In the original RSI model, one exertion is defined as the direct application of force to complete a work task once. Applying this definition for hand-intensive work like dental cleaning would be extremely time-consuming and would inflate the effort per minute variable to an abnormally high value, ultimately overestimating the RSI scores. We attempted to apply the original definition on a small set of video recordings, and all RSI results were over 30. Because the RSI model is designed to consider any score over 10 as ‘hazardous’ (Garg et al., 2017b), exceedingly high RSI scores lack discriminative power for meaningful risk classification. To make the process more efficient and avoid grossly overestimating strain, we defined one exertion as the direct continuous application of a dental scaling instrument with similar intensity and wrist posture patterns until a rest break of at least 15 seconds. The characterization of exertion force and posture patterns are described in the following sections.

In the RSI model, intensity of exertion (e.g., exertion force) represents the percentage of maximum strength (% MVC) required to perform the task once. In the case of dental hygiene work, the intensity of exertion is the amount of force required to hold a dental scaling instrument (static force) and stabilize the instrument against resistance to remove plaque, stains, or calculus from teeth (resistive force). Power-driven tools like ultrasonic dental scalers only require static force to hold the instrument. Conversely, dental scaling using hand-held instruments (i.e., manual scaling) requires resistive force to remove built-up plaque on teeth. Depending on the mouth area, plaque accumulation, and gum pockets’ depth, varying force intensities are needed to perform manual scaling.

We consulted with two dental hygiene experts to characterize dental scaling tasks, who identified three intensity patterns for manual dental scaling: light (L), moderate (M), and heavy (H). The experts and one researcher independently observed and categorized 9 randomly selected video segments. The first round of categorization was conducted without a standardized definition and yielded 70% agreement for categorizing intensity. Following this coding round, the dental hygiene experts and research team developed standardized definitions for each intensity pattern (Table 1). A second coding round using these definitions resulted in 89% agreement. In addition to the three intensity patterns, we added descriptions for ultrasonic scaling (US) and non-scaling (N).

Once we identified the intensity patterns, we conducted live observations to obtain self-reported Borg CR-10 ratings and quantify a standardized exertion intensity for each pattern. In total, we collected 424 manual scaling samples, of which 76 were pattern L, 251 were pattern M, and 97 were pattern H. The average self-reported Borg CR-10 ratings for patterns L, M, and H were 2.63, 3.98, and 5.60, respectively. We assumed a Borg rating of 1.0 for pattern US.

Next, we estimated duty cycles that accounted for static and resistive forces occurring with each exertion. Although the RSI eliminated duty cycle estimation due to high correlations with duration per exertion and effort per minute, we used duty cycles to ensure that defining an exertion that incorporated both resistive force and brief rest would not overestimate strain. We randomly selected 10 video recordings to estimate duty cycles and extracted 2 clips for each exertion force pattern (L, M, H, and US). We timed segments where participants actively applied force to patients’ teeth requiring resistive force (i.e., duty time). The average duty cycle for patterns L, M, H, and US were 83%, 75%, 67%, and 89%, respectively. We rounded the duty cycle percentages to the nearest 5%, resulting in duty cycle percentages for patterns L, M, H, and US of 80%, 75%, 70%, and 90%, respectively.

Lastly, we calculated the duty-cycle-modified intensity ratings using the self-reported Borg CR-10 ratings for the duty time and assuming a Borg rating of 1 for the static force required to hold the instrument during off-duty time. The duty-cycle-modified exertion ratings for patterns L, M, H, and US were 2.30, 3.24, 4.22, and 1.00. The corresponding intensity multiplier (IM) values were 2.93, 3.99, 5.37, and 1.57, respectively. We rounded the RSI multipliers to the nearest 0.5 for simplicity in completing real-time assessments, resulting in final values of 3.0, 4.0, 5.5, 1.5, and 1.0. Figure 2 presents intermediate steps to finalizing the IM values.

We used dental hygiene expert consultation and video observations to establish three general categories of wrist posture: flexion, neutral, and extension. We defined extension posture as wrist extension greater than 30 degrees and flexion posture as any wrist flexion. Because the RSI posture multiplier (PM) is relatively constant at 1.0 for wrist angles between 0 degrees and 30 degrees of extension, we identified this range as neutral. Next, we collected 295 wrist posture samples through video and live observations to validate our categorization and estimate representative wrist angles for each posture category. Data shown in Figure 3 provide the relative distribution of samples aligning with the proposed flexion, neutral, and extension posture categories where positive wrist angle values represent wrist extension and negative values represent wrist flexion.

Of the 295 samples, 162 were neutral, 22 were flexion, and 111 were extension. The mean, median, and mode values for wrist extension were 45.7, 45, and 45 degrees; thus, we chose 45 degrees as the most representative wrist angle for extension posture. The mean, median, and mode values for wrist flexion were −19.3, −15, and −15 degrees. We chose −15 degrees to represent flexion posture instead of the mean value of −19.3 degrees to prevent the overestimation of the posture multiplier. The final PM values for wrist flexion, neutral, and extension were 1.17, 1.00, and 1.06, respectively, all with the same PM of 1.00.

The RSI score is the product of 5 numeric multipliers, including intensity of exertion (IM), efforts per minute (EM), duration per exertion (DM), hand/wrist posture (PM), and hours of work per day (HM). The Composite Strain Index (COSI) algorithm estimates the overall physical stain of multi-task work shift by combining the RSI scores of individual subtasks (Garg et al., 2017a). The calculation of RSI and COSI is straightforward for routinized work where the administrator can identify subtasks (e.g., driving screws and cutting wires) with distinct exertion force, duration, and posture. Yet it is difficult to generalize such procedures to non-routinized work where every exertion is characterized by varying physical exposure parameters. For example, a dental hygienist may alternate between heavy exertion with low frequency and light exertion with high frequency depending area of the mouth and the amount of build-up on the teeth. Therefore, we propose a consolidated definition for subtasks in the RSI model to consider exertions with the same intensity pattern as a subtask. Using this definition, subtask RSI scores of each dental cleaning session can be calculated for each intensity pattern and combined using COSI calculations.

We calculated subtask-level RSI and COSI scores on 5 randomly selected video recordings of dental hygiene patient visits. A typical dental hygiene patient visit in university-run clinics includes assessment, patient education, faculty consultation, instrumentation, and other administrative duties. We only applied the RSI on instrumentation activities because instrumentation consists of the most strenuous work tasks in dental hygiene, including teeth scaling and root planting. One researcher used Observer^® XT (Noldus, Inc.; Wageningen, Netherlands, Version 14.1) video coding software to apply the RSI by manually coding the intensity of exertion and posture patterns. Duration, count, and frequency (i.e., effort per minute) parameters were automatically recorded along with the pattern codes. An Excel calculation spreadsheet (Microsoft, Inc.; Redmond, WA, USA, Version 2016) was created to compute subtask-level RSI and COSI scores.

Table 2 summarizes the COSI and subtask-level RSI results. The highest COSI score was 13.6 (video 2), and the lowest was 7.68 (video 5). The COSI results of videos 1 and 2 exceeded the hazardous threshold of 10 points. Although the remaining 3 videos fall under the ‘safe’ category, it is worth noting that video 4 scored exactly at the threshold of 10.0 points. The average duration of exertion for each pattern varied substantially across the videos, ranging from 38.5 seconds to 102.7 seconds per exertion. Likewise, efforts per minute (frequency) varied significantly. For instance, in video 5, the count of exertion pattern L was 2, resulting in the lowest efforts per minute of 0.02; while the count of exertion for pattern N was 44, resulting in the highest efforts per minute of 0.44. The total duration of work ranged from 0.96 to 1.62 hours across 5 videos. Wrist posture had minimal impact on the final RSI results because of the small multiplier values.

To further examine the validity of the consolidated method, we compared our method with a conventional method that approximates more to the original RSI sampling strategy. The definition of one exertion in the conventional method was the direct application of force with the same apparent intensity pattern or wrist posture, and any change in intensity or posture was coded as a separate exertion regardless of duration. The intensity of exertion patterns L, M, H, US, and N remained, but instead of using duty cycles, the IM value corresponding to the average Borg CR-10 rating was used and holding of the instrument without direct application of force was coded as an independent intensity pattern (i.e., Hold). Regarding wrist posture, the conventional method no longer used neutral, extension, and flexion patterns; instead, the coder recorded wrist angles in 5-degree increments. Table 3 summarizes the key differences between the two sampling methods.

Table 4 presents the COSI and subtask-level RSI results calculated using the conventional method for the same 5 videos as reported using the consolidated method (i.e., table 2). The highest COSI score was 15.7 (video 2), and the lowest COSI score was 8.0 (video 4). Consistent with the consolidated method, videos 1 and 2 exceeded the hazardous threshold of 10 points, and the remaining 3 videos were classified as safe. In contrast to the consolidated method, the conventional coding of video 4 scored around 2 points below the threshold, whereas video 5 scored 1.6 points higher than the consolidated method. The average duration per exertion in the conventional method was significantly shorter than in the consolidated method. The most significant difference was pattern H in video 4, where the duration per exertion decreased from 83.6 seconds in the consolidated method to 18.6 seconds in the conventional method. In terms of efforts per minute, the conventional method’s exertions were two to three times as frequent compared to the consolidated method across all 5 videos. The posture multipliers calculated using the conventional methods were generally higher than the consolidated method, except for video 1. Coding of all 5 videos took about 30 hours using the conventional method (i.e., 4.5 hours per hour of video) versus18 hours (i.e., 2.7 hours per hour of video) using the consolidated method.