To date, it is clear that sedentary behaviour is strongly related to an increased risk of type II diabetes, cardiovascular disease and premature mortality. People suffering from chronic disabilities appear to be particularly susceptible to a sedentary lifestyle and inactivity due to primary disease symptoms. To date, this is an important new research topic in Multiple Sclerosis (MS, \~2.3 million people worldwide, \~10-12.000 diagnosed in Belgium) treatment, since previous research reported a significantly higher prevalence of sedentary behaviour in persons with MS (PwMS) compared to healthy controls (HC). PwMS are reported to have a 40% lower daily step count compared to healthy inactive persons and tend to accumulate their sedentary time in longer bouts. As described above and similar to other chronic conditions, a sedentary lifestyle also makes PwMS more vulnerable to the accumulation of important cardiometabolic comorbidities that seem inactivity-related rather than a direct result of non-reversible tissue injury. Such comorbidities include impaired whole body glycaemic control, an abnormal blood lipid profile, an unhealthy body composition and hypertension. In this respect, it is important to note that corticosteroids, which are often used to treat MS patients pharmacologically, elevate fasting glucose and insulin concentrations and induce insulin resistance in HC therefore probably also increase several cardiometabolic risk factors in MS.

Up to now, research in MS has been focused on structured exercise and its positive effects on functional parameters are well-known (e.g. improvements in cardiorespiratory fitness, muscle strength, balance, fatigue, cognition, quality of life and respiratory function). However, evidence is growing that sedentary time, independent of the (dis)practice of structured exercise, is an important independent health risk factor. Consequently, any strategy that also improves cardiometabolic health may help to further optimize rehabilitation in MS. Breaking up and reducing sedentary time with easy, daily activities such as household activities and other activities which increase light-intensity walking and standing, known as non-exercise physical activity (NEPA) may be such a strategy.

NEPA has already been shown to significantly improve cardiometabolic risk markers in healthy, sedentary subjects, type II diabetes patients and obese adults and it involves lower intensity physical activities that are probably more feasible for PwMS. Moreover, with comparable activity workloads, reducing sitting time by NEPA of longer duration decreases insulin levels and fasting lipid levels more than performing one structured exercise bout of moderate intensity that is usually described in current activity guidelines. So far however, acute exercise bouts and NEPA effects on cardiometabolic health in this population have never been described. Therefore, the aim of this study is to investigate whether (1) cardiometabolic health (glycaemic control, blood lipids, inflammation markers and blood pressure) of persons with MS improves when sedentary time is reduced and (2) NEPA results in better cardiometabolic health parameters than (a shorter daily bout of) moderate-intensity exercise when workload of both activities is identical in this population.