Researchers Roberta Bonfiglioli, MD, Stefano Mattioli, MD, Thomas Armstrong, PhD, Francesca Graziosi, MSc, Francesco Marinelli, MStat, Andrea Farioli, MD, and Francesco Violante, MD, all from the University of Bologna, Italy, with the exception of Armstrong, from the University of Michigan, USA have published a study that they say validates the ACGIH TLV© for HAL (American Congress of Governmental Industrial Hygienists Threshold Limit Value for Hand Activity Level). If so, this could be a ground-breaking study in understanding and preventing work-related carpal tunnel syndrome (CTS), one type of musculoskeletal disorder (MSD).
In their introduction, the authors provide the following evidence related to carpal tunnel syndrome (CTS):
The goal of this study was to “investigate biomechanical load as a causal factor for CTS, using the ACGIH TLV© based on hand activity level (HAL) and normalized peak force (PF).”
The experimental methods for this study are somewhat complex and involved, beyond the scope of this review, and interested readers are encouraged to read the complete research article, cited below and available at no charge at the time of this writing.
The researchers provide this abbreviated description:
"Industrial and service workers were followed from 2000–2003. We classified subjects with respect to action limit (AL) and TLV. Case definitions were: (i) self-reported symptoms; and (ii) combination of symptoms and positive nerve conduction studies. Poisson regression models including age, gender, body mass index, and presence of predisposing pathologies were conducted to estimate incidence rate ratios (IRR) of CTS."
“AL” refers to the Action Limit, one of the components of the ACGIH HAL assessment tool. ACGIH proposed the tool as a means to assess “mono-task” jobs such as “assembly line” and “keyboard” work. This studies authors state that “There is no clear cut-off between mono- and multi-task jobs,” and therefore argue that the tool “…applies to jobs in which there is a regular or predictable pattern of exertions over the course of each work shift.” Readers are again referred to the full article for more information on the ACGIH HAL assessment tool.
The researchers used data from the Occupational Carpal Tunnel Syndrome Observational Prospective Unified Study (OCTOPUS), “a longitudinal study of different industrial and service worker groups started in 2000,” and reported previously by other researchers.
Study results include:
The researchers present much more detail and additional results for interested readers.
What This Might Mean to Ergonomists
This could be one of the more influential studies in the area of MSDs in many years, though time will tell as other researchers review and further replicate and validate, or not, the findings in this study. One of the long-standing criticisms of the ergonomics scientific knowledge base has been the absence of “dose-response” relationships between physical risk factors and MSDs. In the minds of researchers and ergonomics professionals, the lack of risk-factor-to-injury dose-response relationships is due to the difficulties in conducting statistically valid studies. In the eyes of critics, the lack of scientific evidence is indicative that the relationships are weak or non-existent.
It’s a fair question: Why is it so difficult to identify dose-response relationships for Workplace MSDs (WMSDs)? The answer lies in the types of scientific methods and evidence that are necessary to demonstrate such cause-and-effect relationships. Assume for the moment that researchers could control all of the forces, motions and repetitions an experimental subject conducted over the many days, weeks or years it might take to develop an MSD under different conditions. If this were possible, researchers could carefully control their experiments and could expose human subjects to various levels of risk factors like force, repetition and posture to see what exposure levels lead to specific MSDs over time. In essence, they would have to injure a lot of people in order to prove statistically valid cause-and-effect relationships. However, intentionally injuring people for the sake of knowledge is heavily frowned upon in civilized societies, so researchers must instead turn to what are generally known as epidemiology studies.
In epidemiological studies, researchers lack controls over specific risk factor exposures. Instead, they must increase the number of subjects they include in their studies, and then they must attempt to measure and quantify risk factor exposures and MSD outcomes over some extended period of time and look for statistically valid patterns in their data. What they lack in specific controls over some smaller set of subjects in a direct cause-and-effect controlled experiment, they attempt to make up for with a larger subject population and less specific knowledge of the full-spectrum of risk factor exposures their subjects experience. Isolating specific exposures is very difficult, especially in a real-world work environment where people may perform many tasks with different risk exposures.
A primary “take-home” message from the study, however, is that the ACGIH HAL TLV© assessment method, when applied by properly trained assessors, appears to be a good predictor of CTS risk among workers.
Roberta Bonfiglioli, MD, Stefano Mattioli, MD, Thomas Armstrong, PhD, Francesca Graziosi, MSc, Francesco Marinelli, MStat, Andrea Farioli, MD, and Francesco Violante, MD (2012), Validation of the ACGIH TLV for hand activity level in the OCTOPUS cohort: a two-year longitudinal study of carpal tunnel syndrome, Scand J Work Environ Health – online first. doi:10.5271/sjweh.3312. http://www.sjweh.fi/show_abstract.php?abstract_id=3312
This article originally appeared in The Ergonomics Report™ on 2012-07-18.