This cross-sectional survey was conducted to investigate the prevalence of LBP in coal miners and the association of LBP with occupational and personal factors in miners.
The 12-month period prevalence of LBP
The study demonstrated that the 12-month period prevalence of LBP was high in this study, affecting approximately 64.9% of Chinese coal miners. This finding is consistent with a report by Zejda et al., who found the 12-month period prevalences of 62.2% and 66.4% among coal miners at two mines. A recent study by Zhang et al. found that the prevalence of LBP among coal miners in China was 62.9% over 12 months [8, 11]. Sarikaya reported that the prevalence of LBP over the past five years among underground and surface workers was 78% and 34%, respectively . In Limburska's study, the 12-month period prevalence of LBP was 65.2% and 73.2% among coal miners at two mines . It is reported that workers who perform heavy manual jobs have a higher prevalence of LBP and a higher level of absenteeism. Miners have the highest rate of absenteeism due to back pain . This study supported previous reports that LBP is a common occupational health problem among Chinese miners. It is important to pinpoint such hazards to develop prevention strategies.
Factors associated with LBP in coal miners
The causes of LBP in coal miners have not previously been described in detail. Few studies have been conducted on the relationship between occupational factors and LBP among coal miners.
This study demonstrated that advancing age is strongly associated with LBP. In Hagberg's study, the normal degeneration of tissue with age predisposes individuals to LBP. Our research is consistent with the epidemiology literature [15–19]. Notably, at present, it is difficult to distinguish between pathological degeneration and normal changes due to aging .
Epidemiological studies have demonstrated several combinations of physical factors related to LBP. The main physical factors causing musculoskeletal disorders include rapid work pace and repetitive motion patterns, insufficient recovery time, heavy lifting and other forceful manual exertions, non-neutral body postures (either dynamic or static) of the wrists, elbows, or shoulders, concentrations of mechanical pressure, vibration (both segmental and whole-body) and low temperature . Among these factors, weightlifting and postural changes are the most significant factors causing low back pain among workers in heavy industry . This finding highlights four work-related physical ergonomic factors used to categorise coal miners. This conclusion is in accordance with the epidemiology literature [12, 17, 18, 21, 22]. As reported in this study, underground miners have a higher risk of LBP than surface miners because of working-environment characteristics. The underground miners' jobs involve extensive lifting, bending, prolonged standing, and walking in a confined space . The surface workers' jobs involve transporting, servicing, and repairing equipment as well as performing administrative duties at ground level. High physical demand was clearly associated with LBP, which was consistent with previous studies among coal miners [12, 19]. The tasks performed by coal miners involve high-energy demands or require considerable physical strength. Numerous studies have been conducted on the relationship between excessive repetition and work-related musculoskeletal disorders [15, 17, 18, 23]. Some previous studies have demonstrated a strong association between excessive repetition and LBP among the working population [21, 24]. Our study further confirmed this finding among professional coal miners. In accordance with the epidemiological literature [16, 19, 21, 24], extreme bending posture was highly associated with LBP. Among workers in heavy industry, postural changes are the most important risk occupational factor for LBP . Previous studies revealed that working in a bent posture in confined spaces leads to higher intradiscal pressure and increased physical demand as compared to standing [23, 25]. Among underground miners, extreme bending posture in a confined space was associated with LBP. Tasks requiring the arms to be at or above shoulder level represented a significant univariate factor for LBP, but no significant association remained when these factors were examined simultaneously. This may be due to the possible interaction between arms at or above shoulder level and high physical demand. In our study, insufficient recovery time over the course of a working day was strongly associated with LBP. The working times of professional coal miners far exceed those of the general working population; 70.9% of miners reported that they were required to work at least 50 hours each week.
From a methodological perspective, this study has certain limitations. The sample comprised 80.9% of workers who were invited to fill out the questionnaires, including many retired workers. Therefore, there was a high participation rate. Subjects affected by LBP were more inclined to respond to the questionnaires than healthy subjects. The true prevalence would be expected to be lower than estimated by the present study.
Furthermore, because of the limited applicability of diagnostic procedures, the genuine "non-specificity" of pain states or both, epidemiological surveys may include a considerable proportion of cases without an identifiable pathophysiological basis . Pain is a subjective experience, and there is no "gold standard" with which one may calibrate the instrument of measurement . History of LBP was assessed according to the Nordic Musculoskeletal Questionnaire, which has been documented to have acceptable validity and reliability . LBP was diagnosed based on the self-completion questionnaire; the incorporation of a physical examination would improve the quality of future studies.
Because the study had a cross-sectional design, cause-effect relationships regarding the observed associations cannot be established. However, the results of the study were suggestive of a possible relationship between personal and occupational factors and LBP. Because of the nature of this retrospective questionnaire survey, it is difficult to rule out the possibility of recall bias. In other words, respondents with complaints might recall exposure to work-related factors more accurately than respondents without complaints. For example, the workers with pain perceive exposures to be more extreme or adverse than workers without pain Thus, the true associations between the occupational factors examined and LBP may have been overestimated. Further prospective cohort studies would be helpful to confirm or refuse associations found in the present study.