Abstarct: Knee arthritis is a common and chronic disease in the elderly population, which is caused by the erosion of the cartilage of the knee joint. This disease causes a deviation in the alignment of the knee bones and leads to impaired movement, pain and immobility of the knee joint. The treatment of this disease is often done through surgery, but due to reasons such as high cost, postoperative complications, and the increasing number of patients, non-invasive treatment methods such as the use of braces and exoskeletons are recommended today. The joint mechanism of braces and commercial exoskeletons is rolling or rotational, while the kinematics of the knee joint is a combination of rolling and sliding motion. The mismatch of performance in these aids with the kinematics of the knee joint causes more damage and pain in the knee of a person with arthritis. To compensate for this defect and reduce the inconsistency between braces and exoskeleton with the knee joint, this thesis deals with the design of a new extension joint mechanism to be replaced in commercial braces and exoskeletons. The design of this new joint mechanism is baxsed on the study and investigation of simulated biomechanics and kinematics of the knee joint in the Katia software. Inside this joint, a passive auxiliary extension torque generation mechanism is also installed to generate an auxiliary force to straighten the patient's knee joint during knee bending. This auxiliary torque is equivalent to reducing the body weight and loading three parts of the knee at the same time for users. The maximum torque produced at the 90-degree angle of the designed joint is equivalent to 27 kg of body weight reduction, while other braces can reduce up to 20 kg of body weight.................................................