Structural modeling and morphological crimp analysis were used to investigate if collagen fiber recruitment could account for our previous finding, that ligament creep behavior cannot be predicted using inverse stress relaxation behavior directly. Ligament creep behavior was accurately predicted using our simple nonlinear structural model that incorporated collagen fiber creep and collagen fiber recruitment. Collagen fiber creep was modeled using the inverse stress relaxation function and estimated fiber modulus. Collagen fiber recruitment was modeled with a linear variation in crimp over an idealized rectangular ligament cross section. Concomitantly, significant differences in collagen crimp patterns were observed as a result of creep testing; however; no significant changes were observed as a result of relaxation testing. This morphological evidence supported the model assumptions that fiber recruitment occurs during creep and that stress relaxation behavior results from the viscoelastic response of an unchanging group of fibers. Not only was the prediction improved compared to the inverse stress relaxation behavior alone, the model demonstrated that fiber recruitment increased the load-bearing area of the ligament over time and correspondingly stress was redistributed, reducing stress on the fibers initially loaded. These findings may have important implications for both models and experiments on ligament structure and function at low loads.

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