Humans sense a single extremely high amplitude impact that could be damaging through high threshold plantar surface receptors (nociceptors with c-fiber afferents). They sense and avoid lower level impact that may be chronically damaging through SA II mechanoreceptor afferent information. Adequate stimulus for these receptors is a combination of localized vertical and horizontal loading (shear-stress). Footwear by definition have a sole which is attached to the plantar surface. Any intermediate layer attached to the plantar surface substantially attenuates plantar shear stress, therefore it is impossible to design a shoe that will allow protective behavior through SA II mechanoreceptors. Also SA II mechanoreceptor information is used in maintaining optimal stability through precise foot position sense. When humans wear shoes they rely of less precise muscle and tendon receptors for making foot position judgements. Although never as effective as the bare foot in protection from chronic impact, shoes vary considerably in impact with their use, because they can elicit amplified impact in response to instability they produce. With essentially all athletic shoes and most shoes for non-athletic use, an interface of highly resilient material is added which compresses during human locomotion and produces a rapid reaction force resulting in an undamped condition characterized by a series of compressions and overshoots (bouncing) which has negative consequences on stability. Bouncing has both a vertical (up and down) and medial-lateral (side-to-side) component. The medial-lateral oscillation overwhelms muscle and tendon receptors of the foot and ankle that humans use in making foot position judgements. This results in a sense of loss in stability followed by behavior to improve stability that involves amplified impact. Footwear that permit optimal stability will be the best shoe in terms of chronic vertical impact. Stability is negatively and positively related to shoe sole thickness and hardness, respectively. Shoe with thin and firm soles will offer best in terms of stability hence impact. Shoe sole materials with extremely low resiliency (recovery from a standard compressive load of about 5% in 1 second), dampen the oscillatory compression and overshoot inherent to resilient sole materials. This improves stability thereby attenuating chronic overloading that can cause injury.It is possible to design shoes that are substantially better than currently available ones through optimizing stability which in turn lower behavior humans use maintain stable equilibrium that involves amplified impact. These footwear will never be as effective as the bare foot in contact with naturally deposited ground.
Links to relevant published reports: 1998-1.pdf |
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