OBJECTIVE: Our aim was to assess the role of quantitative H and Na MRI methods in providing imaging biomarkers of disease activity and severity in patients with Facioscapulohumeral muscular dystrophy (FSHD). METHODS: We imaged the lower leg muscles of 19 FSHD patients and 12 controls with a multimodal MRI protocol to obtain STIR-Tw images, fat fraction (FF), water T (wT), water T (wT), tissue sodium concentration (TSC), and intracellular-weighted sodium signal (inversion recovery (IR) and triple quantum filter (TQF) sequence). In addition, the FSHD patients underwent muscle strength testing. RESULTS: Imaging biomarkers related with water mobility (wT and wT) and ion homeostasis (TSC, IR, TQF) were increased in muscles of FSHD patients. Muscle groups with FF > 10% had higher wT, wT, TSC, IR, and TQF values than muscles with FF < 10%. Muscles with FF < 10% resembled muscles of healthy controls for these MRI disease activity measures. However, wT was increased in few muscles without fat replacement. Furthermore, few STIR-negative muscles (n = 11/76) exhibited increased wT, TSC, IR or TQF. Increased wT as well as Na signals were also present in muscles with normal wT. Muscle strength was related to the mean FF and all imaging biomarkers of tibialis anterior except wT were correlated with dorsal flexion. CONCLUSION: The newly evaluated imaging biomarkers related with water mobility (wT) and ion homeostasis (TSC, IR, TQF) showed different patterns compared to the established markers like FF in muscles of FSHD patients. These quantitative biomarkers could thus contain valuable complementary information for the early characterization of disease progression.