Progression of physical inactivity in COPD patients: the effect of time and climate conditions – a multicentre prospective cohort study - Boutou, AK; Raste, Y; Demeyer, H; Troosters, T; Polkey, M; Vogiatzis, I; Louvaris, Z; Rabinovich, RA; van, der Molen T; Garcia-Aymerich, J; Hopkinson, N (International Journal of COPD, 2019)