Workplace Ergonomics: Cumulative Stress & Back Injuries

Cumulative Stress to the Spine Plays a Key Role in Assessing Risk of Occupational Back Injuries

workplace ergonomics cumulative stress back injuriesBack pain and back injuries cost companies in the US more than 100 billion US dollars per year, in terms of both treatment costs and in lower productivity. An effective ergonomics program that identifies and reduces all Manual Materials Handling (MMH) risk factors is a key component of any effort targeted at reducing the cost of injuries.

Recently experts have suggested that, as employers recognize and reduce the risks associated with lifting, attention in MMH work has shifted towards reducing the risk associated with pushing and pulling activities. Key to the analysis of push and pull forces has been the determination of maximum acceptable forces, which have been determined by either psychophysical methods or biomechanical methods.

The basis of these analyses is that, so long as the force used to maneuver a cart is less than or equal to the acceptable force when starting a load moving, maintaining movement, turning, or stopping the load, the risk of injury is reduced and the job design is considered acceptable. To determine those acceptable force limits by psychophysical methods, persons performing the push or pull task subjectively judge whether or not a task is acceptable based on the force exerted, the frequency of exertion, the distance the load is moved, etc. In the biomechanical method, static or dynamic models of MMH forces on body structures such as the spine are used to determine acceptable force levels that do not stress the spinal structure beyond its ultimate strength. Biomechanical ultimate strength can be thought of as the maximum stress that a material such as an intervertebral disc can withstand while being stretched or pulled before breaking.

Push-Pull Risk is Underestimated when Elements are Considered in Isolation

However, current practice for both psychophysical analyses and biomechanical analyses of MMH risk is that each element of a push-pull task is considered separately and in isolation. An MMH task might have three elements: the initial force to start a cart moving, the force necessary to sustain movement, and the force necessary to turn the cart. If the initial force is acceptable and the sustained force is acceptable, and the turning force is acceptable, then the task composed of all three of these three elements is considered acceptable. But this current approach to analyses of push-pull forces doesn’t take the interaction or cumulative effect of all these elements of a cart-pushing task into account. That is, the risk of injury may be underestimated if the cumulative effect of the interaction of forces is not considered.

Kumar studied cumulative exposure to spinal loading, where cumulative loading was defined as force (Newtons) multiplied by the duration of the exertion in seconds (N.s). He found that back pain was associated with higher cumulative loading of the spine and suggested that cyclic or repetitive loading could lead to fatigue and a reduction in the stress-bearing capacity of the spine, even for stress loads less than the ultimate strength of the spine. In that study, individuals who did not report back pain had higher average compressive forces on the spine than individuals who did report back pain, but the key difference was that the individuals who experienced pain had much higher cumulative exposures.

S-N Curve Concept Applied to Interaction of Force and Repetition

More recently, Gallagher et al have employed the concept of an S-N curve to describe fatigue failure of biological tissues, such as those in the spine. An S-N curve plots the magnitude of repeated applications of stress (S) against the number of repetitions (N) before failure occurs. If this seems familiar to ergonomics practitioners, it is because it might be thought of as a quantitative statement of the well-known rule of thumb in occupational ergonomics, that the interaction of force and repetition increases the risk of musculoskeletal injury. High-force, high repetition tasks are known to have a greater risk for MSD injury than do either high force, low repetition tasks or low-force, high repetition tasks.

Gallagher and his colleagues at Auburn University have developed a tool, Lifting Fatigue Failure Tool (LiFFT), to evaluate the cumulative fatigue of spinal tissues such as the intervertebral discs in response to the application of repetitive compressive loading to the spine while lifting. The LiFFT model is a cumulative risk model; the additive effect of each stress applied to the spine is taken into account with regard to fatigue and failure of the spinal tissue. While each lift may vary and apply differing levels of stress to the spine, the LiFFT model considers the cumulative effect of all elements of the lifting task towards fatigue failure of the spinal tissues.

It is certainly possible to measure the stress to the spine created by pushing and pulling tasks in the same manner as those created by lifting. Among others, Weston et al have modelled compressive and shear loads to the spine during push and pull tasks, including the initial forces to start the load moving, to sustain movement or to turn the load. These task-dependent stress loads could readily be compared to the ultimate strength of the spine and the likelihood of fatigue failure determined, in the same manner as suggested by Gallagher et al for lifting.

Complex MMH Tasks – Magnitude & Repetition Increases Risk of Fatigue Failure

workplace ergonomics guide darcorIt seems quite plausible that the fatigue failure – cumulative stress approach establishes a common basis for evaluation of injury risk associated with complex MMH tasks, such as those that involve combinations of manually lifting, lowering, carrying, pushing, pulling, turning, and stopping loads. The risk of injury from fatigue failure of the spinal tissues would be determined as a function of the magnitude and repetition of the stresses applied to the spine during performance of those complex MMH tasks.

In conclusion, while evaluating single elements of a push pull task such as initial, sustained or turning forces has proven effective, it likely underestimates the risk of back pain and injury. The cumulative effect of all exertions with regard to fatigue failure of the spine should be considered when assessing the risk of back injuries and back pain during complex MMH tasks.

To learn more about the benefits of industrial ergonomics programs that reduce risks when pushing, pulling and maneuvering carts, download the Guide to Workplace Ergonomics.

Tom Albin PhD is a licensed professional engineer (PE) and a certified professional ergonomist (CPE). He holds a PhD from the Technical University of Delft in the Netherlands. He is a Fellow of the Human Factors and Ergonomics Society.
Tom has extensive experience as a researcher, corporate ergonomist, and product developer. In addition, he has been active in the US and International Standards community. He is accredited as a US expert to several International Standards Organization working groups and is Vice-Convenor of the ISO committee revising the standards for input devices and workstation layout/postures. He chaired the committee that revised and published the American National Standard ANSI/HFES 100-2007 Human Factors Engineering of Computer Workstations and currently co-chairs the committee working on a new revision of that standard.


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