Of 1900 completed questionnaires, 1537 responses with more than 90% of the items completed were included in the analysis. The response rate was 80.9% among the participants. Participants' age was in the range of 19-61 years (35.2 ± 16.7 years). There were 1051 underground workers (68.4%) and 486 surface workers (31.6%).
The overall prevalence of back disorder over the preceding 12 months was 64.9% (n = 977). The prevalence of back disorder among underground workers was higher than among surface workers (67.2% vs 59.4%, P < 0.001). The miners with LBP were significantly older than those without LBP. The miners had been in service for a mean duration of 12.3 years (range: 0.5-41 years). The seniority of coal miners at their current job varied significantly between the groups. Education level was categorized into 1 of 4 groups: very low (5-6 years of school education), low (7-10 years), medium (11-14 years) and high (college education and higher). There were significant differences in education levels between the groups. A significant difference in working hours was found between the groups [Table ].
Characteristics of study subjects
Univariate analyses [Table ] showed a strong association between increasing age and LBP for all workers. A potential relationship between occupational factors and LBP was also identified for all workers. These results demonstrated that LBP was associated with the following factors: standing for long periods (OR 1.2, 95%CI 1.1-1.3), exposure to cold temperatures (OR 1.6, 95% CI 1.3-2.0), tasks characterized by a high degree of repetitiveness (OR 1.5, 95%CI 1.2-1.8), tasks requiring the arms to be at or above shoulder level (OR 1.5, 95%CI 1.3-2.0), tasks characterized by a high level of physical demand(OR1.8, 95%CI 1.4-2.2), posture characterized by extreme twisting(OR1.6, 95%CI 1.3-2.0), posture characterized by extreme bending(OR1.7, 95%CI 1.4-2.1) and static work posture (OR 1.4, 95%CI 1.1-1.7). There was a significantly higher proportion of complaints about having limited time for rest during a working day among those with LBP compared to those without LBP (OR2.0, 95%CI 1.6-2.7). Underground workers had a significantly higher risk of LBP than surface workers (OR1.4, 95% 1.2-1.8). Potential factors were examined simultaneously in multivariate logistic regression. LBP was associated with tasks characterized by a high degree of repetitiveness (OR 1.3, 95%CI 1.0-1.6), tasks characterized by a high level of physical demand (OR1.4, 95%CI 1.1-1.8), posture characterized by extreme bending (OR 1.4, 95%CI 1.0-1.7), and insufficient recovery time (OR 1.4, 95%CI 1.0-1.8). Separate analyses were carried out for underground miners and surface maintenance workers. Factors associated with LBP were a high degree of repetitiveness (OR 1.6, 95%CI 1.2-2.1), tasks characterized by a high level of physical demand (OR 1.6, 95% CI 1.2-2.0), posture requiring extreme bending (OR 1.4, 95% CI 1.1-1.9) and insufficient recovery time (OR 1.5, 95% CI 1.1-2.1) in underground miners (not shown). Factors associated with LBP were exposure to cold temperature(OR 2.0, 95%CI 1.4-2.9), tasks characterized by a high level of physical demand (OR 2.0, 95% CI 1.4-3.0), posture requiring extreme bending (OR 2.1, 95% CI 1.4-3.3) and insufficient recovery time (OR 2.4, 95% CI 1.6-3.5) in surface maintenance workers (not shown).
Univariate and multivariate analyses for potential occupational factors of LBP in coal miners