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  • 1.
    Heiden, Marina
    et al.
    University of Gävle, Faculty of Health and Occupational Studies, Department of Occupational and Public Health Sciences, Occupational health science. University of Gävle, Centre for Musculoskeletal Research.
    Mathiassen, Svend Erik
    University of Gävle, Faculty of Health and Occupational Studies, Department of Occupational and Public Health Sciences, Occupational health science. University of Gävle, Centre for Musculoskeletal Research.
    Garza, Jennifer
    University of Gävle, Faculty of Health and Occupational Studies, Department of Occupational and Public Health Sciences, Occupational health science. University of Gävle, Centre for Musculoskeletal Research. Division of Occupational and Environmental Medicine, UConn Health, Farmington, CT, United States .
    Liv, Per
    University of Gävle, Faculty of Health and Occupational Studies, Department of Occupational and Public Health Sciences, Occupational health science. University of Gävle, Centre for Musculoskeletal Research. Centre for Research and Development, Uppsala University/County Council of Gävleborg.
    Wahlström, Jens
    Department of Public Health and Clinical Medicine, Occupational and Environmental Medicine, Umeå University, Umeå, Sweden.
    A comparison of two strategies for building an exposure prediction model2016In: Annals of Occupational Hygiene, ISSN 0003-4878, E-ISSN 1475-3162, Vol. 60, no 1, p. 74-89Article in journal (Refereed)
    Abstract [en]

    Cost-efficient assessments of job exposures in large populations may be obtained from models in which “true” exposures assessed by expensive measurement methods are estimated from easily accessible and cheap predictors. Typically, the models are built on the basis of a validation study comprising “true” exposure data as well as an extensive collection of candidate predictors from questionnaires or company data, which cannot all be included in the models due to restrictions in the degrees of freedom available for modeling. In these situations, predictors need to be selected using procedures that can identify the best possible subset of predictors among the candidates. The present study compares two strategies for selecting a set of predictor variables. One strategy relies on stepwise hypothesis testing of associations between predictors and exposure, while the other uses cluster analysis to reduce the number of predictors without relying on empirical information about the measured exposure. Both strategies were applied to the same dataset on biomechanical exposure and candidate predictors among computer users, and they were compared in terms of identified predictors of exposure as well as the resulting model fit using bootstrapped resamples of the original data. The identified predictors were, to a large part, different between the two strategies, and the initial model fit was better for the stepwise testing strategy than for the clustering approach. Internal validation of the models using bootstrap resampling with fixed predictors revealed an equally reduced model fit in resampled datasets for both strategies. However, when predictor selection was incorporated in the validation procedure for the stepwise testing strategy, the model fit was reduced to the extent that both strategies showed similar model fit. Thus, the two strategies would both be expected to perform poorly with respect to predicting biomechanical exposure in other samples of computer users.

  • 2.
    Jackson, Jennie A.
    et al.
    University of Gävle, Faculty of Health and Occupational Studies, Department of Occupational and Public Health Sciences, Occupational health science. University of Gävle, Centre for Musculoskeletal Research.
    Mathiassen, Svend Erik
    University of Gävle, Faculty of Health and Occupational Studies, Department of Occupational and Public Health Sciences, Occupational health science. University of Gävle, Centre for Musculoskeletal Research.
    Liv, Per
    University of Gävle, Faculty of Health and Occupational Studies, Department of Occupational and Public Health Sciences, Occupational health science. University of Gävle, Centre for Musculoskeletal Research. Centre for Research and Development, Uppsala University/Region Gävleborg, Gävle.
    Observer performance in estimating upper arm elevation angles under ideal viewing conditions when assisted by posture matching software2016In: Applied Ergonomics, ISSN 0003-6870, E-ISSN 1872-9126, Vol. 55, p. 208-215Article in journal (Refereed)
    Abstract [en]

    Selecting a suitable body posture measurement method requires performance indices of candidate tools. Such data are lacking for observational assessments made at a high degree of resolution. The aim of this study was to determine the performance (bias and between- and within-observer variance) of novice observers estimating upper arm elevation postures assisted by posture matching software to the nearest degree from still images taken under ideal conditions. Estimates were minimally biased from true angles: the mean error across observers was less than 2°. Variance between observers was minimal. Considerable variance within observers, however, underlined the risk of relying on single observations. Observers were more proficient at estimating 0°and 90° postures, and less proficient at 60°. Thus, under ideal visual conditions observers, on average, proved proficient at high resolution posture estimates; further investigation is required to determine how non-optimal image conditions, as would be expected from occupational data, impact proficiency.

  • 3.
    Jackson, Jennie
    et al.
    University of Gävle, Faculty of Health and Occupational Studies, Department of Occupational and Public Health Sciences, Occupational health science. University of Gävle, Centre for Musculoskeletal Research.
    Mathiassen, Svend Erik
    University of Gävle, Faculty of Health and Occupational Studies, Department of Occupational and Public Health Sciences, Occupational health science. University of Gävle, Centre for Musculoskeletal Research.
    Wahlström, Jens
    University of Gävle, Faculty of Health and Occupational Studies, Department of Occupational and Public Health Sciences, Occupational health science. University of Gävle, Centre for Musculoskeletal Research. Department of Public Health and Clinical Medicine, Occupational and Environmental Medicine, Umeå University, Umeå, Sweden .
    Liv, Per
    University of Gävle, Faculty of Health and Occupational Studies, Department of Occupational and Public Health Sciences, Occupational health science. University of Gävle, Centre for Musculoskeletal Research. Centre for Research and Development, Uppsala University/County Council of Gävleborg, Gävle, Sweden .
    Forsman, Mikael
    University of Gävle, Faculty of Health and Occupational Studies, Department of Occupational and Public Health Sciences, Occupational health science. University of Gävle, Centre for Musculoskeletal Research. Institute of Environmental Medicine, Karolinska Institutet, Stockholm, Sweden .
    Digging deeper into the assessment of upper arm elevation angles using standard inclinometry2015In: Applied Ergonomics, ISSN 0003-6870, E-ISSN 1872-9126, Vol. 51, p. 102-103Article in journal (Other academic)
  • 4.
    Jackson, Jennie
    et al.
    University of Gävle, Faculty of Health and Occupational Studies, Department of Occupational and Public Health Sciences, Occupational health science. University of Gävle, Centre for Musculoskeletal Research.
    Mathiassen, Svend Erik
    University of Gävle, Faculty of Health and Occupational Studies, Department of Occupational and Public Health Sciences, Occupational health science. University of Gävle, Centre for Musculoskeletal Research.
    Wahlström, Jens
    University of Gävle, Faculty of Health and Occupational Studies, Department of Occupational and Public Health Sciences, Occupational health science. University of Gävle, Centre for Musculoskeletal Research. Department of Public Health and Clinical Medicine, Umeå University.
    Liv, Per
    University of Gävle, Faculty of Health and Occupational Studies, Department of Occupational and Public Health Sciences, Occupational health science. University of Gävle, Centre for Musculoskeletal Research. Centre for Research and Development, Uppsala University/County Council of Gävleborg.
    Forsman, Mikael
    University of Gävle, Faculty of Health and Occupational Studies, Department of Occupational and Public Health Sciences, Occupational health science. University of Gävle, Centre for Musculoskeletal Research. Institute of Environmental Medicine, Karolinska Institutet.
    Is what you see what you get? Standard inclinometry of set upper arm elevation angles2015In: Applied Ergonomics, ISSN 0003-6870, E-ISSN 1872-9126, Vol. 47, p. 242-252Article in journal (Refereed)
    Abstract [en]

    Previous research suggests inclinometers (INC) underestimate upper arm elevation. This study was designed to quantify possible bias in occupationally relevant postures, and test whether INC performance could be improved using calibration.

    Participants were meticulously positioned in set arm flexion and abduction angles between 0° and 150°. Different subject-specific and group-level regression models comprising linear and quadratic components describing the relationship between set and INC-registered elevation were developed using subsets of data, and validated using additional data.

    INC measured arm elevation showed a downward bias, particularly above 60°.  INC data adjusted using the regression models were superior to un-adjusted data; a subject-specific, two-point calibration based on measurements at 0° and 90° gave results closest to the ‘true’ set angles.

    Thus, inclinometer measured arm elevation data required calibration to arrive at ‘true’ elevation angles. Calibration to a common measurement scale should be considered when comparing arm elevation data collected using different methods.

  • 5.
    Liv, Per
    University of Gävle, Faculty of Health and Occupational Studies, Department of Occupational and Public Health Sciences, CBF. University of Gävle, Centre for Musculoskeletal Research. Umeå universitet, Institutionen för folkhälsa och klinisk medicin, Yrkes- och miljömedicin.
    Efficient strategies for collecting posture data using observation and direct measurement2012Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Relationships between occupational physical exposures and risks of contracting musculoskeletal disorders are still not well understood; exposure-response relationships are scarce in the musculoskeletal epidemiology literature, and many epidemiological studies, including intervention studies, fail to reach conclusive results. Insufficient exposure assessment has been pointed out as a possible explanation for this deficiency. One important aspect of assessing exposure is the selected measurement strategy; this includes issues related to the necessary number of data required to give sufficient information, and to allocation of measurement efforts, both over time and between subjects in order to achieve precise and accurate exposure estimates. These issues have been discussed mainly in the occupational hygiene literature considering chemical exposures, while the corresponding literature on biomechanical exposure is sparse. The overall aim of the present thesis was to increase knowledge on the relationship between data collection design and the resulting precision and accuracy of biomechanical exposure assessments, represented in this thesis by upper arm postures during work, data which have been shown to be relevant to disorder risk.

    Four papers are included in the thesis. In papers I and II, non-parametric bootstrapping was used to investigate the statistical efficiency of different strategies for distributing upper arm elevation measurements between and within working days into different numbers of measurement periods of differing durations. Paper I compared the different measurement strategies with respect to the eventual precision of estimated mean exposure level. The results showed that it was more efficient to use a higher number of shorter measurement periods spread across a working day than to use a smaller number for longer uninterrupted measurement periods, in particular if the total sample covered only a small part of the working day. Paper II evaluated sampling strategies for the purpose of determining posture variance components with respect to the accuracy and precision of the eventual variance component estimators. The paper showed that variance component estimators may be both biased and imprecise when based on sampling from small parts of working days, and that errors were larger with continuous sampling periods. The results suggest that larger posture samples than are conventionally used in ergonomics research and practice may be needed to achieve trustworthy estimates of variance components.

    Papers III and IV focused on method development. Paper III examined procedures for estimating statistical power when testing for a group difference in postures assessed by observation. Power determination was based either on a traditional analytical power analysis or on parametric bootstrapping, both of which accounted for methodological variance introduced by the observers to the exposure data. The study showed that repeated observations of the same video recordings may be an efficient way of increasing the power in an observation-based study, and that observations can be distributed between several observers without loss in power, provided that all observers contribute data to both of the compared groups, and that the statistical analysis model acknowledges observer variability. Paper IV discussed calibration of an inferior exposure assessment method against a superior “golden standard” method, with a particular emphasis on calibration of observed posture data against postures determined by inclinometry. The paper developed equations for bias correction of results obtained using the inferior instrument through calibration, as well as for determining the additional uncertainty of the eventual exposure value introduced through calibration.

    In conclusion, the results of the present thesis emphasize the importance of carefully selecting a measurement strategy on the basis of statistically well informed decisions. It is common in the literature that postural exposure is assessed from one continuous measurement collected over only a small part of a working day. In paper I, this was shown to be highly inefficient compared to spreading out the corresponding sample time across the entire working day, and the inefficiency was also obvious when assessing variance components, as shown in paper II. The thesis also shows how a well thought-out strategy for observation-based exposure assessment can reduce the effects of measurement error, both for random methodological variance (paper III) and systematic observation errors (bias) (paper IV).

  • 6.
    Liv, Per
    et al.
    University of Gävle, Faculty of Health and Occupational Studies, Department of Occupational and Public Health Sciences, CBF. University of Gävle, Centre for Musculoskeletal Research.
    Mathiassen, Svend Erik
    University of Gävle, Faculty of Health and Occupational Studies, Department of Occupational and Public Health Sciences, CBF. University of Gävle, Centre for Musculoskeletal Research.
    The effect of sampling strategy on the accuracy and precision of variance component estimates2010In: Proceedings of the Premus 2010 conference, 2010, p. 207-207Conference paper (Refereed)
  • 7.
    Liv, Per
    et al.
    University of Gävle, Centre for Musculoskeletal Research.
    Mathiassen, Svend Erik
    University of Gävle, Centre for Musculoskeletal Research.
    Sundberg, J
    Svendsen, S W
    Expected and empirical efficiency of different measurement allocation strategies for estimating upper arm elevation among car mechanics2007In: Sixth International Scientific Conference on Prevention of Work-Related Musculoskeletal Disorders: PREMUS, Boston, USA, 2007, p. 119-Conference paper (Other academic)
  • 8.
    Liv, Per
    et al.
    University of Gävle, Faculty of Health and Occupational Studies, Department of Occupational and Public Health Sciences, CBF. University of Gävle, Centre for Musculoskeletal Research.
    Mathiassen, Svend Erik
    University of Gävle, Faculty of Health and Occupational Studies, Department of Occupational and Public Health Sciences, CBF. University of Gävle, Centre for Musculoskeletal Research.
    Svendsen, Susanne Wulff
    Dansih Ramazzini Centre, Herning Hospital, Denmark.
    Accuracy and precision of variance components in occupational posture recordings: a simulation study of different data collection strategies2012In: BMC Medical Research Methodology, ISSN 1471-2288, E-ISSN 1471-2288, Vol. 12, no 1, p. 58-58Article in journal (Refereed)
    Abstract [en]

    Background. Information on exposure variability, expressed as exposure variance components, is of vital use in occupational epidemiology, including informed risk control and efficient study design. While accurate and precise estimates of the variance components are desirable in such cases, very little research has been devoted to understanding the performance of data sampling strategies designed specifically to determine the size and structure of exposure variability. The aim of this study was to investigate the accuracy and precision of estimators of betweensubjects, between-days and within-day variance components obtained by sampling strategies differing with respect to number of subjects, total sampling time per subject, number of days per subject and the size of individual sampling periods.

    Methods. Minute-by-minute values of average elevation, percentage time above 90degrees and percentage time below 15degrees were calculated in a data set consisting of measurements of right upper arm elevation during four full shifts from each of 23 car mechanics. Based on this parent data, bootstrapping was used to simulate sampling with 80 different combinations of the number of subjects (10, 20), total sampling time per subject (60, 120, 240, 480 minutes), number of days per subject (2, 4), and size of sampling periods (blocks) within days (1, 15, 60, 240 minutes). Accuracy (absence of bias) and precision (prediction intervals) of the variance component estimators were assessed for each simulated sampling strategy.

    Results. Sampling in small blocks within days resulted in essentially unbiased variance components. For a specific total sampling time per subject, and in particular if this time was small, increasing the block size resulted in an increasing bias, primarily of the between-days and the within-days variance components. Prediction intervals were in general wide, and even more so at larger block sizes. Distributing sampling time across more days gave in general more precise variance component estimates, but also reduced accuracy in some cases.

    Conclusions. Variance components estimated from small samples of exposure data within working days may be both inaccurate and imprecise, in particular if sampling is laid out in large consecutive time blocks. In order to estimate variance components with a satisfying accuracy and precision, for instance for arriving at trustworthy power calculations in a planned intervention study, larger samples of data will be required than for estimating an exposure mean value with a corresponding certainty

  • 9.
    Liv, Per
    et al.
    University of Gävle, Faculty of Health and Occupational Studies, Department of Occupational and Public Health Sciences, CBF. University of Gävle, Centre for Musculoskeletal Research.
    Mathiassen, Svend Erik
    University of Gävle, Faculty of Health and Occupational Studies, Department of Occupational and Public Health Sciences, CBF. University of Gävle, Centre for Musculoskeletal Research.
    Svendsen, Susanne Wulff
    Danish Ramazzini Center, Department of Occupational Medicine, Herning Hospital, Denmark.
    Theoretical and empirical efficiency of sampling strategies for estimating upper arm elevation2011In: Annals of Occupational Hygiene, ISSN 0003-4878, E-ISSN 1475-3162, Vol. 55, p. 436-449Article in journal (Refereed)
    Abstract [en]

    Objectives

    To investigate the statistical efficiency of strategies for sampling upper arm elevation data, which differed with respect to sample sizes and sample allocations within and across measurement days. The study was also designed to compare standard theoretical predictions of sampling efficiency, which rely on several assumptions about the data structure, with “true” efficiency as determined by bootstrap simulations.

    Methods

    Sixty-five sampling strategies were investigated using a data set containing minute-by-minute values of average right upper arm elevation, percentage of time with the arm elevated less than 15°, and percentage of time with an arm elevated more than 90° in a population of 23 house painters, 23 car mechanics and 26 machinists, all followed for four full working days. Total sample times per subject between 30 and 240 minutes were subdivided into continuous time blocks between 1 and 240 minutes long, allocated to one or four days per subject. Within day(s), blocks were distributed using either a random or fixed-interval principle. Sampling efficiency was expressed in terms of the variance of estimated mean exposure values of 20 subjects, and assessed using standard theoretical models assuming independence between variables and homoscedasticity. Theoretical performance was compared to empirical efficiencies obtained by a nonparametric bootstrapping procedure.

    Results

    We found the assumptions of independence and homoscedasticity in the theoretical model to be violated, most notably expressed through an autocorrelation between measurement units within working days. The empirical variance of the mean exposure estimates decreased, i.e. sampling efficiency increased, for sampling strategies where measurements were distributed widely across time. Thus, the most efficient allocation strategy was to organize a sample into one-minute blocks collected at fixed time intervals across four days. Theoretical estimates of efficiency generally agreed with empirical variances if the sample was allocated into small blocks, while for larger block sizes the empirical, “true” variance was considerably larger than predicted by theory. Theory overestimated efficiency in particular for strategies with short total sample times per subject.

    Conclusions This study demonstrates that when exposure data are autocorrelated within days – which we argue is the major reason why theory overestimates sampling performance – sampling efficiency can be improved by distributing the sample widely across the day or across days, preferably using a fixed-interval strategy. While this guidance is particularly valid when small proportions of working days are assessed, we generally recommend collecting more data than suggested by theory if a certain precision of the resulting exposure estimate is needed. More data per se give a better precision, and sampling larger proportion(s) of the working day(s) also alleviate the negative effects of possible autocorrelation in data

  • 10.
    Liv, Per
    et al.
    University of Gävle, Centre for Musculoskeletal Research.
    Mathiassen, Svend Erik
    University of Gävle, Centre for Musculoskeletal Research.
    Wahlström, Jens
    Hedlund, P
    Between- and within-observer variability in two methods for posture observations2009In: Proceedings of the IEA2009 conference, 2009Conference paper (Refereed)
  • 11.
    Mathiassen, Svend Erik
    et al.
    University of Gävle, Faculty of Health and Occupational Studies, Department of Occupational and Public Health Sciences, Occupational health science. University of Gävle, Centre for Musculoskeletal Research.
    Liv, Per
    Centre for Research and Development, Uppsala University / Region Gävleborg, Gävle, Sweden.
    Influence of task proportion errors on the effectiveness of task-based job exposure modeling2016Conference paper (Refereed)
    Abstract [en]

    Background.Job-based exposure estimation using the occupational mean (JBM) is associated with substantial error. Many studies have therefore estimated job exposures from workers’ tasks, i.e. task-based modeling (TBM), typically by combining individual workers’ task proportions (TP) in the job with a general task exposure matrix. Studies of postures and muscle activity have, however, shown that TBM may be ineffective; one possible reason being that TPs are not correct. The present simulation study investigated the influence of random and systematic TP error on TBM performance.

    Methods.We constructed two virtual two-task jobs with task exposure contrasts of 0.2 and 0.8. In both, TPs and task exposures mimicked likely occupational scenarios. We then simulated four cases of TP error: no error, random error, bias, and bias and random error. For each case, we varied the TP error size, and compared the absolute errors of TBM- and JBM-based job exposures for 10,000 virtual workers.

    Results.For the low-contrast job, TBM with error-free TPs was, on average, only 6% more efficient than JBM, and the probability of TBM leading to a more correct job exposure than JBM was 56%. TP errors had negligible effects on effectiveness. With error-free TPs in the high-contrast job, TPM was 75% more efficient than JBM, and led to more correct job exposures for 71% of all workers. TP errors decreased TBM performance, down to being 34% better than JBM when both random and systematic errors were “large”; 62% of all individuals being more correctly assessed by TBM.

    Discussion.For jobs with limited task exposure contrast, TBM was essentially equivalent to JBM, while TP errors had marginal impact. In high-contrast jobs, TBM was more effec-tive, but was also more sensitive to both random and systematic TP errors. This may feed further discussion of the cost-efficiency of TBM in occupational settings.

  • 12.
    Mathiassen, Svend Erik
    et al.
    University of Gävle, Faculty of Health and Occupational Studies, Department of Occupational and Public Health Sciences, Occupational health science. University of Gävle, Centre for Musculoskeletal Research.
    Liv, Per
    University of Gävle, Faculty of Health and Occupational Studies, Department of Occupational and Public Health Sciences, Occupational health science. University of Gävle, Centre for Musculoskeletal Research.
    Wahlström, Jens
    University of Gävle, Faculty of Health and Occupational Studies, Department of Occupational and Public Health Sciences, Occupational health science. University of Gävle, Centre for Musculoskeletal Research.
    Cost-efficient measurement strategies for posture observations based on video recordings2013In: Applied Ergonomics, ISSN 0003-6870, E-ISSN 1872-9126, Vol. 44, no 4, p. 609-617Article in journal (Refereed)
    Abstract [en]

    Assessment of workingpostures by observation is a common practice in ergonomics. The present studyinvestigated whether monetary resources invested in a video-based posture observationstudy should preferably be spent in collecting many video recordings of thework and have them observed once by one observer, or in having multipleobservers rate postures repeatedly from fewer videos. The study addressed thisquestion from a practitioner’s perspective by focusing two plausible scenarios:documenting the mean exposure of one individual, and of a specific occupationalgroup. Using a data set of observed working postures among hairdressers, empiricalvalues of posture variability, observer variability, and costs for recordingand observing one video were entered into equations expressing the total costof data collection and the information (defined as 1/SD) provided by theresulting estimates of two variables: percentage time with the arm elevated<15 degrees and >90 degrees. Sixteen measurement strategies involving 1-4observers repeating their posture ratings 1-4 times were examined for budgetsup to €2000.  For both posture variablesand in both the individual and group scenario, the most cost-efficient strategyat any specific budget was to engage 3-4 observers and/or having observer(s)rate postures multiple times each. Between 17% and 34% less information wasproduced when using the commonly practiced approach of having one observer ratea number of video recordings one time each. We therefore recommend observationalposture assessment to be based on video recordings of work, since this allowsfor multiple observations; and to allocate monetary resources to repeated observationsrather than many video recordings.

  • 13.
    Mathiassen, Svend Erik
    et al.
    University of Gävle, Faculty of Health and Occupational Studies, Department of Occupational and Public Health Sciences, CBF. University of Gävle, Centre for Musculoskeletal Research.
    Liv, Per
    University of Gävle, Faculty of Health and Occupational Studies, Department of Occupational and Public Health Sciences, CBF. University of Gävle, Centre for Musculoskeletal Research.
    Wahlström, Jens
    University of Gävle, Faculty of Health and Occupational Studies, Department of Occupational and Public Health Sciences, CBF. University of Gävle, Centre for Musculoskeletal Research.
    Cost-efficient observation of working postures from video recordings – more videos, more observers or more views per observer?2012In: Work: A journal of Prevention, Assesment and rehabilitation, ISSN 1051-9815, E-ISSN 1875-9270, Vol. 41, no Suppl. 1, p. 2302-2306Article in journal (Refereed)
    Abstract [en]

    In ergonomics, assessing the working postures of an individual by observation is a very common practice. The present study investigated whether monetary resources devoted to an observational study should preferably be invested in collecting many video recordings of the work, or in having several observers estimate postures from available videos multiple times. On the basis of a data set of observed working postures among hairdressers, necessary information in terms of posture variability, observer variability, and costs for recording and observing videos was entered into equations providing the total cost of data collection and the precision (informative value) of the resulting estimates of two variables: percentages time with the arm elevated 90 degrees. In all 160 data collection strategies, differing with respect to the number of video recordings and the number of repeated observations of each recording, were simulated and compared for cost and precision. For both posture variables, the most cost-efficient strategy for a given budget was to engage 4 observers to look at available video recordings, rather than to have one observer look at more recordings. Since the latter strategy is the more common in ergonomics practice, we recommend reconsidering standard practice in observational posture assessment.

  • 14.
    Mathiassen, Svend Erik
    et al.
    University of Gävle, Centre for Musculoskeletal Research.
    Sundberg, J
    Liv, Per
    University of Gävle, Centre for Musculoskeletal Research.
    Svendsen, S W
    A two-stage bootstrap procedure for examining the ability of exposure samples to determine whether a group mean exposure exceeds a threshold2007In: Sixth International Scientific Conference on Prevention of Work-Related Musculoskeletal Disorders, 2007, p. 118-Conference paper (Other academic)
  • 15.
    Rezagholi, Mahmoud
    et al.
    University of Gävle, Faculty of Health and Occupational Studies, Department of Occupational and Public Health Sciences, CBF. University of Gävle, Centre for Musculoskeletal Research.
    Mathiassen, Svend Erik
    University of Gävle, Faculty of Health and Occupational Studies, Department of Occupational and Public Health Sciences, CBF. University of Gävle, Centre for Musculoskeletal Research.
    Liv, Per
    University of Gävle, Faculty of Health and Occupational Studies, Department of Occupational and Public Health Sciences, CBF. University of Gävle, Centre for Musculoskeletal Research.
    Cost efficiency comparison of four video-based techniques for assessing upper arm postures2012In: Ergonomics, ISSN 0014-0139, E-ISSN 1366-5847, Vol. 55, no 3, p. 350-360Article in journal (Refereed)
    Abstract [en]

    Many video-based techniques for assessing postures at work have been developed. Choosing the most appropriate technique should be based on an evaluation of different alternatives in terms of their ability to produce posture information at low input costs, i.e. their cost efficiency. This study compared four video-based techniques for assessing upper arm postures, using cost and error data from an investigation on hairdressers. Labour costs associated with the posture assessments from the video recordings were the dominant factor in the cost efficiency comparison. Thus, a work sampling technique associated with relatively large errors appeared, in general, to be the most cost-efficient because it was labour-saving. Measurement bias and other costs than labour cost for posture assessment influenced the ranking and economic evaluation of techniques, as did the applied measurement strategy, i.e. the number of video recordings and the number of repeated assessments of them.

  • 16.
    Wahlström, Jens
    et al.
    University of Gävle, Centre for Musculoskeletal Research.
    Liv, Per
    University of Gävle, Centre for Musculoskeletal Research.
    Hedlund, Pernilla
    Department of Occupational and Environmental Medicine, Umea University Hospital, SE-901 85 Umeå, Sweden.
    Forsman, Mikael
    Department of Public Health Sciences, Karolinska Institutet, SE-171 77 Stockholm, Sweden.
    Ahlgren, Christina
    Department of Community Medicine and Rehabilitation, Umeå University, SE-901 87 Umeå.
    Mathiassen, Svend Erik
    University of Gävle, Centre for Musculoskeletal Research.
    Direct measurements of upper arm elevation among female hairdressers with special reference to exposure variability2009In: Proceedings of the IEA2009 conference, Beijing, 2009Conference paper (Refereed)
  • 17.
    Wahlström, Jens
    et al.
    University of Gävle, Faculty of Health and Occupational Studies, Department of Occupational and Public Health Sciences, CBF. University of Gävle, Centre for Musculoskeletal Research.
    Mathiassen, Svend Erik
    University of Gävle, Faculty of Health and Occupational Studies, Department of Occupational and Public Health Sciences, CBF. University of Gävle, Centre for Musculoskeletal Research.
    Liv, Per
    University of Gävle, Faculty of Health and Occupational Studies, Department of Occupational and Public Health Sciences, CBF. University of Gävle, Centre for Musculoskeletal Research.
    Hedlund, Pernilla
    Department of Occupational and Environmental Medicine, Umea University Hospital, SE-901 85 Umeå, Sweden.
    Ahlgren, Christina
    Department of Community Medicine and Rehabilitation, Umea University, SE-901 87 Umeå, Sweden.
    Forsman, Mikael
    Department of Public Health Sciences, Karolinska Institutet, SE-171 77 Stockholm, Sweden.
    Upper arm postures and movements in female hairdressers across four full working days2010In: Annals of Occupational Hygiene, ISSN 0003-4878, E-ISSN 1475-3162, Vol. 54, no 5, p. 584-594Article in journal (Refereed)
    Abstract [en]

    OBJECTIVES: To describe upper arm postures and movements among female hairdressers, including the variability between hairdressers, between days within hairdresser, and between tasks, as a basis for understanding the characteristics of exposures in the job, considering possible sources of variation and recovery, and discussing appropriate exposure assessment strategies. METHODS: Data on upper arm postures were collected using inclinometers during four working days the same week from 28 female hairdressers working in 13 salons. Twenty of the hairdressers noted customer on and off times in a diary, to allow separate analyses of customer tasks (CT) and auxiliary non-customer tasks (AT), including breaks. For a number of posture and movement variables, mean values and variance components between subjects (BS) and within subjects between days (BD) were estimated using restricted maximum likelihood algorithms in one-way random effect models. RESULTS: For the 20 hairdressers with diaries, CT accounted for 279 min (58%) (SD(BS) = 39 min and SD(BD) = 85 min) of the working day and AT and breaks for 207 min (42%) (SD(BS) = 46 min and SD(BD) = 88 min). The hairdressers worked with the right arm elevated >60 degrees for 6.8% of the whole job (SD(BS) = 2.8% and SD(BD) = 2.0%). On average, the hairdressers worked with the right arm elevated >60 degrees for 9.0% of the time during CT, compared to 3.7% during AT, resulting in a contrast between tasks of 0.35. CONCLUSIONS: Hairdressers may be at risk for developing musculoskeletal disorders in the neck and shoulders due to a considerable occurrence of highly elevated arms, especially during CT. On the other hand, we do not find reasons to classify hairdressing as a job with too little variation. Posture variability between days within hairdressers was in the same order of magnitude as that between hairdressers, suggesting that 'typical' workdays do not exist. The exposure contrast between CT and AT for variables describing elevated arm postures indicates that for these variables a simple task-based approach for estimating job exposure could be successful.

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