Repetitive stress injuries, which are common amongst runners result from an accumulation of microdamage from repeated application of stress. This leads to strain, or deformation. At first, this falls within the physiologic range of elastic deformation. With excessive running, bones can be subjected to supraphysiologic plastic deformation and soft tissues can be subjected to supraphysiologic viscous deformation. High strain and high rates of application of that strain can lead to fatigue damage In bone, this manifests as microcracks, and as failures of collagen cross-links in soft tissue. These small-scale failures allow elastic hysteresis, which is the difference between the energy required (stress) to generate a given strain (deformation) and the elastic energy stored for a given cycle of loading. In other words, the accumulation of microdamage makes bones and soft tissues weaker and then less force is required to further the damage.
Hoopes, Daniel M.. "Chronic Injuries Due to Running and a Possible Cure with the Barefoot Style." UNM Orthopaedic Research Journal 2, 1 (2013). http://digitalrepository.unm.edu/unm_jor/vol2/iss1/12