Little is known about the contributions of specific extracellular matrix components of tendon to the tissue’s mechanical properties. Type I collagen, given its abundance and association into long fibrils, is thought to dominate the elastic properties of tendon. Proteoglycans (PGs) are believed to provide elasticity through their potential role in transferring stress between discontinuous fibrils, as well as viscoelasticity via their interaction with water. Previous studies suggest relationships between collagen or PGs and tissue mechanics [1,2]. However, no study to date has isolated the contributions that distinct tendon components make to the elastic and viscoelastic properties of tendon. Recently, transgenic mice with prescribed mutations or deletions of various genes for specific tendon constituents have become available. In this study, we use transgenic mice as a tool to investigate the contributions of tendon components to tendon function based on a previously described approach . In particular, we compare the strain rate sensitivity among fascicles from the tails of mice described in Table 1. We hypothesize that (a) fascicles with alterations in type I collagen (C1TJ8 and C1M8) will have different elastic properties but no difference in strain rate sensitivity than age-matched controls (CTL8), and (b) fascicles with alterations in proteoglycan (DKO8 and CTL3 ) will have different elastic properties and different strain rate sensitivity than CTL8 fascicles.