Impact of home quarantine on physical fitness of school-aged children in Xi’an during COVID-19 lockdown: a cross-sectional study

Data were obtained from the 2019, 2020, and 2021 waves of the Chinese National Survey on Students’ Constitution and Health (CNSSCH), a joint project of the Ministries of Education, Health, Science and Technology, the State Ethnic Affairs Commission, and the State Sports General Administration of the People’s Republic of China. The CNSSCH was conducted in 30 mainland provinces, autonomous regions, and municipalities among school-aged students aged 6–22 years to monitor trends in health, nutrition, and wellbeing [17]. According to the Chinese National Student Physical Health Standard (CNSPFS) (revised in 2014), a stratified cluster sampling method was used to evaluate the body shape, body function, and physical fitness of children and adolescents in Xi’an from 2019 to 2021 [2, 3, 18]. The school and the student that we tested each year were different. All schools are located in Xi’an, Shaanxi Province, China. And all schools were unified under the leadership of the Xi’an Government during the COVID-19.

The participants were randomly selected in each grade. The number of students also changes with the real situation of the school. During the testing period, we only have a lower limit, not an upper limit, for the number of students tested. The sample size was calculated using G-power based on the following assumptions. Tail (s): One, Effect size d: 0.3; an error probability = 0.05; and power (1- β error probability) = 0.8. This gave a sample size of 278 [19]. Finally, 8092 students aged 6–22 years from primary school, middle school, high school and college were contained (Table 1), with physical fitness testing in the autumn of each year (Fig. 1). Primary school students are aged 6–11/12, middle school students are aged 12/13–14/15, high school students are aged 15/16–17/18, and college students are aged 18/19–21/22.

During the testing period, the students were allowed to participate in physical education programms. Although they did not go out during the quarantine period, they still had the opportunity to engage in sports activities through online learning. Physical education major students, as well as athletes, students who are sick, and girls with special conditions will be excluded from the test with a doctor’s permission. During the testing process, students were healthy and tested negative for COVID-19. Two training sessions and a testing handbook were provided to all evaluators to minimize mistakes during the test. The local ethics committee approved the study at Shaanxi Normal University (201,916,001 2019-09). The informed consent was obtained from the students and their parents before each item was tested. Numerical coding of all students’ names was used to avoid disclosing their personal information.

The study used the CNSPFS (revised in 2014) as the test standard, which has received consistent approval from academics [20]. To reduce measurement error, the measurement instruments were calibrated before use. The test for each school was completed on a morning in October to reduce data deviation caused by different test times. Following the Chinese National Student Physical Fitness and Health Research handbook [21], the test projects include 50 m sprint, standing long jump, sit-and-reach, 800 m and 1000 m running, sit-ups, chin-ups, 50 m × 8 shuttle run, and 1 min rope skipping. According to previous studies, the tests were classified into five fitness batteries: endurance, strength, coordination, speed, and flexibility [22]. In addition, our study examined vital capacity and body mass index [16] (Table 2). In order to ensure the fluency of the test, the professional physical education teacher led the students in 30-minute warm-up activities before the test began. During the test, we recommend that height, weight, vital capacity, and visual acuity tests be tested first, followed by strength, flexibility, coordination, speed, and finally endurance.

BMI is a standard index to measure the degree of body fat and whether it is healthy worldwide [23].

The BMI of children and adolescents aged 3–18 years was calculated and categorized according to the “Screening for overweight and obesity among school-age children and adolescents“ [24]. The BMI for adults is calculated as well as categorized according to the WHO (World Health Organization) standards. According to the WHO standard, body mass index includes overweight, obesity, medium, thinness, and severe thinness. Height and weight were used to evaluate students’ growth and development levels [25]. When measuring height, the subjects were barefoot and standing upright on the floor of the height meter (the upper limbs were naturally drooping, the heels were closed, and the toes were separated into 60 degrees). The heel, sacrum, and two scapular areas are in contact with the column, the trunk is naturally straight, the head is straightforward, and the upper edge of the tragus and the lower edge of the orbit are horizontal. Record results in centimeters to one decimal place int. When measuring body weight, subjects were barefooted, and male subjects wore shorts; female subjects wore shorts, short sleeves, or vests and stood in the center of the scale. The reading is in kilograms, accurate to one decimal place.

VC refers to the total amount of gas exhaled after trying to inhale, which can reflect the potential ability of respiratory function in a certain sense [26]. Vital capacity was measured using the electronic spirometer. Electronic spirometer met specific technical criteria, ensuring an acceptable margin of error ≤ 3% [9]. During the test, the subjects take a deep breath, hold the breath, and exhale slowly to the mouth until they can no longer exhale to prevent inhaling from the mouth at this time and not inhale twice during the test [27]. Each subject was measured three times, and the maximum value was selected as the test result. In milliliters, no decimal number is retained.

The strength test mainly includes upper limb strength, lower limb strength, and core strength. The chin-up test results reflected the upper limb strength of the subjects. During the test, the subject jumped up with both hands holding the bar, and both hands were overhanging at shoulder width. After resting, the two arms simultaneously pull the body (the body can not have additional action) and chin-ups to the lower jaw over the upper edge of the horizontal bar for completion [27]. Record the number of leads.

The sit-up test results reflect the core strength of the subjects. During the test, the subjects were supine on the pad, the legs were slightly separated, the knees were bent at an angle of about 90 degrees, and the two fingers were crossed behind the brain. Another partner pressed his ankle to secure his lower limbs. When the subjects sat up, two elbows touched or exceeded both knees to complete once. Two shoulder blades must touch the pad in a supine position. Record the number of completions within 1 min [28].

The standing long jump test results reflect the lower limb strength of the subjects. During the test, the subjects naturally stood on their feet separately. After standing on the take-off line, the toes should not step on the line. Two feet take off simultaneously; There must be no step or jump action. Measure the vertical distance from the trailing edge of the take-off wire to the nearest landing site [28]. Each person tried three times, recording the best one in centimeters.

The 50 m × 8 shuttle run and 800 m / 1000 m test reflect the development level of students’ endurance quality, especially the function of the cardiovascular and respiratory systems and muscle endurance. The subjects were tested in groups of at least two people standing up during the test. They start running when they hear the ‘run’ command. When the subject’s torso crosses the line, the timer stops. Record the test results in minutes and seconds, regardless of decimals [29].

The range of motion that students can reach in the static state of the trunk, waist, hip, and other joints is measured by sitting forward flexion, which mainly reflects the extension and elasticity of joints, ligaments, and muscles in these parts and the development level of students’ physical flexibility. The subjects’ legs were straightened during the test, and the two feet were flat on the ground. The feet were separated by about 10–15 cm, and the upper body was bent forward. Before the arms were straightened, the cursor was gradually pushed forward with the fingertips in both hands until it could not be pushed forward. The inner edge of the longitudinal pedal plate of the tester is 0 point, negative inward and positive forward [28]. Record in centimeters as a unit, and retain a decimal number. Test twice, and take the best score.

The 1 min rope skipping test reflects students’ lower limb explosive force and body coordination ability. Test and record the number of rope skipping times in 1 min.

The 50 m run tests the development level of students’ speed, sensitivity, and nervous system flexibility. The subjects were tested in groups of at least two people during the test. After hearing the command of ‘running,’ the subject starts to run, and the participant’s trunk reaches the vertical plane stop table of the finish line [29]. The researchers recorded the results in seconds, which were accurate to one decimal point.

Visual acuity (VA) is a measure of the ability of the eye to distinguish shapes and the details of objects at a given distance [30]. All the eye measurements were conducted and recorded by a professional eye care doctor. Position the student, sitting or standing, at a distance of 6 m from the chart. Test the eyes one at a time. Ask the student to cover one eye with a plain occluder, card or tissue. The student is asked to point in the direction the ‘legs’ of the E are facing. Record the VA for each eye in the student’s notes. Repeat the whole procedure for the second eye.