A passive ankle dorsiflexion testing system to assess mechanobiological and structural response to cyclic loading in rat Achilles tendon

Tendinopathy is thought to be caused by repeated overload of the tendon with insufficient recovery time, leading to an inadequate healing response and incomplete recovery of preinjury material strength and function. The etiology of tendinopathy induced by mechanical load is being explored with a variety of mechanical load scenarios in small animals. This study establishes a testing system that applies passive ankle dorsiflexion to a rat hindlimb, estimates the force applied to the tendon during cyclic loading and enables the assessment of subsequent structural and biological changes. We demonstrated that the system had no drift in the applied angle, and the registered maximum angle and torque inputs and outputs were consistent between tests. We showed that cyclic loading decreased hysteresis and loading and unloading moduli with increasing cycles applied to the tendon. Histology showed gross changes to tendon structure. This work establishes a system for passively loading the rat Achilles tendon in-vivo in a physiological manner, facilitating future studies that will explore how mechanics, structure, and biology are altered by mechanical repetitive loading.