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Participants
A three-stage stratified whole-group sampling method was used to select and survey the participants of this study [18]. This study began recruiting participants and conducting the test survey in April 2022 and closed in July 2022. First, Henan, Jiangxi, and Fujian in China were selected as the test provinces for this study. Second, two universities in each province were selected as the test schools for this study. Third, in each university, 10 teaching classes were randomly selected in each grade from the first year to the fourth year of university, using the class as the sampling unit. A total of 12,897university students in 240 teaching classes at six universities were sampled for this study. The specific inclusion criteria were: university students enrolled in school, aged 19–22, without major physical or psychological illnesses, and voluntarily accepted the survey for this study. A total of 851 survey forms with incomplete important demographic information, broken survey forms, and response rates lower than 80% were excluded after the survey, and 12,046 valid data were finally returned (boys: 6011, 49.90%). The specific sampling process is shown in Fig. 1.
Flowchart of participant sampling of university students in China
The study was granted with informed consent from the students and parents and written consents were signed. The study was approved by the Ethics Committee of Shangrao Normal University (202,214,045). The survey forms was coded anonymously and the survey forms data were kept strictly confidential to protect the privacy and security of the students.
A body shape index (ABSI)
ABSI is calculated from BMI and WC by using the formula. the formula for ABSI is WC/(BMI2/3×height1/2) [8]. BMI is calculated as weight (kg)/height (m)2. The unit is kg/m2. Height and weight tests are conducted according to the testing instruments and methods specified in the China Student Physical Fitness Survey [18]. The height test was conducted by CSTF-SG3000 height tester, and the test results were accurate to 0.1 cm. The test of weight is conducted by CSTF-TW3000 weight tester, and the test result is accurate to 0.1 kg.
Muscle strength index (MSI)
The MSI of this study includes a total of three parts of muscle strength indexes of upper limbs, abdomen and lower limbs. The grip strength was used to reflect the upper limb muscle strength of university students. The pull-up (boys) and bent-leg sit-up (girls) were used to reflect the abdominal muscle strength of university students. The standing long jump was used to reflect the muscle strength of the lower limbs of university students. The muscle strength indicators reflecting the three body parts were standardized according to age and gender, and the standardized numbers were summed to obtain MSI. The higher the MSI, the stronger the muscle strength of the participants [19, 20]. The formula for calculating MSI in this study is specifically:
MSI(boys)= Zgrip strength+Z standing long jump+Zpull−up
MSI(girls)= Zgrip strength+Z standing long jump+Zbent−leg sit−up
The grip strength test was conducted using a MAX EH106 electronic grip strength meter. The participant was asked to use the strong side of the hand to grip the grip strength meter handle, and the number displayed on the display screen was the test result. Participants were tested twice and the highest one was taken as the test result. The results were retained at 0.1 kg [18].
The pull-up (boys) was evaluated by recording the number of tests performed by the subjects university students. The participants were asked to grab the bar with both hands and pull upwards with both hands at the same time, asking the participant’s head to go over the bar, which was recorded once [18]. The number of stretches was recorded by repeatedly following this requirement [18]. The bent-leg sit-up (girls) required participants to lie flat on a sponge mat, bend their legs at 90 degrees at the knees, place their hands with their fingers interlocked behind their necks, and count the number of times their elbows touched their knees, repeatedly. Participants were asked to test as many times as possible within a minute’s time and record the final count [18].
The standing long jump test requires participants to jump forward as far as possible using a standing jump, requiring a two-footed jump [18]. The vertical distance between the line of jump and the nearest heel is recorded. The test results are in centimeters. Each participant was tested twice and the best result was recorded [18].
Survey of basic information
The survey of basic information mainly included the participants’ region, school, grade, class, college, major, gender, age and other information [18]. The survey of basic information was conducted by means of anonymous numbering.
Quality control
Participants were required to do preparatory activities before the test to prevent sports injuries during the test. Participants were required to take off their shoes when testing height and to empty their bowels when testing weight, and to wear light clothing for the test [18]. Each program was calibrated by special testing staff before each day’s test to ensure the accuracy of the test. The test conditions were the same for all participants. All participants were tested at the same time. Each participant was tested twice consecutively for grip strength and standing long jump, and once for the other events.
Statistical analysis
Continuous type variables in this study were expressed as (mean ± standard deviation).
In this study, the calculated ABSI was stratified by percentile. Stratification was based on different age and gender and was divided into five groups: ABSI < 5th Percentile (A), 5th ≤ ABSI < 25th Percentile (B), 25th ≤ ABSI < 75th Percentile (C), 75th ≤ ABSI < 95th Percentile (D), ABSI ≥ 95th Percentile(E). Comparison of MSI between different ABSI groups was performed using effect sizes, and Cohen’s d was divided into small effect = 0.2, medium effect = 0.5, large effect = 0.8 [21].
The association between ABSI and MSI by gender in this study was analyzed using a curve estimation analysis model with the following equations:
Y = aX2 + bX + c (Y, MSI; X, ABSI)
Where a (non-linear coefficient), b (linear coefficient), and c (intercept) are constants. Curvilinear regression analysis was performed with Y as the dependent variable and X as the independent variable.
SPSS version 25.0 (IBM, Armonk, NY, USA) software was used for data processing and analysis, and graphs were produced using Graph Pad Prism 8.0.2 (Graph Pad Software, Inc., CA), and the level of statistical significance was set at 0.05.
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