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Purpose 23Na MRI demonstrated increased tissue sodium concentrations in a number of pathologies. Acute atrophy results in muscle fibre volume shrinking that may result in a relative increase of extracellular volume and might affect sodium concentration. Thus, we hypothesized that local unloading of the calf muscles would lead to a decrease in muscle volume and an increase in muscle tissue sodium concentration. Method One lower leg of 12 healthy male subjects was submitted to a 60 day long period of unloading using the Hephaistos orthosis, while the other leg served as control. 23Na MRI and 2D PD-weighted Dixon turbo spin echo were obtained from the control and orthosis leg using a 3T scanner. For quantification, a sodium reference phantom was used with 10, 20, 30, and 40 mmol/L NaCl solution. Result Tissue sodium concentration (TSC) increased as an effect of unloading in the orthosis leg. Relative increases were 17.4 ± 16.8% (P = 0.005) in gastrocnemius medialis muscle, 11.1 ± 12.5 (P = 0.037) in gastrocnemius lateralis muscle, 16.2 ± 4.7% (P < 0.001) in soleus muscle, 10.0 ± 10.5% (P = 0.009) in the ventral muscle group, and 10.7 ± 10.0% (P = 0.003) in the central muscle group, respectively. TSC in the control leg did not significantly change. In the orthosis leg, muscle volume decreased as follows: medial gastrocnemius muscle: −5.4 ± 8.3% (P = 0.043) and soleus muscle: −7.8 ± 15.0% (P = 0.043). Conclusion Unloading atrophy is associated with an increase in muscle sodium concentration. 23Na MRI is capable of detecting these rather small changes.