This article is reprinted with permission from The Ergonomics Report™ Archives, with minor changes, where it originally appeared on August 14, 2012.
Well known researcher William S. Marras recently  wrote an article appearing in the journal Human Factors, cited below, that provides an overview of the complexity of low back pain (LBP) and low back disorders (LBDs). In his introduction, Marras provides the following background:
Marras goes on to pose this question:
Do ergonomists understand LBD causality well enough to prevent LBD caused by work exposure?
He notes that researchers have produced few validation studies that examine the relationship between LBP and the assessment tools ergonomists use to estimate risk. Drawing on a quote attributed to Einstein, 'make everything simple as possible, but not simpler,' Marras asks: "Have we made things too simple" when it comes to ergonomics assessments related to LBP? The remainder of his article focuses in on the logic pathways used to understand LBP and LBDs, then considers how well ergonomics assessment tools map to those logical pathways.
The Premise: LBDs and LBP are initiated by stimulation of pain-sensing tissues
Marras states that, "For the purposes of this discussion (and there is good reason to believe), I assume that the source of this stimulation is always force or load. However …"
He also notes that biomechanical loading can be triggered not just by physical work factors, but also by individual, psychosocial and organizational factors. Ultimately it is the relationship between tissue load and tissue tolerance that defines LBD risk.
Traditional tissue loading tolerances have focused on spinal compression forces, and limits determined through cadaver testing have led to assessment tools that set a lower limit for compression force at 3,400 N (770 lb.). However, cadaver tissue failure is not necessarily representative of living tissue, and even among cadaver tissue studies the failure limits can vary widely; Marras encourages further research into such limits.
More recent research suggests that shear loading on the spine in certain working situations forms a lower tolerance limit than compression forces, but there is a lack of research into the specific limits, as well as a lack of assessment tools that can accurately assess shear loading. Marras calls for more research into shear tolerance limits, particularly in multidimensional loading situations, like so many real-world activities are.
Voluntary muscle strength tolerance is another measure used in some ergonomics assessment tools, however, "these muscle strength limits do not document the yield strength of the muscle fibers." Marras also notes that true tolerance levels change over time and they vary from person-to-person based on such influences as age, tissue degeneration, cumulative loading, individual conditioning, genetics, and muscular compensation triggered by pain. He suggests that research that describes the time-dependent behavior of these tissue/material tolerances will be "an important avenue of exploration for future research endeavors." This is a thought echoed by researcher Sean Gallagher in his recent article [subscription required] proposing that a mechanical/tissue fatigue-failure model may be useful to better understand MSDs. There are caveats associated with relying on tissue tolerances, and damage when those tolerances are exceeded, however, since medical diagnostic imaging suggests a relatively small percentage of LBP reports are associated with noticeable tissue damage.
Marras summarizes the tissue tolerance topic by saying, "although researchers have a basic understanding of how mechanical tissue tolerance responds to work, the factors influencing pain perception tolerance are much less understood." Interested readers are directed to the full journal article, referenced below, for a more detailed review of the tissue tolerance question and the role of other factors, such as nerves and pain perception.
Marras provides a brief overview of the evolution of work-related spinal tissue loading from humble beginnings with simple static models in the 1960's to complex dynamic models today, explaining that "Most of the advanced models employ multiple muscle inputs and sophisticated modeling techniques in an attempt to develop person-specific assessments of spinal loading during work … there are several efforts that are approaching the ability to build and use spine models that are specific to an individual worker."
Common Risk Pathways
It's important to recognize that there are non-physical risk factors that can contribute to LBDs. Marras points to a body of studies designed to investigate how psychosocial, psychological, personality and physical loading can interact to increase risk of LBDs. For example, researchers have shown that spinal tissue loading can increase significantly when certain personality types are exposed to demanding mental tasks or challenging psychosocial environments while performing lifting tasks. Others have shown that people with existing LBP experience significantly increased spinal tissue loads while lifting than people without LBP. The common pathway among these non-physical risk factors appears to be coactivation of the muscle groups supporting the spine. "This increased coactivity dramatically increases the magnitude as well as the direction of applied force on the spine." Further, new research shows that certain personality types experience increased inflammatory biochemical reactions under mentally stressful tests, suggesting that pain tolerance may be lowered even as spinal tissue loading increases, compounding the LBP problem.
Frontiers for the Control of Work-Related LBDs
Marras summarizes his review with some key points:
Marras argues that a systems approach that considers mental and physical factors is the basis for wellness, and that practitioners "must begin to integrate a wellness approach into ergonomics approaches."
Marras closes his discussion with: "Given the integrated biomechanical-biochemical-cognitive-psychosocial nature of people, such a wellness approach to ergonomics should have the best chance of improving low back health consistent with the current understanding of causality."
What this Might Mean to Ergonomists
Marras' article is a very important contribution to the field of ergonomics, first because he is a well recognized and highly regarded leader in the field, but also because it subtly ties together several emerging trends in the science and practice of ergonomics. In this review I've used far more direct quotes than I typically would, because I do not want to try to interpret all of his statements.
I recognize several main themes that Marras ties together in this article, whether he does so explicitly, or by implication, or by using terminology that elicits my own thoughts and biases, so this list is a mix of points Marras makes directly, or opinions that have come to my mind through this review:
William S. Marras, (2012), The Complex Spine: The Multidimensional System of Causal Pathways for Low-Back, Human Factors, appearing as an "online first" article, DOI: 10.1177/0018720812452129. The article is available to Human Factors subscribers at http://hfs.sagepub.com/content/early/2012/07/11/0018720812452129.abstract
William S. Marras, September 11, 2013, personal communication.
This article originally appeared in The Ergonomics Report™ on 2012-08-14.