GLOSSARY

Anthropometric Index is a calculated descriptor derived from anthropometric measurements. Its usefulness depends on the formula, intended population and question, not simply the availability of a numerical result.

A body-composition index expresses a component relative to a body-size measure, often height squared, while a ratio compares specified quantities directly. State both numerator and denominator before interpreting the result. Different skeletal muscle index definitions are used in different settings. A convenient normalization does not automatically remove every body-size effect or establish a sport-specific target. Reference values, measurement method and the population being assessed determine whether a comparison is meaningful.

Body-composition models divide the body into components according to a defined level. A two-component model often separates fat mass from fat-free mass. Adding measured water or mineral information can reduce some assumptions in more complex models, but no model removes all error. Molecular, cellular and tissue descriptions are different ways of organizing the same body. Before comparing results, identify the model, measured variables and assumptions rather than treating all reported components as directly measured anatomical tissues.

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.

BMI divides body mass by height squared. It is a screening and population-description measure rather than a direct measurement of body fat, muscle or health. Muscular athletes can have a high BMI without the same adiposity as another person with the same value. Adult classifications do not transfer directly to children, who require age- and sex-appropriate interpretation. Health assessment should consider clinical context, distribution of fat and other relevant findings rather than diagnosing from BMI alone.

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.

Body mass and composition can affect the demands of moving the body, but their relationship to performance depends on the task. Absolute strength, power, technique, endurance, skill and recovery may matter differently between sports. Dividing performance by mass can improve a ratio even when absolute performance declines. A nutrition change should therefore be judged against the actual event and health outcomes. There is no universal body-fat percentage or competition weight that guarantees success across athletes.