*Impact in shod runners is a reaction to maintain balance.

Most injuries associated with running with modern footwear result from excessive repetitive impact. Humans sense impact amplitude poorly, and there is no evidence that behavior is elicited directly in response to it. However, impact during locomotion can vary considerably. I have shown humans respond with intense behavioral responses, that can either amplify or moderate impact. This behavior is a response to both plantar (sole of foot) sensations and sense of stability (sense of falling).  Intense plantar stimulation in the form of localized deformations and shear stress results in behavior that lowers impact. Behavior that amplifies impact is a response to a sense of loss of stability during locomotion. Loss of stability in footwear is caused by loss of foot position awareness – the sense of position and orientation of the plantar surface relative to the support surface. Loss of foot position awareness results from resilient shoe materials of modern footwear creating an underdamped condition when rapidly loaded consisting of a series on compressions and rebounds oscillatory in the medial-lateral plane. The resulting behavior caused by this instability probably is directed at finding a more stable support base. There is evidence that maintaining stability takes priority over foot protection if both are challenged simultaneously.  This conclusion is based on a report that showed that humans, unlike quadrupeds, do not exhibit a withdrawal reflex to noxious plantar stimuli if applied to the plantar surface just prior to the support phase of locomotion.