Abstract
Sheep shearing is a physically demanding occupation, with high energy expenditure, spinal loads and risk of back injury. The cost of injury compensation and rehabilitation for this workforce is considerable. Although research shows the use of a commercially available trunk harness will significantly reduce spinal loads, there has been no investigation of worker skill on spinal loads and risk of injury. A higher skill level is accepted by the wool harvesting industry as improving quality of work and productivity. Others within the industry consider that increased skill lowers risk of injury by improving animal control and working in less demanding postures. Some research has shown a positive effect of skill within other occupations and tasks; such as a reduction in energy expenditure, spinal cumulative loads and asymmetric movements while others have shown no such effect.
The aims of this research are to quantify lumbo-sacral cumulative and peak forces experienced by workers in the wool harvesting industry, and to determine how skill and a history of low back pain requiring clinical intervention (LBP-Clin) impact on these loads. Following ethical approval a total of 140 participants (80 shearers and 60 wool handlers) were recruited and surveyed during formal shearing and wool handling competitions in Southern New Zealand. Each subject was then video-taped while executing 3 to 5 consecutive trials (dependent on skill level and competition requirements) of their normal task cycle. These video clips were analysed by using posture binning and load analysis software (3D Match) that incorporated 3D kinematics, external hand forces and anthropometric data to calculate the peak and cumulative loads on the L4/L5 segment. Cumulative loads were then extrapolated to an 8-hour work day. Correlation analysis was performed to determine collinearity between e xplanatory (independent) variables. Univariate linear regression models were initially used to determine the individual influence of skill and LBP-Clin on cumulative and peak spinal forces while multivariate linear regression models were used to determine the combined influence of skill and LBP-Clin on cumulative and peak spinal forces.
For shearers mean peak lumbo-sacral compression, joint anterior shear, joint anterior reaction shear, and extensor moments for shearers were 3828.7N, 230N, 458.3N, and 185.1Nm respectively. For wool handlers these peak lumbo-sacral loads were 3194.2 N, 189.2 N, 391.4 N and 165.1 Nm. Mean cumulative compression, force weighted compression, joint anterior shear, joint anterior reaction shear, and extensor moments for shearers were 82.6 MN.s, 84.8 MN.s, 5.4 MN.s, 11.8 MN.s and 4.2MNm.s while these mean cumulative scores were considerably less for wool handlers at 48.7 MN.s, 48.9 N.s, 2.53 MN.s, 5.7 MN.s and 0.023 Nm.s. Skill was associated with decreased peak catch and drag compressive force for junior, intermediate and senior shearers and also decreased cumulative extensor moments for junior and senior wool handlers. LBP-Clin was only associated with an increased peak extensor moment during the catch and drag for shearers while LBP-Clin had no significant influence on any peak or cumulative force for wool handlers. The interaction variable for skill and LBP-Clin also showed no significant influence on peak or cumulative forces for either shearers or wool handlers. Although this study demonstrates minimal influence for skill or LBP-Clin (or their interaction) on cumulative and peak cumulative and anterior shear forces, the prevalence of LBP-Clin within each skill level increases considerably (particularly for shearers). Interestingly increased skill is also strongly predictive of a considerable increase in productivity (or tally). Thus increased skill appears to be primarily beneficial in terms of increased wool production and task efficiency.
Further research with a larger within-skill sample size and prospective design is needed to confirm these results. Other biomechanical factors such as body position within working postures, time spent in different postures, harvesting techniques, and non-sagittal postures and forces (medio- lateral shear and reaction forces) may also be linked to skill and LBP-Clin. Exploring the effect of these other biomechanical factors continues within the occupational biomechanics research team at the University of Otago. Similarly personal and psychosocial factors are recognised as being linked to injury and injury risk within the overlapping fields of ergonomics and occupational health. The part they play in injury risk within the wool harvesting occupations is unknown and is also under exploration. A recommendation for the wool harvesting industry is to continue with formal skill training as it does not appear to expose the worker to increased cumulative or peak spinal loading and it is strongly associated with productivity.
However the marked increase in working lifetime prevalence of LBP-Clin in this physically demanding occupation is clearly a problem and it may be that exposure to such high compressive and shear forces (independent of skill) exceeds yet to be determined cumulative loading thresholds that lead to risk of low back injury. While postural demands and non-sagittal forces during traditional shearing also need to be investigated, development of alternative upright posture wool harvesting strategies is an industry identified direction for reduction of injury risk that is biomechanically sound and now under investigation.