CHANGES IN LOWER-LIMB MUSCLE CROSS-SECTIONAL AREA AND TISSUE-FLUID VOLUME AFTER TRANSITION FROM STANDING TO SUPINE

Lower limbs show acute fluid shift in response to transition from upri ght to supine body position. It is hypothesized that this would affect tomographic estimations of muscle mass and composition. Seven healthy subjects were investigated during the initial 120 min of bed rest, us ing repeated computerized tomography (CT) and continuous bioelectrical impedance analysis (BIA). Thigh and calf muscle cross-sectional area (CSA) decreased (P < 0.05) by 1.9 and 5.5 % whereas fat CSA decreased (P < 0.05) by 4.1 and 4.4%, respectively. Radiological density (RD) of muscle showed a simultaneous increase (P < 0.05) by 4.8% in calf but not (P > 0.05) in thigh. No changes occurred (P > 0.05) in muscle or f at CSA or muscle RD in either thigh or calf between the first and seco nd hour of bed rest. Fluid shift, as estimated by BIA, showed an expon ential decay in thigh (tau(th) = 30 min) and calf (tau(c2) = 37 min) b y 2.5 and 8.7%, respectively, from first to 120 min of bed rest. Moreo ver, the calf showed an initial rapid (tau(c1) = 8 s) 2.2% decrease. T he demonstrated short-term changes in leg CSA were more pronounced in the calf than in the thigh. They were similar in muscle and subcutaneo us fat. These fluid shifts merit consideration when tomographic imagin g techniques are used to estimate muscle mass and composition.