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Influence of material handling devices on the physical load during the end assembly of cars
International Journal of Industrial Ergonomics 24 (1999) 657}664
In#uence of material handling devices on the physical load during the end assembly of cars Veerle Hermans*, Mark Hautekiet, Arthur Spaepen, Luc Cobbaut, John De Clerq Laboratory Ergonomics and Occupational Biomechanics, Katholieke Universiteit Leuven, Tervuursevest 101, B-3001 Leuven, Belgium Received 29 August 1997; received in revised form 7 January 1998; accepted 17 July 1998
Abstract The e!ect of using a mechanical device on physical load was analysed during the end assembly of cars. Five tasks, further sub-divided into lifting and positioning, were performed by someone experienced in working with and without the mechanical tool. A practical methodology was used: the subjective walk and talk through method, the NIOSH equation (1991), a three-dimensional goniometer (Back Tracker) and surface electromyography (EMG) of the relevant muscle groups. According to the NIOSH equation, 8 out of 10 of the tasks should only be performed by trained workers and preferably with tools. The Back Tracker revealed that the mechanical tools reduced posture angles signi"cantly (p)0.05) for installing the windows, but not for left latero#exion during pre-assembly of windows. It was clear from the EMG analysis that using tools did not always decrease muscular activity signi"cantly. In general, the appropriateness of each tool needs to be evaluated separately taking the subjective criteria into account. Relevance to industry In this study, the relevance of using a mechanical device to reduce the physical load during car assembly, is evaluated. As the tools are expensive, it is important to the industry whether they are being used and how e$cient they are. 1999 Elsevier Science B.V. All rights reserved. Keywords: Assembly work; Physical load; Back; Upper limbs
1. Introduction The manual handling and lifting of loads by people working in car assembly impose heavy physical demands and biomechanical stresses. These demands may cause musculoskeletal disorders of the upper limbs and lower back. One of
* Corresponding author.
the main reasons for these overuse syndromes is awkward posture. For example, during car assembly, extreme #exions and rotations of the trunk, or working postures with the arms above shoulder level are often held repeatedly for long durations (Punnett et al., 1991; LindstroK m et al., 1994; Nagamachi and Matsubara, 1994). One method to reduce physical load is to use material handling devices. However, these mechanical tools are often di$cult to design and implement and may be
0169-8141/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved PII: S 0 1 6 9 - 8 1 4 1 ( 9 8 ) 0 0 0 7 0 - 5
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V. Hermans et al. / International Journal of Industrial Ergonomics 24 (1999) 657}664
costly. Mathisson et al. (1994) investigated the individual's perception of tools and the psychosocial context. In their study, workers were well aware of the ergonomic risks of not using the tools, but this knowledge did not a!ect usage. Time and stress factors seemed to be very important, since using a tool often took longer than manual lifting. However, Resnick and Cha$n (1996) mentioned that biomechanical stresses in the back are often not well perceived for infrequent exertions or tasks with height peak stress. From the musculoskeletal point of view, Cha$n et al. (1997) found high peak hand forces during acceleration and deceleration of loads using handling devices in a simulation study. When these forces are combined with awkward torso postures (e.g. bending and stooping), high bending moments can be created in the lumbar region during the starting and stopping phases of the motions. Apart from awkward postures and high peak hand forces, Lee et al. (1991) also mentioned the velocity of movement as an important contributor to back stess in pushing and pulling tasks. This study compared the physical load of several real working tasks with and without the use of manual material handling tools during assembly at Volvo Europe Car Industry. Apart from the analysis of back stress, an attempt is also made to analyse the physical load of the upper limbs. The research was prompted by the preference of several employees to work manually, without the supposedly `ergonomica tools, which apparently had disadvantages. A practical methodology was necessary to perform the study in a real working environment.
2. Methods 2.1. Working tasks Five di!erent work processes were analysed: inserting the windows (task 1), pre-assembly of the windows (task 2), installing the battery (task 3), installing the fuel tank (task 4) and assembly of the "fth door (task 5). Each process was divided into two tasks: lifting the object and lowering or installing the object. In task 2, a sub-task was analysed,
relifting the object after lowering. The weight of the object and the horizontal distances during the lifting of the object for each work process are presented in Table 1. For each job, an experienced worker erformed the task ten times both with and without the specialised tool. All the workers had previously been trained in how to use the specialised tools. 2.2. Material and methods Several methods were adopted to gain a global overview of the physical load on the upper limbs and lower back. These methods are summarised brie#y below. 2.2.1. Walk and talk through The worker describes the manual actions necessary in detail while performing the task, (Kirwan and Ainsworth, 1992), and explains why he does or does not use the tool. Important information is recorded in writing. 2.2.2. NIOSH The NIOSH method (1991) was used to analyse the lifting and lowering aspects of each task, taking into account several distance measurements and the lifting frequency. The NIOSH Lifting Index (LI) was calculated from the Recommended Weight Limit (RWL). The LI represents the tolerance of the task without using tools. 2.2.3. Electromyographic analysis (EMG) The muscular activity arising during a task was analysed by a telemetric EMG device (ME3000P, MEGA, Finland), with a sample frequency of 1000 Hz. For each task, the electrical signals of four speci"c muscle groups were registered. The analysed muscles were the right and left part of upper trapezius, deltoid pars anterior and/or erector spinae. The bipolar surface electrodes (type Nikomed, silver}silver chloride, Denmark), were positioned according to standard procedures (Zipp, 1982). In the signal analysing, the average EMG amplitude values (AEMG) were calculated for the two sub-tasks of each job. Average values and standard deviations of these AEMG values were
V. Hermans et al. / International Journal of Industrial Ergonomics 24 (1999) 657}664
659
Table 1 Weight of the objects (kg) and horizontal distances (m) for the "ve work processes
Weight Distance
Inserting window
Preassembly window
Battery
Fuel tank
Fifth door
14.2 2
14.2 2
14}19.9 1
9.6 2
26.8 4
calculated over the ten performances for each job. Before starting the task, subjects were requested to reach maximal voluntary contractions of the speci"c muscles, so that the maximal voluntary electrical activation could be recorded (MVE). The EMG values during the task were normalised to the MVE value, to analyse the relative muscular e!ort for each work task and to compare the results.
2.2.4. Posture analysis Movements and postures were registered with a camcorder. A three-dimensional goniometer (Back Tracker, Isokinetics, USA) was used to calculate the amplitude and velocity of back movements in the frontal, sagittal and vertical planes (#exion/extension, latero#exion right/left and rotations) for two tasks: inserting the windows and the pre-assembly of the windows. This device measures the movements between the sacrum and a thoracal point on the back, by placing a rigid bar on the back. Several studies conducted in cooperation with the manufacturers have reported the validity and reliability of this method (e.g. Magnusson et al., 1994), but we also conducted a small reliability experiment in our laboratory prior to the car assembly analyses. A comparison was made between the data registered simultaneously from the Back Tracker and a three-dimensional movement registration system (Kinemetrix, Orthodata, Germany). A single subject performed a combined movement, which included #exion, rotation and latero#exion and the results of the three-dimensional registrations and of the Back Tracker were comparable (not published). In this car assembly study, the maximal angles in the three planes were calculated for the two sub-tasks.
3. Results 3.1. Walk and talk through For each work process, disadvantages were reported when the tool had to be used. The "ve most reported disadvantages were: E ¹ime consuming: Manipulation of an object with a tool requires more actions, handles have to be manipulated, the tool has to be guided, and positioned, which causes an increase in cycle time. E Not enough space: Manipulating the object with the tool requires more space, which is often very limited. E Not user friendly: Using the tool and manipulating the object at the same time introduces clumsy movements (`you need three handsa). E More di.cult: Manipulating the tool makes it harder to locate the object accurately. E Poorly positioned: The position of the tool hinders the necessary movements of the user. In the section below, the results regarding the movement analysis and muscular e!orts are presented for each task separately. By combining the data of Back Tracker, camcorder, NIOSH and EMG, it was clear which phase of the task caused awkward postures or high muscular e!orts. 3.2. Movement analysis and muscular ewort The NIOSH lifting indices (LI) in Table 3 describe how tolerable the "ve tasks (and their subtasks) are. In eight out of ten cases, the LI is between 1 and 3. According to NIOSH, these tasks should only be performed by trained workers and preferably with some type of mechanical assistance.
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V. Hermans et al. / International Journal of Industrial Ergonomics 24 (1999) 657}664
¹ask 1: Inserting the windows. From the subjective impressions (Table 2) it was clear that using the tool is time consuming, and operating the tool and positioning the window simultaneously cannot be achieved smoothly. However, according to the NIOSH results the use of some type of mechanical device should be considered since the LI is 1.9 (Table 3) in both lifting and positioning. Furthermore, there was a signi"cant decrease (p(0.05) in maximal trunk movements using a tool, both in the frontal and sagittal plane, but only relevant regarding the reduction in right latero#exion. The shoulder muscles are signi"cantly less loaded (Fig. 1). The EMG amplitude of the back muscles decreases enormously during the positioning of the window, but increases slightly during the lifting phase. This means that the use of the tool decreases the load in the heaviest part of the task and slightly increases the load in the lightest part.
¹ask 2: Preassembly of the windows. Slower and more di$cult operation are the most important subjective reasons for not using the mechanical device, although according to NIOSH a tool is preferred, mainly during the lifting (LI"1.9) of the windows (Table 3). No di!erences were found in #exion and extension movements. Furthermore, the high latero#exion left is also found during the use of the tool. Only the decrease in right lateroflexion was signi"cant (p(0.05). A signi"cant decrease in muscular e!ort was found for the four muscles during the three work phases, except for the left back muscle during phase 2 and phase 3 (additional lifting phase in this task). ¹ask 3: Installing the battery. Operating the tool, while holding cables and preventing collision between the tool and the open bonnet hinders and slows down the operation. Furthermore, the range of the tool was too small so that manual lifting was
Table 2 Inserting window
Preassembly window
Battery
Fuel tank
Fifth door
(a) Summary of the subjective impressions regarding the use of a mechanical tool for the "ve assembly tasks Time consuming Not enough space Not user friendly More di$cult Bad position
* * *
*
*
*
*
* * * *
*
* *
(b) The recommended weight limit (RWL) and the NIOSH lifting index (LI) for the "ve tasks, divided into the sub-tasks lifting (lift) and installing (instal), are presented, together with the real weights (kg) of the objects Weight RWL
14.2 7.4 7.4 1.9 1.9
LI lift LI instal
14.2 7.2 12.6 1.9 1.1
14}19.9 20.5 11.1 0.5 0.9
9.6 11.3 5.8 1.2 2.4
26.8 13.4 17.7 2 1.5
Table 3 Results from the Back Tracker for maximal #exion (#ex.), extension (ext.) and latero#exion right (lat.r) and left (lat.l), with and without the use of the tool for tasks 1 and 2. Signi"cant di!erences (p)0.05) between using and not using the tools are presented by * Task 1
Without With
#ex. 19.6 14.1*
Task 2 ext. 5.5 0.9*
lat.r 27.6 4*
lat.l 24.9 18.2*
#ex. 8.1 7.5
ext. 3.9 3.6
lat.r 27.8 8.1*
lat.l 26.1 30
V. Hermans et al. / International Journal of Industrial Ergonomics 24 (1999) 657}664
661
Fig. 1. Replacement of the "fth car door. Left: without using a mechanical device. Right: using the device.
still necessary. Since the LI was smaller then 1 (Table 3), the task could be performed without a tool. No clear advantages or disadvantages can be summarised from the EMG results (e.g. decrease in muscular e!ort for the right trapezius and deltoid during lifting, but increase in the left side). ¹ask 4: Installing the fuel tank. A wave of criticism was reported, summarised in Table 2. A high LI was found during the installation of the tank (LI"2.4), lower during the lifting (LI"1.2). Furthermore, a decrease in EMG amplitude was found using the tool, except for the right deltoid during the installation (22% MVE with tool, 10% MVE without tool, Fig. 1). ¹ask 5: Assembly of the ,fth door. Again several subjective disadvantages were reported for the mechanical device. However, this task involves lifting a window of 26.8 kg a horizontal distance of 4 m, with an LI of 2.0. From the EMG results it is clear that the use of a tool signi"cantly decreases the muscular load of the back muscles. However, it can be seen that the problem is shifted elsewhere: there is a higher increase of trapezius amplitude, due to the lifting of the arms with the tool. From the video registrations it is also clear that the right lateroflexion during the placement of the "fth door changes to a left latero#exion when the tool is used (Fig. 2).
4. Discussion The assembly of a car often involves awkward body positions. Several literature studies mention
repetitive trunk #exions and rotations (Punnett et al., 1991; LindstroK m et al., 1994; Nagamachi and Matsubara, 1994). The aim of this study was to analyse whether the use of mechanical devices decreases the necessity for awkward postures, thereby reducing the physical load on the worker. In order to study the physical load in the working environment, a combined and practical methodology was necessary. Surface electromyography has frequently been used in the working environment to study muscular load and indications of muscular fatigue (e.g. JoK rgensen et al., 1991). Using maximal voluntary electrical activation (MVE) as a reference allows analysis of muscular e!ort. This reference contraction is performed in a "xed position resulting in static muscular e!ort. This is in contrast with occupational tasks where limbs are moving. These changes in posture and velocity in#uence the EMG amplitude and force relationship (Mathiassen et al., 1995). However, we are interested in the comparison of two situations, i.e. working with and without mechanical devices, without focussing on the percentage of maximal e!ort. The Back Tracker was used to analyse speci"c or awkward postures during performance of a work task and to analyse posture over time. Relevant reductions in awkward postures can be found. However, the weight being carried by the subject is not taken directly into account, since no loads or torques are calculated. Therefore, the revised NIOSH lifting equation was used, to assess the physical demands of two-handed manual lifting tasks (Waters et al., 1993). Several criteria are necessary
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V. Hermans et al. / International Journal of Industrial Ergonomics 24 (1999) 657}664
Fig. 2. Relative EMG values (percentages of the maximal voluntary electrical activation (%MVE)) for the "ve tasks (divided into sub-tasks). Both the values without (light grey) and with (dark grey) the tool are presented. Trap"trapezius, Er sp"erector spinae, Del"deltoideus, r"right, l"left.
for this method, which makes it di$cult to apply in real working environments. Recently, a number of modi"cations to the NIOSH equations and procedures have been proposed to improve feasibility of using the method in working situations (Waters, 1997). In this study, the tasks analysed using NIOSH consisted of situations that largely met the criteria. Based on the objective and subjective results of the data analysis, recommendations were made to
the company regarding the use of existing tools for the end assembly of cars. For the installation of the windows, preassembly of the windows and installation of the fuel tank, the tools reduced the load of several muscles, due to improved postures and reduction in the load (the weight of the objects being carried by the tool). Using the tool necessitates altering how the task is carried out, and for this reason practical training sessions are necessary for the workers. For battery installation, advantages
V. Hermans et al. / International Journal of Industrial Ergonomics 24 (1999) 657}664
and disadvantages tend to balance each other out. There is no real reduction in physical load and many subjective disadvantages were reported. The same is true for the tool used during the assembly of the "fth door, where there is a shift in muscular load to other body regions, resulting in a reduction for the back muscles, but not in the upper limbs. This is attributed to the repetitive lifting of the arms above shoulder height. Since it is known that the neck and shoulder region is a prevalent site of work-related pain and disorders (Hagberg, 1984), it is important to evaluate also this region, and not only focussing on back stresses. In the literature it is mentioned that it is necessary to include objective research methods to evaluate the in#uence of material handling devices, since biomechanical stresses in the back are often not well perceived for infrequent exertions or tasks with high peak stresses (Resnick and Cha$n, 1996). This study reveals that a combination of several practical methods can be used to analyse the load of real working tasks. The results of this analysis were reported to the company's medical department and a further action plan was drawn up. A new mechanical device has recently been introduced for window installation, which reduces the number of #exions of the trunk. Furthermore, a new tool for battery installation has been designed, taking the results of this study into account. It is important to mention that these adaptations can only be successful by introducing participatory ergonomics, which increase the workers' motivation.
5. Summary The e!ect of using a mechanical device on physical load was analysed uring the end assembly of cars. Five tasks divided into lifting and positioning were performed by someone experienced in working with and without the mechanical tool. A practical methodology was used: the subjective walk and talk through method, the NIOSH 1991 equation, a three-dimensional goniometer (Back Tracker) and surface electromyography of the relevant muscle groups. According to the NIOSH equation, eight out of ten of the tasks should only be per-
663
formed by trained workers an preferably with tools. The Back Tracker revealed that the mechanical tools reduced posture angles signi"cantly for installing the windows, but not for left latero#exion during pre-assembly of windows. It was clear form the EMG analysis that using tools did not always decrease msucular activity signi"cantly. In general, the appropriateness of each tool needs to be evaluated separately taking the subjective criteria into account.
Acknowledgements We wish to thank the medical department and employees of Volvo Cars Europe Industry, Gent, for their contribution to this study.
References Cha$n, D., Nussbaum, M., Baker, G., Foulke, J., Stump, B., Woolley, C., 1997. Torso modelling of peak exertions required when using materials handling devices. In: SeppaK laK , P., LuopajaK rvi, T., Nyg3ard, C.-H., Matilla, M. (Eds.), From Experience to Innovation IEA'97; Proceedings of the 13th Triennial Congress of the International Ergonomics Association, vol. 3, Finnish Institute of Occupational Health, Helsinki, pp. 502}504. Hagberg, M., 1984. Occupational musculoskeletal stress and disorders of the necka and shoulder: a review of possible pathophysiology. International Archires of Occupational Environmental Health 53, 269}278. JoK rgensen, K., Jensen, B.R., Kato, M., 1991. Fatigue development in the lumbar paravertebral muscles of bricklayers during the working day. International Journal of Industrial Ergonomics 8, 237}245. Kirwan, B., Ainsworth, L.K., 1992. A Guide to Task Analysis. Taylor and Francis, London. Lee, K., Cha$n, D.B., Herrin, G.D., Waikar, A.M., 1991. E!ect of handle height on lower-back loading in cart pushing and pulling. Applied Ergonomics 22, 117}123. LindstroK m, I., Ohlund, C., Nachemson, A., 1994. Validity of patient reporting and predictive value of industrial physical work demands. Spine 19 (8), 888}893. Magnusson, M., Pope, M., Harrington, W., Wilder, D.G., 1994. Accuracy and reliability of a 3-axis goniometer to record trunk motion at the work site. Spine Research Center, University of Iowa. Mathiassen, S.E., Winkel, J., HaK gg, G.M., 1995. Normalization of surface EMG amplitude from the upper trapezius muscle in ergonomic studies } a review. Journal of Electromyography and Kinesiology 5 (4), 197}226.
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Mathisson, B., Ingelgard, A., Johansson, M.I., Nonas, K., 1994. Mechanical aids in manual materials handling } technical context and attitudes. Proceedings of the IEA &94 Conference, vol. 2, pp. 243}245. Nagamachi, M., Matsubara, Y., 1994. The ergonomic implications of a doorless system in an automobile assembly line. Ergonomics 37 (4), 611}622. Punnett, L., Fine, L.J., Keyserling, W.M, Herrin G.D., Cha$n, D.B., 1991. Back disorders and nonneutral trunk postures of automobile assembly workers. Scandirian Journal of Work and Environmental Health 17 (5), 337}346. Resnick, M.L., Cha$n, D.B., 1996. Kinematics, kinetics and psychophysical perceptions in symetric and twisting pushing and pulling tasks. Human Factors 38 (1), 114}129.
Waters, T., Putz-Anderson, V., Garg, A., Fine, L., 1993. Revised NIOSH equation for the design and evaluation of manual lifting tasks. Ergonomics 36, 749}776. Waters, T., 1997. IEA TG proposed procedures for evaluation of manual lifting tasks. In: SeppaK laK , P., LuopajaK rvi, T., Nyga rd, C.-H., Matilla, M. (Eds.), From Experience to Innovation IEA'97; Proceedings of the 13th Triennial Congress of the International Ergonomics Association, vol. 3, Finnish Institute of Occupational Health, Helsinki, pp. 586}588. Zipp, P., 1982. Recommendations for the standardization of lead positions in surface emg. European Journal of Applied Physiology 50, 41}54.
In#uence of material handling devices on the physical load during the end assembly of cars Veerle Hermans*, Mark Hautekiet, Arthur Spaepen, Luc Cobbaut, John De Clerq Laboratory Ergonomics and Occupational Biomechanics, Katholieke Universiteit Leuven, Tervuursevest 101, B-3001 Leuven, Belgium Received 29 August 1997; received in revised form 7 January 1998; accepted 17 July 1998
Abstract The e!ect of using a mechanical device on physical load was analysed during the end assembly of cars. Five tasks, further sub-divided into lifting and positioning, were performed by someone experienced in working with and without the mechanical tool. A practical methodology was used: the subjective walk and talk through method, the NIOSH equation (1991), a three-dimensional goniometer (Back Tracker) and surface electromyography (EMG) of the relevant muscle groups. According to the NIOSH equation, 8 out of 10 of the tasks should only be performed by trained workers and preferably with tools. The Back Tracker revealed that the mechanical tools reduced posture angles signi"cantly (p)0.05) for installing the windows, but not for left latero#exion during pre-assembly of windows. It was clear from the EMG analysis that using tools did not always decrease muscular activity signi"cantly. In general, the appropriateness of each tool needs to be evaluated separately taking the subjective criteria into account. Relevance to industry In this study, the relevance of using a mechanical device to reduce the physical load during car assembly, is evaluated. As the tools are expensive, it is important to the industry whether they are being used and how e$cient they are. 1999 Elsevier Science B.V. All rights reserved. Keywords: Assembly work; Physical load; Back; Upper limbs
1. Introduction The manual handling and lifting of loads by people working in car assembly impose heavy physical demands and biomechanical stresses. These demands may cause musculoskeletal disorders of the upper limbs and lower back. One of
* Corresponding author.
the main reasons for these overuse syndromes is awkward posture. For example, during car assembly, extreme #exions and rotations of the trunk, or working postures with the arms above shoulder level are often held repeatedly for long durations (Punnett et al., 1991; LindstroK m et al., 1994; Nagamachi and Matsubara, 1994). One method to reduce physical load is to use material handling devices. However, these mechanical tools are often di$cult to design and implement and may be
0169-8141/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved PII: S 0 1 6 9 - 8 1 4 1 ( 9 8 ) 0 0 0 7 0 - 5
658
V. Hermans et al. / International Journal of Industrial Ergonomics 24 (1999) 657}664
costly. Mathisson et al. (1994) investigated the individual's perception of tools and the psychosocial context. In their study, workers were well aware of the ergonomic risks of not using the tools, but this knowledge did not a!ect usage. Time and stress factors seemed to be very important, since using a tool often took longer than manual lifting. However, Resnick and Cha$n (1996) mentioned that biomechanical stresses in the back are often not well perceived for infrequent exertions or tasks with height peak stress. From the musculoskeletal point of view, Cha$n et al. (1997) found high peak hand forces during acceleration and deceleration of loads using handling devices in a simulation study. When these forces are combined with awkward torso postures (e.g. bending and stooping), high bending moments can be created in the lumbar region during the starting and stopping phases of the motions. Apart from awkward postures and high peak hand forces, Lee et al. (1991) also mentioned the velocity of movement as an important contributor to back stess in pushing and pulling tasks. This study compared the physical load of several real working tasks with and without the use of manual material handling tools during assembly at Volvo Europe Car Industry. Apart from the analysis of back stress, an attempt is also made to analyse the physical load of the upper limbs. The research was prompted by the preference of several employees to work manually, without the supposedly `ergonomica tools, which apparently had disadvantages. A practical methodology was necessary to perform the study in a real working environment.
2. Methods 2.1. Working tasks Five di!erent work processes were analysed: inserting the windows (task 1), pre-assembly of the windows (task 2), installing the battery (task 3), installing the fuel tank (task 4) and assembly of the "fth door (task 5). Each process was divided into two tasks: lifting the object and lowering or installing the object. In task 2, a sub-task was analysed,
relifting the object after lowering. The weight of the object and the horizontal distances during the lifting of the object for each work process are presented in Table 1. For each job, an experienced worker erformed the task ten times both with and without the specialised tool. All the workers had previously been trained in how to use the specialised tools. 2.2. Material and methods Several methods were adopted to gain a global overview of the physical load on the upper limbs and lower back. These methods are summarised brie#y below. 2.2.1. Walk and talk through The worker describes the manual actions necessary in detail while performing the task, (Kirwan and Ainsworth, 1992), and explains why he does or does not use the tool. Important information is recorded in writing. 2.2.2. NIOSH The NIOSH method (1991) was used to analyse the lifting and lowering aspects of each task, taking into account several distance measurements and the lifting frequency. The NIOSH Lifting Index (LI) was calculated from the Recommended Weight Limit (RWL). The LI represents the tolerance of the task without using tools. 2.2.3. Electromyographic analysis (EMG) The muscular activity arising during a task was analysed by a telemetric EMG device (ME3000P, MEGA, Finland), with a sample frequency of 1000 Hz. For each task, the electrical signals of four speci"c muscle groups were registered. The analysed muscles were the right and left part of upper trapezius, deltoid pars anterior and/or erector spinae. The bipolar surface electrodes (type Nikomed, silver}silver chloride, Denmark), were positioned according to standard procedures (Zipp, 1982). In the signal analysing, the average EMG amplitude values (AEMG) were calculated for the two sub-tasks of each job. Average values and standard deviations of these AEMG values were
V. Hermans et al. / International Journal of Industrial Ergonomics 24 (1999) 657}664
659
Table 1 Weight of the objects (kg) and horizontal distances (m) for the "ve work processes
Weight Distance
Inserting window
Preassembly window
Battery
Fuel tank
Fifth door
14.2 2
14.2 2
14}19.9 1
9.6 2
26.8 4
calculated over the ten performances for each job. Before starting the task, subjects were requested to reach maximal voluntary contractions of the speci"c muscles, so that the maximal voluntary electrical activation could be recorded (MVE). The EMG values during the task were normalised to the MVE value, to analyse the relative muscular e!ort for each work task and to compare the results.
2.2.4. Posture analysis Movements and postures were registered with a camcorder. A three-dimensional goniometer (Back Tracker, Isokinetics, USA) was used to calculate the amplitude and velocity of back movements in the frontal, sagittal and vertical planes (#exion/extension, latero#exion right/left and rotations) for two tasks: inserting the windows and the pre-assembly of the windows. This device measures the movements between the sacrum and a thoracal point on the back, by placing a rigid bar on the back. Several studies conducted in cooperation with the manufacturers have reported the validity and reliability of this method (e.g. Magnusson et al., 1994), but we also conducted a small reliability experiment in our laboratory prior to the car assembly analyses. A comparison was made between the data registered simultaneously from the Back Tracker and a three-dimensional movement registration system (Kinemetrix, Orthodata, Germany). A single subject performed a combined movement, which included #exion, rotation and latero#exion and the results of the three-dimensional registrations and of the Back Tracker were comparable (not published). In this car assembly study, the maximal angles in the three planes were calculated for the two sub-tasks.
3. Results 3.1. Walk and talk through For each work process, disadvantages were reported when the tool had to be used. The "ve most reported disadvantages were: E ¹ime consuming: Manipulation of an object with a tool requires more actions, handles have to be manipulated, the tool has to be guided, and positioned, which causes an increase in cycle time. E Not enough space: Manipulating the object with the tool requires more space, which is often very limited. E Not user friendly: Using the tool and manipulating the object at the same time introduces clumsy movements (`you need three handsa). E More di.cult: Manipulating the tool makes it harder to locate the object accurately. E Poorly positioned: The position of the tool hinders the necessary movements of the user. In the section below, the results regarding the movement analysis and muscular e!orts are presented for each task separately. By combining the data of Back Tracker, camcorder, NIOSH and EMG, it was clear which phase of the task caused awkward postures or high muscular e!orts. 3.2. Movement analysis and muscular ewort The NIOSH lifting indices (LI) in Table 3 describe how tolerable the "ve tasks (and their subtasks) are. In eight out of ten cases, the LI is between 1 and 3. According to NIOSH, these tasks should only be performed by trained workers and preferably with some type of mechanical assistance.
660
V. Hermans et al. / International Journal of Industrial Ergonomics 24 (1999) 657}664
¹ask 1: Inserting the windows. From the subjective impressions (Table 2) it was clear that using the tool is time consuming, and operating the tool and positioning the window simultaneously cannot be achieved smoothly. However, according to the NIOSH results the use of some type of mechanical device should be considered since the LI is 1.9 (Table 3) in both lifting and positioning. Furthermore, there was a signi"cant decrease (p(0.05) in maximal trunk movements using a tool, both in the frontal and sagittal plane, but only relevant regarding the reduction in right latero#exion. The shoulder muscles are signi"cantly less loaded (Fig. 1). The EMG amplitude of the back muscles decreases enormously during the positioning of the window, but increases slightly during the lifting phase. This means that the use of the tool decreases the load in the heaviest part of the task and slightly increases the load in the lightest part.
¹ask 2: Preassembly of the windows. Slower and more di$cult operation are the most important subjective reasons for not using the mechanical device, although according to NIOSH a tool is preferred, mainly during the lifting (LI"1.9) of the windows (Table 3). No di!erences were found in #exion and extension movements. Furthermore, the high latero#exion left is also found during the use of the tool. Only the decrease in right lateroflexion was signi"cant (p(0.05). A signi"cant decrease in muscular e!ort was found for the four muscles during the three work phases, except for the left back muscle during phase 2 and phase 3 (additional lifting phase in this task). ¹ask 3: Installing the battery. Operating the tool, while holding cables and preventing collision between the tool and the open bonnet hinders and slows down the operation. Furthermore, the range of the tool was too small so that manual lifting was
Table 2 Inserting window
Preassembly window
Battery
Fuel tank
Fifth door
(a) Summary of the subjective impressions regarding the use of a mechanical tool for the "ve assembly tasks Time consuming Not enough space Not user friendly More di$cult Bad position
* * *
*
*
*
*
* * * *
*
* *
(b) The recommended weight limit (RWL) and the NIOSH lifting index (LI) for the "ve tasks, divided into the sub-tasks lifting (lift) and installing (instal), are presented, together with the real weights (kg) of the objects Weight RWL
14.2 7.4 7.4 1.9 1.9
LI lift LI instal
14.2 7.2 12.6 1.9 1.1
14}19.9 20.5 11.1 0.5 0.9
9.6 11.3 5.8 1.2 2.4
26.8 13.4 17.7 2 1.5
Table 3 Results from the Back Tracker for maximal #exion (#ex.), extension (ext.) and latero#exion right (lat.r) and left (lat.l), with and without the use of the tool for tasks 1 and 2. Signi"cant di!erences (p)0.05) between using and not using the tools are presented by * Task 1
Without With
#ex. 19.6 14.1*
Task 2 ext. 5.5 0.9*
lat.r 27.6 4*
lat.l 24.9 18.2*
#ex. 8.1 7.5
ext. 3.9 3.6
lat.r 27.8 8.1*
lat.l 26.1 30
V. Hermans et al. / International Journal of Industrial Ergonomics 24 (1999) 657}664
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Fig. 1. Replacement of the "fth car door. Left: without using a mechanical device. Right: using the device.
still necessary. Since the LI was smaller then 1 (Table 3), the task could be performed without a tool. No clear advantages or disadvantages can be summarised from the EMG results (e.g. decrease in muscular e!ort for the right trapezius and deltoid during lifting, but increase in the left side). ¹ask 4: Installing the fuel tank. A wave of criticism was reported, summarised in Table 2. A high LI was found during the installation of the tank (LI"2.4), lower during the lifting (LI"1.2). Furthermore, a decrease in EMG amplitude was found using the tool, except for the right deltoid during the installation (22% MVE with tool, 10% MVE without tool, Fig. 1). ¹ask 5: Assembly of the ,fth door. Again several subjective disadvantages were reported for the mechanical device. However, this task involves lifting a window of 26.8 kg a horizontal distance of 4 m, with an LI of 2.0. From the EMG results it is clear that the use of a tool signi"cantly decreases the muscular load of the back muscles. However, it can be seen that the problem is shifted elsewhere: there is a higher increase of trapezius amplitude, due to the lifting of the arms with the tool. From the video registrations it is also clear that the right lateroflexion during the placement of the "fth door changes to a left latero#exion when the tool is used (Fig. 2).
4. Discussion The assembly of a car often involves awkward body positions. Several literature studies mention
repetitive trunk #exions and rotations (Punnett et al., 1991; LindstroK m et al., 1994; Nagamachi and Matsubara, 1994). The aim of this study was to analyse whether the use of mechanical devices decreases the necessity for awkward postures, thereby reducing the physical load on the worker. In order to study the physical load in the working environment, a combined and practical methodology was necessary. Surface electromyography has frequently been used in the working environment to study muscular load and indications of muscular fatigue (e.g. JoK rgensen et al., 1991). Using maximal voluntary electrical activation (MVE) as a reference allows analysis of muscular e!ort. This reference contraction is performed in a "xed position resulting in static muscular e!ort. This is in contrast with occupational tasks where limbs are moving. These changes in posture and velocity in#uence the EMG amplitude and force relationship (Mathiassen et al., 1995). However, we are interested in the comparison of two situations, i.e. working with and without mechanical devices, without focussing on the percentage of maximal e!ort. The Back Tracker was used to analyse speci"c or awkward postures during performance of a work task and to analyse posture over time. Relevant reductions in awkward postures can be found. However, the weight being carried by the subject is not taken directly into account, since no loads or torques are calculated. Therefore, the revised NIOSH lifting equation was used, to assess the physical demands of two-handed manual lifting tasks (Waters et al., 1993). Several criteria are necessary
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Fig. 2. Relative EMG values (percentages of the maximal voluntary electrical activation (%MVE)) for the "ve tasks (divided into sub-tasks). Both the values without (light grey) and with (dark grey) the tool are presented. Trap"trapezius, Er sp"erector spinae, Del"deltoideus, r"right, l"left.
for this method, which makes it di$cult to apply in real working environments. Recently, a number of modi"cations to the NIOSH equations and procedures have been proposed to improve feasibility of using the method in working situations (Waters, 1997). In this study, the tasks analysed using NIOSH consisted of situations that largely met the criteria. Based on the objective and subjective results of the data analysis, recommendations were made to
the company regarding the use of existing tools for the end assembly of cars. For the installation of the windows, preassembly of the windows and installation of the fuel tank, the tools reduced the load of several muscles, due to improved postures and reduction in the load (the weight of the objects being carried by the tool). Using the tool necessitates altering how the task is carried out, and for this reason practical training sessions are necessary for the workers. For battery installation, advantages
V. Hermans et al. / International Journal of Industrial Ergonomics 24 (1999) 657}664
and disadvantages tend to balance each other out. There is no real reduction in physical load and many subjective disadvantages were reported. The same is true for the tool used during the assembly of the "fth door, where there is a shift in muscular load to other body regions, resulting in a reduction for the back muscles, but not in the upper limbs. This is attributed to the repetitive lifting of the arms above shoulder height. Since it is known that the neck and shoulder region is a prevalent site of work-related pain and disorders (Hagberg, 1984), it is important to evaluate also this region, and not only focussing on back stresses. In the literature it is mentioned that it is necessary to include objective research methods to evaluate the in#uence of material handling devices, since biomechanical stresses in the back are often not well perceived for infrequent exertions or tasks with high peak stresses (Resnick and Cha$n, 1996). This study reveals that a combination of several practical methods can be used to analyse the load of real working tasks. The results of this analysis were reported to the company's medical department and a further action plan was drawn up. A new mechanical device has recently been introduced for window installation, which reduces the number of #exions of the trunk. Furthermore, a new tool for battery installation has been designed, taking the results of this study into account. It is important to mention that these adaptations can only be successful by introducing participatory ergonomics, which increase the workers' motivation.
5. Summary The e!ect of using a mechanical device on physical load was analysed uring the end assembly of cars. Five tasks divided into lifting and positioning were performed by someone experienced in working with and without the mechanical tool. A practical methodology was used: the subjective walk and talk through method, the NIOSH 1991 equation, a three-dimensional goniometer (Back Tracker) and surface electromyography of the relevant muscle groups. According to the NIOSH equation, eight out of ten of the tasks should only be per-
663
formed by trained workers an preferably with tools. The Back Tracker revealed that the mechanical tools reduced posture angles signi"cantly for installing the windows, but not for left latero#exion during pre-assembly of windows. It was clear form the EMG analysis that using tools did not always decrease msucular activity signi"cantly. In general, the appropriateness of each tool needs to be evaluated separately taking the subjective criteria into account.
Acknowledgements We wish to thank the medical department and employees of Volvo Cars Europe Industry, Gent, for their contribution to this study.
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