The evaluation of the knee joint behavior is fundamental in many applications, such as joint modeling, prosthesis and orthosis design. A new test rig for in vitro analysis of the knee joint behavior is presented in this paper. Based on a cable-driven parallel manipulator loading system, the rig can simulate general knee loading conditions, such as clinical tests and common daily activities like walking and sit to stand, in a wide range of flexion angles. The joint natural response in terms of movement is measured by an optoelectronic system. Furthermore, the new rig allows the estimation of the contribution of the principal leg muscles in guaranteeing the equilibrium of the joint. Despite its simplicity and low cost, the rig presents good accuracy, repeatability and versatility that allows its application on a wide range of specimen sizes. It represents an advanced application of cable-driven parallel robots for in vitro motion analysis of the knee subjected to general loads.