InBody Technology

Image Credit: RJL Systems

In 1979, RJL Systems commercialized the impedance meter for the first time and the BIA method began to gain popularity. The device measured impedance by attaching electrodes to the back of the right hand and on top of the right foot. These electrodes then controlled the flow of a 50kHz current through the right half of the subject’s body.          

Prior to this, body composition could only be measured by caliper or underwater weighing. Such methods needed to be carried out by skilled technicians, were uncomfortable, required complicated installation or use of equations, and could not accommodate a wide variety of populations. Alternatively, BIA was easy, fast, less expensive, and non-invasive. Therefore, many body composition researchers, nutritionists, and medical experts began to use BIA.

Sample empirical estimation equation

Source:  Schoeller, D. A., & Kushner, R. F. (1989). Determination of body fluids by the impedance technique. IEEE Engineering in Medicine and Biology Magazine, 8(1), 19-21.

In the 1980’s, research accelerated the evolution of BIA. Studies proved BIA measures had high correlations with gold standard methods, such as underwater weighing and DEXA. However, technical limitations of BIA began to surface in the late 1980s.

Two primary limitations of BIA were its assumption of the human body as a single cylinder and its use of a single frequency (50 kHz). This technique may have worked for users with standard body types, but it was not as accurate for other populations that might not fit a conventional mold, such as fit elderly adults and most medical patients. To increase the accuracy of results, researchers derived various population-specific equations for determining body composition. These equations were based on what is known as empirical data.

Sample empirical estimation equation

Source: Lukaski, H. & Bolonchuk, W.W. (1989). Estimation of body fluid volumes using tetrapolar bioelectrical measurements. Aviation Space and Environmental Medicine, 59(12), 1163-1169.

Empirical data is knowledge acquired by means of observation or experimentation. By collecting data from a sample population deemed to represent the expected characteristics of the entire population, researchers can derive equations that may be used to predict outcomes. In body composition, researchers have identified trends in muscle and fat mass and have used this data to predict body composition based on specific variables.

In 1986, research was published in which the impedance index was combined with factors such as body weight and gender into empirical equations. Over time, numerous other equations were developed based on additional factors such as age, ethnicity, and body type.

Although empirical estimations have the potential to provide an accurate estimate of a healthy individual’s body composition, significant problems arise when they are used for medical purposes in which accurate and precise assessments are a requirement. 

For instance, age is a common factor in empirical equations used for body composition. In general, most individuals tend to lose lean body mass with age due to a sedentary lifestyle. Based on this trend, empirical equations often skew lean body mass up for younger individuals and down for older individuals. However, such data manipulation can cause inaccuracies and significant misassessments regarding health risks in population outliers such as obese youth or fit older adults.

Suppose a device that relies on empirical equations to estimate body composition is used on two people who have the same amount of lean body mass, but one person is 30 years old and the other is 40 years old. Because most individuals tend to lose lean body mass with age, even though the two individuals have the same amount of lean body mass, the empirical equations will skew the 40 year old’s lean body mass down, resulting in a higher percent body fat. 

In the late 1980s, Japanese manufacturers released various types of BIA body composition devices for general public use. Gradually, BIA devices became more popular for personal use rather than professional medical assessments due to technological constraints (mentioned in the previous section). Some devices measured the impedance between both feet as the user stands on the scale, while others measured the impedance between both hands while holding the device.

In 1992, Dr. Robert Kushner proposed that the technical limitations of BIA could be improved by measuring the human body as five separate cylinders (right arm, left arm, torso, right leg, left leg) instead of one. Each of these cylinders have different lengths and cross sectional areas, resulting in varying impedance values.

When considering the single cylinder model, the thinness and smaller cross-sectional area of the limbs reduce their impact on whole body impedance. However, since the torso makes up 50% of lean body mass, measuring the impedance of the torso separately is crucial to accurately determine body composition.

According to Kushner, measuring segmental impedance alone would not be sufficient; instead, all five body cylinders would also need to be measured at different frequencies to distinguish intracellular, extracellular and total body water. This distinction would allow for a better understanding of fluid distribution, providing an accurate measure of the hydrated state of lean mass. 

In other words, the technical limitations of BIA could be overcome by measuring the different body segments at different frequencies.

In 1996, Dr. Kichul Cha developed InBody, the world’s first 8-point tactile electrode system with direct segmental analysis to measure impedance in the five different body cylinders using multiple frequencies.

By doing so, the impedance in the limbs and torso were measured separately, yielding highly accurate results without using empirical data based on factors like age, gender, ethnicity, athleticism, and body shape. Thus, the InBody DSM-MFBIA body composition analyzer is a precision medical device.

Many BIA products today provide segmental measures of muscle and fat mass, but most of these products are still unable to take segmental impedance measurements, particularly in the torso. The InBody measure each segment separately and shows the impedance values of all five cylinders of the body at each frequency in the Impedance Section of the InBody Result Sheet.