Muscle atrophy refers to the thinning of muscles losing their tone and strength. Knee rehabilitators are usually based on a 4-bar system due to its simplicity, and the passive movement sought the rehabilitation of joints, seeking comfort in the patient. This research determines the critical movements and the correct selection of the 4-bar mechanism, considering the restrictions according to the rules of the flat mechanisms. In addition, a kinematic analysis based on geometric models is carried out for validation through a computer program for its subsequent selection of the actuator to obtain the desired movement according to the initial conditions of the knee rehabilitator that considers the rehabilitation of the patient in a sedentary position. The result shows a mechanism with an isokinetic movement employed for knee rehabilitation.