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(I have copied this over from the plantar pressure thread to start a new discussion thread).

All biological tissues are viscoelastic in that they do not behave like inert materials such as a steel bar, a polypropylene rope or a sheet of glass. What this means is that, due to their fluid component, the stress-strain curve http://darkwing.uoregon.edu/~struct/courseware/461/461_lectures/461_lecture24/461_lecture24.html
of biological tissues will change depending on the strain rate (i.e. the rate that the bone, ligament, tendon, muscle or cartilage is being compressed or elongated). With increasing strain rate, an increased slope of the stress-strain curve will result and, thus, increase the stiffness of the material. Slower strain rates will decrease the slope of the stress-strain curve and will decrease the stiffness of the material.

Viscoelastic tissues also exhibit a mechanical characteristic in which they will lose energy to heat during deformation so that when they are compressed or elongated, they will not return the same amount of energy as the energy put into them in order to deform them. This is in contrast to a purely elastic material (ideal) that will load and unload along the same straight line on the stress-strain curve. The term hysteresis refers to the energy loss that happens to a viscoelastic tissue (or other material) when it is loaded and deformed and then unloaded and released back to its original length/shape.

Viscoelastic tissues also exhibit two time-dependent phenomena called creep response and stress-relaxation response. In creep response, a tissue subjected to a loading force will tend to deform more rapidly to a certain contant force level but will then tend to deform more slowly as the force is continued over a period of time. This is the probable physiological reason for the therapeutic efficacy of plantar fasciitis night-splints in relieving the post-static dyskinesia of plantar fasciitis.

The stress-relaxation response occurs when a tissue is subjected to a deformation and then held at that deformation level (i.e. held at a certain length). The material will tend to have a decrease in stress over time, or a "relaxation", as the material is held at a certain length.

Engineers design the structural sizes and types of materials used within a building or bridge so that they will operate within their elastic range on the stress-strain curve. In this way, engiineers can be assured that the material is not subjected to such large magnitudes of stress that could cause disastrous plastic deformation and/or failure under the loading forces that the structural material is likely to be subjected to.

In the same way, the structural components of the body such as bone, ligament, tendon, muscle and cartilage, are meant to function within their elastic range on the stress-strain curve. In this way, injury of that tissue is less likely to occur. When too much stress occurs in one of these structural components of the body (such as in the increased tensile stress within the posterior tibial tendon with posterior tibial dysfunction), then plastic deformation, partial or complete failure of that tissue may occur. So, when we make orthoses and recommend shoes or recommend stretching or strengthening exercises for our patients, we are basically trying to bring the tissues of the body back to operating within their elastic range on their stress-strain curve to prevent plastic deformation or tearing/fracture of the biological tissue and to ensure decreased pain and more rapid healing to the painful musculoskeletal pathology that is present within the patient.

 

Here is a nice article from the University of Michigan on the structure and function of ligaments and tendons including the subjects of stress-relaxation and creep. There is also an interesting section on the differences in mechanical properties in ligaments and tendons in between young and old animals, on the effects of immobilization vs. exercise and on the mechanical properties of healing ligaments and tendons.

http://www.engin.umich.edu/class/bme456/ligten/ligten.htm