Collagen is the primary structural component in soft connective tissues. Collagen molecules are organized into fibers which appear crimped or wavy when the tissue is in a stress-free state . As the tissue is stretched, the crimping pattern diminishes as the fibers align in the direction of the applied load . It is generally accepted that the sequential straightening of collagen fibers is responsible for the mechanical behavior exhibited by soft tissues. Several mathematical models incorporate this uncrimping phenomenon to describe the characteristic nonlinear behavior of biological tissues [3,4,5]. Although the models describe the mechanical behavior well, it remains unknown exactly how the collagen fiber orientation changes with strain.