From The Ergoweb® Learning Center

The Complex Spine: Understanding and Preventing Low Back Pain and Disorders

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 [2012] 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:

  • 80% of USA adults experience low back pain (LBP) at some point during their life;
  • between 15% and 29% of Americans report LBP each year;
  • the USA spends more than $90 billion per year on LBP treatment, roughly the same spent on cancer treatment;
  • LBP is the second most common reason to visit a physician in the USA;
  • LBP is the second greatest contributor to disability in the USA;
  • more than 100 million lost work days are attributed to LBP related disabilities;
  • expenditures are estimated to be 60% greater for LBP issues than for other illnesses;
  • in an extensive literature review published in 2001 researchers concluded that LBP is indeed linked to work exposures, with a wide range, between 11% and 80%, attributed to physical work factors;
  • a series of recent literature reviews contends there is no association between LBP and work exposure, prompting researcher Stuart McGill to comment that “Another review of nonspecific back pain related to nonspecific activity guarantees a null finding”
  • some researchers recognize that the more specifically the nature of workplace exposures are documented, the greater the ability to recognize risk associations;
  • however, "critics of ergonomics have continued to challenge the ability of ergonomic interventions to control LBP and LBDs [Low Back Disorders]"
  • if LBP or LBDs are not to be addressed or prevented through ergonomics interventions, the alternative to preventing LBDs becomes treatment after the fact;
  • however, medical treatments have proven "less than effective," for example:

    • surgical discectomies have only a 42.6% success rate compared to 32.4% success for non-operative treatments;
    • "Another recent study notes a 629% increase in Medicare expenditures for epidural injections, a 423% increase in opioids, a 307%  increase in magnetic resonance imaging, and a 200% increase in spinal fusion surgeries in recent years without any population level improvements in LBP patient outcomes or disability rates."

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 …"

  • the relationship between loading and stimulation can take many forms;
  • the relationship between loading and stimulation can vary within a single person, and from person to person; and
  • understanding this within and between person variation is critical to LBD causality, risk, and intervention.

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.

Tissue Tolerances

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.

Tissue Loading

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:

  • a multidimensional set of factors, both physical and mental, greatly influence the loading of the  spine tissues
  • researchers need to improve their understanding how tolerance limits change over a person's life span due to both controllable (e.g., workplace exposure) and uncontrollable (genetics) factors
  • "… loading and tolerance factors exist in a delicate balance and how something as subtle as a certain personality matched with a psychosocially challenging situation can lead to increased risk for certain individuals. It should be clear that this is a call for greater understanding of the spine system behavior and not a call for worker discrimination."
  • many current low back assessment techniques may oversimplify the risk, and while the techniques may be appropriate for certain individuals, we need to expand them to include "… population characteristics so that the tools apply to more of the population variance. Doing so will increase the sensitivity and specificity of these tools."
  • to effectively prevent LBDs we need a better understanding of mechanisms that may affect both the load and the tolerance sides of LBD risk
  • evidence shows that simply focusing on the load exposure does not fully control LBP and LBDs
  • "… there is abundant evidence that participatory ergonomics greatly increases the effectiveness of the intervention," and it may be "… that participatory 'factors' contribute a cognitive balance to the intervention that includes emotional, intellectual, and social environmental features."
  • "Given the subtleties associated with muscle recruitment patterns of tissue loading and the biochemical responses associated with pain perception tolerance, it is reasonable to expect that many of these participatory ergonomics factors serve a function to preserve a state of balance (or wellness) within the individual worker. This balance would then serve to minimize the tissue load (by mediating muscle recruitment patterns) as well as optimize the tissue tolerance (via biochemical pathways) and the resulting risk of LBD for an individual."
  • the musculoskeletal system and the mental system are not independent, but instead are "inextricably integrated and interactive," making it imperative that we take a systems approach that "must consider the worker’s perception of his or her environment … to mediate both his or her biomechanical and biochemical responses."
  • "Hence, one needs to consider the social, occupational, spiritual, physical, intellectual, emotional, financial, mental, and medical aspects of the environment if one is to truly minimize the risk of low-back problems."

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:

  • we don't have a complete understanding of all contributing factors to LBP and LBDs, and they continue to appear in working populations even after various attempts to control the physical factors we know to be responsible for their development and severity; [as Marras explains in a personal communication, "While general risk factors have been well established research needs to figure out how the interacting factors set the stage for who gets LBP and who does not."]
  • we know there are mental and biochemical factors that influence LBP and LBD, but we have few if any applicable tools that consider their contribution, let alone direct us in our control efforts;
  • there is a research trend moving our understanding of the physical contributions to MSDs toward the tissue and biochemical level, as opposed to the biomechanical approaches that consider gross joint and muscle loading;
  • there are research and applied trends that broaden the systems approach (and good ergonomics is always a systems approach) to include both physical and mental contributions to MSDs;
  • there is a trend to recognize the non-work related risk factors, such as genetics and personal fitness, as part of the MSD equation;
  • there is a growing mix of methods, politics and terminology tying "ergonomics" together with "wellness";


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

William S. Marras, September 11, 2013, personal communication.

This article originally appeared in The Ergonomics Report™ on 2012-08-14.