Data
The present study evaluated data from the 2014–2017 Korea National Health and Nutrition Examination Survey (KNHANES). The different participants were selected every year. These annual cross-sectional surveys are performed using multi-stage probability samples that are representative of the noninstitutionalized civilian Korean population. The data of selected participants has specific sampling weights factors because each data does not have equal probability of being selected. The data were analyzed after adjusting with specific sampling weights factors for each participant.
Subject selection
During 2014–2017, a total of 2,988 individuals participated in the KNHANES. The present study included participants aged 10–18 years; participants with missing data were excluded. Thus, 2,304 participants (1,227 boys and 1,077 girls) were included in the analysis. The participants were categorized by age and sex. The subjects were categorized into 9 age groups by each year, from ages 10 to 18 years.
Measurement of anthropometric and laboratory variables
Systolic blood pressure (SBP, mmHg) and diastolic blood pressure (DBP, mmHg) were measured to the nearest 2 mmHg using a mercury sphygmomanometer (Baumanometer Wall Unit 33(0850)). Height was measured to the nearest 0.1 cm using a stadiometer (SECA 225, Seca GmbH & KG.), and weight was measured to the nearest 0.1 kg using a balance beam scale (GL-600-20, G-tech). Waist circumference was measured to the nearest 0.1 cm using a measuring tape (SECA 200, Seca GmbH & KG.). The body mass index (BMI) and waist-to-height ratio (WtHR) were calculated from the measured height and weight of participants (kg/m2 and waist (cm)/height (cm) × 100, respectively). Levels of fasting glucose (mg/dL), aspartate aminotransferase (AST, U/L), and alanine aminotransferase (ALT, U/L), triglyceride (TG, mg/dL), cholesterol(mg/dL), high-density lipoprotein (HDL, mg/dL), were measured using a Hitachi Automatic Analyzer 7600.[17]
Handgrip strength measurement
The Takei digital grip strength dynamometer (Model T.K.K.5401, Takei Co., Ltd., Ishioka, Japan) was used to measure handgrip strength. The dynamometer were calibrated according to standardized protocol and special investigator checked the measurement whether it was done properly. Measurement of handgrip strength was performed after rest and light exercise which could helped performing maximum strength of handgrip. All participants, except those with a history of wrist surgery within 3 months or any wrist discomfort, underwent the handgrip strength test. The handgrip strength testing procedures has been done according to muscle strength procedures manual by Centers for Disease Control and Prevention.[18, 19] Handgrip strength was measured in a standing position with the arm and wrist in the anatomical position. Participants were asked to exhale and apply a maximal grip for 3 seconds, for a total of 3 repetitions each, starting with the dominant hand. The left and right hands were alternated. Sixty seconds of rest was allowed between each measurement. The highest handgrip strength value (in kg) between both hands was recorded and included in the analysis.[11] (http://knhanes.cdc.go.kr/) Handgrip strength was calculated as a ratio (handgrip-to-weight, HGtW, (hand grip, HG / weight) × 100) and it was used in the analysis.[11, 14]
Statistical analysis
The SPSS software (version 23.0; IBM Inc., Armonk, NY) was used for statistical analyses of all data. Data was adjusted with sampling weight factors because of the complex survey design of the KNHANES which small number of participants are representative of the general Korean population. Continuous data was analyzed and expressed as mean ± standard error, applying weight factors. P for trend was analyzed by linear regression with applying weight factors of survey design.