Load adaptation mechanisms in tendons are determined by the hierarchical assembly

Tendons are hierarchically structured, transferring forces across multiple length scales. Loads are partitioned from the whole tendon through fascicles, fibers, and fibrils down to the tropocollagen molecules. To elucidate the structural deformation of tendons at different hierarchical levels, this study investigated the simultaneous tissue, sub-tendon, fibrillar, and molecular response to tensile load by combining in situ loading, digital image correlation and, small- and wide-angle X-ray scattering. To investigate differences in mechanical properties stemming from their hierarchical arrangement and possibly their proximity to either muscles or bone, bovine deep digital flexor tendons (DDFT) were separated into two groups: tendons close to the muscle junction (TM), and tendons close to the bone junction (TB). Samples were loaded at a strain rate of 0.01 s^-1 in cyclic loading (6%, 10 cycles), stress relaxation (8%, 600 s), and load to failure. The TB group exhibited higher elastic modulus than its TM counterpart across all mechanical tests. Viscoelastic properties differed between TM and TB groups at the tissue level, while at the fibril level there was no apparent difference. This suggests that components at larger length scales such as fibers and fascicles and their arrangement could be responsible for the difference observed at the tissue-level. Strain partitioning was observed at all hierarchical levels as a fraction of the applied strain on the tissue with the sub-tendon (52%), fibril (10%), and collagen molecule (12%), experiencing lower levels of strain. The surface strains were heterogeneous, with areas of localized higher strain. This study highlights the importance of multiscale analysis when investigating deformation mechanisms in large animal models of hierarchical tendon tissues. The methodology developed here is well suited for future studies of human tendons. STATEMENT OF SIGNIFICANCE: Achilles tendons are prone to injuries, however injury occurs suddenly. Due to their hierarchical structure understanding how the loading is taken up by the tissue is extremely complex. Understanding the hierarchical structural response and its relation to tendon function is crucial to aid in rehabilitation. We combine the use of digital image correlation, synchrotron small- and wide-angle X-ray scattering with simultaneous in situ loading of bovine Achilles tendons to understand the relation between the loading of the whole tendon, the distribution at the tendon surface and down to the structural adaptations of the collagen fibrils and collagen molecules, experienced at the nano- and angstrom-scale. The proposed methodology aids in understanding the relative partitioning between length scales, and deformation mechanisms occuring during tendon loading and rupture.