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Visoelastic Materials

- Discussion:
    - exhibit stress / strain behavior that is time-rate dependent, and varies w/ the material, that is a function of the material's internal friction;
    - visoelastic materials are stiffer and stronger at high strain rates than at low strain rates;
           - for example in bone-ligament interface, slow rate of loading will result in avulsion frx of bone but a fast rate of loading will cause 
                   ligament failure;
                   - hence, bone has a higher modulus of elasticity at higher strain rates;
    - ligaments are viscoelastic - that is, their stress-strain behavior is time-rate dependent, w/ elongation of ligament being more likely to occur 
           with slower loading conditions;
           - rate of loading affects ultimate load to failure.
    - isotropic materials:
           - possess the same properties in all directions;
    - anisotropic materials:
           - have mechanical properties that vary w/ orientation of the loading;

- General Ligament Tensile Properties:
    - ligaments display visoelastic characteristics (time-rate dependent), with slow loading allowing more ligament elongation);
     - load elongation curve of bone ligament complex contains an initial low stiffness region (toe region) followed by a high stiffness region;
           - similar loading properties are found in the tendon;
    - ligaments and tendons have a non linear, strain stiffening structural response;
           - this may result from the crimp pattern collagen fibrils which elongate when small tensile loads are applied;
           - once the crimp pattern is unwound, larger forces are required to produce strain;
    - creep:
           - ligaments also possess characteristic creep properties (progressive elongation or change in strain w/ constant load over time);
    - stress relaxation:
           - stress-relaxation properties (amount of stress measured in preloaded ligament decreases w/ time);
           - for  ACL, stress relaxation has been noted to stabilize at 80 % of the initial stress over time;
           - this property is demonstrated in scoliosis correction in which peak distraction forces can be reduced by 50% because of vertebral 
                   soft tissue creep;

- Effects of Immobilization:
    - after immobilization, the resumption of joint motion leads to a slow reversal in the effects ofi mmobilization on the structural properties of 
           the FMTC and FATC;
    - these ultimate load and energy absorbed at failure of these two complexes reach 80% to 90% of control at one year;
    - histologic evidence of new bone formation at ligament insertion reveals that time required to return to normal is much longer than the 
           immobilization period;
           - in contrast, material properties of the MCL substance returns to normal after nine weeks of remobilization

 Viscoelastic properties of muscle-tendon units:  The biomechanical effects of stretching.

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