Patients with acute spinal cord injury (SCI) with paralysis experience rapid and marked muscle atrophy below the level of the lesion. Muscle is lost above the lesion due to enforced bed rest associated with immobilization. Presently, there is no viable method to quantify muscle loss between the time of injury to the initiation of rehabilitation and re-mobilization. Furthermore, to assess the efficacy of any physical- or pharmacological-intervention necessitates the ability to accurately determine the impact of these treatments on muscle mass and function. Our results are presented from measurements of regional potassium (K) in the legs of persons with chronic SCI. The intracellular body K, comprising about 97% of the total body K, is indicative of the metabolically active cell mass, of which over 50% is located in the skeletal muscle (SM). To assess regional variations in SM mass in the legs, a partial body K (PBK) system designed for this purpose was placed on a potentially mobile cart. The SM mass measured by PBK in an able-bodied control cohort (n=17) and in patients with chronic SCI (n=21) was 17.6±0.86 kg, and 11.0±0.65 kg, respectively, a difference of about 37.5%. However, the difference in the lean tissue mass of the legs obtained by dual-energy absorptiometry (DXA) in the same cohorts was 20.5±0.86 kg and 15.5±0.88 kg, or about 24.4%. PBK offers a novel approach to obtain regional K measurements in the legs, thus allowing the potential for early and serial assessment of muscle loss in SCI subjects during the acute and sub-acute periods following paralysis. The basic characteristics and performance of our PBK system, and our calibration procedure are described in this preliminary report.