## 1 Introduction  
Stroke is one of the leading causes of death and disability among adults worldwide, with its relative incidence doubling every ten years after age 55 [1; 2; 3]. According to the World Health Organisation, the number of people aged 65 and over will increase by 207% until 2050 [2]; therefore, it is expected that the economic and health burden related to stroke will continue to rise. The reduction or loss of functional abilities is one of the most common consequences of stroke. It can cause, among others, motor and movement impairments, such as loss of coordination, strength and skills, reduced ability of fractionated movement, abnormal muscle tone, and paresis [1; 4; 5; 6; 7]. Even though a stroke usually affects both extremities on one side of the body, regaining or improving the upper limb function is more problematic than for the lower limb. A decreased upper limb function occurs in 85% of stroke patients. At 3-6 months post-stroke or even upon completing the standard rehabilitation program, its function is still not sufficiently restored in 50-80% of the patients, further limiting their activities of daily living (ADL) [1; 8; 9; 10; 11; 12]. Meanwhile, improvements in the lower limb function and regaining the ability to walk are expected in 75-83% of patients [8]. Particularly problematic in the recovery of upper limb function is spasticity--its prevalence varies between 4% and 46% in different post-stroke stages [6; 13; 14; 15]; however, it is most common in the chronic stage [13]. Physiotherapy plays a vital role in the post-stroke rehabilitation process. It primarily aims to maintain joint mobility, prevent loss of strength and changes in muscle tone,and improve voluntary movement control [16]. There are various approaches to the rehabilitation of motor and functional abilities of the upper limb post-stroke; however, it has been shown that the implementation of protocols that include regular, high-intensity, task-specific exercises, and focus of high repeatability of the movement is one of the most effective approaches [17; 18; 19; 20; 21; 22; 23; 24]. Exercises involving a prolonged stretch of the affected muscles should also be included to reduce spasticity [25; 26]. Commonly deployed rehabilitation exercises for the wrist and hand are the flexion-extension of the fingers, grasping of different objects, and flexion-extension of the wrist while keeping the fingers extended [27]. Providing high-intensity and task-specific rehabilitation exercise protocols with highly repetitive movements can be, despite their effectiveness, difficult or sometimes even impossible to ensure for all patients. Namely, such protocols are physically exhausting and time consuming for the therapists, as they require manual interaction and one-on-one work with each patient every day for several weeks [17; 28]. Thus, there is a trend of increased use of robotic devices in stroke rehabilitation protocols because not only can they reduce labour costs [29] and therapists' physical exertion [30; 31], but they can also provide customised, high-intensity, repeatable, and task-oriented interactive exercises and enable an objective and reliable monitoring of the patient's progress [17; 22; 24; 32]. Another great advantage of using robotic devices for rehabilitation is that they can accurately and systematically control the amount of force applied to the patient and gradually change the level of assistance or resistance, therefore adjusting the exercise intensity to the patient's abilities [33]. Most robotic rehabilitation devices usually offer different training modalities: passive, assistive, active, and resistive [22; 32]. The training modalities are classified as in other forms of conventional therapy and describe the patient's activity level while interacting with the device [22].