STORED ELASTIC ENERGY AND VISCOSITY

The ability to generate stored elastic energy (SEE) is proportionate to the tensile strength of the tissue. Tensile strength is the maximum stress that a material can withstand before it breaks. The ductility (how malleable a substance is) decreases as a material reaches its tensile strength failure; conversely, the amount of SEE increases as a material reaches its tensile strength failure.

This is the concept of viscosity: the property of an object that demonstrates that a body at rest tends to stay at rest unless acted upon by an outside force. Many tissues of the human body exhibit constricting, congealing, and thickening characteristics when not exposed to outside forces. The viscosity of a tissue is its resistance to the force: The greater the viscosity, the greater the force and time required to cause deformation.

To understand this concept, we pull a rubber band in two opposite directions. The more that we pull, the harder it is to pulL For example, if we pull the rubber band one inch, it gains 5 units of SEE; if we pull one more inch, it produces 10 additional units of SEE (15 total); if we pull one final inch, it results in 20 more units of SEE (35 total). The increase is exponentiaL The more we pull the rubber band, the farther it will fly when we release one side.

Tissues adapt to both the intensity and the duration of the stress placed upon them. So two things can occur. In the rubber band example, too much stress can cause the rubber band to snap, or the rubber band can begin to deform permanently.

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when changing stress, tan(phi)=elastic stress/viscous stress

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