The aim of this study was to analyse the convergence of two methods by comparing exposure and the assessed risk of developing musculoskeletal disorders at 18 repetitive task workstations. The already established occupational repetitive actions (OCRA) and the recently developed upper limb risk assessment (ULRA) produce correlated results (R = 0.84, p = 0.0001). A discussion of the factors that influence the values of the OCRA index and ULRA's repetitive task indicator shows that both similarities and differences in the results produced by the two methods can arise from the concepts that underlie them. The assessment procedure and mathematical calculations that the basic parameters are subjected to are crucial to the results of risk assessment. The way the basic parameters are defined influences the assessment of exposure and risk assessment to a lesser degree. The analysis also proved that not always do great differences in load indicator values result in differences in risk zones.
Another difference is that OCRA species organisational factors such as daily duration of repetitive tasks and a pattern of recovery periods. These factors affect the values of the OCRA index at workstations. In this study, these factors were considered constant not inuencing either the OCRA index or the RTI. ULRA considers those aspects indirectly, rather than directly. The concept adopted in ULRA considers all activities performed during the work day. ULRA assesses the work load related not only to repetitive tasks but also to all tasks performed during the work day. So, in this way, this issue is addressed when work at the examined workstations consists of various types of tasks, not just repetitive tasks.
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The fact that OCRA index depends on the number of activities per minute to a greater extent than RTI can be responsible for cases when OCRA has higher values and the results are not in step. If the number of actions per minute is the source of discrepancies between methods, differences between the number of actions would be proportional to differences in indexes (Table 5). However, it is not. The greatest differences between OCRA and RTT occurred for the Controller operator workstation for the right upper limb (Table 5). For that case, there were no differences between the methods in the number of actions per minute. On the other hand, there were cases where those differences were over 20% without strong differences in the values of the indicators. In seven cases, the differences between the number of actions per minute counted according to RTT and OCRA were below 20%. This supports the hypothesis that differences between the counted number of actions per minute could have been one of
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Figure 2. Relationship between the results obtained with OCRA and ULRAs RTI. Ergonomics 1591 Table 5. Differences in indicators of upper limb exposure obtained with OCRA and ULRA (values with indicate that values obtained with OCRA are higher than the values of RTT: RTT 2.2 * RTI 2 1.06).
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Workstation FFF (%) RTT/OCRA (%) FFF (%) RTT/OCRA (%) Circuit board assembly Preliminary sewing Circuit board operator Controller Programming option 1 Manual assembly Sewing headrest Unpacking circuit board Sewing leather Scanning Sewing car seat Final check Programming option 2 Socket assembly 56 31 87 22 62 50 12 34 24 51 10 46 58 16 6 38 2 61 12 15 61 18 2 7 31 2 10 2 29 0 8 20 43 31 3 2 1 68 25 14 7 12 11 37 17 45 6 2 49 2 6 1 2 111 41 10 2 14 2 28 15 2 31 15 22 46 13 Note: RTT/OCRA, differences between RTT and OCRA; FFF, differences in the number of activities during one minute calculated according to OCRA (the number of technical actions per minute) and ULRA (the number of cycle phases multiplied by 60 and divided by cycle duration). (Table 5). However, this parameter is not the only factor responsible for differences between the OCRA and ULRA results; other factors are meaningful too.
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