Leg length and sitting height reference data and charts for children in the United States

Population-specific reference data are required to interpret growth measurements in children. Sitting height and leg length (standing height minus sitting height) measurements are indicators of proportionality and can be used to evaluate children with disordered growth. NHANES III recorded sitting height and standing height measurements in a strategic random sample of the United States population from 1988 to 1994, and we have previously published reference charts for sitting height to standing height ratio in this population. In this study, we have developed separate sitting height and leg length reference charts for Non-Hispanic Black, Non-Hispanic White, and Mexican-American children in the United States. In addition, we provide mean (SD) and LMS data to support the use of these reference charts in clinical care.


Data Description
The National Health and Nutritional Examination Survey (NHANES) is performed periodically in the United States and aims to enroll a representative sample of the United States population [ 1 , 2 ]. NHANES III, performed in 1988-1994, is the last survey to include sitting height measurement in children. Standing and sitting-height measurements performed during this survey were used to generate these data.
For standing height measurements, the subject was required to stand on the floor-board of the stadiometer with both heels together. His or her heels, buttocks and scapulae were touching the vertical backboard and the subject's arms were allowed to hang freely by their side, with palms facing thighs. When sitting height was measured, the subject was seated on a measurement box with their back and buttocks touching the backboard of the stadiometer, knees directed straight ahead, arms and hands resting at their side. For both standing and sitting height measurement, the head was in the Frankfort Horizontal Plane [3] . Leg length was calculated by subtracting sitting height from standing height.
Figs. 1 and 2 show sex-and ancestry-specific centile charts for sitting height and leg length respectively and can be used to track growth of these parameters over time. Tables 1 and 2 provide sex-and ancestry-specific mean (standard deviation) values for sitting height and leg length in this population, respectively. This provides an additional tool for interpreting these measurements in children. Tables 3-6   Tables 7 -10 provide these data for leg length, and will support integrating these growth charts to electronic health records or calculating an individual child's Z-score for each measurement.

Experimental design, materials and methods
Sitting height and standing height were measured and leg length was calculated for each subject with available measurements aged 2-18 years. In order to adjust the data from study participants to reflect the United States population, weighting was assigned to each subject (designated WTPFEX6 in the NHANES dataset [4] ). This weighting was used to generate populationspecific reference charts and all analyses, as described elsewhere [5] .
Non-Hispanic black (NHB) children in the United States were found to have differences in body proportions when compared with non-Hispanic white (NHW) and Mexican-American chil- dren [6] . To account for these differences, separate sex-and ancestry-specific reference charts for NHB and NHW/Mexican American male and female children were generated.
LMS Chartmaker Pro (Harlow Printing Ltd., Tyne and Wear, UK) was used to generate sitting height and leg length reference charts. The LMS method [7] uses a Box-Cox transformation to obtain normality, and separate smooth curves are generated for skewness, median and variability. These are constrained to smooth changes over time and combined in one graph. As per software recommendations, these were adjusted until the fit of the curve was visually optimized. LMS files were generated for each chart.

Ethics statement
For participation in the NHANES III survey, parental consent was provided for all children under 18 years of age and the protocol was approved by the National Center for Health Statistics Research Ethics Review Board.