Most Manual Materials Handling are Compound Tasks – Existing Assessments Underestimating Risks
No matter where you work – aerospace, health care, high tech, or other industries – manual materials handling (MMH) is an everyday part of the job. MMH plays a causal role in the development of low back pain and injuries; low back disorders continue to be among the most common and most costly musculoskeletal disorders seen in the workplace. It is estimated that more than 80 percent of all adults will develop back pain at some point in their lives and lost time back cases account for about one-third of all workers’ compensation costs.
Identifying musculoskeletal injury risk factors associated with MMH is an important component of any industrial ergonomics program that targets the reduction of workplace injuries. MMH tasks are often compound tasks or aggregates of several different tasks. For example, an MMH task might consist of loading boxes onto a cart, pushing the cart to a first location, maneuvering the cart around corners, unloading a few boxes, pushing the cart to a second location, and unloading or loading boxes there. Compound MMH patterns of this type are routinely seen in industrial workplaces.
Currently, separate tools are used to individually evaluate each simple component of MMH tasks, such as each instance of lifting, carrying, pushing, and pulling. Applying the tools to a compound task yields several separate risk assessments, e.g. one for the lifting component and another for the pushing component. How should the combination of these individual analyses be evaluated to estimate the risk of injury for the compound task which includes multiple tasks, e.g. both lifting and pushing?
Assumptions during MMH Risk Analysis may result in Overlooking Possible Risks
Suppose that we determine that 75 percent of females are able to exert the push force necessary to move a cart and that 75 percent of females are able to exert the force necessary to lift materials to place them onto the cart. Are we safe in assuming that 75 percent of women are able to perform both the lifting and pushing actions?
Although it seems counterintuitive, I would argue that it likely isn’t a safe assumption, because it all depends on the correlation between lifting strength and pushing strength. The same 75 percent of females would be able to perform both tasks only if lifting and pushing strengths have a perfect correlation value of 1.0. While the different types of strength used to push and lift are correlated, they aren’t perfectly correlated.
A study of US Army recruits noted that female recruits’ lifting and pushing strengths were only weakly correlated, with a correlation value of 0.15. Without going into detail, a mathematical calculation tells us that, rather than 75 percent of females being capable of performing both the required pushing and lifting exertions, only about 59 percent of females are capable of doing both. Relying on separate analyses of the lifting and the pushing exertions can lead to a serious overestimation of the acceptability of the compound task.
One solution to protect 75 percent of females would be to design the task so that about 85 percent of females are capable of exerting the pushing force and 85 percent are capable of exerting the lifting force. Then the math tells us that, when the lift and push are combined in the compound task, about 75 percent of females will be capable of exerting both the required lifting and pushing forces, even though about 85 percent of females are capable of doing one or the other.
Complexity of Compound MMH Task Means Taking a Closer Look at Risk Analysis to Reduce Workplace Injury Risks
Back injuries related to MMH are common and costly. Evaluating the safety of MMH tasks is critical to reducing workplace injuries. However, workplace MMH tasks are often complex with different amounts of pushing, pulling, lifting and carrying. Relying on separate risk analyses of each lift, push, pull and carry task to evaluate the safety of an MMH task may lead to serious over-estimation of acceptable force levels for compound MMH tasks. Research into the development of improved methods of assessing the risk associated with compound MMH tasks is urgently needed.
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