Becker muscular dystrophy (BMD) is a genetic neuromuscular disorder leading to muscle weakness and degeneration. Quantitative MRI has been widely proposed to characterize skeletal muscle tissue structure of BMD and revealed increased intramuscular fat fraction associated with functional decline. In this study, we measured the extracellular volume fraction (ECV) of skeletal muscle tissues using MR fingerprinting with water and fat separation. We evaluated this variable as an early imaging biomarker of skeletal muscle tissue alterations. Compared to healthy controls, we showed that the muscles of BMD subjects present elevated ECV before fatty replacement occurs.