Guidance on the Use of The Shoulder Tool

The Shoulder Tool is an easy-to-use ergonomics risk assessment tool developed by researchers at Auburn University for evaluating risk associated with shoulder intensive tasks. It is based on fatigue failure theory, which provides a means for estimating the accumulation of damage in material subjected to repeated stress. There is considerable evidence that musculoskeletal disorders (MSDs) result from an accumulation of damage (or “cumulative trauma”) in musculoskeletal tissues as the result of repetitive stress (Gallagher and Schall, Jr., 2017). The Shoulder Tool has been validated against a cross-sectional epidemiological database (Sesek, 1999) in which the Shoulder Tool’s cumulative damage measure showed strong dose-response relationships with both shoulder pain and clinic visits for shoulder/neck symptoms.

The purpose of The Shoulder Tool is to determine the cumulative shoulder load experienced by a worker during a typical workday. The tool’s cumulative shoulder damage measure provides an estimate of the probability that the worker would experience shoulder musculoskeletal symptoms. If multiple tasks are performed, The Shoulder Tool will be able to identify tasks that are most responsible for the risk of shoulder outcomes.

Using The Shoulder Tool

All that is needed to use The Shoulder Tool are three pieces of information for each shoulder task being analyzed: 1) the weight held, or force exerted by the hands; 2) the greatest horizontal distance from the acromion (the flat bone on the top of the shoulder) to the center of the hand or load during the task (using a measuring tape); and 3) the total number of repetitions of the task performed during the workday. Figures 1-3 below demonstrate how to measure the maximum distance from the shoulder joint to the load. Note that the measuring tape should be held horizontally for manual handling tasks (Figure 1), but vertically when assessing tasks involving forward pushes or backward pulls (Figure 3).

The weight of the item should be divided between the hands. In many cases, the weight may be evenly divided, but there will also be cases where the analyst should divide the weight unevenly if one shoulder is bearing more of the weight of the object. This may have to be estimated by the analyst.

When measuring lever arms for both shoulders it is important to measure the maximum lever arm for each shoulder during the task. The maximum lever arm for the left shoulder may occur at a different time than the maximum lever arm for the right shoulder.

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Assessing Mono-Task Jobs

The simplest application of The Shoulder Tool is for mono-task jobs (i.e., jobs involving repetition of the same shoulder intensive task throughout the workday). Assume a job involves handling a 2-pound load with the right shoulder having a lever arm of 16 inches repeated 2,880 times during the workday. Figure 5 below shows a screenshot of The Shoulder Tool for this mono-task job, which demonstrates this job’s probability for a right shoulder outcome is 20.8%. For the shoulder tool the outcome is defined as symptoms severe enough that the worker seeks medical attention. Note that while The Shoulder Tool can evaluate the risk to both shoulders, if the analyst only wants to evaluate one shoulder, the tool will accommodate such an analysis as well.

Now, let’s assume another monotask job involves an operator lifting a 10 lb. panel from a conveyor to a rack using both hands (with even weight distribution). This task is performed 480 times per shift and the peak lever arm when handling the load is 18 inches. Note that in this example, the since the weight is evenly distributed and the lever arms are the same, the risk to each shoulder is identical (the tool does not differentiate “handedness” dominant hand strength). The analysis is shown in Figure 6 below:

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Assessing Jobs with Multiple Shoulder Tasks

For jobs involving multiple exertions during a workday, each individual task should be analyzed using the same methods shown above. The cumulative damage associated with each task will be summed by The Shoulder Tool to determine the "daily dose" of cumulative damage for both left and right shoulders. An example is provided in Figure 7 below.

As can be seen in Figure 7, The Shoulder Tool identifies the percentage of the total damage associated with each task. Note that the first task represents a symmetric lift with equal moments about the left and right shoulders and the same number of repetitions. In this example, task 2 was associated with the majority of the cumulative damage. Therefore, task 2 should be given priority for ergonomic intervention.

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Assessing Highly Variable Shoulder Jobs

If a large number of highly variable shoulder intensive tasks are to be performed (for example, warehouse picking), a "binning" procedure can be used. In this procedure, tasks within the same range of peak moments are summed together and the amount of cumulative damage is determined using the number of shoulder tasks within each "bin". For example, let’s assume a warehouse picker was to lift items that have the following shoulder moments (for the right shoulder):

Table 1. Right shoulder data for binning example

Task	Lever Arm (inches)	Load (pounds)	Moment (ft.lbs.)	Repetitions
1	12	8	8	30
2	18	14	21	25
3	20	15	24	35
4	21	10	17.5	50
5	15	4	5	55
6	16	8	10.7	32
7	24	14	28	45
8	15	7	8.8	72
9	18	10	15	40
...

We can reasonably create three "bins" for these data based on the shoulder moments: one from 0-10 ft. lbs., one from 11-20 ft.lbs., and one from 21-30 ft.lbs. The first bin would include tasks 1, 5, and 8. Similarly, the second bin would include tasks 4, 6, and 9 and the third bin would include tasks 2, 3, and 7.

For the first bin, we would input the data for the first bin into first task (i.e., first line) of The Shoulder Tool. This task would have a 10 ft.lb. moment (12 inch lever arm and 10 lb. load) and 157 repetitions (the sum of the repetitions for tasks 1, 5, and 8). For the second line, we would input a 20 ft.lb. moment (12 inch lever arm and 20 lb. load), sum the repetitions for tasks 4, 6, and 9 (50+32+40 = 122), and input this value as the repetitions for this bin. The procedures for the third bin would follow the same steps as above. Figure 8 shows the results of the binning analysis example.

It should be noted that the binning procedure described above might result in a somewhat inflated probability of shoulder outcomes. However, the estimates should be reasonably close. In this regard, it should be recognized that when using the binning technique, it is important to use the narrowest range for the bins possible. While our example uses bins that are 10 ft.lbs. wide, there is no reason that bins with a width of 5 ft.lbs. could not be used. In fact, it is recommended that the analyst use the smallest bin size that can be reasonably used and completely cover the range of shoulder moments observed in the job.

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Limitations

The shoulder is a complex structure and it is difficult in a simple risk assessment tool to account for all possible loading situations the shoulder might encounter that might increase risk. One example is the case where an arm is simply held directly above the shoulder. Such a posture will create a low moment, but requires substantial muscle contraction, will be physiologically fatiguing, and will undoubtedly incur some injury risk. Similarly, there may be tasks where the applied forces go directly through the shoulder joint, such as pushing forward at shoulder height with the arms straight in front of (and in line with) the shoulder. Such an activity creates no moment, but will create stress in the shoulder. These situations are not currently addressed in the tool. We hope that future versions will be able to address these limitations. However, the tool in its current form demonstrates strong associations with shoulder pain and worker clinic visits for shoulder and neck complaints.

Excel™ Version of The Shoulder Tool

An Excel™ version of The Shoulder Tool will soon be available for download. This version provides additional functionalities, including the ability to save your analyses in an Excel™ worksheet. This will allow the analyst to create a record of all analyses performed at a particular worksite, including information regarding the job and worker. The Excel™ version allows the analyst to save analyses of both left and right shoulders for a worker, and will save all relevant information to inform the risk associated with each shoulder and which tasks are contributing most to the workers overall risk.

Other Fatigue Failure Tools

The Shoulder Tool is the third risk assessment tool developed by ergonomists at Auburn University, all based on the fatigue failure model of musculoskeletal injury development. The others are the Lifting Fatigue Failure Tool (LiFFT) (Gallagher et al., 2017) and the Distal Upper Extremity Tool (DUET) (Gallagher et al., 2018). The former can be used to assess the risk associated with lifting and lowering tasks, while the latter can be used to evaluate the risk of upper extremity disorders. All three can be used in conjunction with one another to get a comprehensive assessment of risk to the low back, upper extremities, and shoulders for workers.

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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., Sesek, R. F., Schall, Jr., M. C., & Huangfu, R. (2017). Development and Validation of an Easy-to-Use Risk Assessment Tool for Cumulative Low Back Loading: The Lifting Fatigue Failure Tool (LiFFT), Applied Ergonomics 63, 142-150.

Gallagher, S., Schall, Jr., M. C., Sesek, R. F., & Huangfu, R. (2018). An Upper Extremity Risk Assessment Tool Based on Material Fatigue Failure Theory: The Distal Upper Extremity Tool (DUET). Human Factors, 60(8), 1146-1162.

Marras, W.S., Allread, W.G., Ried, R.G. (1999). Occupational Low Back Disorder Risk Assessment Using the Lumbar Motion Monitor. In: W. Karwowski and W.S. Marras, eds. The Occupational Ergonomics Handbook (CRC Press: Boca Raton, FL).

Sesek, R. F. (1999). Evaluation and refinement of ergonomic survey tools to evaluate worker risk of cumulative trauma disorders. Unpublished doctoral dissertation.