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