2.1. Patient selection and outcomes measurement
The patients and outcomes included in this study have been previously described as part of an ongoing natural history study of DMD [9,10,22]. Included boys were enrolled at 11 tertiary neuromuscular care centers in Italy between January 2008 and June 2010. Inclusion required boys to have genetically confirmed DMD, to be aged 5 years or older, to be able to walk independently for at least 75 meters, and to be free of moderate or severe learning difficulties or behavioral problems. All boys fulfilling these criteria at the study centers were enrolled. The present study sample includes 96 boys with 3 years of annual 6MWD assessments conducted at the same study center at intervals of 12 ± 3 months. 6MWD was assessed at enrollment and annually according to the American Thoracic Society guidelines [23], with the modification of