Variation in spinal load and trunk dynamics during repeated lifting exertions
Introduction
Ergonomic task analyses often assume a task is performed in a biomechanically repeatable manner. Most ergonomic tools and biomechanical models predict identical spinal load in repeated exertions given the same workplace parameters. However, evidence from motor control analyses and musculoskeletal measurements indicate the potential for significant variability during the repeated performance of a specific task. This variability may influence the interpretation of the results of ergonomic risk assessments.
Variability in biomechanical performance and spinal load during lifting tasks may influence the risk of low-back disorders (LBDs) associated with the task. Mirka and Marras [1] concluded that biomechanical variability influences the relative number of repeated exertions that might exceed the lifting guideline limits [2]. Fig. 1 illustrates the distributions of compressive load associated with two hypothetical tasks. One task may possess an average spinal load and a narrow distribution of loads resulting in a very low probability of any exertion exceeding spinal tolerance levels. A similar task may be associated with identical mean spinal load but a wide distribution, resulting in a significant percentage of the repeated exertions that exceed spinal tolerance. However, because the mean values were similar, traditional ergonomic assessments may interpret the two tasks as equally safe.
Clearly, the risk associated with lifting exertions must be associated with the variability of the lifting biomechanics as well as the mean value. Therefore, workplace factors that influence the biomechanical variability associated with a lifting task subsequently influence the risk of exceeding injury tolerance and associated LBD risk. Although a specific workplace parameter may marginally reduce the spinal compression, if it also dramatically augments the variability, the benefits may be questionable.
To improve our understanding of the etiology of spinal loading and low-back pain risk, it is necessary to identify the biomechanical variability associated with lifting exertions as well as the workplace factors that influence that variability. A companion paper assesses the variability associated with performance of a model currently used to estimate spinal loads [3]. The objectives of this research were twofold. First, quantify the variability in lifting motions, trunk moment, trunk muscle activities, and spinal loads associated with repeated dynamic lifting exertions. Second, identify the workplace factors, including experience, that influence the variability associated with lifting tasks. We included work experience as a variable to evaluate whether more experienced workers were more consistent, i.e. reduced variability, in their lifting motions and spinal loads than inexperienced subjects.
Section snippets
Methods
Twelve healthy males with no prior history of LBD volunteered to participate in this study. The subject population included seven college students and five experienced manual materials handler (MMH) warehouse selectors from a local distribution center. The subjects’ ages ranged from 22 to 34 years with an average age of 26.1 years. The average (SD) stature of the subject population was 179.2 (4.5) cm and the average weight was 74.7 (7.0) kg. Experienced and inexperienced subjects were similar
Analysis of means
Mean values of trunk motion dynamics and spinal loads demonstrate that the results agree with previously published values. This supports the validity of the analyses of motion and spinal load variability data which follow. Analyses of the mean data trends provides biomechanical insight into the associated variability.
Statistical ANOVA results indicate that the lifting task parameters influence the three-dimensional dynamic motion patterns of the trunk. Post-hoc analyses identified the
Discussion
The analysis of the lifting tasks performed in this study demonstrated that workplace factors influence both spinal load magnitudes and the variability in spinal loads. Dynamic spinal compression values for the tested conditions averaged 5790 N, with a standard deviation of 1480 N. Task analyses might observe that the mean compressive load on the spine was below the NIOSH MPL limit of 6400 N [2]. Such conclusions would overlook the fact that over 20% of the lifts exceeded the recommended MPL
Conclusions
Lifting kinetics, kinematics, and spinal load demonstrate significant variability under identical and repeated lifting conditions. Workplace factors such as the magnitude of the load, task asymmetry, and worker experience can influence variability. Ergonomic assessments considering only the mean value of spinal load overlook the fact that a large fraction of the lifts may exceed recommended levels despite mean levels safely below tolerance limits.
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