BIA Body Composition Measurement Principles and Four / Eight-Electrode Selection

When choosing a body composition solution, it is easy to jump straight to “four or eight electrodes” and “single or multiple frequency” - but those choices all come back to the same underlying physics. Once you understand how BIA measures, what impedance really is, and why electrodes are arranged the way they are, selection becomes straightforward: which outputs you need, what product form you are building, and what cost you can accept largely determine the solution. This article follows that order, ending with a selection decision and the matching BestHealth modules.

How BIA measures body composition

Body composition describes what the body is made of - fat, muscle, bone, water, and so on. Compared with weight or BMI alone, it describes physical status far more accurately.

BIA (Bioelectrical Impedance Analysis) works by exploiting the large difference in conductivity between water and fat. A safe, tiny alternating current is passed through the body: water-rich tissue (muscle, body fluid) conducts well and has low impedance; fat tissue holds almost no water, conducts poorly, and has high impedance. So lower measured impedance usually means more body water and muscle and a lower fat percentage; higher impedance means a relatively higher fat percentage.

Think of body water as a highway and the current as a car on it. If the highway has only that one car, there is almost no resistance (low impedance); but the more non-conductive “obstacles” such as fat and bone, the more resistance the current meets (high impedance). BIA measures how clear that highway is. In a BestHealth module, weight and impedance are measured by the module, while age, gender, and height are sent by the host over UART / USB or the specified interface; the built-in algorithm then outputs fat mass, muscle mass, water, and other body composition parameters.

What impedance actually contains: resistance, reactance, and phase angle

Many sources lump the BIA reading together as “impedance,” but impedance is the vector sum of two parts:

• Resistance (R): mainly from body fluid (intra- and extracellular water) opposing the current. More water means lower resistance.
• Reactance (Xc): mainly from the capacitive effect of cell membranes. The more intact the membranes and the more cells there are, the higher the reactance.

These do not add up simply - they combine as vectors 90° apart to form impedance (Z). From them you can also derive an important metric, the phase angle: the phase difference between the resistance produced as current flows through body fluid and the reactance produced as it crosses cell membranes. Phase angle reflects cell membrane health and cell structural stability; a lower phase angle generally indicates poorer cell health.

Single or multiple frequency: which water you actually measure

Frequency is the other key variable in BIA, because current at different frequencies takes different paths:

• Low frequency (e.g. 20kHz): current struggles to cross cell membranes and flows mainly through extracellular water (ECW).
• High frequency (e.g. 100kHz): current penetrates cell membranes, so total body water (TBW) participates in conduction.

So for the same person, high-frequency impedance is usually lower than low-frequency impedance. The algorithm uses the impedance trend across different frequencies to estimate extracellular water, total body water, and water distribution, then supports advanced parameters such as edema trend, cell status, and segmental water.

In practice:

• Single frequency (commonly 50kHz): low cost, simple to implement, enough for basic body composition such as whole-body fat, water, and muscle - the mainstream for home body-fat scales.
• Dual / multi-frequency (e.g. 20kHz and 100kHz): better separates intra/extracellular water and supports more complete water, segmental, and professional assessment parameters. Phase-angle output still depends on whether the module supports complex-impedance / reactance measurement and built-in algorithm output.

Why a four-electrode setup is required, and why two won’t do

With only two electrodes, the same pair must both inject the current and sense the voltage, so the skin-electrode contact impedance is measured in series with the body impedance. Contact impedance varies enormously with skin moisture, calluses, and pressure - far more than the body itself - making the result unusable.

The four-electrode method (also called tetrapolar) solves this by separating the excitation current electrodes (I) from the voltage sense electrodes (V). Current is injected through one pair, and voltage is sensed through a separate, independent pair. Because almost no current flows through the sense side, no voltage drop forms across its contact impedance, and the measured voltage reflects only the true impedance of that internal body path.

That is why every proper BIA solution - four-electrode or eight - follows the “separate current and voltage” tetrapolar principle. Four electrodes are the baseline of BIA; eight electrodes simply apply this principle to each limb and then combine the results.

Four-electrode: measuring a single path

A four-electrode solution uses 4 electrodes to measure the overall impedance of one path. There are two typical types:

• Two-leg (TwoLegs): step onto a body-fat scale with one electrode pair under each foot (one excitation, one sense). Current goes up one leg, through the pelvis, and down the other, measuring the overall “leg-to-leg” impedance, commonly at 50kHz (sometimes 50kHz & 100kHz dual frequency). Products: home / commercial body-fat scales.
• Two-arm (TwoArms): a handheld device with one electrode pair per hand. It measures “arm-to-arm” impedance, commonly at 50kHz. Products: wristbands, watches, treadmill handles, shared body composition kiosks, and other devices that can only contact the hands.

Four electrodes measure the overall impedance of a single path and cannot directly measure the segmental impedance of each limb and the trunk, so whole-body results depend more on single-path modeling and user profile data such as height, age, and gender. Weight is measured by the module, and the user profile is sent by the host over UART / USB or the specified interface. An eight-electrode solution measures more real segmental impedance and does not need age or gender empirical compensation to make up for missing segmental paths.

Matching BestHealth module: BMH05104-2 (four-electrode, integrating BIA + weight + heart rate over a UART interface).

Eight-electrode: segmental measurement

An eight-electrode setup places two electrodes on each limb (one pair each for left/right hand and left/right foot, 8 total). By switching which two electrodes inject current and which two sense voltage, it measures multiple path impedances, for example:

• Leg-to-leg (TL) and arm-to-arm (TA)
• Left body (LB) and right body (RB)
• Right arm to left leg (RALL) and left arm to right leg (LARL)

From these combined paths, the module’s built-in algorithm integrates multiple impedance measurements to produce segmental results for the left arm, right arm, left leg, right leg, and trunk. This makes the eight-electrode approach suitable for products with both hand and foot electrodes, such as stand-on scales with handgrips and professional body composition analyzers, when limb and trunk segment outputs are required.

With segmental impedance, an eight-electrode solution can output items that four electrodes cannot: segmental fat / muscle (each limb plus trunk) and left-right balance. If the selected module and its built-in algorithm support complex-impedance / reactance measurement, it can also output phase angle; with multiple frequencies it can also provide intra/extracellular water.

Matching BestHealth modules:

• BMH05108: eight-electrode dual-frequency body composition analysis module, supporting segmental measurement for stand-on scales with handgrips and body composition analyzers.
• BMH05109: eight-electrode 8-frequency body composition module for professional analysis products that need more frequency impedance data, phase angle and finer water assessment.

Four-electrode vs eight-electrode: the difference at a glance

How to choose: work backward from product form and required outputs

Selection should start not from “how many electrodes” but from your product form and which data you must show:

1. Home / commercial body-fat scale, only whole-body fat, water, muscle, cost-controlled → four-electrode TwoLegs (BMH05104-2).
2. Wearable or handheld device that can only contact the hands (wristband, watch, treadmill handle, shared device) → four-electrode TwoArms (BMH05104-2).
3. Need segmental limb fat / muscle, trunk data and professional reports → eight-electrode dual-frequency solution (BMH05108).
4. Need more frequency impedance data, phase angle, intra/extracellular water or water-distribution assessment → eight-electrode 8-frequency solution (BMH05109); actual outputs follow the module specification and enabled version.

If you are unsure, confirm two things first: which items the terminal must display, and whether the device lets the user contact the hands, the feet, or both. Settle those, and the electrode count and frequency are essentially decided.

How to frame accuracy

Be clear: BIA is an indirect body composition measurement method. In ordinary health-management products, it provides trend reference; in Class II medical-device or other medical-grade complete products, use must follow the product’s certification / registration scope, intended users, and instructions for use. In either case, accuracy depends on complete-product structure, electrode contact, posture, and measurement timing, so even the same device should be used at a fixed time under fixed conditions, looking at long-term trends rather than a single absolute value.

When the product structure and measurement conditions meet requirements, BestHealth body composition modules have been compared against the DEXA gold standard by a third-party institution: the four-electrode solution shows about 0.93 correlation, while the eight-electrode solution can reach a correlation above 0.98. These figures represent the ceiling after the module completes weight measurement, impedance measurement, and built-in algorithm calculation. Customers do not need to design the measurement front end or body composition algorithm again; they need to follow the datasheet application circuit to connect power, the load cell, hand / foot electrodes, and UART / USB interfaces, then send age, gender, and height over the specified interface to read the module’s body composition outputs. BMH05108 / BMH05109 support customer medical-device certification projects, and multiple customers have already completed certification using these modules; for certification materials, project cases, or integration guidance, please contact us. Whether the final product reaches that ceiling depends on the customer’s structure design, electrode contact, posture control, and cable routing. If the electrodes, cabling, or contact are faulty, no algorithm can recover it. How to troubleshoot that during the debugging phase is covered in the companion article, How to Use Impedance Data to Troubleshoot BIA Body Composition Measurement.

FAQ

Which is more accurate, four or eight electrodes?

For BestHealth modules, the eight-electrode solution has a higher accuracy ceiling. A four-electrode solution measures a single path and uses height, age, and gender profile data to estimate whole-body results; when product structure and contact conditions meet requirements, its correlation against DEXA is about 0.93, enough for home and commercial body composition products. An eight-electrode solution measures segmental impedance across the hands, feet, and trunk, so it is theoretically closer to the body’s real impedance distribution; when the module’s built-in weight measurement, impedance measurement, and algorithm capability are allowed to perform by the product structure, contact conditions, and user posture, BestHealth eight-electrode solutions can reach correlation above 0.98 against DEXA. In short, eight electrodes can be more accurate, but the complete product must let the module performance show.

Why are most home body-fat scales four-electrode?

Because a home scale’s core need is whole-body fat percentage, water, and muscle, which four-electrode TwoLegs covers - at low cost, with the user just stepping on, and with little sensitivity to posture. Eight electrodes require gripping handles and proper posture, better suited to scenarios chasing professional data.

Is multi-frequency mandatory, or is single frequency enough?

It depends on the required outputs. For basic body composition, 50kHz single frequency is usually enough. Phase angle itself does not require multi-frequency measurement; the key is whether the module hardware measures the reactance component and whether the software algorithm outputs it. To separate intra/extracellular water and provide more complete water and professional segmental assessment, dual / multi-frequency measurement (such as 20kHz and 100kHz) is needed.

Are eight electrodes always better than four?

Eight electrodes have a higher measurement ceiling and are better suited to professional segmental assessment, but that does not mean every product should use them. They require simultaneous hand and foot contact, with higher cost, electrode-structure complexity, and posture requirements. If the product only needs basic whole-body composition, the four-electrode BMH05104-2 is the better fit.

What is phase angle and what is it for?

Phase angle is the phase difference between body-fluid resistance and cell-membrane reactance, reflecting cell membrane health and structural stability. It varies with gender, age, and body type and is commonly used to assess cell vitality and nutritional status. To output phase angle, the solution must actually measure the reactance component.

Can a BIA reading be used for clinical diagnosis?

That depends on whether the complete product, not just the module or algorithm, has completed the required medical-device certification / registration. BMH05108 / BMH05109 support customer medical-device certification projects, and multiple customers have already completed certification using these modules; for certification materials, customer cases, and project-specific guidance, please contact us. In an ordinary health-management product, BIA results should be used for health management and trend observation. If a complete product has passed Class II medical-device certification, it can be used as a medical-grade application within the approved product scope and instructions for use. Users with implanted electronic devices such as cardiac pacemakers should follow the product instructions, contraindications, and physician guidance.