H. Sapiens separated from a common ancestor perhaps 300,000 years ago, probably on the savanna of sub-Saharan Africa. They likely lived there for almost two-thirds of their existence before migrating widely. This is a vast plain of naturally deposited loam soil that supports expanses of grasslands, and consequently immense herds of grazing animals - a reliable source of protein and essential fatty acids that are essential to human existence. Loam soils are sedimentary deposits composed of clays, gravels and weathered stones. They drain well, yet retain sufficient moisture and nutrients to support lush surface vegetation. Humans and many other species of animals facilitate locomotion through dense grassland via paths - vegetation free channels formed through compaction and root fracturing from hooves and feet. These were the highways of human movement through grasslands. They were, and remain, essentially rigid when dry, with a random array of relatively small weathered (rounded rather than sharp-edged) protruding stones. It provides moderate frictional resistance with respect to the bare foot. This is the naturally deposited ground on which human locomotion seems particularly efficient and safe.
Naturally deposited ground, man made surfaces and interfaces
Consider natural deposited ground as rigid, and presenting a surface profile containing a varying array of rigid protrusions of varying sizes. This is probably the support surface humans have adapted to best through natural selection, therefore it allows efficient and safe mobility, although they retain the ability to adapt to other surfaces reasonably well probably through plantar tactile receptors sensing charges in support surface characteristics.
Most man-made surfaces are recent technological developments possessing surfaces that are both rigid and smooth compared to naturally deposited ground. Interfaces are layers placed between the bare foot and support surface. Mats and carpets are interfaces that are fixed to the support surface, whereas shoes are interfaces that are attached to the foot. Interfaces of all types typically are used to alter support surface physical properties vis-a-vis the plantar surface of the foot - perhaps lowering surface rigidity, raising static and dynamic coefficient of friction or thermal insulation. All lack the random array of surface protrusions typical of most naturally deposited surfaces.
During human locomotion natural surfaces the plantar surface is vertically loaded. These load are concentrated locally via surface irregularities - protrusions that deform the plantar surface. Frictional resistance with the support surface applies shear-stress to the plantar surface which is also concentrated at vertical deformations. It seems probable that these intense localized vertical and horizontal loads with surface formations would damage the plantar surface if they were presented in the same array. This does not occur because naturally deposited ground presents a random array of irregularities. When barefoot on man made surfaces designed for outdoor use, such as concrete and asphalt, frictional resistance with the plantar surface is extremely high (probably exceeding what humans normally experience on natural surfaces), but lack of localized deformations cause more uniform plantar surface loading. With interfaces in the form of footwear, plantar shear is markedly reduced as these forces are transmitted substantially to the "shoe upper" - essentially a system that attaches the shoe to the foot. Shoes attenuate overall plantar shear-stress but most importantly localized vertical loading and shear stress, which are adequate stimuli of SA II mechanoreceptors of the plantar skin that humans use to maintain stable equilibrium and optimize gait mechanics in terms of energy efficiency. The result is both unsafe and inefficient mobility.
When the bare human foot is rapidly loaded during locomotion, most of the energy is absorbed and returned slowly so as to maintain an over-damped condition - minimize oscillation (bouncing). Most shoes, and many other interfaces are made of rubber and expended polymer foam materials. These materials, which do not resemble any surface found in nature, produce an under-damped condition consisting of a series of compressions and rebounds (bouncing) when the human foot enters the support phase. This oscillation has both vertical but also intense medial-lateral (side-to-side) components, which we have shown destabilizes humans through both challenging stability directly and by entering with mechanisms use to make foot position judgements. Instability produces behavior which amplifies impact thereby encouraging injury.
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