CFTR and cystic fibrosis: Design: RCTInclusion criteria: a confirmed diagnosis of CF (i.e. a typical clinical picture and at least two positive sweat tests or CF-relevant mutations on both alleles of the CFTR gene), an age of 12 years or older, an FEV1 of 35% or higher of predicted and the ability to perform PAExclusion criteria: non-CF-related chronic diseases and CF-related conditions posing an increased risk to the patient when exercising