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.
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