Isokinetics devices are widely used in the field of knee rehabilitation. This kind of apparatus often solicits the knee joint in the sagittal plane with one degree of freedom. With the development of exoskeletons and intelligent prostheses, more precise control applications to assistive robotics for rehabilitation are possible. Our aim is to design a new apparatus for the 3 dimensional control of knee joint movements for sports training and rehabilitation. The first step to design such apparatus is to specify its kinematics. This paper presents a kinematical model of the knee joint that will be employed to specify the future Device kinematics and a preliminary control scheme.