Health Measurement Glossary — Understand the Terms Behind Your Percentile

A

Abdominal circumference: See Waist circumference.
Adipose tissue: Body fat. Two main types: subcutaneous (under the skin, relatively benign) and visceral (around organs, metabolically dangerous). Excessive visceral adipose tissue is a major risk factor for type 2 diabetes, cardiovascular disease, and metabolic syndrome.
Anthropometry: The scientific study of human body measurements including height, weight, circumferences, and skinfold thicknesses. The foundation of the field since the 19th century.
Arm circumference (MUAC): Mid-upper arm circumference, measured at the midpoint between the shoulder and elbow. Used as a quick nutritional screening tool and muscle mass indicator.

B

BMI (Body Mass Index): Weight in kilograms divided by height in meters squared (kg/m²). WHO categories: <18.5 underweight, 18.5–24.9 normal, 25–29.9 overweight, ≥30 obese. Developed in the 1830s by Adolphe Quetelet. Limitation: cannot distinguish muscle from fat or indicate fat distribution.
Body composition: The proportions of fat, muscle, bone, and water in the body. More informative than total body weight alone. Measured via DXA, bioelectrical impedance, or estimated via formulas like Boer.
Body Roundness Index (BRI): A newer metric (Thomas et al., 2013) that models body shape as an ellipse using waist circumference and height. Values range from 1 (very lean) to ~16+ (very round). Validated as a mortality predictor in JAMA 2024. Better than BMI at capturing central obesity.
Boer formula: A formula to estimate lean body mass using height, weight, and sex. Developed by Boer (1984). LBM(men) = 0.407×weight + 0.267×height − 19.2. LBM(women) = 0.252×weight + 0.473×height − 48.3.

C

Cardiometabolic risk: The combined risk of developing cardiovascular disease (heart attack, stroke) and metabolic conditions (type 2 diabetes, metabolic syndrome). Abdominal obesity is a key modifiable risk factor.
CDC: Centers for Disease Control and Prevention. The US federal agency that conducts NHANES and publishes population health statistics.
Central obesity: Fat accumulation specifically around the abdomen and trunk. Also called abdominal obesity or "apple-shaped" body type. Associated with higher health risk than fat stored on hips and thighs ("pear-shaped"). Measured via waist circumference, WHtR, or WHR.
Confidence interval: A statistical range that likely contains the true population value. When we say "median grip strength for men aged 30–39 is 47 kg (95% CI: 45–49)", it means we're 95% confident the true median falls between 45 and 49 kg.

D

Dynamometer: A device that measures force or torque. Hand-grip dynamometers (like the Jamar) are the clinical standard for measuring grip strength. They measure the maximum force you can squeeze.
DXA (Dual-energy X-ray absorptiometry): A medical imaging technique considered the gold standard for measuring body composition (bone density, fat mass, lean mass). NHANES uses DXA for a subset of participants.

F

Frailty: A clinical syndrome in older adults characterized by decreased physiological reserve and increased vulnerability to stressors. Five Fried criteria: unintentional weight loss, exhaustion, weakness (low grip strength), slow walking speed, low physical activity. Having 3+ criteria = frail.
Functional biomarker: A measurable indicator of physiological function rather than disease state. Examples: grip strength (neuromuscular function), walking speed (integrated physical function), resting heart rate (autonomic function). Often predict health outcomes better than traditional biomarkers like cholesterol.

G

Gait speed: Walking speed, typically measured over a short distance (4 meters) at usual pace. A validated functional biomarker. Cutoffs: ≥1.0 m/s normal, 0.8–1.0 intermediate, <0.8 m/s slow (elevated fall/disability risk), <0.6 m/s significant impairment. See also: walking speed.

H

Hazard ratio (HR): A measure of how much a factor affects the risk of an outcome over time. HR=2.0 means twice the risk. In the Leong 2015 grip strength study, each 5 kg decrease had HR=1.16 for all-cause mortality (16% higher risk).

L

Lean body mass (LBM): Total body weight minus fat mass. Includes muscle, bone, organs, and water. Higher LBM is generally associated with better metabolic health and longevity. Estimated using formulas like Boer or measured via DXA.

M

Metabolic syndrome: A cluster of conditions: abdominal obesity, high blood pressure, high blood sugar, high triglycerides, and low HDL cholesterol. Having 3+ of these 5 criteria = metabolic syndrome, which significantly increases cardiovascular and diabetes risk.
Mortality risk: The probability of death from any cause (all-cause mortality) or specific causes (cardiovascular mortality, cancer mortality) over a defined period.
MUAC: See Arm circumference.

N

NHANES: National Health and Nutrition Examination Survey. The CDC's continuous health survey that combines interviews, physical exams, and lab tests on a nationally representative sample of ~5,000–6,000 US adults per 2-year cycle. The gold standard for US population health data and the source of our percentile calculations.

P

Percentile: A value below which a given percentage of the reference population falls. Example: if your grip strength is at the 75th percentile for your age and sex, you're stronger than 75% of comparable US adults and weaker than 25%. Not a "score" — it's a comparison to the population distribution.
Population norm: The distribution of a measurement across a defined population (e.g., US adults aged 18–85). Used to determine what's "typical," "above average," or "below average" for that population.

R

Reference population: The group of people used to generate percentile comparisons. Our calculators use NHANES survey participants as the reference population, stratified by age and sex. The reference population matters — norms from one country may not apply to another.
Resting heart rate (RHR): The number of heartbeats per minute while at rest. Normal adult range: 60–100 bpm. Lower RHR generally indicates better cardiovascular fitness. Athletes may have RHR as low as 40 bpm. Persistently elevated RHR (>80) is associated with higher cardiovascular risk.

S

Sarcopenia: Age-related loss of muscle mass, strength, and function. Affects ~10% of adults over 60 and ~50% over 80. Key diagnostic criteria include low grip strength (<27 kg men, <16 kg women per EWGSOP2) and low muscle mass. Preventable and partially reversible with resistance training and adequate protein intake.
Subcutaneous fat: Fat stored directly under the skin. Generally less harmful than visceral fat and may have some protective metabolic effects. Pinchable fat (like on the hips and thighs) is mostly subcutaneous.

V

Visceral adipose tissue (VAT): Fat stored deep in the abdominal cavity, surrounding internal organs. Highly metabolically active — releases inflammatory compounds and free fatty acids directly into the portal circulation. Strongly linked to insulin resistance, type 2 diabetes, cardiovascular disease, and non-alcoholic fatty liver disease. Measured accurately only via CT or MRI; estimated by waist circumference or WHtR.

W

Waist circumference: The measurement around the abdomen at the level of the navel or midpoint between lowest rib and iliac crest. WHO cutoffs for increased risk: >94 cm (37") men, >80 cm (31.5") women. Cutoffs for substantially increased risk: >102 cm (40") men, >88 cm (34.6") women.
Walking speed: See Gait speed. Also called gait speed. A functional test where a person walks a measured distance at their usual pace. Research by Studenski et al. (2011, JAMA) on 34,485 older adults found walking speed strongly predicted remaining life expectancy.
WHR (Waist-to-Hip Ratio): Waist circumference divided by hip circumference. A measure of body fat distribution. WHO cutoffs: high risk >0.90 men, >0.85 women. The INTERHEART study found WHR was the strongest single predictor of myocardial infarction risk among all measured variables.
WHtR (Waist-to-Height Ratio): Waist circumference divided by height (both in same units). Simple rule: keep waist < half your height (<0.5). Ashwell et al. (2012) meta-analysis of 200,000+ adults found WHtR outperforms BMI for detecting cardiometabolic risk.

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