle injury."21 One of the most touted benefits of today’s running shoes – the ability to provide shock absorption – also deserves a close look. Modern running footwear is well endowed with cushioning purportedly to reduce impact forces absorbed by the body. However, there exists no scientific study providing evidence that cushioning has a significant effect on in-vivo impact forces. On the other hand, there is evidence that an increase in impact forces is associated with softer shoes.22,23 Combine this evidence with the previously mentioned sensory deprivation aspect of shoe cushioning and the role of athletic footwear as a protective device must be questioned. There is some evidence that barefoot running is less fatiguing than shod running, at least for one important muscle. A recent study looked at the tibialis anterior muscle which is responsible for dorsiflexing the foot before impact. Since the shod foot exhibits more dorsiflexion at touchdown than the bare foot, it is not surprising that electromyographic signals indicated greater muscle activity before heel-strike in the shod condition. Since this muscle activity must conclude quickly to release the forefoot, it is likely to incorporate a high percentage of fast twitch muscle fibers. Given that fast twitch muscle fibers fatigue quicker than slow twitch fibers, this important muscle will undoubtedly fatigue sooner when runners wear shoes.24 Finally, how does running economy compare between the barefoot and shod state? Oxygen consumption has been shown to be 4.7% higher while wearing shoes (approximately 700 g per pair) and running at 12 km/h.20 Reasons for this include the mass of the added footwear requiring additional energy to move the shoes through each stride, energy being absorbed by the shoe’s cushioning, and the energy expense of flexing the sole of the shoe. When these energy drags are combined with the previously detailed loss of a stretch reflex from the lower leg it becomes understandable that barefoot running is more economical. Conclusion Dr. Benno Nigg, founder of the Human Performance Laboratory at the University of Calgary, believes barefoot running reveals the body’s preferred movement pattern. The body’s locomotor system adheres to the same pattern even when shoes, inserts, orthotics, or other interventions are introduced. The neuromuscular system automatically prevents straying from this preferred pattern. Footwear that does not support the natural pattern will have a deleterious effect on a runner’s performance.25,26 Unfortunately most of us are wearing shoes that are not in sync with our body’s preferred movement patterns. The result is that we do not experience normal gait and propulsion. Podiatric surgeon Ray McClanahan offers, "Shoes and their construction have been hypothesized to be the single most important identifiable feature that separates our long distance runners from those who grew up in countries where their feet and legs developed normally."11 In the absence of the "perfect" shoe, barefoot running deserves serious consideration. The likelihood that all shoe-wearing runners will immediately abandon their footwear and take up fulltime barefoot running is remote. Yet increasing the amount of time we run, or walk, barefoot should be beneficial. In their paper on running-related injury prevention, Robbins and Hanna concluded, "The solution to the problem of running-related injuries could be as simple as promoting barefoot activity."6 Resources 1. Carrier David R. "The Energetic Paradox of Human Running and Hominid Evolution." Current Anthropology, 25 (1984): 483. 2. Cook, Gareth. "The Running Animal." Boston Globe, 9 April 2002: C1. 3. De Wit, Brigit, Dirk De Clercq, and Peter Aerts. "Biomechanical analysis of the stance phase during barefoot and shod running." Journal of Biomechanics, 33 (2000): 269-278. 4. Rossi, William A. "Footwear: The Primary Cause of Foot Disorders." Podiatry Management, February 2001: 129-138. 5. De Clercq, D., et al. 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"Changes in EMG signals for the muscle tibialis anterior while running barefoot or with shoes resolved by non-linearly scaled wavelets." Journal of Biomechanics, 36(2003): 1169-1176. 25. Nigg, Benno M. "The Role of Impact Forces and Foot Pronation: A New Paradigm." Clinical Journal of Sports Medicine, 11(2001): 2-9. 26. Wright, Karen. "Watching Your Steps." Scientific American, 283.3(2000): 52+.