NHANES National Youth Fitness Survey

Overweight and obesity is a major public health problem among children and adolescents in the United States. Regular physical activity promotes health and fitness and may help to prevent obesity. As the part of the NHANES National Youth Fitness Survey (NNYFS) specific data collection effort, the body measures data are used to study the associations between body weight and measures of physical activity and fitness, and to estimate the prevalence of overweight and obesity in U.S. children and adolescents.

All survey participants aged 3 to 15 years were eligible for the body measures component.

The NNYFS body measures data were collected in the Mobile Examination Center (MEC) by trained health technicians. Generally, one technician completed the measurements and data recording. Occasionally, one technician conducted the measurements and another technician assisted in data recording. Standing height was measured using a portal stadiometer. Weight was measured using a portal digital weight scale. Arm length and arm, waist, and calf circumferences were measured using a steel measuring tape. Calf, triceps, and subscapular skinfolds were measured using a skinfold caliper. Arm and calf measurements were made on the right side of the body. If a participant had a medical condition or medical appliance, such as a cast, that prevented measurements from being taken on the right side of the body, the health technician took measurements on the left side. All measurements were entered into the NNYFS database by the health technician.

Please refer to the NNYFS Body Measures Procedures Manual (https://www.cdc.gov/nchs/nnyfs/manuals.htm) for further details on obtaining body measures.

The NNYFS health technicians completed at least 12 hours of training with survey staff and an expert anthropometrist. The training included an overview of the component, using the NHANES III anthropometry video, and demonstrations conducted by the expert examiner with volunteer subjects. The expert examiner reviewed and demonstrated the proper technique to use for each measurement. Supervised practice exercises followed, conducted with several volunteer subjects aged 3 to 15 years. Health technician performance was monitored using direct observation, data reviews, and periodic expert examiner (gold standard comparison) evaluations.

The body measures examination areas in each of the MECs were identical with respect to layout and equipment. Scheduled equipment calibration was performed by the health technicians. The NNYFS Body Measures Procedures Manual includes detailed descriptions of the quality assurance and quality control measures that are used in the NNYFS body measures component.

The NNYFS body measures data were reviewed for unusual and erroneous values. Review criteria were based on NHANES 1999–2010 body measures data for children ages of 3 to 15 years. During the data review, values that were above the 99th percentile or below the 1st percentile, for a particular age or age-gender group, were flagged for review. When records were flagged, the entire body measures record was reviewed for reasonableness. Subject characteristics such as height, weight, age and gender were taken into consideration. Values that were determined to be unrealistic were deleted from the file. None of the original body measures data were changed and there are no imputed values in this file.

Body Mass Index (BMXBMI):
Body Mass Index (BMI) was calculated as weight in kilograms divided by height in meters squared, and then rounded to one decimal place.

BMI Category – Children/Adolescents (BMDBMIC):
This variable was created for all participants aged 3 to 15 years. Cutoff criteria are based on the Centers for Disease Control and Prevention's sex-specific 2000 BMI-for-age growth charts for the United States. Age in months at examination was used to match age in months from BMI growth chart data, separately for males and females. There are four codes:

1. Underweight (BMI < 5^th percentile)
2. Normal weight (BMI 5^th to < 85^th percentile)
3. Overweight (BMI 85^th to < 95^th percentile)
4. Obese (BMI ≥ 95^th percentile)

Component status code: A final body measures component status code (BMDSTATS) provides analysts with a quick method of identifying survey participants with complete or partial body measurement data.

Unusual values: Unusual body measures values were noted during the review of the data. Typically, unusual values occurred when a subject was extremely short, tall, overweight or underweight. Analysts should examine the distributions of the body measurements carefully and consider whether or not it is appropriate to include or exclude extreme values in a given analysis.

Comment codes: Comment codes were added by the health technicians, during data collection, to document problems or situations that arose during the body measures examination. For example, the variable BMIWT is a comment code for the body weight measurement. If a participant did not wear light shorts and t-shirt or did not change into the NNYFS lightclothing, a code of BMIWT of “3” denoting “regular clothing worn” was made in the record. Analysts should review the comment code information for each of the body measures prior to data analysis.

Weight status classification: BMI, expressed as weight in kilograms divided by height in meters squared (kg/m²), is commonly used to classify weight status. The age-and sex-specific 5^th, 85^th, and 95^th percentiles of the 2000 CDC growth charts are usually used as cutoff criteria for children and adolescents. The variable BMDBMIC provides weight status categories for participants aged 3 to 15 years at examination. Information about age in years at screening (RIDAGEYR) and at examination (RIDEXAGY) is available in the NNYFS Demographic data file.

The analysis of NNYFS body measures data must be conducted using the appropriate survey design variables, sample weights, and the basic demographic variables. Please refer to the NNYFS Analytic Guidelines for details on the use of sample weights and other analytic issues.