The complete protocol of this cross-sectional study (Registration # ChiCTR1900021700). was approved by the academic board of the China Institute of Sports Science (No. 201818-21). All methods were performed in accordance with the guidelines and regulations of the institution. Before the study, written informed consent was obtained from each study subject and their parents. We excluded all students who refused to participate in the study. During the analysis, all participants' names and other identifiers have been removed from all sections of the manuscript.
From September 2014 to September 2016, the middle schools in Xicheng District of Beijing were divided into demonstration, ordinary, and general schools. Three schools were selected by random sampling at each level. At each school, three classes (Junior 2, Senior 1, and Senior 2) were selected by simple random sampling. A cluster survey was conducted in the selected classes to test spine morphology and function.
Before the test, all students and parents were given an informed consent form and questionnaires (the main contents of the questionnaires included whether the student had a definite diagnosis of spinal diseases, whether there had been back or lumbar pain in the past month, whether there had been an acute back injury in the past month, whether there had been sports-related injuries to other body parts in the past month, and so on).
In the pre experiment, which was 34 males and 28 females were included, the mean value of TKA in boys is 37.3°and 35.4°, respectively. And the standard deviation is 10.2 for the whole subjects. With he type I error rate was setting at 0.05 and power at 80%, the study used the following formulas to compute sample size :
As a result, each group should have at least 450 subjects. Considering the 10% of rejection rate. We design to include at least 500 subjects in male and female.
The subjects were given screening questionnaires and were examined by physical therapists. The exclusion criteria were as follows: a. history of spine fracture, spine-related surgery, shoulder joint motion injury, pelvis-related injury, or definitive diagnosis of spine-related diseases (such as cauda equina syndrome, lumbar disc herniation, spinal stenosis, congenital scoliosis, idiopathic scoliosis, and so on); b. pain in the spine in the past month; c. acute spine-related injuries in the past month; d. limb fracture, joint trauma, and other phenomena in the past month; e. structural kyphosis with obvious structural abnormalities; f. positive Adams flexion test (to exclude possible scoliosis); and g. anomalous kyphosis with thoracic deformity
Test methods for spinal morphology and function
The gold standard of spinal morphology testing is X-ray irradiation, but the radiation produced is contrary to the ethical requirements. In this study, a non-invasive device (Spinal Mouse) was used to test the spine morphology and function; this device has good reliability and validity for replacing X-ray use to some extent[13-15].
The test methods were as follows: the subjects were asked to take off their shoes and their jackets and expose the entire back from the seventh cervical vertebra (C7) to the third sacral vertebra (S3). The surface localization of all spinous processes from C7 to S3 was marked with a fluorescent pen. The test protocol included standing sagittal plane, maximum flexion, maximum extension, and Matthiass test[16, 17]. All body position tests first determined all the spinous process markers on the body surface and then used the receiver to test the spine based on the spine markers. The specific testing methods for each body position were as follows: 1. upright position, the study participants were instructed to stand upright in a casual position with feet shoulder-width apart bearing equal weight, arms by their sides, and looking straight ahead; 2. forward bending position, the study participants were instructed to stand with their feet shoulder width apart and to bend the torso forward while keeping the legs straight and allowing the arms to fall naturally; 3. extended position, the study participants were instructed to stand with their feet shoulder-width apart bearing equal weight, arms by their sides or supported by the hips, looking straight ahead, with the jaw close to the chest. the participants were asked to stretch the torso backwards as far as possible; 4. Matthiass test, the study participants were instructed to stand with their feet shoulder-width apart, looking straight ahead. When asked, the subjects pushed their arms forward horizontally while holding a certain amount of weight until their arms created a 90° angle with the shoulders. Spinal curvature and pelvic tilt were measured. The participant was instructed to retain this posture for 30 seconds; then, the second spinal curvature and pelvic tilt measurements were taken.
The test indexes included the thoracic kyphosis angle (TKA), lumbar lordosis angle (LLA), sacral/hip angle (SA), and incline (INA) of the sagittal plane while the participants were in the standing and sitting positions.
The TKA is the Cobb angle from T1 to T12, and the LLA is the Cobb angle from L1 to S1. SA reflects pelvic positioning (the angle between the surface contour line and the vertical line of the sacrum) and pelvic mobility as a result of the limited movement of the sacroiliac (SI) joint. The incline line is the line between T1 and S1. The angle between the incline line and the vertical line is called the INA. The ROMs of the left and right thoracic and lumbar spine in the frontal plane were recorded. The ROMs of the sacrum, thoracic spine, and lumbar spine from the fully flexed position to the fully extended position in the sagittal plane, which can reflect the overall mobility of the spine, were also recorded. The Matthiass test was used to measure the changes in the TKA, LLA, SA, and INA upon loading.
Normal ranges for the TKA and LLA
Research agrees that the normal range for the TKA should be 20°–40° [18, 19].
However, few studies have reported the normal range of LLA among adolescents. Jean et al. reported a normal LLA range of 48.0° ±11.7° . In this study, the normal range for the LLA was considered to be 48.0° ± 11.7°. We used the mean value ± 2 standard deviations based on the results of Jean et al.’s study.
We make groups by sex and grade. Because gender difference and the fact that the students in different grades have great diversities on pressure and life schedule among Chinese students. Using the SPSS 19.0 statistical software, an independent sample t test was carried out on the basic indicators of height, weight, and age among the groups to test whether there were significant differences among the groups. An independent samples t-test was used for all indicators of the same sex and age groups. The likelihood ratio chi-square test was used to analyse whether there were differences in the incidence of abnormal thoracic kyphosis and lumbar lordosis among the different groups. Simple correlation analysis was used to analyse the correlation between age and spinal morphology and function. The difference was statistically significant with P<0.05.
SAS 9.4 was used for data cleaning and analysis. The CANCORR process was used to analyse 4 indexes (sacral INA, thoracic kyphosis angle, lumbar lordosis angle, INA) reflecting spinal morphology and 8 indexes (sacral activity, thoracic activity, lumbar activity, INA activity, sacral inclination load, thoracic kyphosis load, and lumbar lordosis load) reflecting spinal function. Canonical correlation analysis was carried out to further explore the internal relationship between the two variables of the adolescent spine, with P < 0.05 indicating statistical significance.