How minimalist (“barefoot”) shoes cause so many injuries (originally posted April 2013- revised May 2016) 

SUMMARY

All shoes designed for distance running, whether traditional, minimalist or somewhere in between, are inherently injurious because they all incorporate a shoe sole that is attached to the foot which attenuates plantar sensory feedback.[1-4] This feedback is used to moderate aggregate load (peak ground reaction force), localized overloading of foot structures and to maintain optimal stable equilibrium. In blind testing, aggregate loading varies between shoes through differences in behavior that can amplify impact in relation to differences between shoes in instability they produce.[5-8] However, the greatest differences between injury incidence between shoes is accounted for by relative "vigilance" when they are used. Vigilance is behavior that moderates impact in response to fear of injury. It is attenuated strongly by deceptive advertising suggesting protection, and indirect deception caused by shoe cost and novel technology which has been associated with deceptive advertising that suggests greater protection to users.

THE NEUROPHYSIOLOGY

The neurophysiology of these processes are understood. Footwear attenuate mainly plantar skin shear, which combined with localized vertical loading is the adequate stimulus for SA II mechanoreceptors of the plantar surface.[9]  SA II afferent information is used to moderate aggregate load. Fragile foot structures are also protected from damage from excessive loading though differences in avoidance in response to plantar surface location. Fragile structures have a low pain threshold. This directs intense load the robust calcaneus, moderate load the durable distal digits and near total avoidance of the fragile metatarsal-phalangeal joints. [5-7] This explains the reported reduced damage to the metatarsal-phalangeal joints in habitually barefoot humans, and also less plantar fasciitis in this group, since plantar fascia tension is reduced via arch support by muscle through plantar-flexing the digits to avoid loading of the metatarsal-phalangeal joints. Distance runners are injured following the neuropathic model, however footwear cause attenuated sensory feedback rather than a disease state.

VIGILANCE

Vigilance is considered here to be behavior in response to fear of injury that moderates plantar loading under conditions when plantar sensory feedback is inadequate, such as when wearing shoes or with disease states that cause peripheral neuropathies involving the plantar surface. Vigilance was shown to exist in an experiment in which barefoot subjects landed from a 4.5 cm high platform onto a bare force platform, or that platform covered with identical sole material made to appear different and deceptively advertised as either from "expensive shoes" or "cheap shoes", and of "old technology" or "latest  technology." Impact varied significantly between conditions. It was lowest when no interface, highest with the “expensive shoe” and "latest" technology interfaces, intermediate with the “cheap shoe” and old technology interface. This indicates that humans elicit behavior that can protect when when they fear injury when sensory information is limited, although vigilance protects less well than the bare foot in direct contact with the substrate. Deceptive advertising inferring protection has a strong influence on vigilance.This explains why expensive running shoes, which are advertised as offering greater protection and novel technology than cheap ones, are particularly injurious,[13] This also explains why Vibram FiveFingers^TM shoes, until recently perhaps the most deceptively advertised shoe in the history of footwear, caused ten times more injuries than more traditional running shoes.[14] Also why in another report a novel and expensive semi-minimalist shoe made by NIKE accounted for twice the injury rate of Vibram FiveFingers^TM shoes, and why both were much more injurious than traditional shoes. Although the presentation of shoes to subjects was never mentioned by the authors of this report, the NIKE shoe had to be presented to subjects as more advanced protection than the then venerable Vibram shoe. This must have resulted in subjects feeling that they were more protected in the novel NIKE shoe than the Vibram one. Subjects probably thought that both protected better than traditional running shoes.

SHOE DESIGN RELATED TO INJURY LOCATION

Within the tactile sensory poor environment inherent to footwear, shoe design variables affect location of injuries. Minimalist shoes typically are designed with essentially no midsole or heel flare, so when the foot is dosi-flexed, the posterior aspect of the calcaneus which is beyond the protection afforded by the fat pad of the calcaneus experiences high amplitude painful focal loading. This induces wearers of minimalist shoes to avoid foot dorsi-flexion at contact. This can only be accomplished by making foot contact with the foot in the sagittal plane near parallel to the support surface. This also requires a shorter stride length than wearers of traditional shoes. In this way wearers of minimalist somewhat resemble barefoot runners. The shorter stride length accounts for a lower incidence of patello-femoral pain syndrome in minimalist shoe wearers compared to wearers of traditional shoes due to less knee extension when running, though more than with barefoot runners who have greater sensory feedback through no interface between the posterior calcaneus and support surface..

However, because plantar sensory feedback of minimalist shoes resembles that of traditional shoes, users of minimalist shoes are directed to apply forefoot load to the metatarsal-phalangeal joints rather than the protected distal digits in the case of barefoot runners. Since minimalist shoes have no midsole to diffuse this impact compared to traditional shoes, fragile metatarsal-phalangeal joints tend to be severely overloaded when minimalist shoes are worn. This accounts for the remarkably high incidence of injuries to these structures in wearers of minimalist shoes. Unlike shod runners, the barefoot runner is well protected by a low pain threshold at the metatarsal-phalangeal joints which directs forefoot load to the distal digits which are protected with fat pads.

In summary, users of minimalist shoes are spared knee disorders when running, but less so than barefoot runners. They are subject to far more injuries to the region near the metatarsal-phalangeal joints than users of traditional shoes because while both overload this region, traditional shoes offer some resilient midsole to diffuse this load, thereby reducing risk of overloading injuries. Despite these shoe differences in injury location and perhaps incidence, the highest rates of injuries are undated to shoe differences. They are accounted for deceptively advertising shoes in a way that suggests protection. This attenuates user vigilance.

REFERENCES

[1] Robbins, S.E., Hanna, A., Jones, L.A. (1988) Sensory attenuation induced by modern athletic footwear. (1988) Journal of Testing and Evaluation, 16(4), 412-416.

[7] Robbins, S.E., Waked, E., Gouw, G.J., & McClaren, J. (1994). Athletic footwear affect balance in men. British Journal of Sports Medicine, 28(2), 117–122.

[8] Robbins, S.E., Waked, E., McClaren, J. (1992 ) Shoe Sole Thickness and Hardness Influence Balance in Older Men. Journal of the American Geriatrics Society, 40(11), 1089-1094.

[9] Robbins, S.E., Waked, E., McClaren, J. (1995) Propriocepion and stability: Foot position awareness as a function of age and footwear. Age and Ageing, 24, 67-72.

[10] Pollard JP, Le Quesne LP, Tappin JW. Forces under the foot.  J Biomed Eng 1983: 5; 37-40

[11] Inglis JT, Kennedy PM, Wells C, Chua R. The role of cutaneous receptors in the foot. Adv Exp Med Bio 2002: 508; 111-117 

[12] Kennedy PM, Inglis JT. Distribution and behaviour of glabrous cutaneous receptors in the human foot sole. J Physiol: 2002: 38; 995-1002

[13] Zipfel B, Berger L. The emergence of forefoot pathology in modern humans? Foot 2007 17:205-213. 

[14] Marti B. Relationship between running injuries and running shoes – Results of a study of 5000 participants of a 16 km run – The May 1984 Berne “Grand Prix”. In Segesser B, Pforringer W, eds. The shoe in sport, Chicago: Year Book Medical Publishers, 1989: 256-65.

[15] Ridge S, Johnson A, Mitchell, U et al. Foot bone marrow edema after 10 week transition to minimalist running shoes. Med Sci Sports Exerc 2013.

[16]  Ryan M, Elashi M, Newsham-West R, Taunton J. Examining injury risk and pain perception in runners using minimialist footwear. Br J Sports Med 2014: 48; 1257-62