Measuring full-field strain of the muscle-tendon junction using confocal microscopy combined with digital volume correlation

The muscle-tendon junction (MTJ) is a specialised interface that facilitates the transmission of force from the muscle to the tendon which has been implicated in muscle strains and tears. Understanding the transmission of forces and the strain generated in the MTJ is therefore important. For the first time, we report the 3D full-field strain distribution across the muscle-tendon junction (MTJ) using in-situ tensile testing and confocal microscopy coupled with digital volume correlation (DVC). This approach allowed us to measure the mechanical behaviour of the MTJ at the fibre/fascicle level. Acridine orange (AO) in 70% ethanol was used to enhance the contrast of the mouse Achilles-gastrocnemius MTJ, and the specimens were rehydrated prior to the tensile testing, which was performed using custom made tensile rig that fit under confocal microscopy. The 3D full-field strain distribution was obtained using DVC, where the strain changes were measured from confocal images taken with the MTJ under preload (0.4 N) and load (0.8 N and 1.2 N) representing 2.7- and 4-times body weight. High strain concentration was observed at the junction for both 0.8 N and 1.2 N loads. At the junction, the first principal stain (ε_p1), shear strain (γ) and von Mises strain (ε_VM) reached 15.2, 34.2 and 19.2% respectively. This study allowed us to measure fascicle level strain distribution at the MTJ. Using histology, microtears at the MTJ were seen in specimens loaded with 1.2 N which were associated with von Mises strain concentration in the adjacent region. The microtears occurred in regions where the strain level was between 8 and 15%. This study developed a methodology to determine high-resolution strain distribution at the MTJ and has the potential to be used to analyse the strain at the cellular level using higher magnification objectives.