Clear explanations of the physical and measurement concepts behind bioelectrical impedance analysis and related research. Established principles come first; the limits of interpretation follow each section.
Bioelectricity is electrical activity generated by living tissue, mainly through the movement of ions across cell membranes.
Ions such as sodium, potassium, calcium and chloride are unevenly distributed across the membrane. Their movement creates potential differences and currents. Familiar clinical examples include the electrocardiogram (ECG), electroencephalogram (EEG) and electromyogram (EMG).
Bioelectricity is generated by the tissue. Bioimpedance is the passive opposition of tissue to a current applied from outside.
Both are measured with electrodes and both involve ionic conduction, but they answer different questions. A standard BIA device applies an external current and measures the response, so it does not record the tissue's own signals.
Key idea. Active bioelectric signals are produced by tissue. Impedance measurements apply a small external signal and record the passive response.
A physiological measurement is a quantitative record of a biological function or structure, made with a defined method, unit and set of conditions.
Every measurement has a method, a unit and an uncertainty. A reading is only useful if it is valid (it measures what it claims to measure) and reliable (repeated readings agree within known limits).
| Measurement | What it records | Key considerations |
|---|---|---|
| Electrocardiogram (ECG) | Electrical potentials generated by the heart, recorded with surface electrodes | Electrode placement, skin contact, interference and standard lead configurations |
| Electromyogram (EMG) | Electrical activity of skeletal muscle | Electrode type, muscle activity state and movement artefacts |
| Blood pressure | Pressure in the arterial system, measured by cuff or invasive methods | Cuff size, posture, rest period and device validation |
| Body temperature | Core or surface temperature, depending on the site and method | Site, timing and ambient conditions |
| Bioelectrical impedance | Response of a body segment or the whole body to a small applied alternating current | Hydration, temperature, posture, electrode placement and device model |
Tissues conduct current through ionic solutions, and cell membranes act as capacitors. Their combined response changes with the frequency of the applied current.
In simplified textbook models, low-frequency current flows mainly through the extracellular fluid, because the membrane acts as an insulator. At higher frequencies the membrane's capacitive path becomes more conductive, so current also passes through cells. The impedance therefore varies with frequency.
This is a simplified picture. Real tissues contain many compartments, and the models used to interpret measurements are approximations.
Resistance dissipates energy and is in phase with the voltage. Reactance stores and returns energy and shifts the phase of the current relative to the voltage.
Resistance (R) arises from ionic conduction. Reactance (X) arises from capacitive (and, in other systems, inductive) elements. For an ideal capacitor, the reactance magnitude is Xc = 1 / (2πfC), so it falls as frequency increases.
Impedance is the complex sum of resistance and reactance: Z = R + jX.
The magnitude is |Z| = √(R² + X²), and the phase angle is φ = arctan(X / R). By convention capacitive reactance is negative, although many device reports quote its magnitude, so sign conventions should always be stated.
It produces a controlled stimulus or records a physiological signal, converts it into digital data, and processes and displays the result. Each stage can add error, so each must be calibrated and validated.
Accuracy describes closeness to a true or accepted reference value. Reproducibility describes how consistently a measurement gives the same result under stated conditions.
A device can be reproducible but inaccurate, and an accurate device can still be poorly reproduced if the procedure varies. Both must be assessed.
Repeatability is tested with repeated measurements under the same conditions. Reproducibility is tested across operators, devices, sites or times.
Common approaches include the coefficient of variation, intraclass correlation coefficients and Bland–Altman analysis of agreement. These should be reported before a difference between two measurements is interpreted.
This section is important for anyone reading measurement studies in homoeo-bio-engineering.
Not by itself. A change in a measured value is one piece of information. It must be interpreted within a study design that can separate treatment effects from other causes.
Several things can change a measured value without any treatment effect. These include hydration, time of day, recent food or exercise, skin temperature, electrode position, the measurement device, and the expectations of participants and observers. Study design is the main protection against these explanations.
Check editions and page numbers against the publisher or a library copy before citing.
