Summaries of Our Applied Research: Manual Materials Handling
Summary: Though biomechanically determined guidelines exist for lifting, existing recommendations for pushing and pulling were developed using a psychophysical approach. This study establishes objective hand force limits based on the results of a biomechanical assessment of lumbar spine forces during occupational pushing and pulling activities. It was found that these limits can be up to 30% lower than existing psychophysically determined pushing and pulling recommendations. Practitioners should consider implementing these guidelines in both risk assessments and workplace designs. Guidelines based on this research are available online, for free.
Summary: Information is lacking on spinal loading when using lift assist systems for airport baggage handling. We conducted a laboratory study to evaluate a vacuum lift system for reducing lumbar spinal loads during baggage loading/unloading tasks. Use of this system resulted in significant reductions in spinal loading (39% for compressive force; 25% in AP shear force) in the lumbar region for loading and unloading a baggage cart to the aircraft cargo hold in the ramp area. If manual baggage lifting is necessary, lifting baggage from the conveyor or belt loader to the top shelf of a baggage cart should be avoided. Similarly, if the vacuum lift assist system is to be used, loading baggage to the top shelf of a baggage cart should be prioritized among other tasks to reduce peak risk exposures.
Summary: Several factors can impact lumbar spine loads during lifting, including weight knowledge and weight magnitude. The results of this study suggest that subjects changed their lifting manner when handling unknown light weight, which increased spine loads to levels equivalent to handling heavier weights. This may be important for high frequency lifting tasks common in modern distribution centers.
Summary: The risks that carrying poses to the spine are still not fully understood. This study analyzed forces on the lumbar spine for various realistic carrying styles. Most carrying tasks resulted in relatively low levels of spine loading. However, two conditions (loads carried in the front of the body) significantly increased lumbar spine (A/P shear) compared with other carrying styles. This increase in load appeared to be due to the large moment arms imposed in front of the body during carrying.
Summary: Pushing tasks have become common in manual materials handling, and these tasks impose different tissue loads compared to lifting. Industry has commonly used psychophysical tables to determine job acceptance. However, due to the biomechanical complexity of pushing tasks, psychophysics may be misinterpreting risk. This study has shown that psychophysics may not provide an adequate estimation of low back injury risk during pushing. Subjects did not change their psychophysical response relative to spinal loading patterns; rather the response was associated with tactile sensation translated to the upper extremities from the hands. As a higher degree of control was needed for pushing tasks requiring navigation of the cart into a target, some of the A/P shear spinal loads rose above suggested tolerance limits, which increases the risk for low back disorders. Therefore, caution should be taken when using psychophysics for the evaluation of pushing tasks.
Summary: Masonry workers face some of the highest physical demands in the construction industry, where large bags of masonry material weighing 42.7 kg (94 lbs.) are commonly handled by mason tenders who mix the mortar, distribute mortar and bricks/blocks, and erect/dismantle scaffolding throughout the day. The objective of this study was to determine the effectiveness of using half-weight bags (21.4 kg / 47 lbs.) on reducing the biomechanical loading, physiological response, and perceived exertions. Based on the peak spine loads, the A/P shear and compression spine loads were reduced by 25% for the small bags. Bag weight was only one contributing factor, as the mass of the worker’s trunk plays a significant role in the resulting loads.
Summary: Low back loading and risk associated with pushing activities have been poorly understood. Twenty subjects were tested to examine how various physical factors might influence spine loads during pushing tasks that workers might experience in industrial settings. Load magnitude, speed of push, required control, and handle height were varied while pushing both carts and overhead suspended loads. This study confirms that pushing and pulling is not as simple a task as once believed, since it entails a complex biomechanical activity. Spine shear forces result from a complex coactivation of trunk muscle activities and spine orientations that are influenced by several occupational factors. This study may help explain why low back pain rates in some work environments associated with lifting may not be reduced even when lifting interventions (that change the task from lifting to pushing) are employed.
Summary: Discomfort surveys are commonly used to assess risk in the workplace and prioritize jobs for interventions before an injury or illness occurs. However, discomfort is a subjective measure and the relationship of discomfort to work-related factors is poorly understood. The objective of this study was to understand how reports of discomfort relate to work-related low back risk factors. It was found that the perception of discomfort is strongly influenced by work experience level and minimally related to spine loads. Novices consistently reported higher discomfort levels than experienced subjects, who reported similar levels of discomfort for all levels studied. Discomfort for novices increased significantly over the duration of the trial and once a certain acceptable threshold of moment was exceeded. These strong experience interactions indicate that caution must be used when interpreting studies that involve inexperienced subjects to apply the findings to experienced workers. In addition, discomfort should only be used as a supplement to objective measures, such as spinal loading, to assess low back pain risk.
Summary: Stooped, restricted, kneeling, and other awkward postures adopted during manual materials handling (MMH) have frequently been associated with LBP. However, lift assessment tools have focused on materials handling performed in an upright, or nearly upright standing postures. Many of the tools designed to analyze standing postures are not easily adapted to jobs requiring restricted postures. The objective of this study was to evaluate spinal loading during MMH in kneeling postures and determine if those loads can be predicted using simple regression. Spine loads in kneeling postures were found to be independently sensitive to both the weight lifted and the final destination height. Increasing both weight and destination height increased spinal compression and AP and lateral shear. These effects can be used to estimate spinal loading using easy-to-measure variables such as subject height, lift destination height, and load weight.
Summary: Injuries to airline baggage handlers cost millions of dollars annually. Two low-cost administrative means of changing their working conditions were explored. (1) Weight class ID tags, to help prepare the employee for the effort necessary for each lift. While this was not found to significantly reduce spinal loads, it could act as a procedural aid helping to determine bag storage location, so that heavier bags could be slid into place on the bottom with lighter bags being stacked on top, thus reducing the cumulative load experienced by the baggage handler. (2) The use of a tipping motion rather than sliding or lifting the bags. This was shown to reduce spinal loads. This demonstrated that simple, low-cost solutions that could be readily incorporated into current baggage handling practices can result in potentially significant benefits, such as lower spinal loads, to airline baggage handlers.
Summary: No guidelines have previously been developed to address the issue of recurrent low back disorders due to materials handling when injured employees return to the workplace. This study compared spine loads of low back pain patients and asymptomatic controls, for weights varying from 4.5-11.4 kg (10-25 lbs.) at four vertical heights, two horizontal distances and five task asymmetries that, collectively, represent common industrial lifting situations. Guidelines for safe lifting based on this research are available online, for free.
Summary: This study documented three-dimensional spinal loading during lifting from an industrial bin. Two lifting styles (one- vs. two-hands) and two bin design factors (location of the load in the bin, bin height) were examined. It was found that spinal loading was reduced when individuals:
- Lifting from the lower regions or upper back region of a bin used one-handed support;
- Lifting from all bin regions maintained contact with the floor with both feet;
- Supported their body weight on the side of the bin with one hand; and
- Started lifting from the upper front region within a bin.
Summary: This study evaluated spinal loads associated with lifting and hanging heavy mining cable in four work postures (standing, stooping, kneeling on one knee, or kneeling on both knees) and two cable load levels. It was determined that:
- Changes in posture and cable load both influenced trunk muscle recruitment and spinal loading.
- Increased cable load resulted in significantly increased activity of all trunk.
- Kneeling on both knees was the least stressful posture in terms of spine loading, and stooping was most stressful, while standing and kneeling on one knee involved a level of spinal loading intermediate between these two.
- The magnitudes of spine compression and shear loading on the spine were always quite high when lifting the cable, regardless of the load condition or posture.
- Efforts should be made to provide mechanical assistance to perform this task.
Summary: High low back disorder rates are common in distribution centers and warehouses, where order selectors manually lift boxes from storage bins to mobile pallets. This study explored the effect of box features and box location on spine loading. It was found that the position from which the selector lifted a box on a pallet had the most profound effect on spine loading, while handling boxes to the lower level of the pallet produced the greatest load. Box weight did not appear to be a feasible means of controlling spine loading unless its position on the pallet could also be controlled. In addition, the inclusion of handles on boxes had an effect similar to reducing the box weight by 4.5 kg (~10 lbs.), whereas box size did not affect spine loading.
Summary: Two-person or team lifting is a popular method for handling materials under awkward or heavy lifting conditions. This study explored the spine loading characteristics of one- and two-person lifting teams when lifting during several sagittally symmetric and asymmetric conditions. It was found that the preferable number of team members involved in lifting depends on many factors:
- In general, one-person lifting is beneficial when lifting under symmetric lifting conditions.
- Lateral spine shear forces may become problematic when two-person teams place a load in a specific asymmetric location compared with allowing a one-person lifter to place a load in an asymmetric “non-fixed” location. Thus, precision of placement is a variable that must be considered when lifting.
- The detrimental effects of two-person lifting can be significantly mediated, especially at asymmetric destinations, by training the lifting team to lift synchronously.
- A significant trade-off is associated with one- versus two-person carrying tasks. Spine compression is greater with one-person carrying, whereas lateral shear is greater with two-person carrying. The degree to which the destination of the load placement is specified also affects this trade-off.
Summary: It is widely believed that depalletizing operations in manufacturing and service environments substantially increase the risk of occupationally related low back disorders (LBDs). This study assessed spinal loading and LBD risk as a function of box weight and its location on the pallet. Experienced order selectors transferred boxes of different weights (40, 50, and 60 lb.) from six different locations (regions) of a pallet to a pallet jack. They were monitored for their trunk motion characteristics trunk muscle activity. It was found that the location of the box on the pallet is a more important indicator of LBD risk and spinal loading than the weight of the box. The lower regions of the pallet were identified as most problematic areas for lifting.
Summary: A challenge for ergonomists has determining the types of trunk motions and how much trunk motion contributes to the development of occupationally related low back disorders (LBDs). A study was performed to assess the contribution of three-dimensional dynamic trunk motions on LBD risk during lifting in industry. Over 400 lifting jobs were studied across 48 industries. Medical records were examined to identify specific jobs historically categorized as either low, medium, or high LBD risk. Use of a multiple logistic regression model indicated that a combination of five trunk motion and workplace factors (lifting frequency, load moment, trunk lateral velocity, trunk twisting velocity, and trunk sagittal angle) predicted well both medium and high LBD risk. Increases in the magnitude of these factors significantly increased risk. These factors could be used as quantitative, objective measures to redesign the workplace so that occupationally related LBD risk is minimized.