An observational method for Postural Ergonomic Risk Assessment (PERA)
Introduction
The World Health Organization (Luttmann et al., 2003) defines musculoskeletal disorders (MSDs) as health problems of the locomotor apparatus, including all forms of ill-health ranging from light, transitory disorders to irreversible, disabling injuries. If the MSDs are induced or aggravated by work and the circumstances of its performance, they are considered work-related.
The impact of work-related musculoskeletal disorders (WMSDs) on the health of the working population is quite significant. According to a report published by the European Agency for Safety and Health at Work (EASHW) in 2010, MSDs are the most common occupational disease among the recognized occupational diseases, accounting for 59% of the total (Schneider et al., 2010). The 5th European Working Conditions Survey (EWCS), 2012, reported that more than 40% of the workers suffered from a backache and/or muscular pains. Repetitive hand or arm movements and tiring or painful positions (awkward postures) are the most common physical risks in the workplace, with about 63% and 46% of the workers being exposed to these risks, respectively, for at least a quarter of the time (Parent-Thirion et al., 2012). This data corresponds with the high incidence of backache and muscular pain. It also motivates the indications provided by the Machinery Directive 2006/42/EC (The European Parliament and the Council of the European Union, 2006), with reference to ISO 11226 (2000) and EN 1005-4 (2008), and the significant literature available for the assessment of postural ergonomic problems at the workplace.
Li and Buckle, 1999, David, 2005 and Takala et al. (2010) exhaustively reviewed literature methods for ergonomic risk assessment and discussed the requirements of practitioners from these methods. Among occupational safety and health professionals, observational techniques, either through pro forma sheets or computer software programs, are considered the most suitable methods (David, 2005).
Ovako Working Posture Analysing System (OWAS) (Karhu et al., 1977) describes the whole body posture using a four digit code, indicating the position of the back (4 options), arms (3 options), legs (7 options) and the load to be handled (3 options). The method provides a look-up table for translating the four digit code into four action categories. OWAS is generally used to analyze single snapshots of postures instantly. A possible weakness is that the posture categories of OWAS could be quite broad to provide an accurate description of posture (Keyserling, 1986).
Plan för Identifiering av Belastningsfaktorer (PLIBEL), also known as “Method for the identification of musculoskeletal stress factors which may have injurious effects” (Kemmlert, 1995), uses a checklist to identify ergonomic hazards in the workplace rapidly. The list of items consists of questions concerning awkward postures (including neck/shoulders, back, arms, hips, and legs), tiring movements, poor design of tools or workplace, and stressful environmental or organizational conditions. The author of the method advised against a quantitative measure after the completion of the checklist and recommended to conclude the assessment by short verbal description. Although the method is general and straightforward to use, most of the questions in the checklist are subjective, requiring strong ergonomics knowledge from the user to assess the ergonomic conditions, especially in the presence of multiple hazards (Li and Buckle, 1999).
Rapid Entire Body Assessment (REBA) (Hignett and McAtamney, 2000) can assess a variety of postures. The method allows for scoring 144 possible combinations of posture (including the trunk, neck, legs, upper arms, lower arms, and wrists). The additional factors considered are load, coupling, and frequency. After the analysis, the method provides an overall score and classification into five action levels of ergonomic intervention. However, the user must identify the critical work activity to assess, which could be difficult, depending upon the body part and the risk being assessed (Takala et al., 2010).
The three methods, OWAS, PLIBEL, and REBA, lack indications for combining risks from multiple sources. Moreover, they do not consider duration in their analyses, making their use difficult for evaluating cyclic work.
European Assembly Worksheet (EAWS) (Schaub et al., 2013) is an ergonomic risk assessment method to assess cyclic work holistically. The method was developed based on the following four criteria:
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Physiological and biomechanical criteria
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Medical/epidemiological data
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Psychophysical factors
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Compliance with other internationally accepted methods and standards
The method assesses work cycles under five sections. Four of these (general, postures, action forces, and manual material handling) are combined to produce a whole body score, while the fifth section (repetitive load of the upper limbs) is scored separately. Thus, the method allows for a holistic evaluation of the work cycle considering the combination of risks from different sections, along with their durations in a work cycle. The final score of the EAWS (higher of the whole body and upper limbs) can be classified into the ‘traffic light' scheme of 3 levels (green, yellow and red), which is supplemented with recommendations for ergonomic intervention. Although the authors of the method note that the application of EAWS is complex and requires intensive training, the EAWS is disseminated in several companies in Europe (Schaub et al., 2013).
The purpose of this work is to propose a simple method for Postural Ergonomic Risk Assessment (PERA) of cyclic work. Apart from the EAWS, the literature review revealed a gap in the existing methods for the ergonomic risk assessment of cyclic work. The EAWS has many strengths and was considered a benchmark tool for this work due to its widespread use in the industry (Schaub et al., 2013). The key target for the proposed method was usefulness for the industries. This goal was to be achieved by developing a simple method, accounting for the relevant standards (ISO 11226 and EN 1005-4), and considering the simultaneous impact of the posture, the force applied by the operator, and the duration of the tasks on the ergonomic risk to the operator. The method analyses every work task of the work cycle, which helps in the quick identification of high-risk work tasks, along with providing an overall averaged score and corresponding recommendations for ergonomic intervention.
This paper describes the method and its development and validation, followed by discussion and conclusion.
Section snippets
Development of the method
A trial and error method was used to develop the method iteratively. Fig. 1 shows the steps followed to develop the method. The current section explains these steps.
Postural Ergonomic Risk Assessment (PERA)
Just as in the cube method proposed by Kadefors, the three parameters considered in PERA are posture, force, and duration, and they are divided into three demand levels of low risk, medium risk, and high risk. Fig. 3 shows a graphical representation of the cube method. The cube method was adapted to evaluate cyclic work, characterized by awkward static postures and light assembly work using hand tools or partners. PERA was tested for cycle times from 25 s to 250 s. For very short cycle times,
Discussion and limitations
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All the evaluations had been performed using video recordings of the work cycles, and the same data was used in analyses by all the methods. The videos were of real work cycles in the factories (or a part of the work cycle), and they were made by the company analyst, specifically for this work.
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The key target of this work was to ensure compliance with the current standards. This compliance was to be achieved through a simple model for ergonomic risk assessment. A simple model compromises on the
Conclusions
PERA is mainly focused on postural ergonomic risk assessment, but offers industrial relevance for work cycles dominated by such risk.
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PERA achieved a 100% success rate with respect to the evaluations by the EAWS. The nine work cycles, comprising 88 different work tasks, offered a substantial variety. The cycle time ranged from 25 s to 250 s. Although most of the analyzed work cycles were from the automotive sector, the work cycles included risks to the trunk, the shoulders, the elbows, and the
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