Mathematical modeling approaches are frequently used to characterize and predict the mechanics of biological soft tissues. Structurally-based continuum models, which describe the relationship of the constituents’ properties (i.e., collagen fibers, matrix) to overall tissue properties, require knowledge of the relationship between microscopic (fiber) and macroscopic (tissue) deformation. The most common and straightforward approach is the use of an affine model, which assumes that local fiber kinematics follow the global tissue deformation. Although the affine assumption is often used in constitutive modeling, several studies have reported non-affine fiber behavior in soft tissue testing [1–2]. Our recent work has quantified the anisotropic and inhomogeneous mechanical and organizational properties of human supraspinatus tendon (SST) [3–4]. We have also utilized a fiber dispersion model to examine SST [5]; however the relationship between macroscopic and microscopic deformation in this tendon remains unknown. Therefore, the purpose of this study was to examine the affine assumption in human SST fiber kinematics by comparing experimentally-measured fiber alignment to the affine model prediction.

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