E.Balestri , M.Ambroni, S.Dall'Ara, A.Miano “ Efficacy of physical exercise for mucus clearence in patients with cystic fibrosis” Pediatric Pulmonology Suppl.27, 2004 Citato in: Elkins M, Jones A, van der Schans CP. Positive expiratory pressure physiotherapy for airway clearance in people with cystic fibrosis. Cochrane Database of Systematic Reviews 2006