Guidance on the Use of the Distal Upper Extremity Tool (DUET)

The DUET risk assessment tool is an easy to use method of evaluating the risk associated with upper extremity tasks. It is based on fatigue failure theory, which is the theory that assesses the cumulative damage of materials subjected to repeated stress. There is substantial evidence that upper extremity disorders (and other musculoskeletal disorders) are the result of an accumulation of damage in musculoskeletal tissues due to repetitive stress (Gallagher and Schall, Jr., 2017). The DUET tool has been validated against a cross-sectional epidemiological database in which the DUET cumulative damage measure showed strong associations with upper extremity outcomes (Gallagher et al., 2017).

Using DUET

The purpose of the DUET tool is to determine the cumulative upper extremity load experienced during a workday. Based on DUET’s cumulative damage measure, a probability of an upper extremity outcome (specifically a first time office visit due to upper extremity symptoms) is calculated (Gallagher et al., 2017).

All that is needed to use the DUET tool are two pieces of information on each upper extremity task: 1) a rating of the intensity of the exertion for the task; 2) the number of repetitions of the task during the workday. The rating of exertion intensity can be obtained in different manners. One method is to obtain a subjective rating of exertion by the individual performing the task. Using this method the worker should be shown a copy of the OMNI-RES scale, and instructed as follows (modified from Robertson et al., 2003):

“Please give your subjective intensity of effort, strain, discomfort, and/or fatigue that you feel during performance of this task guided by the descriptors on the 10-point scale provided.”

Observers can also evaluate the intensity of exertion using the descriptors below. While a professional ergonomist should always use their best judgement when performing the rating, we generally recommend the following relationship:

Observation	OMNI-RES Score
Extremely Easy	0
Easy	2
Somewhat Easy	4
Somewhat Hard	6
Hard	8
Extremely Hard	10

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Assessing Mono-Task Jobs

The simplest example of applying DUET to analyze upper extremity task demands is that of a mono-task job (i.e., a job involving the same upper extremity task all-day long). In this example, the job involves an upper extremity task with an exertion intensity rating of “Somewhat Easy” (4 on OMNI-RES scale), which is performed 1350 times during the workday. Figure 1 below shows a screenshot of the DUET tool for this mono-task job, which demonstrates that this job has approximately a 32.1% probability of experiencing distal upper extremity symptoms.

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Assessing Jobs with Multiple Exertions

For jobs involving multiple exertions during the workday, each individual task should be analyzed using the methods shown above. The cumulative damage associated with each task will be summed by the DUET tool to determine the daily dose of cumulative upper extremity damage. An example is provided in Figure 2 below.

The example above shows that the combination of these three tasks leads to a combined probability of 48.2% for distal upper extremity symptoms. Further, note that task #3 accounts for approximately 92.5% of the total damage for the workday. This task would be a prime candidate for redesign. For example, if task #3 exertion level were reduced to “Somewhat Easy”, the probability that the overall job is high risk would be reduced to 29.5%.

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Assessing Highly Variable Tasks

If a large number of highly variable upper extremity tasks is performed, a “binning” procedure can be used. In this procedure, upper extremity tasks with the same exertion level would be summed together and the cumulative damage score is developed using the number of repetitions within each “bin”. Once again, the cumulative damage associated with each exertion level will be summed to establish the cumulative daily load for this combination of tasks. An example is shown in Figure 3.

While all of the “bins” individually result in low to moderate risk, the overall probability that these tasks would lead to distal upper extremity symptoms is greater than 60%. As can be seen from Figure 3 above, more than 90% of the total cumulative damage is due to the 230 exertions in the “Hard” and “Extremely Hard” categories.

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References

Gallagher, S., Schall Jr., M.C. (2017) Musculoskeletal disorders as a fatigue failure process: evidence, implications and research needs, Ergonomics, 60: 255-269.

Gallagher, S. Schall, Jr., M.C., Sesek, R.F., Huangfu, R. (2017). Validation of A Fatigue Failure-based Risk Assessment Tool For Distal Upper Extremity MSDs. Proceedings of the Annual Meeting of the Human Factors and Ergonomics Society, Volume: 61 issue: 1, page(s): 911-913.

Robertson, R. J., Goss, F. L., Rutkowski, J., Lenz, B., Dixon, C., Timmer, J., Frazee, K., Dube, J., Andreacci, J. Concurrent Validation of the OMNI Perceived Exertion Scale for Resistance Exercise. Med. Sci. Sports Exerc., Vol. 35, No. 2, pp. 333–341, 2003.