Observers rank partly visible postures on video frames differently than fully visible postures, but it’s not clear if this is due to differences in observer perception. This study investigated the effect of posture visibility on between-observer variability in assessments of trunk and arm posture.  Trained observers assessed trunk and arm postures from video recordings of 84 pulp mill shifts using a work sampling approach; postures were also categorized as ‘fully’ or ‘partly’ visible.  Between-worker, between-day, and between-observer variance components and corresponding confidence intervals were calculated. Although no consistent gradient was seen for the right upper arm, trunk posture showed smaller between-observer variance when all observers rated a posture as fully visible. This suggests that, partly- visible data, especially when observers disagree as to the level of visibility, introduces more between-observer variability when compared to fully visible data.  Some previously-identified differences in daily posture summaries may be related to this phenomenon.

Introduction: A cost-efficient alternative to measuring working postures directly could be to build statistical models for predicting results of such measurements from cheaper data, and apply these models to samples in which only the latter data are available. The present study aimed to build and assess the performance of statistical models predicting inclinometer-assessed trunk and arm posture among paper mill workers. Separate models were built using administrative data, workers’ ratings of their exposure, and observations of the work from video recordings as predictors.

Methods: Trunk and upper arm postures were measured using inclinometry on 28 paper mill workers during three work shifts each. Simultaneously, the workers were video filmed, and their postures were assessed by observation of the videos afterwards. Workers’ ratings of exposure, and administrative data on staff and production during the shifts were also collected. Linear mixed models were fitted for predicting inclinometer-assessed exposure variables (median trunk and upper arm angle, proportion of time with neutral trunk and upper arm posture, and frequency of periods in neutral trunk and upper arm inclination) from administrative data, workers’ ratings, and observations, respectively. Performance was evaluated in terms of Akaike information criterion, proportion of variance explained (R²), and standard error of the model estimate (SE). For models performing well, validity was assessed by bootstrap resampling.

Results: Models based on administrative data performed poorly (R²≤15%) and would not be useful for assessing posture in this population. Models using workers’ ratings of exposure performed slightly better (8%≤R²≤27% for trunk posture; 14%≤R²≤36% for arm posture). The best model was obtained when using observational data for predicting frequency of periods with neutral arm inclination. It explained 56% of the variance in the postural exposure, and its SE was 5.6. Bootstrap validation of this model showed similar expected performance in other samples (5^th-95^th percentile: R²=45-63%; SE=5.1-6.2).

Conclusions: Observational data had a better ability to predict inclinometer-assessed upper arm exposures than workers’ ratings or administrative data, but they are typically more expensive to obtain. The results encourage comparisons of the cost-efficiency of modeling based on administrative data, workers’ ratings, and observation.

Background. Arm posture is a recognized risk factor for occupational upper extremity musculoskeletal disorders and thus often assessed in research and practice. Posture assessment methods differ in cost, feasibility and, perhaps, bias. An attractive approach could be to build statistical models for predicting results of expensive direct measurements of arm posture from cheaper or more accessible data, and apply them to large samples in which only the latter data are available. We aimed to build and assess the performance of such prediction models in a random sample of paper mill workers.

Methods. 28 workers were recruited to the study, and their upper arm postures were measured during three full work shifts using inclinometers. Simultaneously, the workers were video filmed, and their arm posture and gross body posture were assessed by observing the video afterwards. Models for predicting the inclinometer-assessed duration (proportion of time) and frequency (number/min) of periods spent in neutral right arm posture (<20°) were fitted using subject and observer as random factors, measured shift (1, 2 or 3) as fixed factor, and either observed time in neutral right arm angle or observed gross body posture as predictor.

Results. For the proportion of time spent in neutral arm posture, the best performance was achieved by using observed gross body posture as predictor (explained variance: R2=26%; standard error: SE=9.8). For the frequency of periods spent in neutral arm posture, the corresponding model fit was R2=60% and SE=5.6. Bootstrap resample validation of the latter model showed an expected performance in other samples of R2=59-60% and SE=5.5-5.6 (5th-95th percentile).

Discussion. Surprisingly, we found that observed gross body posture was a better predictor of variation in arm posture than observed arm angles. The findings suggest that arm posture during paper mill work can be cost-efficiently assessed using simple observations.

Objectives: The present study assessed full shift trunk and upper arm postural exposure amplitudes, frequencies, and durations among Swedish airport baggage handlers, and aimed to determine whether exposures differ between workers at the ramp (loading and unloading aircraft) and baggage sorting areas.

Methods: Trunk and upper arm postures were measured using inclinometers during three full work shifts on each of 27, male baggage handlers working at a large Swedish airport. Sixteen of the baggage handlers worked on the ramp and 11 in the sorting area. Variables summarizing postures and movements were calculated, and mean values and variance components between subjects and within subject (between days) were estimated using restricted maximum likelihood algorithms in a one-way random effect model.

Results: In total, data from 79 full shifts (651 hours) were collected with a mean recording time of 495 minutes per shift (range 319-632). On average, baggage handlers worked with the right and left arm elevated >60° for 6.4% and 6.3% of the total workday, respectively. The 90^th percentile trunk forward projection (FP) was 34.1° and the 50^th percentile trunk movement velocity was 8°s^-1. For most trunk (FP) and upper arm exposure variables, between-subject variability was considerable, suggesting that the flight baggage handlers were not a homogeneously exposed group. A notable between-days variability pointed to the contents of the job differing on different days. Peak exposures (>90°) were higher for ramp workers than for sorting area workers (trunk 0.6% ramp vs 0.3% sorting; right arm 1.3% ramp vs 0.7% sorting).

Conclusions: Trunk and upper arm postures and movements among flight baggage handlers measured by inclinometry were similar to those found in other jobs comprising manual material handling, known to be associated with increased risks for musculoskeletal disorders. The results showed that full-shift trunk (FP), and to some extent peak arm exposures, were higher for ramp workers compared to sorting workers.

Background. Previous studies have shown that video-based observation of postures that are only partly visible leads to different daily summary values than when postures are fully visible. However, the source of these differences is unclear. The purpose of this study was to estimate the between-observer variance of trunk and arm posture estimates (relative to within- and between-worker variance), and to investigate the effect of visibility on this observer variability of trunk and arm postural exposure estimates.

Methods. Video recordings were made of 28 pulp mill workers for three full shifts each. Trunk and arm postures were then estimated by trained observers using a work sampling approach; posture images were also assessed as being “fully” or “partly” visible. REML techniques were used to estimate the between-worker, between-day and betweenobserver components of variance at different visibility levels; Wald-based confidence intervals and p-values were used to determine sources of variation.

Results. Estimates of partly visible postures (as agreed upon by all observers) were lower than fully visible postures. However, more than 90% of trunk observations and 85% of arm observations did not have full agreement on visibility between observers. Right upper arm posture showed smaller between-observer variance when all observers rated a posture to be fully visible, as compared to all observers agreeing it was only partly visible. This suggests partly visible data introduces more methodological (i.e. between-observer) variability when compared to fully visible data. However, no significant differences in between-observer variability were found for the trunk, suggesting that other factors explain the reported differences in estimated postures between fully and partly visible data in this case.

Discussion. Future studies involving concurrent direct measurement would determine whether there is a true difference in posture between partly visible and fully visible periods, or whether the difference between fully and partly visible periods are related to observer performance.

We investigated the extent to which observers rate clearly visible postures on video differently from postures that are only partly visible, and whether this would have an effect on full-shift posture summaries. Trunk and upper arm postures were observed from 10,413 video frames representing 80 shifts of baggage handling; observers reported whether postures were fully or only partly visible.  Postures were summarized for each shift into several standard metrics using all available data, only fully visible frames, or only partly visible frames. 78% of trunk and 71% of upper arm postural observations were inferred.  When based on all data, mean and 90^th percentile trunk postures were 1.3° and 5.4° lower, respectively, than when based only on fully visible situations.  For the arm; differences in mean and 90^thpercentile were 2.5° and 8.1°.  Daily posture summaries can, thus, be significantly influenced by whether partly visible postures are included or not

Objectives: This study compared the cost-efficiency of observation and inclinometer assessment of trunk and upper arm inclination in a population of flight baggage handlers, as an illustration of a general procedure for addressing the trade-off between resource consumption and statistical performance in occupational epidemiology.

Methods:  Trunk and upper arm inclination with respect to the line of gravity were assessed for 3 days on each of 27 airport baggage handlers using simultaneous recordings by inclinometers and video.  Labour and equipment costs associated with data collection and data processing were tracked throughout.  Statistical performance, in terms of the inverse of the standard deviation and root mean squared error of the group mean exposure, was computed from the variance components within and between workers, and bias (with inclinometer assumed to produce ‘correct’ inclination angles).  The behavior of the trade-off between cost and efficiency with changed sample size, as well as with changed logistics for data collection and processing, was investigated using simulations.

Results:  At similar total costs, time spent at trunk and arm inclination angles greater than 60 degrees as well as 90^th percentile arm inclination were estimated at higher precision using inclinometers, while median inclination and 90^th percentile trunk inclination was determined more precisely using observation.  This hierarchy persisted in a scenario where the study was immediately reproduced in another population, while inclinometry was more cost-efficient than observation for all three posture variables in a scenario where data were already collected and only needed to be processed. Observations showed to be biased relative to the –assumed to be correct – inclinometer data, and so inclinometry became the most cost-efficient option for all posture variables and irrespective of scenario when statistical performance was measured by bias and precision combined.

Conclusions: Inclinometers were more cost-efficient in use than observation for two out of three posture metrics investigated when statistical performance was measured only in terms of precision. Since observations were biased, inclinometers consistently outperformed observation when both bias and precision were included in statistical performance. The presented general model for assessing cost-efficiency may be used for designing exposure assessment strategies with considerations not only to statistical criteria, but even to costs. The empirical data provide a specific basis for planning assessments of working postures in occupational groups.

Objectives. Data processing contributes a non-trivial proportion to total research costs, but documentation of these costs is rare. This paper employed a priori cost tracking for three posture assessment methods (self-report, observation of video, and inclinometry), developed a model describing the fixed and variable cost components, and simulated additional study scenarios to demonstrate the utility of the model. 

Methods. Trunk and shoulder postures of aircraft baggage handlers were assessed for 80 working days using all three methods. A model was developed to estimate data processing phase costs, including fixed and variable components related to study planning and administration, custom software development, training of analysts, and processing time.   

Results. Observation of video was the most costly data processing method with total cost of € 31,433, and was 1.2-fold more costly than inclinometry (€ 26,255), and 2.5-fold more costly than self-reported data (€ 12,491). Simulated scenarios showed altering design strategy could substantially impact processing costs. This was shown for both fixed parameters, such as software development and training costs, and variable parameters, such as the number of work-shift files processed, as well as the sampling frequency for video observation.  When data collection and data processing costs were combined, the cost difference between video and inclinometer methods was reduced to 7%; simulated data showed this difference could be diminished and, even, reversed at larger study sample sizes. Self-report remained substantially less costly under all design strategies, but produced alternate exposure metrics. 

Conclusions. These findings build on the previously published data collection phase cost model by reporting costs for post-collection data processing of the same data set.  Together, these models permit empirically based study planning and identification of cost-efficient study designs.

Improved assessment of whole body vibration exposure is needed for epidemiological studies investigating the causes of low back disorders. Vibration was measured on 54 worker-days in five heavy industries, with data collected on observed and self-reported driving conditions, demographics, and vehicle characteristics. Variables significant at p<0.1 in simple linear regressions (20 of 34) were retained for mixed effects multiple regressions to determine the best prediction of rms vibration level and 8-h equivalent vibration exposure. Vibration was measured, on average, for 205 min per work shift (SD 105). Means and standard deviations in ms-2 were: x-axis 0.35 (0.19); y-axis 0.34 (0.28); z-axis 0.54 (0.23); vector sum 0.90 (0.49); and 8-h equivalent vector sum 0.70 (0.37). The final three regression models retained only 2 or 3 of the 34 variables (driving speed (<20 km/h and/or 20–40 km/h) and industry and/or vehicle type and explained up to 60% of the variance (R2=0.26–0.6))

Background. Documentation of posture measurement costs is rare and cost models that do exist are generally naïve. This paper provides a comprehensive cost model for biomechanical exposure assessment in occupational studies, documents the monetary costs of three exposure assessment methods for different stakeholders in data collection, and uses simulations to evaluate the relative importance of cost components.  Trunk and shoulder posture variables were assessed for 27 aircraft baggage handlers for 3 full shifts each using three methods typical to ergonomic studies: self-report via questionnaire, observation via video film, and full-shift inclinometer registration.  The cost model accounted for expenses related to meetings to plan the study, administration, recruitment, equipment, training of data collectors, travel, and onsite data collection.  Sensitivity analyses were conducted using simulated study parameters and cost components to investigate the impact on total study cost.

Results. Inclinometery was the most expensive method (with a total study cost of € 66,657), followed by observation (€ 55,369) and then self report (€ 36,865). The majority of costs (90%) were borne by researchers.  Study design parameters such as sample size, measurement scheduling and spacing, concurrent measurements, location and travel, and equipment acquisition were shown to have wide-ranging impacts on costs. 

Conclusions. This study provided a general cost modelling approach that can facilitate decision making and planning of data collection in future studies, as well as investigation into cost efficiency and cost efficient study design. Empirical cost data from a large field study demonstrated the usefulness of the proposed models.