Daily physical activity (PA) is a predictor of survival in the general population and has become a significant outcome measure in patients with COPD. Recent studies have demonstrated an independent association between PA, hospitalizations and mortality. Increased PA has significant benefits for patients with COPD. In most studies PA has only been assessed at one time point. Little is known about longitudinal changes in PA and data on determinants of changes in PA are scant. Moreover, information on best approaches to enhance PA in a population with decreasing PA is missing. To evaluate the long-term effect of a coaching program which is aimed at helping patients with severe COPD (FEV1<50%) to enhance their level of daily physical activity, in addition to usual care. Furthermore, this study aims to evaluate mechanisms that are responsible for changes of physical activity over time such as sociodemographic factors, lung function, comorbidities, vascular function, exacerbations, and medication.