1 Harefield Respiratory Research Group, Royal Brompton and Harefield Clinical Group, Guy's and St Thomas' NHS Foundation Trust, London, UK.
2 Division of Physiotherapy, Dept of Health Sciences, College of Health, Medicine and Life Sciences, Brunel University, London, UK.
3 Dept of Respiratory Medicine, King's College Hospital, London, UK surinder.birring@nhs.net.
4 Centre for Human and Applied Physiological Sciences, School of Basic and Medical Biosciences, Faculty of Life Sciences and Medicine, King's College London, London, UK.