## 1 Introduction  
An active lifestyle, especially professional sport, which puts a heavy strain on the musculoskeletal system, as well as civilization-related diseases that are usually associated with a lack of physical activity, such as obesity, diabetes, etc., all contribute to damaging the knee. The knee is the largest joint in the human skeleton. Because of its complex structure and the stresses of everyday movement, it is highly susceptible to injury. Diseases and injuries of the knee joint generally exclude or severely restrict people from performing their daily activities due to the impaired motor function associated with a reduced or lost ability to move fully or partially. The main complaints in the knee joint are usually associated with meniscus injuries, sprains or dislocations, and damage to the cruciate ligaments. These conditions are usually temporary and can be reversed with appropriate rehabilitation. Many times, in order to make a proper diagnosis, a procedure is practiced to view the knee joint, called arthroscopy. Chronic joint inflammation is much more problematic from a rehabilitation point of view. This can be caused by advanced osteoarthritis of the knee or rheumatoid arthritis. The joint surfaces that cover the bones, which have been lost or mechanically damaged, need to be replaced in whole or in part by artificial implants that precisely restore motor function and eliminate pain. The treatment uses total endoprostheses, single-compartment endoprostheses, or bicompartmental endoprostheses. The medical procedures used to replace a total or partial knee are called total or partial knee replacement (also called knee arthroplasty) [1, 2]. The above surgical procedures cause patients discomfort after surgery due to pain, swelling, and tenderness of the joint. Patients' limitation of motor function due to joint pain paradoxically leads to an increase in stiffness and thus a reduction in the initial mobility within the joint (Range of Motion--ROM). Most physio-therapists recommend continuous passive motion (CPM) devices immediately after knee surgery. Continuous passive motion (CPM) is a rehabilitation therapy that uses a machine to move a joint for a patient [3; 4]. Such a device performs a controlled movement within a pre-set range to increase the initially limited ROM of the knee joint and restore muscle strength. The CPM device is powered by an actuator that provides continuous motion of the joint, performing regular flexion and extension of the limb at the knee joint without any effort from the patient. As a result, the knee joint tissue is better nourished by the rehabilitation device and therefore regenerates more quickly, reducing pain and minimizing the potential for new inflammation [5]. Various modifications of the CPM device are being used in many research centres around the world for mechanical kinesitherapy of various human upper and lower limb joints [6; 7; 8]. The Department of Automatic Control and Robotics at the Bialystok University of Technology, Poland has also developed a similar device for rehabilitation after arthroscopy and knee arthroplasty [9; 10]. However, a problem with the drive of the ARTROBOT CPM device motion apparatus by an electric actuator coupled to a mechanical gearbox was identified during further research and the prototype solution was tested. From the analytical calculations carried out to select the right actuator for the solution, it became clear that the actuator selected would have to generate high torques, which would result in a high purchase cost for the motor with a gearbox, as well as its increased size and the mass of the solution. When it comes to mobile CPM solutions for use in independent rehabilitation, the mass of a geared actuator is an important parameter.