GLOSSARY

Whole-Body BIA uses a whole-body measurement configuration to estimate relevant compartments. The current path and model determine how local tissues contribute to the overall result.

Bioelectrical impedance methods measure the body's opposition to an applied electrical current and use models to estimate water and body-composition compartments. Hydration, recent food, exercise, skin temperature and electrode placement can change the result. Extra frequencies or additional displayed compartments do not eliminate modeling assumptions. Compare measurements under standardized conditions on the same validated system. A consumer scale's apparent daily change in muscle or body-fat percentage should not be read as a direct tissue measurement.

Resistance reflects the resistive component of electrical opposition, while reactance reflects capacitive behavior. Impedance combines these components, and phase angle describes their relationship at a specified frequency. Interpretation depends on device and protocol as well as the individual. Phase angle is not a direct measure of muscle strength, hydration or a universal cellular-health score. Changes require context, and a single number cannot identify the exact nutritional problem or prove that a supplement has improved cell function.

Fat-free mass includes water, protein, minerals and other nonfat constituents. It therefore includes organs and bone as well as skeletal muscle. Lean body mass, fat-free mass and lean soft tissue can have different technical meanings, although terms are sometimes used loosely. A short-term rise in a device's lean-mass estimate can reflect hydration or glycogen rather than new contractile protein. Check the device definition and testing conditions before calling a change muscle growth.

For example, a bioimpedance scale can report a different muscle estimate after a sweaty session or a large drink. That is a reason to review measurement conditions, not evidence that muscle was rapidly lost or created. Repeat testing under the same protocol and interpret the result together with the broader training and nutrition trend.

Isotope dilution estimates a distribution space after an administered tracer mixes with body water, with appropriate sampling and corrections. Converting total body water to fat-free mass requires assumptions about hydration of the fat-free compartment. This differs from doubly labeled water, which estimates expenditure from isotope elimination over time. Confusing these methods can turn a water measurement into an unsupported calorie estimate. Testing conditions, tracer handling and the population being studied determine how the result should be interpreted.

Validity concerns whether a method estimates the intended quantity accurately, while reliability concerns consistency on repetition. A method can repeat well while remaining biased. Technical error and prediction error also describe different sources of uncertainty. Small changes should be interpreted relative to relevant measurement error, with the same protocol used over time. Agreement at group level does not guarantee close agreement for every individual athlete, and switching instruments can create an apparent change that is methodological rather than biological.

Repeated measurements become more informative when timing, clothing, scale, hydration and recent intake are comparable. An average can reduce random daily noise, but it can also conceal rapid changes that need attention. Keep individual values available and relate the trend to symptoms, food and training. Monitoring should have a clear purpose and appropriate frequency. More frequent weighing or scanning is not inherently better, especially if it increases distress or encourages restrictive eating without improving decisions.