This dissertation is composed of three general sections:​ first, the analysis and modeling of tendon mechanical behavior;​ second, the analysis and modeling of tendon behavior following diffuse damage (induced by overstretch);​ and third, the analysis of tendon mechanical behavior through the use of ultrasound.

In the first section, the tendon was subjected to stress relaxation at various strains to determine rate strain-dependence, as well as stress relaxation followed by recovery at a low but nonzero strain to investigate recovery behavior. Relaxation rate in tendon was found to increase with increased input strain, and the rate of recovery was found to be much slower than that of the preceding relaxation and also slower than that of a relaxation at the same low strain. The relaxation and recovery behavior of tendon was well described by Schapery's nonlinear viscoelastic model through the physiologic strain range (0-6% strain).

In the second section, diffuse damage was induced in the tendon by subjecting it to an overstretch strain (exceeding the elastic limit) of 6.5, 9, or 13% strain. Stress relaxation and cyclic testing were performed on the tendon prior to and following overstretch such that the ratio of several parameters (such as peak stress, decrease in peak stress during cyclic testing;​ max stress, decrease in stress during relaxation testing) could be calculated and compared. Diffuse damage induced laxity in the tendon, resulting in decreased stress at a given strain. Damage also reduced the viscoelastic response of the tissue, resulting in less viscoelastic change at a given strain. This behavior led to the development of an "equivalent strain" model, in which the tendon was modeled as if it were being subjected to a lower strain. With this model, strain-dependent parameters could be calculated and inserted into a constitutive model. In this study, Schapery's nonlinear viscoelastic model was used and found to be predictive of post-damage mechanical behavior.

In the final section, the ability of ultrasound, specifically the ability of acoustoelastic (AE) theory, to characterize mechanical properties was tested. AE theory predicts that as a material is deformed, its acoustic properties are altered, and thus the amplitude of the reflected ultrasound echo (or, the echo intensity) would be changed. In this study it was demonstrated that under steady-state conditions, echo intensity is linearly related to strain and nonlinearly related to stress. During viscoelastic testing, echo intensity increased in a time-dependent fashion. In order to test the ability of the tool to detect mechanical compromise, tendons were subjected to the same diffuse damage model and echo intensity changes were compared before and after damage. Results found that the laxity introduced by diffuse damage resulted in reduced echo intensity changes, further reinforcing the concept of effective strain (echo intensity changes decrease as input strain decreases). A second damage model was performed when a tendinitis-like focal defect was created by injecting collagenase (digesting collagen in a small region of the tendon midsubstance). Focal damage induced in this manner resulted in local alterations in echo intensity changes.