From The Ergoweb® Learning Center

Force? Repetition? Posture? Duration? Which Risk Factors are Most Important in MSDs?

The Ergonomics Report™ has recently published summaries of several important research articles that delve into the question of physical ergonomic risk factors and their relationships in the development or aggravation of musculoskeletal disorders (MSDs). Ergonomists have long recognized that certain factors, including force, posture, frequency/repetition and duration of exposure are primary players in the MSD "equation," but exactly what that equation looks like, and the scientific evidence to support it, has been a challenge. However, recent research articles appear to be narrowing down the scientific evidence, strengthening our confidence in the cause-and-effect relationships between certain risk factors and injuries or discomfort.

In December 2011, we published An Equation to Predict Maximum Acceptable Efforts for Repetitive Tasks, which summarizes research by Jim R. Potvin. After performing a meta-analysis of various psychophysical studies (e.g., Snook et al) that looked at the maximum acceptable efforts for a variety of tasks, Potvin proposed an equation that could have significant impact on the way ergonomists estimate risk for repetitive tasks. He found that Duty Cycle (DC), which is essentially the percentage of a job cycle that force or torque is exerted, can be used to predict maximum acceptable force and torque efforts for upper extremity tasks (hands/arms/shoulders). In other words, he found a strong correlation between force (or torque/moment) and duration, the length of time an exertion is held. Potvin also tested the relationship between repetition and maximum acceptable effort, but found that it was only able to account for roughly half the variation in the data set, while DC accounted for greater than 85% of the variation. 

This month we published two more research reviews that bring additional clarity to the risk factor questions:

  1. Research Review: Force and Repetition Combine to Affect MSD Risk

  2. Researchers Identify Dose-Response Relationship Between ACGIH Hand Activity Level TLV and Risk of Carpal Tunnel Syndrome

In the first, researcher Sean Gallagher summarizes findings he and study co-author John Heberger reached after reveiwing 12 studies that evaluated the relationship between force and repetition. Gallagher notes that many current ergonomics tools and guidelines consider the factor of repetition to have the same impact on MSD risk no matter the level of force involved, but they found that it may be the way these two factors are combined — the way they interact — that is most important in the development of MSDs, rather than the effect of one or the other on its own.

In the second study, researchers Bonfiglioli et al found a dose-response relationship between the ACGIH TLV© for HAL and Carpal Tunnel Syndrome. The HAL method "is based on the frequency of exertions and the duty cycle (distribution of work and recovery periods), while the PF is the peak effort exerted by the hand during each regular work cycle." That is, repetition (frequency), duty cycle (duration), recovery periods and force combine to form a predictive relationship with Carpal Tunnel Syndrome (in their article, the researchers suggest the results may extend to other MSD condition, but those relationships were not sufficiently investigated in this particular study).

Together, these studies hone in on force, duty cycle and repetition as key factors in the MSD "equation." But what about posture? Gallagher summarizes its contribution by stating that "We also suggest that a third MSD risk factor may be relevant to the force-repetition interaction – awkward postures.  Awkward postures usually increase forces experienced by musculoskeletal tissues, so it’s therefore conceivable that a large reason that awkward postures have been identified as an MSD risk factor is simply that these postures increase forces experienced by musculoskeletal tissues." If you've ever developed or applied a biomechanical model, you know this to be true (e.g., Ergoweb's 2D Biomechanical model, part of the Job Evaluator Toolbox suite, or The University of Michigan's 3DSSPP model, or others). 

In practice, ergonomics professionals have recognized these factors for many years and applied various assessment methods to real world working conditions in an effort to understand, prevent or control work-related MSDs (WMSDs). Some of those assessment methods are better than others at capturing the complexity of WMSDs. Some are evidence based, meaning they have their roots in the scientific literature. Others are not, and represent someone's "best guess," or worse, some organization's "collective wisdom," rooted in sales and marketing, or worse yet, someone's personal, anecdotal perspective, often also rooted in sales and marketing.

Then again, which comes first in the pursuit of knowledge: basic science; or experience and practice? For MSDs, like most medical cause-and-effect questions, it's a balancing act. However, in the end, good science holds the ultimate answer, so it's great that the research is making strides to better quantify the MSD "equation." It's a complicated dance, if you will, and "it takes two to tango" (practitioners and scientists, in this case).

With this background in mind, are you using assessment tools that represent the state-of-the-art in capturing, and better yet, quantifying WMSD risk factors? As the saying goes, "if you can't measure it, you can't improve it."

On the topic of measurement, what other measures might ergonomics risk factors impact in a real production environment? The above discussion, and most physical ergonomics marketplace applications, center primarily on WMSDs. While no doubt important, WMSDs and worker discomfort are not always the biggest concerns to an organization, and a singular focus on them may well detract from the greater value ergonomics can bring. Things like productivity, waste, quality, and various human resource measurements (e.g., absenteeism, turnover) are often more important organizational drivers, and unless and until the physical ergonomics marketplace expands its value to recognize and impact such measures, its contributions will likely remain marginalized.

This article originally appeared in The Ergonomics Report™ on 2012-07-26.