Data source
The present study is based on deidentified data from the 2012 NNYFS; a detailed description of the study design and methods are available elsewhere [24]. In brief, the 2012 NNYFS was conducted by the Centers for Disease Control and Prevention’s National Center for Health Statistics, to obtain data on physical activity and fitness levels of US youth aged 3 to 15 years. Similar to NHANES, the NNYFS population included a stratified, multistage probability sample representative of the civilian noninstitutionalized population in the US. The survey included interview followed by a physical fitness tests and the body measurements conducted by trained examiners. NNYFS procedures include automated quality control checks for each assessment, which include pop-up reminders prior to data entry; these are detailed in the referenced procedures manual [24]. Methods relevant to the current study are described briefly below.
Participants
Participants included in this analysis were children who participated in the NNYFS and were aged 6–11 years at time of data collection. Participant characteristics were obtained via parent/guardian proxy report and included a brief health history, race/ethnicity, and other demographics. Physical measures included height, weight, and calculated body mass index (BMI) according to standard NHANES procedures; age- and sex-specific BMI percentile was determined using the 2000 CDC growth charts. The current study was ruled exempt according to the researchers’ Institutional Review Board because it was a secondary analysis of publicly available data (IRB #19-E-226).
Measures
Measures of fitness included in the NNYFS 2012 varied according to child age between 3 and 15 years. These included core muscle strength (plank), lower body muscle strength (LBMS), upper body muscle strength (hand grip), and aerobic fitness. A dietary recall was also completed for this age range. Only core muscle strength, LBMS, upper body muscle strength, and aerobic fitness completed by children aged 6–11 years were included in our analysis.
Cardiorespiratory fitness
Aerobic fitness was assessed via maximal treadmill test and expressed as treadmill time to exhaustion (seconds). NNYFS staff demonstrated use of the treadmill for each participant prior to allowing participants to familiarize themselves with walking on the treadmill. The protocol for all 6–11-year-old children progressed in a graded fashion with progressive increases in speed and/or grade until voluntary exhaustion. Children were encouraged to exercise as long as possible and to notify examiners if they experienced any pain, dizziness, or nausea. The treadmill protocol included a 1-min warm-up walk and subsequent stages progressed in 2-min increments according to a protocol varying by age; the full protocol has been published elsewhere [25]. Test outcome was reported as maximal treadmill time (seconds) by NNYFS. The treadmill used in this protocol (Quinton TM55) was calibrated for speed, incline, and heart rate weekly. We created age- and sex-specific z-scores for maximal endurance time to account for expected variation in endurance capacity and in order to control for the differing treadmill protocols by age categories.
Muscular fitness
The NNYFS isometric grip strength protocol was used to estimate upper body strength. This protocol utilizes a digital hand dynamometer (Model 5401; Takei Scientific Instruments Co., Ltd., Niigata-City, Niigata-Pref, Japan). Participants were asked to maximally squeeze the hand dynamometer, adjusted for hand size, alternately three times in each hand. Values are reported in pounds of force created from the sum of the highest value from each hand [24]. Data were available for children aged 6–15 years only; children aged 6–11 years were included in this analysis. Each participant had three trials for the grip strength test in the NNYFS. Grip strength is known to be strongly correlated with body weight [26, 27]. Therefore, we report the grip strength relative to body weight (kg) (relative hand grip strength). Upper body muscle strength was also assessed via modified pull-up; these data are not included in the present analysis.
Assessment of LBMS was completed using a hand-held tension dynamometer to assess maximal isometric knee extension force in the sitting position. Body position was maintained using a chair built specifically for the NNYFS assessments, which included straps to secure the participants’ hips, thigh, and upper body. Participants pushed their legs as hard as possible against a strap passed through the dynamometer and around the chair; participants performed three repetitions alternately with each leg and the highest values from each leg were summed for analysis [24]. Similar to grip strength, each participant had three trials for each leg during the fitness test. LBMS is also highly correlated with body weight [26]; therefore, we report the LBMS relative to child’s body weight (kg).
Core muscle endurance (trunk and pelvis) was assessed by plank hold. Participants started lying prone on the floor and pushed up into plank position with the arms resting on the forearms and weight balanced on the toes. Children were directed to hold the position as long as possible with their back straight and without the stomach dropping or hips rising up [24]. Plank hold performance has moderate negative correlations with child’s height and body weight [26, 28] and a positive correlation with cardiorespiratory endurance. Plank hold performance was found to be linearly correlated to children’s weight status in this cohort [26, 27], we normalized the value for child’s body mass index (BMI: kg/m2).
Household income and child’s age
The NNYFS collected information about participating youths’ household income, federal income-poverty ratio (FIPR) and age in years. The FIPR is calculated by dividing family income by a poverty threshold that is specific to family size. As such, using FIPR as an indicator of SES automatically adjusts for the number of people in the household. Federal assistance programs, including the free-and-reduced school lunch program, HeadStart, and supplemental nutrition assistance program (SNAP) use FIPR to determine program eligibility. For income categories, we used family income of less than 130% FIPR (equivalent to eligibility for free-and-reduced school lunch) as low income, between 130 and 349% FIPR as middle income, and greater than 350% FIPR as high income. Child’s sex was dichotomized as male or female and age at time of screening is expressed in years.
Data analysis
Descriptive statistics are reported using frequency tables and group differences are estimated using Rao-Scott chi-square test. Mean and standard errors are reported for all fitness tests by sex and household income category using PROC SURVEYMEANS procedures. With the exception of the treadmill test, which is adjusted for age according to the protocol, least square means and standard errors for the mean were estimated using PROC SURVEYREG procedures after controlling for age. Between-group differences were evaluated by computing t-statistics at p < 0.05 significance using the PROC SURVEYREG procedure. Plank hold performance and LBMS values were square root transformed because of skewed distributions. Linear and quadratic trends for income gradient were assessed using orthogonal polynomial contrasts with significance accepted at p < 0.05. Sample weights that account for the unequal probabilities of selection, oversampling, and nonresponse and complex survey design were included in all analyses. All statistical analyses were conducted using SAS University Edition (SAS Institute, Inc., Cary, NC).