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Direct Segmental Multi-frequency Bioelectrical Impedance Analysis (DSM-BIA) separately measures the impedance of the trunk, arms, and legs of our body. This separates the flows of currents and electric potential. This new measurement concept measures the resistance of the cross point of two flows and takes the resistance values of the arms, legs and the body trunk. This technology was first developed by Biospace, and is protected by patents in various countries in the world. Conventional equipment, as shown in the figure below, uses the whole body impedance measurement method that measures one impedance value that included the arms, legs, and trunk with electrodes that were applied to the arm and leg.

The human body is composed of five cylinders: the arms, the legs and the trunk. With the technology of the InBody, the trunk is directly measured. The trunk is occupied by various internal organs and its metabolic characteristics are different from the other parts of the body. It is important to precisely and directly measure the trunk, although difficult, since the trunk has very low resistance compared to the arms and legs. Resistances of arms and legs are normally between 200~500Ω while those of body trunks are between 20~30Ω. Despite its low resistance, the trunk accounts for 50% of total body weight. Essentially, an error of 1~2Ω in arms or legs has a minimal effect upon final measurement, but a 1~2Ω error in the trunk will cause major error in the final value for this complex region. Therefore the overall body composition can be accurately calculated only when the slightest changes in impedance of the trunk can be detected.

	Electric current has a different penetration force depending on frequency. Low frequencies travel down the flow of the external surface water and are unable to penetrate the cell wall, thus measuring the water content outside of the cell. A higher frequency will measure the water content inside of the cell because it penetrates the cell wall. By using a diverse set of frequencies, the InBody can accurately measure water contents inside and outside the cell separately. Also, with the usage of several different frequencies, you are able to provide more information on the body composition because more resistance values are attained. The InBody520 is based on six different frequencies that measure each segment, providing a total of 15 separate impedance values, thus providing accurate body composition information.

When measuring impedance through any type of electrode, contact resistance occurs, and it is important to control the contact resistance in order to correctly measure the resistance in the body. The InBody uses a unique method of electrode placement. The 8-point tactile method separates the current and voltage starting point, thus measurement always begins at a fixed point in the wrists and ankles. This provides high reproducibility and correct body impedance measurement because the contact resistance from the skin has been removed.

Empirical estimations are values that are achieved through observation. This data is used for calibrating accuracy in typical BIA that measures body composition through whole body impedance and is useful while measuring the typical body type. The problem occurs when measuring those that are not ideal in terms of body composition, which is the majority of the population that requires body composition analysis. Separate impedance measurements of the arms, legs, and trunk can sufficiently reflect characteristics from body system density or distribution of body composition of the person receiving an examination. With the InBody, output values will be correct regardless of age, sex, race, and body type. Acknowledged for its accuracy and reproducibility by the inherent technology which correctly measures segmental impedance, specifically isolating the trunk, Biospace does not use empirical data in calculating results. Therefore, the InBody always provides correct results regardless of sex, age, race, and particularly, non-average body types.