The micro-structural strain response of tendon

Tendons are multi-level fibre-reinforced composites, designed to transmit muscle forces to the skeleton. During physiological loading, tendons experience tensile loads, which are transmitted through the structure to the cells, where they may initiate mechanotransduction pathways. The current study examines the structural reorganisation and resulting local strain fields within the tendon matrix under tensile load. It uses confocal microscopy to photobleached a grid onto the collagen and image its deformation under the application of incremental tensile strain. Six parameters are used to quantify fibril and fibre movement and examine the mechanisms of extension employed by fascicles. Results demonstrated an inhomogeneous strain response throughout the matrix and large variability between samples. Local strains in the loading axis were significantly smaller than the applied values. However, large compressive strains, perpendicular to the loading axis, were recorded. The average Poisson's ratio (0.8) suggested cells may experience significant compression during loading. Deflection of the grid lines, indicating sliding between collagen fibres, and rotation of the grid were also recorded. These data highlight the non-homogenous strain environment of fascicles and provide further evidence for fibre sliding under tensile load. They also suggested a rotary component to tendon response, which may indicate a helical organisation to the tendon matrix.