*Why runners from rich countries are injured, and those from developing countries are durable.

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.  
 
 

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