Cystic fibrosis (CF) is characterized by various extrapulmonary manifestations, including altered skeletal muscle function, with both quantitative (e.g. reduced muscle mass) and qualitative (e.g. impaired oxidative function) impairments that may have a negative impact on exercise tolerance and quality of life. These abnormalities have traditionally been related to disease (e.g. systemic inflammation) or behavioral factors (e.g. increased physical inactivity). However, most of the studies that observed these abnormalities and tried to shed light on the underlying factors were either small or conducted before the widespread of CFTR (Cystic fibrosis transmembrane conductance regulator) modulators that have profound impact on the trajectory of the disease. While several studies suggested that the major recent improvements in therapeutics, including highly effective CFTR modulators, may have positive effects on skeletal muscle function, either directly (e.g. improved mitochondrial function) or indirectly (e.g. reduction in physical inactivity), no studies to date have thoroughly investigated this issue in a representative sample of people with CF. The absence of recent data on muscle function and physical activity levels casts doubt on the relevance of recommendations on exercise training in this population that were published before the widespread use of highly effective CFTR modulators. This study aims to compare muscle function, measured according to the latest recommendations of the European Cystic Fibrosis Society (Saynor et al., 2023), and physical activity of children and adults with CF under CFTR modulators, compared to age- and sex-matched healthy individuals. We hypothesize that the strength, endurance, muscle power, and physical activity levels of individuals with cystic fibrosis, treated with CFTR modulators, remain reduced compared to healthy individuals.