Running has become an increasingly popular and efficient way to achieve fitness and promote long-term exercise. Running and jogging participation in the United States has increased 10.3% in the past 2 years, totaling 35.5 million, according to the National Sporting Goods Association (www.nsga.org/files/public/2010_Participation_Alphabetically_4Web_100521.pdf). Participation varies from recreational to competitive, with race distances ranging from the 5K to the marathon. Other people may participate in running for fun or as a functional part of their lives or occupations (43).
Footwear has evolved considerably over the years humans have been running. Early humans either went barefoot or wore protective and insulating foot coverings in the form of sandals or moccasins (42). Advances in footwear offered improved traction and performance and eventually provided support and cushioning for the foot. Changes in construction methods and the availability of new materials allowed for improved breathability, comfort, and durability (42). The current selection of running shoes offers a vast array of stability and cushioning features from numerous shoe brands.
Despite the advances in shoe technology providing for increased cushioning and motion control, there has been a recent movement promoting running barefoot or in light “minimalist” shoes. Advocates of barefoot running believe that returning to the way our primal ancestors ran may result in fewer running-related injuries. The Barefoot Runners Society, founded in the United States, has nearly 2000 members internationally and is growing annually. Barefoot running has been the topic of numerous books, journal and magazine articles, as well as news reports. The purpose of this article is to discuss the biomechanical differences between barefoot and shod (wearing shoes) running and to present a preparatory exercise program for the runner interested in transitioning from a traditional running shoe to the barefoot style. Focus will be placed on preparing the lower extremity for the demands required by the biomechanics of barefoot running. The proposed benefits and risks to barefoot running will briefly be discussed as will an appraisal of the available evidence.
RUNNING GAIT CYCLE
Running gait is comprised of 2 basic periods: stance and swing (8). Stance begins when the foot is in contact with the ground, whereas the swing phase begins as the foot moves into toe-off and prepares to leave the ground. Stance makes up approximately 40% of the cycle and swing comprises the other 60% (7). Running gait is characterized by single-leg support and double-leg float periods. During walking, however, one foot is always in contact with the ground. The impact landing of one foot from an unsupported position during running results in transmission of forces as much as 5 times the body weight throughout the lower limb (3). The lower extremity must control and absorb these impact forces efficiently to avoid potential injury.
The stance period of running gait can further be divided into initial contact, midstance, and toe-off. From initial contact to midstance, the lower extremity actively decelerates the forward-swinging leg and passively absorbs the shock of ground reaction forces. In midstance, the foot makes full contact with the ground and body weight begins shifting from the rearfoot to the forefoot. From midstance to toe-off, there is a relative lengthening of the lower extremity with concentric muscle contraction of the hip and knee extensors to prepare the foot for the propulsive push-off, in which the weight is shifted to the toes and the foot leaves the ground (3). The swing period of gait can be further divided into initial swing, midswing, and terminal swing. During initial swing and midswing, the foot advances forward in the air and in terminal swing positions itself for heel strike and weight acceptance.
Running gait has been described as a spring-mass system of the leg in which the joints of the lower extremity lower the center of mass and absorb energy much like a spring compresses. This occurs during the stance phase of running. The energy absorption is quickly followed by energy generation as the limb moves into extension, similar to the recoil of a spring, allowing for propulsion during the toe-off phase (10,11). The longitudinal arch of the foot has been described as an “impact dampening structure” during the loading (stance) phase of the gait cycle (32). With each foot strike, the lower limb endures significant impact force to the musculoskeletal structures. The impact at landing is created through collision of the shoe, foot, and lower leg mass. Ground contact style and cadence also affect the impact imposed on the lower extremity at landing. The way in which a runner absorbs and generates energy at each foot strike differs in barefoot and shod running because of variations in biomechanics. Knowledge of these key differences will aid the strength and conditioning professional in preparing runners interested in transitioning to barefoot running.
BIOMECHANICAL DIFFERENCES BETWEEN BAREFOOT AND SHOD RUNNING
One primary difference between running barefoot and in shoes is noted at the foot during the initial contact phase. The barefoot running technique uses a midfoot to forefoot striking pattern when compared with a rearfoot heel strike pattern of the shod runner (Figure 1) (7,21,36). This foot striking position results in a shorter stride length and a higher step frequency (cadence) in barefoot runners. These stride differences may possibly reduce initial impact forces by allowing higher preactivation of plantar flexors before braking at impact (9). The higher preactivation of the gastrocnemius and soleus decreases impact forces by anticipating the shock with landing. The foot switches to a forefoot strike and allows for the ankle plantar flexors to eccentrically lower the body in a more controlled manner (9). Lower peak torques at the hip, knee, and ankle have also been reported in barefoot versus shod running, most prominently at the hip and knee (2). Ultimately, barefoot runners demonstrate decreased ground contact time, flight time, and stride duration because of the higher cadence (2,7,21,36). This increased cadence reduces step length, produces less vertical excursion of the center of mass, and reduces braking impulse and impact transient forces. In addition, an increased step rate of less than 10% does not alter metabolic costs and reduces impact load on the body because of the reduced vertical center of mass velocity at landing (16).
The flatter foot placement of the barefoot style at contact results from a larger plantar flexion range of motion at the ankle. This causes a more vertical position of the lower leg and results in a larger amount of knee flexion to soften the impact load (7). An overall greater joint excursion at the ankle has also been identified when barefoot, suggesting that the ankle absorbs impact as well (36). The flatter foot position also limits pressure at the heel, where sensation of mechanical inputs and pain is well established in the foot sole (7). It should be noted, however, that calcaneal and tibial movement patterns do not differ substantially between barefoot and shod running despite the increased range of motion seen at the ankle (37).
Another key difference found when running barefoot versus in shoes involves the proprioceptive ability of the foot. Barefoot running allows for direct contact with the ground and for increased proprioceptive feedback. The glabrous epithelium of the plantar surface of the foot is equipped to withstand potential abrasive injuries when barefoot because of its higher pain threshold and ability for sensory feedback (31). It has been demonstrated that approximately 600% greater abrading loads are required to reach pain threshold in the plantar skin of the foot when compared with hairy skin of the thigh (26). The sensory feedback from the sole of the foot activates a series of muscle contractions in the intrinsic foot musculature that allows for shock absorption and diminishes impact transmission (32). A well-trained foot disperses pressure to a wider area, functionally avoiding injury. Barefoot running removes the external passive support of a shoe and replaces it with internal active support by the foot musculature. However, untrained foot muscles rely heavily on the support provided by a shoe.
Running in shoes, however, offers several advantages that barefoot running does not. The shoe functions to protect the plantar surface of the foot from harmful terrain, extreme weather conditions, and infectious agents. Additional functions of the shoe include providing for motion control, cushioning, stabilization, shock distribution and traction between foot and the ground (5,26). These shoe design factors aid in decreasing the high impact forces of a rearfoot heel strike at initial contact (5,26). The wearing of shoes and shoe inserts while running has also been associated with reduced impact loading rate and reduced latency between the maximum external force and internal forces of the lumbar musculature (27). Thus, going barefoot may result in both an increase in puncture wounds to the foot as well as overuse of muscles, tendons, and ligaments throughout the lower extremity and low back. The additional cumulative loading that results from the increased step rate and forefoot striking pattern when barefoot could also be considered as a potential source for injury, such as metatarsal strains and stress fractures (22). Although case reports cannot be generalized, 2 cases of metatarsal stress fracture have been documented in runners who have adopted training in footwear simulating the barefoot condition (14).
Running performance may be impacted by the wearing of shoes versus running barefoot. Heart rate and relative perceived exertion have been found to be significantly lower in the barefoot condition (15). When running barefoot over ground or on a treadmill, the associated oxygen cost has been found to be 5.7% lower than while running shod (15). It has been found that at 70% of V[Combining Dot Above]o2max pace barefoot running is more economical than running shod, both overground and on a treadmill (15). Additional studies have found maximum oxygen uptake values to be 1.3% lower when running barefoot than when running in shoes (36). More than a 10% increase in step rate has been associated with an increased relative perceived exertion; however, no significant increase in oxygen consumption or heart rate ensued (16). These findings suggest that running barefoot is more efficient than shod running. Future research is needed to determine the effects of barefoot running on competitive performance.
A shod runner may first opt to run in a “minimalist” shoe before making the full transition to barefoot running. A popular minimalist shoe that has been studied in the literature is the Vibram FiveFinger (Vibram SpA, Albizzate, Italy). Research reports that the minimalist shoe may offer similar biomechanics as running barefoot, including a forefoot striking pattern, lower ground contact time, higher step rate, and lower peak impact forces compared with the traditional running shoe (36). The minimalist shoe effectively mimics barefoot conditions while providing small amount of protection, yet still sits between foot and the ground and may desensitize and weaken the foot intrinsics (36). The use of minimalist shoes, however, has been considered a possible causative factor for stress injury to the metatarsals. This may be because of the need for gait alterations from a heel strike pattern to a midfoot striking pattern when running in the minimalist shoes (14). Nevertheless, use of the minimalist shoe may prove to be useful in the overall transition to barefoot running.
It should be noted that not all runners may be candidates for the barefoot running technique. Numerous anatomical factors have been associated with running injury, including cavus (high-arch) foot, leg length discrepancy, and muscle weakness (4,20,25,38). Specifically, weakness of the hip abductors and hip flexors has been associated with running-related injury, including iliotibial band syndrome (12,25). Structural abnormalities in the lower extremity may lead to biomechanical problems during the running gait cycle. Additionally, runners with diminished sensation in the foot as seen in peripheral neuropathy are not good candidates for barefoot activity because of the loss of protective sensation. A thorough evaluation of lower extremity strength and gait biomechanics should be conducted before transitioning to the barefoot style of running. Careful preparation and a gradual pace should be implemented when transitioning a runner to the barefoot technique.
PREPARATORY PROGRAM BASICS
Various sources have presented transition to barefoot running programs. Certainly, the transition should be gradual and over a period of no less than 4–8 weeks because muscular adaptation to training accounting for strength gains requires this period (23,33). In addition to strengthening exercises for core and hip musculature, an evidence-based preparation program should consist of activities and exercises that target the key biomechanical differences the barefoot runner will experience when compared with being shod (Table 1). These key differences include: plantar sensitivity adaptation, foot strike pattern and related changes in stride rate and length, lower extremity proprioceptive ability, ankle joint flexibility, intrinsic foot strength, and eccentric strength of the lower limb to control impact forces. Learning the barefoot style, namely, a reduced heel strike, is fundamental in the transition to barefoot running.
PLANTAR SENSITIVITY ADAPTATION
Because of the high concentration of sensory receptors on the plantar surface of the foot, sensitivity adaptation in the form of increased barefoot activity should be the first component of the transition to a barefoot running program. Suggested mechanisms to facilitate the foot's adaptation include increasing total barefoot activity, walking both indoors and outdoors with bare feet, running indoors with bare feet, and eventually running barefoot outdoors on soft surfaces followed by harder surfaces (32). Adaptations to the plantar skin will take 3–4 weeks of barefoot running at 30 minutes daily before an increased velocity in running speed will be tolerated (31).
RUNNING FORM DRILLS
Because the foot strike pattern of the barefoot technique is located more at the forefoot to midfoot, drills should be incorporated to enhance and learn the proper landing techniques and to reinforce the resulting increase in stride frequency with subsequent shorter step length (36). Barefoot running drills done in the grass using a metronome at a 5–10% faster cadence could be beneficial in training a runner for the demand of increased stride rate when barefoot (16). Drills should focus on the increased step frequency combined with a shorter step length while maintaining a forefoot landing (Figure 2). The author recommends aiming for a cadence close to 180 steps per minute in accordance with the high step rate found in the barefoot technique.
LOWER EXTREMITY PROPRIOCEPTIVE EXERCISES
Because of the increased neuromuscular control required by the lower limb in controlling impact forces, proprioceptive exercises should be incorporated into the preparatory transition program. Exercises that have been cited in the literature to improve lower limb proprioception include: ankle range of motion exercises on fixed surfaces followed by wobble board with eyes opened and closed; single-leg stance activities using an ankle disc (Figure 3) (34), balance board (41), or mini-trampoline (19); and static kicks using resistive bands (1) (Figure 4). Performance of these activities with increased weight-bearing through the forefoot should train the foot more specifically for the forefoot loading used in barefoot running.
ANKLE FLEXIBILITY EXERCISES
As an increased ankle joint excursion is required by the barefoot runner, flexibility exercises to improve ankle range of motion should be performed. Traditional calf stretching against a wall or off the edge of a step may be performed (Figure 5). Focus should be placed on maintaining a neutral arch throughout the duration of the stretch. The stretch is typically held for 30 seconds and repeated 3–5 times for each leg. Additionally, proprioceptive neuromuscular facilitation techniques, including contract–relax and agonist-contract stretching, have been found to be a useful training modality for increasing ankle joint range of motion (29).
INTRINSIC FOOT STRENGTHENING EXERCISES
Because of the apparent weakening of the foot intrinsics that occurs in the habitually shod runner, strengthening of these muscles is a critical component of a transitional training program. Traditional exercises, such as towel curls, picking up objects, single-limb balance activities, and the short-foot exercise, have been used to strengthen the intrinsic foot musculature (18,24). The towel curl exercise is used to strengthen the flexor digitorum longus and brevis, lumbricales, and flexor hallucis longus (18). The short-foot exercise, however, has been found to be superior to the traditional toe curl exercise in activating the abductor hallucis, the largest foot intrinsic muscle found most medial within the first layer of the foot intrinsic muscles (Figure 6) (18). This muscle contributes to increased arch height and helps to control pronation when activated. The muscle had been found to be more activated while performing the short-foot exercise in the 1-legged standing position versus seated (18). To effectively perform this exercise, the patient attempts to draw the heads of the metatarsals toward the calcaneus while avoiding extraneous motion. Tactile input can be provided by the clinician and verbal reinforcement to avoid toe curling (Figure 7).
ECCENTRIC STRENGTHENING OF THE LOWER LIMB WITH PLYOMETRICS
As the knee and ankle become more responsible for controlling the impact loading during barefoot striking, lower extremity plyometric exercises should be incorporated into the training program to prepare the lower limb for this activity. Before beginning eccentric lower extremity training, however, the runner should have sufficient strength in the core and hip musculature to provide proximal stability for the distal extremity. Although the recommended plyometric exercises have been studied in shoes, the authors recommend that the activities be done barefoot to better prepare for barefoot running. Beginning these exercises on a mini-trampoline allows for the stretch-shortening cycle mechanism to produce greater maximum leg power and acts to reduce the impact forces on the body during jump training, thus reducing the potential for injury (6). Plyometric training exercises include hops, jumps, bounding in horizontal and vertical planes, squat jumps (Figures 8, 9), split scissor jumps, double-leg bounds, alternate leg bounds (Figure 10), single-leg forward hops, depth jumps, double-leg hurdle jumps, and single-leg hurdle hops. These specific plyometric activities have been found to improve distance running performance (28,35). The athlete may progress to performing these activities on field grass and progressively harder surfaces. General progression guidelines for plyometric activities should be followed while monitoring for muscle soreness and skin integrity of the bare foot.
BAREFOOT RUNNING PROGRAM PROGRESSION
Once a runner has prepared the lower extremity for the demands of barefoot running through preparatory exercises, the runner should be ready to increase mileage while barefoot or in minimalist shoes. Some runners may exclusively run barefoot or in minimalist shoes, whereas others may opt to train barefoot only for certain types of runs and shod for others. Some may choose only to perform running drills barefoot and continue to run in shoes for training runs. Ultimately, the runner will need to decide what his or her goals are for implementing barefoot training.
No studies to date have demonstrated the safest or most effective method for implementing a barefoot running program (17). General recommendations advise for a very gradual increase in barefoot running activity for successful implementation to allow for musculoskeletal and cutaneous adaptation (17). The barefoot running transition program begins with barefoot activity including daily walking and the aforementioned preparatory exercises. The author recommends running no more than a quarter mile to 1 mile every other day during the first week of barefoot running. This may be performed independently or added onto a regular training run. For example, a runner may do 3 miles of shod running on the road and then a quarter mile barefoot on a grassy field. When increasing the training distance, it is recommended to increase barefoot running by no more than 10% per week (Table 2). Should muscles remain sore, mileage should not be increased but rather maintained instead for an additional week. In our experience, sore and tired muscles are to be expected; however, careful attention should be paid to bone, joint, or soft tissue injury because this may signal the presence of injury. A grassy field or a rubberized track may be the preferred surfaces to begin running barefoot or in minimalist shoes. This could then be followed by smooth paved trails and roads while paying careful attention to debris when going barefoot. Patience will be required because it may take months to make the transition to a full-time barefoot runner.
Several key biomechanical differences between barefoot and shod running have been identified in the literature (Table 3). These differences are primarily found during the stance phase of gait and directly impact the step length and step frequency of the running cycle. Running barefoot uses a forefoot to midfoot landing and, thus, creates a shortened step length with resulting increase in step frequency. In contrast, initial contact while shod is at the heel and results in a longer step length and reduced step frequency. Additionally, the proprioceptive ability of the foot is greater when barefoot because the foot makes direct contact with the ground. This may allow the foot musculature to react to the ground impact forces and to control shock absorption. However, the shoe allows for the protection, cushioning, stabilization, and shock absorption that barefoot running does not. The forefoot contact, reduced step length, increased step frequency, and increased proprioception while running barefoot may contribute to reduced impact forces and decreased injury rates in the lower extremity.
Despite the lack of research studies comparing injury rates in barefoot versus shod populations in developed countries, it is proposed that runners using the barefoot running style will encounter less impact-related injuries. Yearly incidence of long-distance running injuries in recreational and competitive runners is high with variability ranging from 19.4 to 79.3% (39). Two of the most recent studies found incidence rates of 54.85 and 59%, more than half of runners (30,40). Injury rates are as high as 90% in runners training for a marathon (13). Higher training mileage per week in male runners and a history of previous running injury have been identified as risk factors for injury (39). Running injuries typically manifest in the lower extremity and can affect the bone, ligaments, tendons, and muscles. Frequently reported running injuries include ankle sprain, plantar fasciitis, tibial stress syndrome/shin splints, iliotibial band tendinitis, Achilles tendinitis, and peripatellar pain (13,39). Runners are particularly interested in learning ways to reduce the possibility of injury. Barefoot activity has been found to spare the plantar fascia from impact forces as the foot intrinsic muscles activate to control impact loads (32). In addition, where shod and unshod populations coexist, the injury rate is higher in the shod population (32). Unshod lifestyles are also associated with a lower frequency of lower extremity osteological pathology, such as bony lesion, osteophyte formation, and fracture (44).
The recent resurgence of barefoot running may be a result of the growing belief that barefoot running is better for the body than using supportive footwear. The expectation is that injury rates will decrease as runners encounter lower impact-related forces when barefoot. To date, most supportive reports for barefoot running have been anecdotal. Future research is indicated to examine the effects of barefoot running on both injury reduction and performance.
Numerous studies demonstrate the profound biomechanical gait differences seen in those running barefoot compared with shod individuals. These differences should be accounted for in preparing a runner for the barefoot style of running. Several studies support the use of barefoot running for the proposed advantages of improved sensory feedback and proprioception and reduced impact forces; however, no evidence exists that these factors result in reduced injuries or improved performance. Some evidence exists for improved foot intrinsic strength in the foot musculature and improved physiological economy when running barefoot, but no evidence is linked to injury reduction or improved performance. Clearly, much more research is needed on barefoot running, especially in the areas relating to injury rates and performance. Although an absence of evidence does not imply an evidence of absence, those individuals involved in exercise prescription must recognize the difference between evidence-based information and that which is based on an ad novitatum premise.
Nevertheless, runners may be curious to experiment with the barefoot style of running for the purported benefits of injury reduction and performance enhancement. Making the transition from shod to barefoot running should be gradual and ideally supervised by a knowledgeable strength and conditioning professional. Carefully selected training exercises, such as those outlined in this article, may prepare the runner for the new demands placed on the barefoot lower extremity and should minimize adverse effects during the transitional period. Sufficient patience and time may be required to adapt to the new style because pain or discomfort may be present due to running in a completely different way. New barefoot runners should be prepared to initially run slower while barefoot because of the change in running form and increased need for attention to the ground. Continued supervision and guidance from the strength and conditioning professional may aid the runner in a successful transition to the barefoot style.
1. Baltaci G, Kohl HW. Does proprioceptive training during knee and ankle rehabilitation improve outcome? Phys Ther Rev 8: 5–16, 2003.
2. Braunstein B, Arampatzis A, Eysel P, Bruggemann GP. Footwear affects the gearing at the ankle and knee joints during running. J Biomech 43: 2120–2125, 2010.
3. Brotzman S, Wilk KE. Clinical Orthopedic Rehabilitation. Philadelphia, PA: Mosby, 2003. pp. 513.
4. Brunet ME, Cook SD, Brinker MR, Dickenson JA. A survey of running injuries in 1501 competitive and recreational runners. J Sports Med Phys Fitness 30: 307–315, 1990.
5. Butler RJ, Hamill J, Davis I. Effect of footwear on high and low arched runners' mechanics during a prolonged run. Gait Posture 26: 219–225, 2007.
6. Crowther RG, Spinks WL, Leicht AS, Spinks CD. Kinematic responses to plyometric exercises conducted on compliant and noncompliant surfaces. J Strength Cond Res 21: 460–465, 2007.
7. De Wit B, De Clercq D, Aerts P. Biomechanical analysis of the stance phase during barefoot and shod running. J Biomech 33: 269–278, 2000.
8. Dicharry J. Kinematics and kinetics of gait: From lab to clinic. Clin Sports Med 29: 347–364, 2010.
9. Divert C, Mornieux G, Baur H, Mayer F, Belli A. Mechanical comparison of barefoot and shod running. Int J Sports Med 26: 593–598, 2005.
10. Farley CT, Glasheen J, McMahon TA. Running springs: Speed and animal size. J Exp Biol 185: 71–86, 1993.
11. Farley CT, Gonzalez O. Leg stiffness and stride frequency in human running. J Biomech 29: 181–186, 1996.
12. Fredericson M, Cookingham CL, Chaudhari AM, Dowdell BC, Oestreicher N, Sahrmann SA. Hip abductor weakness in distance runners with iliotibial band syndrome. Clin J Sport Med 10: 169–175, 2000.
13. Fredericson M, Misra AK. Epidemiology and aetiology of marathon running injuries. Sports Med 37: 437–439, 2007.
14. Giuliani J, Masini B, Alitz C, Owens BD. Barefoot-stimulating footwear associated with metatarsal stress injury in 2 runners. Orthopedics 34: e320–e323, 2011.
15. Hanson NJ, Berg K, Deka P, Meendering JR, Ryan C. Oxygen cost of running barefoot vs. running shod. Int J Sports Med 32: 401–406, 2011.
16. Heiderscheit BC, Chumanov ES, Michalski MP, Wille CM, Ryan MB. Effects of step rate manipulation on joint mechanics during running. Med Sci Sports Exerc 43: 296–302, 2011.
17. Jenkins DW, Cauthon DJ. Barefoot running claims and controversies: A review of the literature. J Am Podiatr Med Assoc 101: 231–246, 2011.
18. Jung DY, Kim MH, Koh EK, Kwon OY, Cynn HS, Lee WH. A comparison in the muscle activity of the abductor hallucis and the medial longitudinal arch angle during toe curl and short foot exercises. Phys Ther Sport 12: 30–35, 2011.
19. Kidgell DJ, Horvath DM, Jackson BM, Seymour PJ. Effect of six weeks of dura disc and mini-trampoline balance training on postural sway in athletes with functional ankle instability. J Strength Cond Res 21: 466–469, 2007.
20. Korpelainen R, Orava S, Karpakka J, Sirra P, Hulkko A. Risk factors for recurrent stress fractures in athletes. Am J Sports Med 29: 304–310, 2001.
21. Lieberman DE, Venkadesan M, Werbel WA, Daoud AI, D'Andrea S, Davis IS, Mang'eni RO, Pitsiladis Y. Foot strike patterns and collision forces in habitually barefoot versus shod runners. Nature 463: 531–535, 2010.
22. Milgrom C, Finestone A, Sharkey N, Hamel A. Metatarsal strains are sufficient to cause fatigue during cyclic overloading. Foot and Ankle 23: 230–235, 2002.
23. Moritani T, deVries HA. Neural factors versus hypertrophy in the time course of muscle strength gain. Am J Phys Med 58: 115–130, 1979.
24. Newsham KR. Strengthening the intrinsic foot muscles. Athl Ther Today 15: 32–35, 2010.
25. Niemuth PE, Johnson RJ, Myers MJ, Thieman TJ. Hip muscle weakness and overuse injuries in recreational runners. Clin J Sport Med 15: 14–21, 2005.
26. Nigg BM, Segesser B. Biomechanical and orthopedic concepts in sport shoe construction. Med Sci Sports Exerc 24: 595–602, 1992.
27. Ogon M, Aleksiev AR, Spratt KF, Pop MH, Saltzman CL. Footwear affects the behavior of low back muscles when jogging. Int J Sports Med 22: 414–419, 2001.
28. Paavolainen L, Häkkinen K, Hamalainen I, Nummela A, Rusko H. Explosive-strength training improves 5-km running time by improving running economy and muscle power. J Appl Physiol 86: 1527–1533, 1999.
29. Rees SS, Murphy AJ, Watsford ML, McLachlan KA, Coutts AJ. Effects of proprioceptive neuromuscular facilitation stretching on stiffness and force-producing characteristics of the ankle in active women. J Strength Cond Res 21: 572–577, 2007.
30. Ristolainen L, Heinonen A, Turunen H, Mannström H, Waller B, Kettunen JA, Kujala UM. Type of sport is related to injury profile: A study on cross country skiers, swimmers, long-distance runners and soccer players. A retrospective 12-month study. Scan J Med Sci Sports 20: 384–393, 2010.
31. Robbins S, Gouw GJ, McClaran J, Waked E. Protective sensation of the plantar aspect of the foot. Foot Ankle 14: 347–352, 1993.
32. Robbins SE, Hanna AM. Running-related injury prevention through barefoot adaptations. Med Sci Sports Exerc 19: 148–156, 1987.
33. Sale DG. Neural adaptation to resistance training. Med Sci Sports Exerc 20: S135–S145, 1988.
34. Sheth P, An K-N, Laskowski ER, Yu B. Ankle disk training influences reaction times of selected muscles in a simulated ankle sprain. Am J Sports Med 25: 538, 1997.
35. Spurrs RW, Murphy AJ, Watsford ML. The effect of plyometric training on distance running performance. Eur J Appl Physiol 89: 1–7, 2003.
36. Squadrone R, Gallozzi C. Biomechanical and physiological comparison of barefoot and two shod conditions in experienced barefoot runners. J Sports Med Phys Fitness 49: 6–13, 2009.
37. Stacoff A, Nigg BM, Reinschmidt C, van den Bogert AJ, Lundberg A. Tibiocalcaneal kinematics of barefoot versus shod running. J Biomech 33: 1387–1395, 2000.
38. Thijs Y, Tiggelen DV, Roosen P, De Clercq D, Witvrouw E. A prospective study on gait-related intrinsic risk factors for patellofemoral pain. Clin J Sport Med 17: 437–445, 2007.
39. van Gent RN, Siem D, van Middelkoop M, van Os AG, Bierma-Zeinstra SMA, Koes BW. Incidence and determinants of lower extremity running injuries in long distance runners: A systematic review. Br J Sports Med 41: 469, 2007.
40. Van Middelkoop M, Kolkman J, Van Ochten J, Bierma-Zeinstra SMA, Koes B. Prevalence and incidence of lower extremity injuries in male marathon runners. Scan J Med Sci Sports 18: 140–144, 2008.
41. Verhagen E, Bahr R, Bouter L, Twisk J, van der Beek A, van Mechelen W. The effect of a proprioceptive balance board training program for the prevention of ankle sprains: A prospective controlled trial. Am J Sports Med 32: 1385, 2004.
42. Werd MB, Knight EL. Athletic Footwear and Orthoses in Sports Medicine. New York, NY: Springer, 2010. pp. 3–4.
43. Wilk BR, Muniz A, Nau S. An evidence-based approach to the orthopaedic physical therapy: Management of functional running injuries. Orthop Phys Ther Pract 22: 213–216, 2010.
44. Zipfel B, Berger LR. Shod versus unshod: The emergence of forefoot pathology in modern humans? Foot 17: 205–213, 2007.