Dependence of tendon multiscale mechanics on sample gauge length is consistent with discontinuous collagen fibrils

While collagen fibrils are understood to be the primary load-bearing elements in tendon, controversy still exists on how fibrils functionally transmit load from muscle to bone. Specifically, it's unclear whether fibrils are structurally continuous along the tendon length and bear load independently, or if they are discontinuous and transfer load through interfibrillar shear forces. To address this question, we investigated whether the multiscale mechanics of rat tail tendon fascicles is dependent on sample gauge length. We hypothesized that as the grip-to-grip length is reduced and approaches the length of the collagen fibrils, tendon fascicles will adopt a multiscale mechanical response consistent with structurally continuous fibrils. Our findings show that, for gauge lengths of 20 mm or greater, the local fibril strains are less than the bulk tissue strains, which can be explained by relative sliding between discontinuous collagen fibrils. In contrast, at a 5 mm gauge length, the fibril strains are equivalent to the applied tissue strains, suggesting that the collagen fibrils are structurally continuous between the grips. Additionally, the macroscale tissue modulus is increased at gauge lengths of 5 and 10 mm. Together, these data support the hypothesis that collagen fibrils in rat tail tendon fascicles are discontinuous and also suggest that their length is between 5 and 10 mm. This fundamental information regarding tendon structure-function relationships underscores the importance of the tissue components that transmit load between fibrils and is critical for understanding tendon pathology as well as establishing structural benchmarks for suitable tissue engineered replacements.