Quality of life research is conducted in many areas of human functioning. It is undertaken by representatives of various scientific disciplines and theoretical approaches. Despite various definitions of quality of life, referring to the relatively universal WHO1 approach, it can be assumed that quality of life concerns a person's perception of his or her position in life. The assessment of quality of life here is subjective, which emphasizes its psychological dimension and simultaneously is embedded in a cultural and social context. The person's values and goals, as well as standards and expectations provide the specific context for the assessment in this approach1. Among the numerous, more specific conditionings of quality of life, a person's health status occupies a very important place. Hence, in the field of health disciplines, the concept of health-related quality of life has been gradually developing2. It is of particular relevance to the functioning of patients with long-term chronic diseases, including MS. It also refers to the overall well-being of the patient - which includes psychological, social and physical dimensions - influenced by the course of the disease and the treatment process3. It is emphasized that various psychosocial factors determine the health-related quality of life as well4, 5, 6, 7, 8, 9.

Undertaking of physical activity is one of the factors induced by the patients’ intentional action which can increase his or her quality of life. It refers to body movements that increase energy expenditure and involve skeletal muscles10. In broad terms, it includes sport, recreation, work, etc. Exercise, on the other hand, falling within the scope of physical activity, refers to planned and repetitive activities to improve or maintain physical fitness10. The health and psychosocial benefits of appropriately undertaking regular physical activity are now well known. Its importance in reducing the risk of cardiovascular disease, osteoporosis, diabetes, depression, colorectal cancer, breast cancer, etc. is highlighted. It also plays an important supporting role in the rehabilitation of patients with various diseases, including MS11. However, in analyses of physical activity in people with MS, it has been shown that patients were less physically active than a group of healthy adults12. Despite the various reasons for this, it has been repeatedly emphasized that more active MS patients had better physical and psychosocial functioning than less physically active patients. For example, a meta-analysis by Motl, Sandroff13 indicated that planned physical training in patients could positively influence patients' balance and walking efficiency, experience of fatigue, efficiency of cognitive function, alleviation of depressive symptoms as well as improvement in quality of life. The effects obtained in the study were not very strong, although the authors emphasized the supportive importance of physical activity for patients' functioning13. Similar conclusions were reached in the meta-analysis conducted by Contrò et al.14 indicating that undertaking physical activity by patients with MS could improve their resistance to fatigue, alleviate depressive symptoms and increase their sense of quality of life.

However, undertaking physical activity in the form of intentional exercise and thereby increasing quality of life requires significant psychological and social resources from patients. Among the personal factors that increase the chance of engaging in health behaviours, including physical activity, the importance of HLoC is often emphasized. It refers to the patient's belief about the impact of his or her own actions on life events15, 16. Given the dynamic nature and low predictability of the disease in patients with MS, HLoC seems particularly relevant to taking or not taking personal health-related actions and subsequently improving or reducing quality of life. Patients with an internal locus of control believe that they have a personal influence on their behaviour and life events. Thus, they have beliefs that their health status depends on actions they can take themselves, which are preceded by their own decisions. In contrast, people with an external locus of control claim that events in their lives are beyond their control and dependent on external factors. For example, patients with a high level of external HLoC believe that their health depends mainly on the people who support them in this regard (i.e. medical personnel) or purely random factors15. The number of studies on the relationships between locus of control and health-promoting behaviour, including the uptake of physical activity by patients with MS, is quite limited. However, it is possible to predict the directions of these relationships based on studies on people with other disease entities. For example, intrinsic HLoC in cardiac patients was associated with more frequent physical activity undertaking. In contrast, patients' belief that their health was determined by chance was associated with lower levels of physical activity17. Among patients with MS associations between the HLoC and quality of life and its correlates have been demonstrated. It has been reported that internal HLoC generally promoted high health-related quality of life in patients18. In addition, patients with an internal HLoC were more effective in their therapeutic interventions, had a greater awareness of their illness, and received more social support. In contrast, external HLoC was associated with experiencing depressive symptoms, anxiety, stress and greater levels of disability19. It has also been shown that patients with internal HLoC tend to use more effective coping strategies that are associated with improvements in their mental health20.

Despite previous research on the relationship between HLoC and the presented psychosocial correlates of quality of life in patients with MS (e.g. depressive symptoms, experience of stress, etc.), relatively little work has addressed the potential explanation of these relationships. Given the rationale and findings presented here, we hypothesize that patients' HLoC is related to patients' health-related quality of life via health-promoting behaviors, in particular via patients' engagement in physical activity. We hypothesize that patients with an internal HLoC, having a sense that their health depends on their own actions, are more likely to intentionally engage in health-promoting physical activity. In turn, undertaking physical activity increases the chance of experiencing higher levels of health-related quality of life by patients. We also hypothesize that patients with an external HLoC are less likely to undertake physical activity, which ultimately reduces the chance of experiencing a higher health-related quality of life. In addition, we hypothesize that the importance of physical activity for the psychosocial functioning of patients with MS may be dependent on the degree of disease progression associated with disease duration. Hence, we hypothesized that the relationship between HLoC and physical activity, the relationship between physical activity and health-related quality of life, and the relationship between HLoC and quality of life are dependent on disease duration. It is therefore possible that physical activity is more important for the effective psychosocial functioning of people with longer disease duration associated with greater disease severity. These issues will be addressed in the present study.