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Most injuries associated with running with modern footwear result from excessive repetitive impact. Humans sense impact amplitude poorly, and there is no evidence that behavior is elicited directly in response to it. However, impact during locomotion can vary considerably. I have shown humans respond with intense behavioral responses, that can either amplify or moderate impact. This behavior is a response to both plantar (sole of foot) sensations and sense of stability (sense of falling). Intense plantar stimulation in the form of localized deformations and shear stress results in behavior that lowers impact. Behavior that amplifies impact is a response to a sense of loss of stability during locomotion. Loss of stability in footwear is caused by loss of foot position awareness – the sense of position and orientation of the plantar surface relative to the support surface. Loss of foot position awareness results from resilient shoe materials of modern footwear creating an underdamped condition when rapidly loaded consisting of a series on compressions and rebounds oscillatory in the medial-lateral plane. The resulting behavior caused by this instability probably is directed at finding a more stable support base. There is evidence that maintaining stability takes priority over foot protection if both are challenged simultaneously. This conclusion is based on a report that showed that humans, unlike quadrupeds, do not exhibit a withdrawal reflex to noxious plantar stimuli if applied to the plantar surface just prior to the support phase of locomotion.
From an evolutionary perspective, human locomotion was an activity for designed for safe and efficient movement on naturally deposited ground. Surface protrusions in the form of weathered stones fail to perforate the plantar surface, yet deform the plantar surface (sole of foot) and apply localized shear stress (horizontal load - abrading force) - adequate stimuli for SA II mechanoreceptors and noiceptors with c-fibre afferents. The combination of these loads easily produce sensations up to the level of pain even at amplitudes well below the levels required to damage the plantar surface. These sensations elicit a behavioral response that is directed at minimize discomfort. The behavior consists of transferring loads to less sensible areas across the plantar surface, and reduction of overall vertical impact by such means as increasing hip flexion when landing. Furthermore, a running technique is instituted which reduces abrasion of the plantar surface. Stability is best when barefoot on natural surfaces because information about the position and orientation of the support surface is provided by tactile receptors.
Yet modern man usually performs barefoot activity on outdoor and indoor man made surfaces. With respect to concrete and asphalt, compared to the natural conditions, localized deformations are minimal and shear stress is intense but diffuse. Safety must be less than with the natural system, although there is evidence that locomotion can be reasonably safe. Increase risk compared to natural surfaces may result from higher vertical impact due to lowered plantar sensibility from attenuated plantar surface deformation, resulting in higher risk of foot damage and injury from excessive impact. Lacking plantar surface deformations, there would be less inducement to transfer load to less sensible areas of the plantar surface. It is difficult to produce intense plantar surface pain from diffuse shear stress and minimal plantar deformations, hence there would less tendency to lower impact as a consequence of avoiding plantar discomfort. Stability would be suboptimal but relatively high when barefoot on man made surfaces, with either tactile or muscle receptors used in making foot position judgments. There would be minimal amplified impact as a behavioral response to instability.
Modern shoes, particularly those with soft sole materials, such as most athletic shoes (including “minimalist” shoes), insulate the sole of the plantar surface from tactile (touch) information from plantar receptors. Information about support surface position and orientation (required for stable equilibrium) is now provided by receptors of muscles that actuate the foot and ankle. This is inherently less precise than tactile information and easily become overwhelmed by medial-lateral oscillation caused by high resiliency materials in shoe soles. Instability, caused by shoes results in amplified impact, a behavior that appears to be an intense search for a stable support base in order to avoid falling.
Links to relevant published reports: 1987-1.pdf 1988-1.pdf 1989-1.pdf 1992-1.pdf 1993-1.pdf 1994-1.pdf 1995-1.pdf 1997-1.pdf 1997-4.pdf 1997-5.pdf
All current running shoes, including "minimalist" ones, do not allow localized plantar deformations and shear stress - adequate stimuli of SA II mechanoreceptors and nociceptors with c-fibre afferents. This plantar tactile information is required to elicit behavior that moderates impact, achieves optimal stability and protects the plantar surface when barefoot. Despite this, impact might not be excessive so long as balance was optimal, since instability elicits behavior that amplifies impact. With low tactile information, humans can maintain suboptimal but adequate stable equilibrium though muscle receptor sensing of position and orientation of the foot. Unfortunately, all athletic footwear, and most non-athletic footwear are made with materials that strongly destabilize humans because they incorporate high resiliency sole material. High resiliency is the property of rapid rebound after deformation from compressive load - some manufacturers ironically advertise this as a desirable property of "energy return." This material underfoot transforms the normal highly damped state of the bare foot to an underdamped one consisting of rapid medial-lateral and vertical oscillation, surpassing the muscle receptors ability to provide reliable information about foot position and orientation. This results in both a sense of instability and actual increase risk of falling. Instability elicits an intense behavioral response that includes amplified impact which appears to be an innate reaction to prevent falling through seeking a firm support base. The instability elicited by these materials is a negative function of their thickness and positive function of hardness. Even relatively thin layers of high resiliency material of hardness usually found in shoes is capable of destabilizing humans considerably.
Man made surfaces provide barefoot humans lessened sensory feedback compared to natural surfaces because they lack rigid surface projections that deform the plantar surface. Outdoor surfaces, such as finished concrete and asphalt, provide intense frictional resistance, thereby transmit shear stress to the plantar surface with bare foot interaction, but do not provide localized shear stress from surface projections, therefore sensory feedback substantial but less intense compared to naturally deposited ground. Most indoor man made surfaces are designed for appearance and ease of cleaning. They lack small surface projections of natural surfaces and have lower frictional resistance with the bare foot (less overall shear stress) than the outdoor surfaces, resulting in further reduction in sensory feedback during barefoot locomotion. Shoes provide minimal sensory feedback because there are no localized plantar surface deformations, shoe sole materials conform to the plantar surface thereby distributing vertical and horizontal load more evenly to a larger surface area. Furthermore, all shoes have an upper structure to attach the foot which transfer horizontal load to the dorsum of the foot, thereby reducing plantar surface shear stress.
Under natural conditions (bare foot on naturally deposited ground), deformations and localized shear stress provide adequate stimuli for SA II mechanoreceptors and perhaps nociceptors with c-fibre afferents. This information is used to moderate impact but also provides foot position awareness (the sense of position and orientation of the plantar surface relative to the support surface) - information required for stable equilibrium. The exceptions to this is when the foot is anesthetized by cold, and in the late middle aged and elderly because of receptor decline, where less precise information from muscle receptors is used for this sense with decline in stability. When barefoot on man made outdoor surfaces, tactile information is less intense. Presumably it remains sufficient for adequate foot position judgment based on SA II mechanoreceptor afferent information. It is probable that tactile information is attenuated sufficiently on indoor man made surfaces, such as typical urethane finished gym floors and polished linoleum, whereby muscle receptors are used exclusively for foot position sense with decline in stability. Muscle receptors are used for foot position sense under all footwear conditions, regardless whether soles are rigid or resilient - thin as in "minimalist shoes" or as thick as with modern "training" shoes. Sole thickness and hardness as well as resiliency influence foot position judgments. Through amplifying medial-lateral oscillatory foot movements during the support phase of locomotion, which interferes with muscle receptors information used for foot position judgments. Thin and hard soles are superior to thick and soft ones in terms of foot position judgments. High resiliency sole materials impair foot position sense whereas even soft low resiliency material underfoot may actually improve it through dampening medial-lateral foot oscillation.
Stability is a positive function of foot position sense, whereas impact is a negative function of this sense. Accordingly, impact is greatest with athletic footwear, and lowest with the barefoot on natural surfaces, with other surface and interface conditions intermediate.
Links to relevant published reports: 1988-2.pdf 1989-1.pdf 1992-1.pdf 1993-1.pdf 1994-1.pdf 1995-1.pdf 1997-4.pdf 1997-2.pdf
Naturally deposited ground interacting with the barefoot elicits a specific foot contact pattern due to local differences in pain thresholds across the plantar surface. Pain thresholds to small deforming objects is highest at the heel, are intermediate at the distal digits and extremely low at metatarsal-phalangeal joints (MTPJ, or "balls of the foot"). Accordingly, when forefoot contact is required, barefoot humans intensely avoid contact with the MTPJ through extreme plantar flexion of digits and loading the pads of the toes which are far less sensible. Plantar flexion of the digits is accomplished through both intrinsic and extrinsic foot muscle that span the medial arch, and plantar interosseous muscles. People that normally wear shoes (including "minimalist shoes") have no sensory feedback inducing this avoidance. When normally shod individuals commence barefoot activity on natural surfaces, intense plantar flexion of the digits results in raising of the medial arch thereby shortening the overall foot length and consequent tilting of the foot laterally (to the outside). Furthermore the forefoot width increases as interosseous muscles become more robust. People that normally wear shoes use the plantar fascia as their main arch support, which accounts for the high incidence of plantar fasciitis in shod runners. The medial arch is supported by foot muscle in barefoot running which accounts for the absence of this condition in normally barefoot populations. Furthermore, arthritic MTPJs are typical of shod populations yet much less apparent in normally barefoot groups. Many of the same adaptations appear with running barefoot on man made surfaces, such as concrete and asphalt, but presumably these foot adaptations are somewhat less profound because of few protruding deforming surface irregularities.
When wearing shoes, risk of damage from plantar surface abrasion is minimal, but when barefoot it is high without altered running mechanics. Abrasion risk on man made surfaces is extremely high due to high frictional resistance between the plantar surface and concrete and asphalt, yet is somewhat lower on naturally deposited ground. Barefoot running alters running mechanics so as to protect the plantar surface in response to uncomfortable intense shear stress. Running mechanics is adjusted when barefoot so as to minimize shear stress. Knee and hip flexion precedes foot contact during barefoot locomotion so as to match foot velocity to overall forward movement. This minimizes shear stress, reducing risk of abrading injury. This appears to be one the most difficult adaptations to acquire as normally shod individuals commence barefoot locomotion.