Source and study population
The study involved people from North, East, central, South, Southwest, northwest and northeast China, It involves 15 industries or working groups related to WMSDs, such as automobile manufacturing, shoemaking, biomedical manufacturing, electronic equipment manufacturing, ship and related equipment manufacturing, petrochemical, construction, furniture manufacturing, coal mine cleaning, animal husbandry, medical personnel, automobile 4S shops, vegetable greenhouses, civil aviation flight attendants, toy manufacturing, etc. Inclusion criteria of the study population: persons who have served for 1 year or more; Exclusion criteria: congenital spinal malformations and patients without WMSDs, such as trauma, infectious diseases, and malignancy, etc. This study has passed the ethical review of the Ethics Review Committee of The Chinese Center for Disease Control and Prevention, and the respondents were informed and consent. Statement that all methods of this study were performed in accordance with relevant guidelines and regulations. The data handling and storage were compatible with the law. All protocols was carried out in accordance with the Helsinki Declaration.
Sample size determination and sampling procedures
In this study, stratified cluster sampling method was adopted to select all on-duty employees meeting the inclusion criteria from representative enterprises in key industries in North China, East China, Central China, South China, Southwest China, northwest China and Northeast China. A total of 64052 people were surveyed and 61034 questionnaires were received, with a response rate of 94.6%; A total of 57501 valid questionnaires were collected with effective rate of 94.2%. The total number of respondents was 57501, including 37240 males and 20261 females.
Data collection tool and procedure
The incidence of WMSDs among occupational groups in key industries in different regions of China was investigated by using the electronic questionnaire system of <Chinese version of musculoskeletal disease questionnaire> compiled by our research group. This electronic questionnaire system is based on Nordic Musculoskeletal Disorders Questionnaire (NMQ) . After appropriate modification, it has been proved to have good reliability and validity and can be used for Chinese occupational population. The survey contents include: ① general information such as age and years of service; ② Occurrence of musculoskeletal symptoms; ③ Work type, the organized form of work, working posture, etc.
The survey was conducted in the form of 1: N. One investigator conducted a face-to-face survey on N respondents. The respondents scanned the QR code of the electronic questionnaire and answered the questions online. After submitting the questionnaires were directly uploaded to the cloud database.
Criteria for low back pain
The criteria of NIOSH for musculoskeletal injury  were adopted: pain, stiffness, burning, numbness or tingling and other uncomfortable symptoms, which were consistent with ① discomfort in the past year;② After accepting the current job, I began to feel uncomfortable;③ No previous accidents or sudden injuries (local effects and discomfort);④ If discomfort occurs every month or lasts for more than 1 week, it is judged as musculoskeletal disease in this part.
Data quality control
In order to ensure the scientific nature of research conclusions and the authenticity, validity and reliability of data, quality control runs through the whole research process, including design, implementation, data collection and data collation.
Refer to relevant literature, clarify the research purpose and investigation methods and other key aspects, take appropriate measures to control the possible bias in research design.
On-site investigation and measurement
Before the investigation, the investigators should be trained strictly so that they can fully understand the purpose and significance of the research and master the investigation and monitoring methods. During the survey, the investigators will explain the purpose, significance and requirements of the survey, and conduct a face-to-face survey with the investigators. The respondents will fill in the questionnaire and submit it on the spot to ensure the authenticity, integrity and high retrieving rate of information sources.
Investigators monitored the completion of questionnaires to ensure that all surveyed information was from the respondents themselves. Electronic questionnaire has logical error correction to avoid unreasonable information. If there are blank items that cannot be submitted, you can negotiate with the investigator to fill in the blanks to ensure that the information is complete.
Data management and analysis
After the survey data were exported from the backend database, the data were statistically processed by Spss20.0 statistical software. The measurement data adopts x̄±s indicators, and the single factor analysis of WMSDs adopts χ 2 test method, multivariate analysis was analyzed by unconditional logistic regression model.