Background Peripheral muscle weakness and exercise intolerance are prevalent (56%) in cystic fibrosis (Trooster et al, 2009). Physical inactivity is likely to be an important underlying factor. Those conditions are associated with a poor prognosis (Nixon et al, 1992). The effect of intravenous antibiotherapy on peripheral muscle and physical activity remains unclear. Wieboldt et al showed that quadriceps strength at the hospital admission for an exacerbation was lower than before and one month after hospitalisation while Burtin et al showed that individual changes in quadriceps force were correlated with daily time spent activities of at least moderate intensity (Wieboldt et al, 2012; Burtin et al, 2013). The effect of intravenous antibiotherapy itself is poorly known in cystic fibrosis. Moreover, they did not study the impact on muscular strength of hospitalisation versus home treatment. In contrast, in COPD, it is well known that exacerbations are associated with a decline of muscle mass and strength and that repeated exacerbations lead to a more rapid decrease in fat free mass (Spruit et al, 2003; Jones et al, 2015; Joppa et al, 2016).