We need to define a shoe in terms of how footwear of all types interfere with plantar sensory feedback
Through natural selection humans adapted for safe barefoot locomotion on naturally deposited ground primarily through the use of sensory information from tactile (touch) receptors on the foot. Footwear interfere with these there protective mechanisms in relation to shoe sole hardness, thickness and resiliency.
In an attempt to allay the concern about the negative effects of contemporary footwear on safety, manufacturers have marketed an array of products referred to as “minimal” or “minimalist” shoes, with the direct of implied message that do not alter the natural mechanics of barefoot locomotion. One manufacturer of these class of shoes has paid an investigator to present pseudoscience in Nature indicating that the sensible runner could chose either running barefoot or with a minimal shoe because they appear to him to be equivalent.
Scientific reports refute this position, however presenting them clearly would be aided by a definition of footwear that encompasses all available products.
A shoe is a device that is used in human locomotion that consists of an interface placed between the foot plantar and support surfaces, and a fixation system which mitigates shear (horizontal) forces so as to keep the plantar surface positioned over the interface.
The interface may be a single material or consist of a series of layers, such as a sockliner, insole and outer-sole, typically found on most modern shoes. It may be of uniform thickness, or thicker in the heel region as with most current footwear. High heeled shoes of current fashion can be seen as having an interface that is considerably thicker, and rigid at the heel region, and tapers to a narrow base near the support surface.
Whereas interfaces have received a considerable attention, the fixation system has not. A fixation system is required on all footwear, without exception, because the interface would otherwise be lost during non-support phases of locomotion, and is essential to keep the foot relatively centered on the interface during weight-bearing phases of locomotion. Footwear displacement vis-a-vis the interface is a consequence of frictional resistance between the interface and supporting surface being greater than between the plantar surface and interface. When walking at modest speeds, a fixation system that controls forward displacement of the foot relative to the interface is sufficient. Forces promoting backward displacement caused by muscular propulsive work in walking are modest, and might not require posterior fixation such as a "heel counter". Perhaps the simplest example of forward displacement fixation is “thong” placed between the great and second toe - a fixation method seen in footwear artifacts 5,000 years old.
When running, the forces displacing the foot amplify, and all footwear require a fixation system that limits anterior, posterior, medial and lateral displacement vis-a-vis the interface. Most modern shoes, and with few exceptions all footwear used in running use a fixation system that encases the foot, thereby fixes the foot relative to the interface by transmitting forces to the dorsum of the foot. A rarely used alternative uses the above “thong”, but adds lashing of the shoe to the leg to prevent posterior and lateral foot displacement relative to the interface.
Since all shoes possess a fixation system, all attenuate plantar surface horizontal forces sustained during locomotion through transmitting this force to other foot or leg structures such as the dorsum of the foot. Consequently, any protective adaptation initiated through horizontal plantar loading would be diminished with footwear, regardless of type.
Using the definition to understand why shoes interfere with protective adaptations
Man evolved barefoot. Our understanding of the mechanics of barefoot locomotion and control of impact and stability has advanced considerably. We know that the plantar receptor of interest is the SA II mechanoreceptor. Adequate stimulation of this receptor is localized vertical and horizontal plantar surface loading, but both applied simultaneously at the same location is the particularly effective stimulus. This would happen when a small surface irregularity is encountered during locomotion. Information from this receptor is used for skin protection but it also provides information essential to impact control and stability.
Interfaces vary in terms of allowing small surface objects to deform the plantar surface. Shoes with thick yielding sole material allow essentially no localized skin deformation while extremely thin soled varieties might allow modest deformation. When it comes to horizontal localized plantar surface loading, all footwear are similarly reduce shear considerably, as has been shown using in-shoe transducers.
In conclusion, all footwear must interfere with protective adaptations because they all reduce sensory information through limiting horizontal plantar forces through the fixation system. Footwear are variable in how they attenuate localized vertical deformations, but all, including minimal ones, also reduce it considerably.
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