What is Bioelectrical Impedance Analysis (BIA)?
The impedance is used to determine total body water (TBW), which can then be used to derive your fat-free mass—the portion of your body that does not contain fat, including your muscle and bone—and then body fat.
How Bioelectrical Impedance Analysis (BIA) Works
To understand how bioelectrical impedance analysis works, it is important to understand the core concepts of resistance and reactance and how they are used to calculate impedance.
When an electrical current is sent through your body, components such as body water, fat, muscle, and bone present varying levels of resistance. Body water is highly conductive and makes up a significant percentage of your body. Therefore, the greater the amount of body water, the less resistance there is.
For example, your muscles contain a high percentage of water, resulting in less resistance. Body fat, on the other hand, contains very little water and presents much higher resistance compared to muscle mass or body water.
To better illustrate this concept, imagine the flow of cars in traffic. The cars on the highway represent the current, and the number of lanes on the highway represents the amount of water in the body. A highway with more lanes allows for the cars to go faster, resembling the case where there is more water or muscle mass, and the current can flow more easily. If we close several lanes in the highway (or have less water), the same number of cars will take longer to move through the highway (higher resistance).
The difference in the resistance to the electrical current between water, muscle, and fat cells is known as reactance.
In addition to the resistance described above, the human body presents another type of resistance, called reactance.
The body is composed of trillions of cells, each of which is protected by a cell membrane that separates the inside of the cell (intracellular) from the outside (extracellular) environment.
Impedance: The combination of resistance and reactance
Impedance is the vector sum of resistance and reactance. It is what BIA devices uses to determine your body composition and is measured in ohms (Ω).
BIA defines the human body as a cylinder and uses two mathematical concepts to describe the relationship between impedance and body water:
- Volume of a cylinder (Volume = Length x Area)
- Characteristic of impedance: Impedance is inversely proportional to cross-sectional area and directly proportional to the length of the cylinder (Impedance = Length ➗ Area)
Using the impedance and length of the cylinder (or height of the individual), BIA can determine the volume of total body water. From there, body composition is determined, including muscle mass and body fat percentage.
Now that you understand how bioelectrical impedance analysis (BIA) works, keep scrolling to learn how BIA technology has evolved through the years.
The History of BIA Technology
1969 - Hoffer et al. and the Impedance Index
In 1969, Hoffer et al. carried out a series of experiments to prove that total body water and bioelectrical impedance were highly correlated, suggesting that impedance measurements could be used for determining total body water.
Impedance of the right half of the body was measured, including the right arm, torso, and right leg. The squared value of this length measure divided by impedance had high correlations with total body water (r = 0.92). This correlation was more in agreement with the gold standard technique when compared to other indices, including body weight. The equation Hoffer et al. showed to be correlated to body water is the impedance index (Height2➗ Impedance) used in BIA today.
1979 - RJL Systems and the First Impedance Meter
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.
1980's - Discovering limitations to BIA with Empirical Data
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.
1980's - Development of Empirical Equations
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.
1980's - Home-use BIA devices
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.
1992 - Kushner and the Proposal of Multi-frequencies with Segmental Analysis
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.
1996 - Dr. Cha creates the InBody Body Composition Analyzer
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.
Revolutionizing BIA Technology with InBody
InBody’s medical-grade body composition analyzers rely on four pillars of technology to provide clients with accurate and precise direct segmental measurement multi-frequency bioelectrical impedance analysis (DSM-MFBIA) results that have been extensively validated to gold-standard methods.
8-Point Tactile Electrode System
Direct Segmental Measurements
No Empirical Estimations
BIA Tech Problem
The ability to distinguish between extracellular and total body water is important to identify fluid imbalances related to acute inflammation or edema. Many BIA devices use only one frequency at 50 kHz to measure impedance. However, since frequencies at 50 kHz do not fully pass through the body’s cells, accurate measurement of total body water is not possible. As a result, patients with increased extracellular water may be misidentified as being healthy.
InBody uses a combination of low and high frequencies to determine extracellular, intracellular, and total body water. The use of multiple frequencies allows InBody devices to achieve a high level of precision. Medical practitioners can use InBody for measurements of body composition and fluid status.
Total body water (TBW) is stored throughout the body and can be separated into 2 compartments:
- Intracellular water (ICW) – aka cytosol; water located inside cells of muscles, bones, organs, etc., comprising the majority of TBW.
- Extracellular water (ECW) – water in the blood and interstitial fluids
Early BIA devices used a single 50 kHz frequency to calculate TBW. However, the 50 Khz frequency is not strong enough to fully pass through the body’s cells and therefore cannot give an accurate measurement of ICW. Therefore, ICW was estimated proportionally based on the ECW. This estimation was used to determine TBW, lean mass, and fat mass.
The estimation of intracellular water was based on the assumption that the ratio of ICW to ECW in healthy adults is about 3:2. However, individuals with body compositions that differ from conventionally healthy adults, such as elderly, obese or chronic disease patients, often have a higher ratio of ECW. Thus, in these patient populations, relying on the 3:2 ICW:ECW ratio could result in significant error.
InBody uses multiple frequencies ranging from 1 kHz to 1 MHz to provide precision body water analysis. Electrical currents interact differently with the cells at different frequencies, which allows the InBody to quantify the different fluid compartments. Low frequencies are better suited for measuring ECW, while high frequencies can pass through cell membranes to measure ICW and therefore TBW.
An accurate measure of TBW and the ability to analyze ICW versus ECW allows for a deeper analysis of individual body composition. Compartmental water measures can be used to properly quantify and identify changes in fluid balance to reflect nutritional status and fitness progress.
BIA Tech Problem
If the starting measurement position changes, the length of the measured cylinder also changes. This directly impacts impedance and introduces error.
8-Point Tactile Electrode System with Thumb Electrodes
8-Point Tactile Electrode System with Thumb Electrodes
When the human body comes in contact with an electrode, resistance occurs. To accurately measure the resistance in the human body, it is important to control the measurement location. Competitors’ products usually lack the thumb electrode or have the hand electrodes close together. These designs can cause measurements to start in the palm, which has a high impedance and can cause inaccuracies, or lead to inconsistent measurement starting points, reducing the reliability of results.
The anatomical design of the hand electrode creates a simple holding position that is easy to reproduce. Utilizing the anatomical characteristics of the human body, when an InBody user grasps the hand grip, current flows from the palm electrode and the electrical energy, or voltage, is initiated at the thumb electrode.
When current and voltage overlap, impedance can be measured. By separating current and voltage into the hand and foot electrodes, the point of overlap can be controlled to isolate the five cylinders of the body (limbs and torso) and consistently start at the same location on the wrists and ankles for reproducible results. With this design, the point of measure stays the same even when the user changes the holding position of the hand electrode or the contact points on the hands and feet.
BIA Tech Problem
Traditional BIA views the human body as one cylinder. However, the torso of the body needs to be measured separately because its short length and large cross-sectional area mean that even a small measurement mistake can lead to substantial error.
Direct segmental measurement bioelectrical impedance analysis regards the human body as five cylinders: left arm, right arm, torso, left leg, and right leg. InBody independently measures each cylinder to provide accurate measurements for the entire body.
Direct Segmental Multi-frequency Bioelectrical Impedance Analysis (DSM-BIA)
Traditional BIA systems viewed the human body as a single cylinder, using whole-body impedance to determine total body water.
However, this method had a number of flaws:
- It assumed the distribution of lean body mass and body fat across all segments of the body is consistent.
- The shape and length of the arms, legs, and torso differ; as such, the body should not be considered one cylinder, but rather as five separate parts.
- Since impedance is based on length and cross-sectional area, the calculation of TBW is inaccurate because each segment of the body has different length and cross-sectional area.
One of the biggest problems with the single cylinder method is the lack of a separate torso measurement. The torso has the shortest length and highest cross-sectional area, which results in a very low impedance (typically 10-40 ohms). However, the trunk comprises about 50% of an individual’s lean body mass (LBM). Therefore, small errors in torso impedance have significant impact on body composition results.
With whole-body impedance measurement, the torso impedance is not observed separately and thus, changes in torso impedance cannot be quantified. Because of the large amount of lean mass in the torso, small variability in impedance measures can have a drastic effect on how the results are interpreted. For example, when isolating the torso, a change of just 3 ohms can lead to a difference of several pounds of lean mass; however, using a single cylinder model, this significant distinction would be observed as less than 1% difference in the whole-body impedance measurement.*
*Based on hypothetical example. Differences and percentages may vary based on the individual.
Some BIA devices avoid the torso measurement entirely. For example, with many BIA scales, only the impedance of your legs and a small part of your torso are measured. Similarly, with handheld BIA devices, only the impedance of your arms and a small portion of your torso are measured. With this design, the rest of the body must be estimated. Such devices that exclude the torso and estimate the majority of the body’s impedance may result in great errors in total body water and, in turn, lean body mass and fat mass.
With InBody’s direct segmental technology and 8-point tactile electrodes, these potential errors are removed, providing more accurate body composition results.
BIA Tech Problem
In many bioimpedance technologies today, empirical equations are incorporated to compensate for technological flaws, including the lack of torso impedance (due to whole-body impedance measurement), single frequency measurements (which are unable to differentiate between water compartments), and lack of reproducibility (from electrode placement or positioning). These empirical equations utilize data, such as age, gender, and ethnicity, to calculate body composition based on common trends rather than relying solely on the individual’s actual body composition.
InBody measures body composition without relying on empirical assumptions based on age, gender, ethnicity, or body shape, producing accurate and precise results that are validated to gold standard methods. Put simply, InBody provides individualized feedback for better tracking of progress to help you achieve your goals.
No Estimations or Empirical Equations
Because many BIA devices today use whole-body and single frequency measurements, they commonly incorporate empirical equations to calculate a user’s body composition. These equations help compensate for the lack of torso impedance measurement and ability to differentiate between body water compartments by plugging in empirical data based on factors, such as age, gender, and ethnicity.
Empirical equations can give a somewhat accurate estimate of a user’s body composition if the user has a typical body shape for their specific age, gender, and ethnicity. For example, these equations may take into consideration that muscle mass generally decreases with age and that males tend to have more muscle mass than females. This expectation is then reflected in the results. Therefore, the problem with relying heavily on empirical estimations is that your results are predetermined, regardless of your actual body composition.
Testing on the InBody will give a user the same body composition measurements whether that user tests as a male or female because the InBody does not use empirical estimations based on factors of age, gender, ethnicity, athleticism, or body shape in its measurements. Instead, your body composition is determined based on the impedance values found from each of the body’s 5 cylinders and the distribution of body water. In other words, direct measures of your impedance and water distribution are used to determine your individualized results.
High Correlation to Gold Standard Methods
Because of its technology, InBody has been found to be one of the most accurate BIA devices on the market. In fact, it has been found to have a high correlation of 0.99 to DEXA for lean body mass in a population of normal weight adults.
Article: Accuracy of direct segmental multi-frequency bioimpedance analysis in the assessment of total body and segmental body composition in middle-aged adult population
Interested in learning more about how InBody can fit into your practice?
InBody devices are used by leading professionals around the world to give their clients results they can trust and track.