ErgoSAM: A Preproduction Risk Identification Tool

J. LaringDepartment of Product and Production Development/Human FactorsEngineering, Chalmers University of Technology, Göteborg, Sweden

M. Christmansson and R. KadeforsNational Institute for Working Life–West, Göteborg, Sweden

R. ÖrtengrenDepartment of Product and Production Development/Human FactorsEngineering, Chalmers University of Technology, Göteborg, Sweden

ABSTRACT

Simulating work in a workstation with computer manikins is for many companies too expensive toacquire and master. An alternative method to detect high musculoskeletal loads early in the plan-ning process is ErgoSAM.

This article describes the users’ and potential users’ view of ErgoSAM and the suggestions ofpossible and desired improvements. Some improvements were introduced in a new version ofErgoSAM and validated at Volvo Car Corporation, Sweden. The new version demonstrates im-proved capacity to predict the occurrence of high loads on the operator when performing an assem-bly task, described in the MTM method SAM and in an assembly environment. © 2005 WileyPeriodicals, Inc.

1. INTRODUCTION

1.1. Method Time Measurement and Ergonomics

There are many methods that assess the risk for excessive musculoskeletal load by observ-ing workers’ task performance in an existing workplace. However, there are few methodsfor risk assessment of a task in a planned but not yet existing workplace.

The method time measurement (MTM) method was developed by H.B Maynard in theUnited States in the 1940s as one of many predetermined time systems (PTS) (Maynard,Stegmerten, & Schwab, 1948). As an alternative to time measurements, PTS offers thepossibility to calculate expected time consumption for a planned task, not yet observable,or as a neutral calculation independent of individual differences in the execution of atask. Sequence-based activity and method analysis (SAM) was developed in Sweden in1983 (Luthman, Bohlin, & Wiklund, 1990); it groups several MTM-1 movements intoone SAM movement with the exclusion of some special cases. The analysis is simplified

Correspondence to: Jonas Laring, Bergsvagen 33, SE-43360 Savedalen, Sweden. E-mail: jonas.laring@gothnet.nu

Human Factors and Ergonomics in Manufacturing, Vol. 15 (3) 309–325 (2005)© 2005 Wiley Periodicals, Inc.Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/hfm.20028

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resulting in shorter times both to learn and to use the system. Sequence-based activity andmethod analysis, as well as other similar high-level MTM derivatives such as the univer-sal analyzing system (UAS) and Maynard operation sequence technique (MOST), is usedin production primarily for method improvement and for balancing line production. How-ever, MTM performs just as well in many other applications involving repetitive humanwork (Maynard, 2004). At Volvo Car Corporation (VCC) the assembly line pace is set asa percentage of the time predicted by SAM.

Attempts to use the information received from MTM systems for ergonomic evalua-tions have been and are being carried out by some research teams. Kühn and Laurig (1990)studied the possibility to calculate the distribution of the work task over the differentbody regions by means of MTM-1, giving the possibility to detect an uneven balancebetween load on the right and left hand during work. Peterson, Winkel, Björing, and Math-iassen (1996) studied the relationship between some MTM activities and electromyo-graphic (EMG ) amplitude. Neither of these two studies, even though they contributed toincreased knowledge in the field, led to a useful tool for practical purposes. ErgoMOST(H.B. Maynard and Company, Inc., Pittsburgh, PA) is a computer program that can usethe job description made in MOST for an ergonomic analysis. The MOST code is importedinto ErgoMOST and through a series of “clickable” menus and graphic representationsthe force, posture, and grip for each movement is defined. One job sequence as well as acombination of different jobs during a work shift may be analyzed.

At the Institute of Ergonomics headed by Professor Kurt Landau at Darmstadt Univer-sity of Technology there is a research project that represents the most ambitious effort sofar in this line of study and is similar to ErgoSAM.

. . . to create an ergonomic risk assessment tool, based on MTM UAS, that could be applied in anearly production process phase, where the first time studies were available. . . The UAS-Ergo pro-gram code realized for the feasibility allows to evaluate working postures during MTM-UAS pick& place task operations by means of an enlarged MTM-UAS code. An additional “ergonomicscode” besides the UAS code, which is prompted from the user, contains ergonomic relevant infor-mation that is not kept in the UAS code. (Institute of Ergonomics, 2004)

1.2. The Cube Model

The basis for the musculoskeletal load analysis in ErgoSAM is a three factors methodcalled the cube model developed by Kadefors (1997). The methodology is mentioned byPutz-Anderson (1988) who points out the need to consider a combination of influentialfactors when estimating risks: “When the recovery time is insufficient . . . and when highrepetition is combined with forceful and awkward postures, the worker is at risk of devel-oping a CTD” (Cumulative Trauma Disorder). A general methodology for a multifacto-rial approach was suggested by Tanaka and McGlothlin (1993) and Kumar (1994). Themodel suggests a combined evaluation of risk for injury encompassing the three funda-mental factors posture, force, and time. Kumar suggests a general model with the factorsload, exposure time, and motion on logarithmic scales. Tanaka and McGlothlin’s appli-cation focus is carpal tunnel syndrome (CTS). Posture is limited to wrist angle, the appli-cable force is internal force exerted by the finger-hand-wrist-forearm complex, and thetime factor is repeatability. The authors suggest a formula for risk prediction that essen-tially is these three factors multiplied. In the cube model, the three factors are posturalstrain, external force exertion in pushing/pulling or in manual materials handling, and

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repetitious loads. According to the theory behind the cube model (Kadefors, 1997) cri-teria may be formulated to identify in operative terms what constitutes low, medium, andhigh demands for each factor. From an ergonomic evaluation point of view, these basicfactors are heavily interrelated and thus suggest the total load on the operator is a functionof the three factors multiplied.

1.3. ErgoSAM

The prototype to ErgoSAM was created at Lindholmen Development in Göteborg, Swe-den, in the mid-1990s to offer a tool for the production engineer when there is a need totake ergonomic considerations in a workplace design. The envisaged use was in the pre-production stage when the product design is under way but no product yet exists in realityand the workplace is to be designed. ErgoSAM is SAM with a module added that alsopredicts the physical workload on the operator, making it possible to identify points ofhigh load during a work cycle (Laring, Forsman, Kadefors, Örtengren, 2002). The eval-uation is made by an algorithm written in Excel macros that makes the calculation basedon a small knowledge base in a matrix that covers the combination of all the SAM ele-ments and body postures as well as limits for handled weights and frequencies of theupper limbs. The analysis is presented as a mean value for the work cycle analyzed and adiagram showing the load over the work cycle making it possible for the analyst to iden-tify tasks with high loads (see Figure 1). This prototype has been evaluated in three sep-arate studies and these are the point of departure in the present study.

1.3.1. ErgoSAM Validation 1 and 2. A first evaluation study was carried out atVolvo Car Corporation (VCC) and ITT Flygt Company (Laring et al., 2002) where theoutput was compared to the results from an ambulatory EMG monitoring equipment calledMyoGuard (Kadefors, Sandsjö, & Öberg, 1992) and a video-based system for operator

Figure 1 The diagram output from ErgoSAM showing the momentary cube value (Y-axis) overthe work cycle. Up to 6 is green, 6 up to 9 is yellow, 9 and up is red. The diagram makes it possibleto pinpoint the tasks with the highest musculoskeletal loads.

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assessment called VIDAR (Kadefors & Forsman, 2000). The engineers at each companywere asked to pick out three manual assembly stations representing light, medium, andheavy physical load on the operator. The engineers, trained in SAM methodology, weregiven a lesson in how to add the extra information needed; they then performed theErgoSAM analysis. In the second evaluation at VCC, the ErgoSAM output was comparedto the evaluation made by an experienced industrial ergonomist (Christmansson et al.,2000). Six balances (workstations) along the assembly line were analyzed on event level,i.e., SAM code level. Both reports from the evaluations concluded that ErgoSAM has theability to classify musculoskeletal load in a manual assembly setting. However, becausethe base for body posture is a simple bipolar description of hands in the inner or outerzone, many special postures such as extreme wrist or neck angles are invisible to ErgoSAM.Another problem is that the limit values for repetitive activities are low and need to berevised. A further and more refined evaluation as well as redevelopment of the methodwas thus recommended.

1.3.2. ErgoSAM Validation 3. The third evaluation of ErgoSAM was also made atVCC assembly line in Göteborg (Amprazis, Christmansson, & Falck, 2001). The evalu-ation was made on five balances totaling 17.4 minutes of assembly work. A productionengineer with schooling in ergonomics made an ErgoSAM analysis and an experiencedoccupational ergonomist made an analysis on each one of the five balances. The evalu-ation was made at SAM code level using a video recording of the work at the balance. Toadapt ErgoSAM to the Volvo standard some adjustments were made in the database refer-ring to weight levels, body posture and total assessment levels, thus creating a Volvoversion of ErgoSAM. This work is the basis for the further development of ErgoSAMdiscussed below.

The conclusion of the evaluation by Amprazis et al. (2001) is that “ErgoSAM is a goodmethod, with which it is easy to analyze a workplace by assessing the physical workload.The method does not take into account all ergonomic parameters, but gives nonetheless agood picture of the degree of physical workload in an analyzed work sequence” (ourtranslation). The authors also state that the extra time needed for data input in ErgoSAM,in addition to what is needed in traditional SAM analysis, was only about 5%. In general,the ErgoSAM Volvo version showed a somewhat lower degree of risk for injury in itsassessment than the ergonomist did. It was stated that this depended both on differencesin method and in user input. The user input differences consisted mainly of the produc-tion engineer’s and the ergonomist’s occasional differing opinions in the registration offorce. The differences in method were identified as:

• A number of physical workload parameters not considered in ErgoSAM such asspecific postures in specified joints, influence of how the objects are gripped, andstatic loads and forces assessed according to VCC specifications

• The manner in which the work area and weight limits are classified• The number of levels in the classifications

Amprazis et al. (2001) also give some recommendations how to improve ErgoSAM.They mainly concern the task of correctly assessing the work posture based on the datapresent in ErgoSAM. ErgoSAM limits the input to one figure describing the position ofthe hand in the inner or outer zone. This excludes many cases where the object or com-ponent is kept in the inner zone but, for instance, the neck or the hand is held in an awkward

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angle. Amprazis et al. suggest a widened definition of the two zones encompassing aspecified range of cases of body postures. The VCC version of ErgoSAM is not treatedfurther in the present article, which encompasses the original plus a developed versionthat has taken some of the suggestions from the VCC professionals into account. Seefurther comments on the relations between the versions in the Discussion section.

2. STUDY GOAL

ErgoSAM is a preproduction tool for the production engineer in the design of worksta-tions and in the analysis of manufacturability of new products. Our goal in the presentstudy is (a) to identify and correct inconsistencies in ErgoSAM found during evaluations,so that ErgoSAM can more accurately evaluate the risk for musculoskeletal disorders inmanual assembly work, and (b) to evaluate the revised version. The discussion toucheson further development of the tool such as enhancing the scope and increasing the usabil-ity of ErgoSAM.

3. MATERIAL AND METHODS

3.1. Interviewed Users and Potential Users

The study employs a user-centered approach, involving a number of organizations andindividuals that in various ways have familiarized themselves and worked with ErgoSAM.

• ErgoSAM is used to a certain extent at VCC.• ErgoSAM is taught at Chalmers University of Technology, Göteborg, Sweden.• Lernia, a professional training service, provides adult occupational education courses

on ErgoSAM.• The Nordic MTM Association as well as the International MTM Directorate are

showing interest in the method and are considering its inclusion in their standards.• One company, MVV Data (Skövde, Sweden), offering services in production tech-

nology, is considering including ErgoSAM in Casat, their computer tool for timecalculations and balancing.

To study the experience of the users and their views on function and usability ofErgoSAM six users in these organizations were interviewed. The main focus was to deter-mine to what extent they had used ErgoSAM, for what purpose, how well it stood up totheir expectations, and whether they had any opinion on how to improve ErgoSAM froma user perspective.

3.2. Method of Validation

To validate ErgoSAM results have to be compared to those of an existing and establishedmethod. The lack of existing and relevant nonobservational methods limits the validationto the use of observational ones. The analysis thus has to be performed on existing stations.

In the third evaluation a method developed by Munck-Ulfsfält, Falck, Forsberg,Dahlin, and Eriksson (2003) at VCC was chosen. The checklist for load ergonomics for

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manufacturing engineers was developed to function in the type of environment present atthe VCC final assembly line; it can be considered more accurate and extensive than othermethods applied here. It is an observation method, and the parameters fall into the samethree main categories as those in the cube model: work posture, weight and/or force, andmovement frequency. Within these categories, however, the Volvo method encompassesmore details than ErgoSAM, using 22 parameters in 8 levels. The posture category con-tains an evaluation of eight different body parts; the weight/force category has six factorsto evaluate, e.g., type of grip, hand-held tool category, and movement frequency of eightdifferent body parts. Beside this evaluation, the VCC standard lists five special cases toconsider, applicable at the use of certain tools or components. The ergonomist used avideo recording of the work at the balance and performed an evaluation at SAM codelevel. At each balance, an average value from the ergonomist’s evaluation was also cal-culated and compared to the average value from ErgoSAM. The result of the ergonomist’sevaluation was plotted parallel to that of ErgoSAM (see Figure 2).

The differences in the analysis results were identified and categorized into those depend-ing on method and those depending on differences in opinion between the ergonomist andthe engineer. An example in differences in opinion is that a certain posture held by theoperator during a work task was rated by the ergonomist according to what she could noteby observing experienced operators. But the engineer rated according to how the operatorwas instructed to act and consequently, how the SAM code was written. Measurements offorce were made during the evaluation, when considered necessary but in some cases theergonomist and the engineer still differed in opinion over what were the forces involvedin a task or what they should be. To focus on method in this study, the engineer’s analysiswas adapted to the ergonomist’s, when the differences were depending on matters of opin-ion. In a real industrial setting, the differences would have had to be settled by a discus-sion where the engineer and the ergonomist would have to use their experience and agreeon the content of the planned work sequence to estimate expected postures.

After having introduced the changes in ErgoSAM described below, thus creating a newversion, both the new and the old version of ErgoSAM were compared to the evaluationmade by the industrial ergonomist.

Figure 2 The diagram output from ErgoSAM parallel to that of the ergonomist showing the momen-tary cube value (Y-axis) over the work cycle. The ErgoSAM’s analysis is marked in a thin line andthe ergonomist’s reference analysis in a bold.

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4. RESULTS

4.1. Interviews

In general, the users and potential users appreciated that ErgoSAM allows them to makerough ergonomic evaluations in an engineering workplace, without the need to makedetailed observations. The extra time needed beyond an ordinary SAM analysis was notconsidered discouraging and the basics of both SAM and ErgoSAM were considered easyto learn. After familiarization with the system, however, there was a desire to broaden theuse outside the original intention and to have a more user-friendly interface.

ErgoSAM has not yet been put to use in preproduction planning, although SAM is usedat VCC years before a model goes into production to estimate assembly times for eachcomponent. This is done simultaneously with a virtual product and process evaluationwhen analyzing design solutions. At this stage, the opportunity to introduce changes andnew solutions may be less expensive than at later stages. But the framework for assemblyis rather fixed because new models are introduced in the old assembly lines. A number ofproduction engineers have been educated in the use of the program and started to use it inthe planning of changes at the assembly line. The graph from the output of ErgoSAM alsoproved to facilitate the discussion with operators and other involved professionals whencontemplating the work content of each balance. The description of the balance in thisform adds visualization to the discussion. It becomes possible to show the expected resultfrom a suggested change side-by-side with an analysis of the present situation. It thusbecomes as much a tool for communication as it is for assessment. At VCC, they proposethat ErgoSAM be used for input on changes in work organization. Used on observedassembly work at the Göteborg plant, their experience is that ErgoSAM works very welland gives assessments of musculoskeletal load at the balances close to those made by anexperienced industrial ergonomist and the operators themselves.

At Chalmers University of Technology an introduction to ErgoSAM is given to someundergraduate students when doing master thesis work within production planning. Thestudents use it to analyze the expected physical workload in planned workplaces as wellas to compare the planned workplaces with existing ones. They appreciate ErgoSAM forits ability to pinpoint problematic tasks and serve as a basis for discussion on improvements.

Lernia, a professional training company is providing courses in SAM and has a sup-plement with a description of ErgoSAM in the literature. Because ErgoSAM is notdesigned for manual (paper and pencil) SAM analysis in the sequence according to thestandard methodology, no actual training is given to students, who are employees pri-marily from larger companies or recently graduated production engineers who are newlyemployed.

MVV Data, a production management consultancy firm in Skövde, Sweden, with mainlySwedish customers, has studied ErgoSAM and is considering integrating it into their pro-gram Casat. Their computer tool is used mainly for managing time data in production andbalancing line production. The opportunity to optimize not only over time consumptionbut also physical workload is considered attractive to their customers. Their customersare mainly middle-sized and small enterprises, with production engineers few of whomhave any deep ergonomic knowledge. In most cases, SAM is used when preparing for anintroduction of new products that are slightly changed from older products, and in run-ning production. To be able to identify ergonomic problems with the help of ErgoSAMwould be very appealing. ErgoSAM with its simplicity in use and short extra work input,in addition to an ordinary SAM analysis, is considered vital.

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4.2. Suggestions for Improvements

In the evaluations referred to above and during the interviews, a number of suggestionson how to improve ErgoSAM were made. They can be separated into two main groupsaccording to when it is used: in preproduction or during running production.

4.2.1. Preproduction.

1. Adaptation to Common Practice. To speed up a SAM analysis it is common thatthe user groups identical tasks into one repeated sequence, even if these tasks inreality are spread out during a work cycle. When calculating only time it is oflittle importance, even if it is not recommended for making it more difficult toretrace the analysis when making changes. When calculating physical load in anErgoSAM code movement, information is used from the preceding movement. Ifthe preceding line in an analysis does not correspond to reality, an error may beintroduced. A solution may be found in either changing the method of analysis orthe method in which the user is allowed to supply the input (Ulf Rogbrant, per-sonal communication, May 27, 2004).

2. Some corrections to adapt ErgoSAM to common practice are also suggested in theevaluation made at VCC, mainly concerning the supplementing input of weight/force (Amprazis et al., 2001)

3. Limits for Upper Limb High Frequency Movements. The limits for repeatabilityare considered by the users to be set at an unrealistically low level. The effect is thatone of the three factors almost constantly has the highest value, limiting its contri-bution to the analysis by being a constant and not a variable (Amprazis et al., 2001;Ann-Christine Falck, personal communication, June 5, 2005)

4. Revision of Knowledge Base. During the latest evaluation at VCC, some irregu-larities in the knowledge base and some differences in opinion in the ergonomicevaluation of the SAM code were identified (Amprazis et al., 2001).

4.2.2. Running Production.

1. Larger Scope. During observations in running production, there are more relevantdata present on work postures than can be used in an ErgoSAM analysis. Thus, itwould be advisable to include this information in ErgoSAM to do a more detailedanalysis. A larger list of input for a defined number of joints and a clear instructionfor limits in each case would increase the possibility for ErgoSAM to detect unaccept-able musculoskeletal load (Amprazis et al., 2001).

2. Balancing Load. After the completion of a series of ErgoSAM analyses along aproduction line, the result is that most likely some stations are causing more phys-ical load than others are. Naturally, it would best to balance this load over the oper-ators in the same way that the time needed for production is balanced. ErgoSAMwould thus be a tool for job design and work organization (personal communica-tions on May 25, 2004, with Joakim Amprazis, Ulf Rogbrant, and Ann-ChristineFalck).

5. CHANGES INTRODUCED

Weighing the solutions to the suggestions above in terms of what is currently possible,changes to ErgoSAM according to the preproduction suggestions 2 and 3 above were

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introduced. The remaining recommendations have been left for further research anddevelopment.

The knowledge base in ErgoSAM is based on a consensus discussion with several indus-trial ergonomists over theoretical situations. When the result of an analysis made byErgoSAM shows a constant discrepancy with an analysis of the same task in a practicalapplication made by an experienced industrial ergonomist, it is a strong indication that anupdate is necessary. Based on a number of such considerations concerning the evaluationof body posture in combination with SAM code, some changes were introduced in theknowledge base of ErgoSAM to allow the evaluation to more closely follow theergonomist’s assessment. An example is given in Figure 3.

In light of the evaluations carried out, the limits for frequency (repetition) can be con-sidered too low. The result is that in the assessment of a work cycle, the frequency factoris in effect constantly attaining the value of 3. Consequently, initiatives in improving thework cycle by lowering the number of hand/arm movements often have no effect on theErgoSAM output. Some update on repetitivity can be found in a draft CEN standard RiskAssessment for Repetitive Handling at High Frequency (European Committee for Stan-dardization [CEN], 2003; Ringelberg & Koukoulaki, 2002). This draft standard containslimits for frequency that follows the OCRA method (Occhipinti, 1998). The limits of theOCRA Checklist for arm movements are considerably higher than what is used inErgoSAM. Here we have changed the limits according to the suggestions laid down inthis draft standard (See Table 1).

Figure 3 Example of changes (framed) in the ErgoSAM knowledge base. “Code” is the SAMbasic element of movements, “Fakt” is the standard time for this movement, and I and U stands forthe inner or outer zone. The numbers in the matrix are the estimated values supplied to the cubeanalysis as posture factor.

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6. EVALUATION

After having introduced the changes in the knowledge base of ErgoSAM, the balancesanalyzed in the VCC evaluation (Amprazis et al., 2001) were evaluated once again. Theresults are shown in Table 2.

After the changes were made, the ErgoSAM analyses correlate better or are unchangedcompared to the ergonomist’s template. The mean value is closer to the ergonomist’s inbalance 1, 2 and 5, but further away in balance 3 and 4. The differences between ErgoSAMand the ergonomist are large in the case where the load on the operator is considered low,but within reasonable distance from the ergonomist’s evaluation where the load is con-sidered medium or high.

A detailed view of the two ErgoSAM versus the ergonomist’s analyses of balance 4,“Side Panel” shows where the differences occur (Figure 4). The differences are dividedinto two categories:

• Method, i.e., physical workload parameters not considered in ErgoSAM some ofwhich are specific Volvo standard

• Method development, i.e., differences between original and developed version ofErgoSAM

TABLE 1. Limits for Repetitivity in the Old and New Version of ErgoSAM in Movements/Min

Lowold

Lownew

Mediumold

Mediumnew

Highold

Highnew

Fingers 0–20 0–20 �20–200 �20–200 �200 �200Hand/lower arm 0–1 0–15 �1–10 �15–30 �10 �30Upper arm/shoulder 0–0, 1 0–15 �0, 1–2, 5 �15–30 �2, 5 �30

TABLE 2. The Five Balances at VCC With Overall Assessments Made by the Ergonomist,ErgoSAM Original Version, and ErgoSAM Developed Version

No Balance ErgonomistOriginal

ErgoSAM % diff RDevelopedErgoSAM % diff R

1 Tank lid 3,2 green 4,6 green �44 0,79 4,5 green �41 0,802 Start 4-ling 6,0 yellow 6,5 yellow �8 0,79 6,3 yellow �5 0,893 St. wheel axis 7,7 yellow 8,1 yellow �5 0,81 8,4 yellow �9 0,924 Side panel 7,7 yellow 7,9 yellow �3 ,88 8,2 yellow �6 0,915 CD panel 11,3 red 9,6 red �15 0,77 10,3 red �9 0,76

Note: The table gives mean cube value and corresponding color, the difference to the ergonomist’s evaluation,and the correlation factor between the two curves.

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Figure 4 Analysis of balance 4 “Side Panel” with ErgoSAM in its original version in the upperfigure and the new version in the lower figure compared to the ergonomist’s analysis, the same inboth figures. The most dominant differences and changes are numbered and referred to in the text.

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The differences, as shown in Figure 4, may be described as follows:

1, 5 and 9 Lowest value in the original ErgoSAM was 3 due to the fact that the eval-uation of frequency constantly remained value 3. In the new version, thisproblem is remedied, resulting in a varying value for frequency.

2, 3, 8 This difference between ErgoSAM and the ergonomist’s evaluation refersto both the fact that the ergonomist rates the existence of static load and anunfavorable type of grip, where ErgoSAM does not, and uses a scale witheight levels where ErgoSAM only uses three. In the redeveloped ErgoSAM,the changed rating of posture over working zone and SAM code resulted ina smaller difference to the ergonomist’s evaluation.

4, 7 The differences between the original and developed versions of ErgoSAMrefer to the upgrading in evaluation of the SAM code BEND while havinghands in the outer zone and also in the SAM code “machine time” MT withhands in the outer zone. The two over-estimations at seconds 35 and 39 inthe new version of ErgoSAM are explained by the fact that Volvo ratesweights at higher limits than ErgoSAM.

6 The new version of ErgoSAM lowers the rating because of higher limitsfor frequency.

A proposed use of ErgoSAM is as a musculoskeletal load detector, or as an “earlywarning system.” When a high load is identified, a deeper analysis may reveal what causesthe load. In other words, no harm is done if ErgoSAM detects a high load that a deeperanalysis reveals is false (false positive). But a failure of ErgoSAM to detect a high mus-culoskeletal load that actually is there could be a critical failure (false negative). A detailedstudy of the evaluations made by ErgoSAM and the ergonomist side-by-side, and secondby second, reveals the occasions when ErgoSAM misses a risk that the ergonomist iden-tifies. The number of seconds analyzed is 751, which is the total time of active workduring the 17.4 minutes of balanced time. During 29 of these seconds, ErgoSAM regardsthe musculoskeletal load as being “conditionally acceptable” with the color yellow whereasthe ergonomist’s evaluation is “not acceptable,” or red. During 19 seconds, ErgoSAMregards the load as “acceptable” or green, when the ergonomist evaluation is “condition-ally acceptable,” or yellow. This is 6% of the total time analyzed. Figure 5 illustrates thedistribution of each analyzed second in a diagram.

There are three main reasons for this discrepancy, each explaining about one third ofthe occasions:

• General Differences Between Methods. The ergonomist used a larger specificationover what is considered an awkward body posture and used company standards con-cerning grip ability and hand tools. Three instances also appear where the underly-ing knowledge matrix of ErgoSAM evaluates a certain task as causing a lower loadthan the ergonomist does.

• Differences in Resolution Between Methods. The VCC method, used by theergonomist, had eight levels between 0 and 3 for each factor.

• User-Dependent Differences. The ergonomist estimated averages of load over someshort cycled sequences to save analysis time—a preference not shared by ErgoSAM.

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The distribution of the relative difference between the ergonomist and ErgoSAM foreach second is demonstrated in Figure 6. It is obvious that ErgoSAM slightly overesti-mates the occurrence of musculoskeletal load. The arithmetic mean value is 0.33. Thetype value is 2 and is dependent on the fact that ErgoSAM’s evaluation of frequency isbased on a longer time span than the ergonomist’s evaluation. On many occasions, whenthe load is low, the ergonomist’s value is 1 while the frequency factor of ErgoSAM is stillat 3 because an activity just terminated.

7. DISCUSSION

The simple assessment of musculoskeletal load in ErgoSAM was originally meant to iden-tify problematic design features in a work place design not yet put into practice. Its intendeduse is when the product exists only in a design phase, as a digital or physical prototype,and the production is being planned. The production engineer is interested in defining thenecessary resources for production in terms of production time, workspace, equipment,and personnel. ErgoSAM is designed to assist in optimizing the workplace in terms of

Figure 5 A diagram showing the correlation between the evaluation made by ErgoSAM and theergonomist for each second. The size of the circle is proportional to the number of incidents at thepoint. Zone 1 is where the ergonomist considers the cube value to be red (not acceptable), butErgoSAM’s evaluation is yellow (conditionally acceptable). Zone 2 is where the ergonomist con-siders the Cube value to be yellow, but ErgoSAM’s evaluation is green (acceptable).

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production time and physical load on the operator. To secure the best possible analysisresponse it is essential that the person doing the data input into ErgoSAM not only istrained in SAM methodology, but also is well oriented in ergonomics. The user needs tobe able to supply realistic work postures and weight/forces when the data input is carriedout. The quality of the output always corresponds to that of the input.

The only established alternative method at hand to perform a musculoskeletal loadanalysis in production planning at VCC was the use of the computer manikin tool Tran-som JackTM. However, this program was not appropriate and it was decided to do theanalysis on existing workstations, thus making it possible to use observation methods.But the involved production engineer and ergonomist had information that would notapply in the production-planning situation. While performing the analysis, details on howto increase the level of detail in ErgoSAM were discussed. The risk is that the knowledgeof how straining a task is considered by the operator, the engineer, and the ergonomistinfluenced the engineer to unconsciously adapt the input in a way to get a more “correct”ErgoSAM analysis. The production engineer thus has the ability to impact analyses thatwould not be possible in a planning situation.

The observation method used to validate ErgoSAM is a method developed at VCC bya group of industrial ergonomists with extensive experience at VCC. It uses establishedknowledge and focuses on the type of assembly and prevalent injury statistics at the plant.But the observer is only human, making it probable that human errors were introduced inthe validation method. Using intuition, preconceptions, and hidden knowledge as well astaking shortcuts and attention lapses may very well have influenced the analysis result ofthe validation method. The deviations between the two different methods may thus beexplained by shortcomings in the validation method as well as in ErgoSAM.

A goal in the present development has been to keep the extra input, over what is neededto do the SAM analysis, to a minimum. The production engineer has to consider manydifferent aspects when doing preproduction planning. Simplicity in combination with a

Figure 6 A diagram showing the relative difference between the ergonomist and ErgoSAM foreach of the 753 seconds. A positive value signifies that ErgoSAM’s estimate is higher that theergonomist’s.

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little extra time to do the ErgoSAM analysis makes it more probable that it is being usedat all. The 5% extra time over an ordinary SAM analysis was considered by the produc-tion engineers at VCC well within the limits of being a minimal charge for a high benefit.Despite its simplicity, the analysis result is close to that of an experienced ergonomist inperforming an observation analysis. In a planning situation, the ergonomist may do acoarse judgment of a planned task and use CAD and VR tools, but aided by a surveymade by ErgoSAM he is able to save time concentrating on identified tasks with highloads.

Using ErgoSAM in job design, however, is an application that was not considered inthe original development of the program. If the focus instead is on the operator over awork shift, other considerations are at hand. When balancing load, instead of eliminatingit, it is vital to track load on specific body parts. A more detailed analysis has to be madeto be able to discriminate between loads on specified parts of the body. In a future devel-opment of ErgoSAM, it may be possible to add a systemized method of supplying andtracking data on identified body parts. A solution may be possible with a map of repre-sentations of joints and angles and a method of how to mark them to include them in theassessment.

Volvo Car Corporation personnel, when performing the evaluations of the first proto-type of ErgoSAM, suggested higher limits for weight handled or force exerted. We havedecided to stay with the original values however (see Table 3), especially since the VCCversion of ErgoSAM, with these higher limits, rated work with low levels of weight in theouter zone (low risk weight, high risk body posture) too low in comparison with theratings of the ergonomist.

Referring to the differences in the internal order in the work sequence between realityand the SAM analysis, the value of an improved common practice should be noted. Whenassessing the physical workload content of a workplace design, it is of interest to reach anaverage load level of the work carried out in the workplace. It is also vital to identifyisolated tasks with high load that may cause problems and may need special attention. Itis of less importance when in time these occur, because a SAM analysis is seldom used inapplications where work cycles are longer than 10 minutes. (Ulf Rogbrant, personal com-munication, May 27, 2004). The error in absolute terms we consider small and may varygiving both positive and negative contributions. But when using ErgoSAM in runningproduction for job design purposes, it is obvious to any user that the work sequence in theanalysis must closely follow reality and thus give a better analysis. This connection betweencommon practice and analysis results must be clearly pointed out to the user. The habit ofgathering similar motions occurring at different times during a work cycle into one lineevolved when paper and pencil were the only tools available. As computers are increas-ingly used, the time saved using this method is less significant, and habits changed for theimproved analytic result gained. The expansion of ErgoSAM into a job design tool is apath well worth being explored.

TABLE 3. Limits for Weight/Force Used by VCC and ErgoSAM

Low Medium High

VCC �2 kg �2–7 kg �7 kgErgoSAM (Kadefors, 1997) �1 kg �1–2.3 kg �2.3 kg

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In ergonomics it is of vital importance to know the body measurements of the operatorwhen designing a workstation. In ErgoSAM, it is left to the user to supply the informationon body posture in a simple statement, whether the operator has the hands in the inner orouter zone, thus taking into account the expected body dimensions of the operator. Thisleaves the result open to the experience and the competence of the user, who has to knowhow the type of work that is being analyzed is carried out in general terms. The ergo-nomic knowledge to correctly estimate the resulting postures and forces in the executionof a planned task is thus a necessity.

8. CONCLUSION

The three evaluations referred to in this article demonstrate that under the circumstancespresent at the VCC assembly line, ErgoSAM has a high potential for estimating manyrisks for musculoskeletal disorders—matching the analysis of a qualified ergonomist. Theexperience gained from the evaluations pointed out some desired changes of ErgoSAM.After including those that were possible without further research, ErgoSAM showed animproved correlation to the evaluation of an experienced ergonomist.

To fully explore the potential of ErgoSAM to supply an early ergonomics analysis andassessment, when a change in design is still possible and the workplace can still be altered,there is a need to adapt the methodology and the user interface to the systems used inindustry. Integration into the commonly used computer tools is crucial if the benefits ofan early ergonomics assessment are to be achieved.

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