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Evaluation of lifting tasks frequently performed during fire brick manufacturing processes using NIOSH lifting equations
International Journal of Industrial Ergonomics 25 (2000) 423}433
Evaluation of lifting tasks frequently performed during "re brick manufacturing processes using NIOSH lifting equations Min K. Chung!,*, Dohyung Kee" !Department of Industrial Engineering, Pohang University of Science and Technology, Hyoja San 31, Pohang 790-784, South Korea "Department of Industrial Engineering, Keimyung University, Taegu 704-701, South Korea Received 1 December 1998; accepted 23 June 1999
Abstract A "re brick manufacturing company with a high prevalence of low back injuries was selected for analysis of lifting tasks using the 1991 revised NIOSH lifting equations. We analyzed several manufacturing processes: forming, heating and packing processes involving frequent lifting and lowering in asymmetric postures. A questionnaire survey showed that weight of the load signi"cantly in#uenced the incidence of back injuries and that workers who reported to have experienced back injuries were older than those who did not experience them. Composite Lifting Indices (CLI) based on the 1991 revised NIOSH lifting equations were calculated for 14 tasks of the forming process and "ve tasks of the heating/packing processes. Calculated CLIs for the tasks ranged from 0.86 to 8.8 (average 2.73) in the forming process and from 3.7 to 18.9 (average 11.12) in the heating/packing processes. The majority of the lifting tasks in this company exceeded the recommended weight limit (RWL). The results suggest that the tasks should be redesigned ergonomically to eliminate the risk factors that may cause low back injuries. Most of the jobs under study could be redesigned to lessen the biomechanical stress simply by making horizontal locations closer to a worker or by reducing the asymmetric angles. Relevance to industry During "re brick manufacturing processes, asymmetric lifting activities are frequently performed in Korea. It is crucial and bene"cial to both management and labor to evaluate such tasks ergonomically based on the 1991 revised NOISH lifting equations for identifying risk factors that may cause musculoskeletal disorders. ( 2000 Elsevier Science B.V. All rights reserved. Keywords: Low back injuries; Asymmetric lifting; Revised NIOSH lifting equations; Lifting index; Recommended weight limit; Fire brick manufacturing
1. Introduction Manual materials handling (MMH) tasks have been one of the most important topics in ergono* Corresponding author. Tel.: #82-562-279-2192; fax: #82562-279-2870. E-mail address: [email protected] (M.K. Chung)
mics, biomechanics and related subjects for the last 40 years. Although today the tasks or processes are being mechanized or automated as the technology has advanced, many tasks are still performed manually in several industrial settings. In fact, approximately one third of all industrial jobs in the US involves some form of MMH such as lifting, lowering, holding, carrying, pushing or pulling (Cook
0169-8141/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 8 1 4 1 ( 9 9 ) 0 0 0 4 1 - 4
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and Neumann 1987; NIOSH 1981). It is also estimated that more tasks are manually performed in Korea, where labor-intensive industries are more prevalent and industrial processes are less automated. Manual handling and lifting are major causes of work-related low back pains (LBP) and impairments (Waters et al. 1993b). Therefore, it is important to design MMH tasks ergonomically and safely from an economic and ethical point of view so that industry-related accidents including LBP can be prevented. The National Institute for Occupational Safety and Health (NIOSH) developed an equation in 1981 and revised it in 1991 to assist safety and health practitioners in evaluating lifting demands (NIOSH 1981; Waters et al. 1993b). These lifting guidelines are being widely used by occupational health practitioners because they provide an empirical method for evaluating manual lifting tasks. In this study, we surveyed the potential hazards of manual lifting performed in one of manufacturing industries in Korea, and proposed improved methods to overcome the potential risks found from a job analysis. Based on a statistical survey of compensated low back injury cases reported in the Pohang region, a "re brick manufacturing company with a high prevalence of low back injuries was selected for the study (Kee and Chung 1995). We administered a questionnaire survey to workers performing the jobs selected for this study, and evaluated the lifting indices for the lifting tasks based on the 1991 revised NIOSH lifting equations. The company manufactures "re bricks used in the blast furnace of the iron and steel industry. Eighty-eight occupational injury cases were reported in this company from 1985 to 1993, and 28% (25 cases) of the reported cases were low back injuries. Most of the manufacturing processes are performed by typical MMH activities such as lifting and carrying. The weights of the manufactured "re bricks ranged from 1.2 to 90 kg: 60% of the products are 7}8 kg and 20% are 10}15 kg. MMH tasks are largely employed in the three "re brick production processes: forming, heating and packing. Most of the low back pain complaints were reported by the workers performing these tasks.
2. Questionnaire survey To examine the status of low back injuries in an industrial setting, a questionnaire survey was conducted taking into account the following information: (1) employee's personal information including gender, age, anthropometric data, somatotype, department, type of shift; (2) job-related information including size and weight of objects handled, job description, handling frequency, working hour and posture, trunk twisting; and (3) LBP-related information including LBP experience, injury date, place of origin, pain description, causes of LBP, e!ect of safety training program. The safety personnel of the participating company conducted a questionnaire survey. Questionnaires were distributed to 60 workers engaged in the forming, heating and packing processes, and 43 workers responded (response rate: 71.7%). From these respondents, we analyzed the data from 37 workers excluding 6 workers who did not answer the questionnaire item of whether or not LBP was experienced. The LBP group and the non-LBP group were compared and analyzed according to several questionnaire items. 2.1. LBP classixed by department Twenty-six out of 37 workers responded &yes' to the question of whether or not LBP was experienced, indicating that they had indeed experienced pain in the lower back for at least three consecutive days. Among them, 12 workers belonged to the packing process group, 11 workers were in the forming process group, and the remaining 3 workers in the heating process group. 2.2. LBP classixed by gender The twenty-six workers experiencing LBP consisted of 20 males and 6 females. All of the females who experienced LBP worked only in the packing process, and half of the LBP-experienced workers were shown to be female.
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Comparing the 26 LBP-experienced workers with the remaining 11 non-LBP workers (Table 1), we found that the mean age of the former group was older than the latter by about 4 yr though the di!erence was not statistically signi"cant (p"0.426). Anthropometric data including stature and body weight showed no signi"cant di!erence between the two groups (p"0.248 and p"0.752, respectively). However, the mean weight of the bricks handled by the former was signi"cantly larger than that of the latter by 15 kg or more (p"0.009).
had mild pain initially and felt more painful later, and "ve had general back discomfort. It is postulated that many LBPs might not be reported for compensation claims because, in Korea, LBP was recognized as an occupational injury only when it had occurred at work. The average service length of the LBP-experienced workers was 5.9 yr with a standard error of 2.9 yr, and its maximum value was 11 yr. This indicates that low back injuries in the manufacturing industry are mainly chronic due to repetitive trauma on the lumbosacral disc. This agrees with the comment of the safety sta! in the company that workers engaged in lifting longer than 5 yr are more prone to low back injuries.
2.4. Task type and working posture
2.6. Age and location during LBP experience
We found that occurrence of LBP was not significantly a!ected by the tasks of lifting, carrying, pushing, pulling or lowering, nor by working postures such as standing, seating, kneeling, squatting and stooping. On the other hand, 21 out of 28 workers (78.5%) performing tasks that require trunk twisting were reported to have experienced LBP, while 5 out of 9 workers (55.6%) performing tasks that do not require trunk twisting were reported to have experienced LBP. However, s2-test revealed that the trunk twisting did not show a statistically signi"cant e!ect on the occurrence of LBP (p"0.267).
Twenty one out of the 26 workers (80.7%) experiencing LBP were in an age group of 25}44 years old (Table 2). This concurs with the previous study that the percentage of back injury compensation was highest in the 30s, second in the 20s, and third in the 40s in the Pohang region of Korea (Kee and Chung 1995). 24 out of 26 workers (92%) in the LBP group answered that they experienced LBP at work, and the remaining two stated that they experienced LBP at home.
2.3. Anthropometric data and weight of object handled
2.5. Level of low back pain and length of service When workers "rst experienced LBP, 14 workers (54%) reported to have felt pain suddenly, three
2.7. Training E!ects of safety training, exercise for preventing LBP and use of mechanical aids, etc. did not show any signi"cant di!erence between the LBP-experienced and the inexperienced. Although the safety training program was administered every two
Table 1 Age, anthropometric data, and weight of bricks lifted for the LBP group and the non-LBP group!
LBP Group Non-LBP Group
No. of cases
Age
Stature (cm)
Body weight (kg)
Weight of bricks lifted (kg)
26
38.6 (7.4)
167.8 (6.5)
62.9 (7.2)
28.1 (24.0)
11
34.9 (8.6)
170.2 (5.3)
63.5 (4.2)
13.3 (9.7)
!The value in the parenthesis represents standard deviation.
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Table 2 Age distribution of LBP group Age group
20}24
25}34
35}34
45}54
Over 55
Number in LBP Group
2 (7.7%)
9 (34.6%)
12 (46.1%)
3 (11.5%)
0 (0.0%)
months in the company, the program was not shown to be e!ective for preventing LBP in the industrial sites.
performs several tasks, the multi-task procedure was used when calculating LI. 3.1. Forming process
3. Ergonomic evaluation of 5re brick manufacturing processes In this study, 14 jobs in the forming process and "ve jobs in the heating/packing processes were selected for analysis based on the 1991 revised NIOSH equations. The jobs under study were reported to have high lifting frequency and long duration according to the questionnaire survey. The heating and packing processes were analyzed together since the task variables in the two processes are similar. The only di!erence between the two processes is that objects are lifted in the heating process while they are lowered in the packing process. Since the NIOSH lifting equations were developed based on American workers, it may not be appropriate to directly apply the equations to Korean workers with di!erent anthropometric characteristics and physical capacities. It was reported in previous studies that arm and leg strengths of Korean workers are greater than those of American workers although their torso strength is much less (Chung et al. 1992). The maximum acceptable weight of loads (MAWL) obtained by the young healthy male Korean students were reported to be about the same as American workers (Lee and Park 1995). Such "ndings suggest that the young, healthy, male Korean population can be well protected by the NIOSH lifting equation. Because of the potential inaccuracies that can occur when task variables are averaged for multitask assessments, NIOSH provided two procedures for calculating LI: a single-task and a multi-task procedure. Since a worker in the selected company
The shape of "re bricks is formed during the forming process. In this process, a worker manually "lls up a press mold with raw materials reduced to powder from ore or stone and actuates the press. Then he moves the formed "re bricks to the worktable, and moves them again onto the pallet in three tiers (Fig. 1). Twisting and bending are necessary when picking up and putting down the "re bricks on the pallet. Since the worker is free to take one or two steps in the workplace to get closer to the object on the pallet, only one layer in depth from the front of the pallet is analyzed. Because the job consists of more than one distinct task and the task variables often change, the multi-task lifting analysis procedure with three tasks should be used. Each job in the process is divided into three tasks representing the three tiers of bricks. Since bricks are continuously stacked at the rate of 3/min, it was assumed in the multitask lifting analysis that one brick was stacked per tier, thereby each task frequency becoming 1 lift/min/tier. The information needed in the multi-task procedure is presented in Table 6 (appendix), where the weight of bricks lifted, lifting frequency and duration vary according to jobs being performed, and the remaining variables are constant. The procedure for calculating the composite lifting index (CLI) of Job 1 was summarized in Table 7, where CLI was shown to be 1.85, indicating the job to be stressful for a majority of the workers. Table 7 shows that the frequency-independent lifting index (FILI) value of Task 3 exceeds 1.0, indicating that this task is stressful on a biomechanical
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427
Fig. 1. Workplace layout of forming process.
Table 3 Task variables and CLI of the remaining 13 jobs in the forming process Job No.
Weight of bricks (kg)
Lifting frequency (lifts/min)
1 2 3 4 5 6 7 8 9 10 11 12 13 14
6.0 10.0 12.0 9.0 10.0 12.0 25.0 40.0 7.1 8.6 9.0 3.8 4.0 6.0
1.0 0.17 0.17 0.17 0.2 0.3 0.14 0.04 0.14 0.14 0.06 0.2 0.7 0.3
basis, and that all tasks are stressful if they are performed individually, i.e., all single task lifting index (STLI) values are above 1.0. Table 3 exhibits task variables and CLIs of the 14 jobs in the forming process, which ranged from 0.86 to 8.8 with an average of 3.02. Therefore, most of the jobs except Job 12 (CLI"0.86) were found to be stressful, which could lead to low back pain and other musculoskeletal injuries. 3.2. Heating and packing processes During the heating process, the formed "re bricks were treated under high temperature to
Lifting duration (h)
8h
CLI
1.85 2.3 2.8 2.1 2.3 3.0 5.7 8.8 1.7 2.0 2.0 0.86 1.2 1.5
enhance their strength. The bricks moved onto a palletized cart were lowered and stacked again on another pallet, which is used in the heating process. The bricks were laid up to 120 cm when stacked in 3 tiers, and then three types of footstep with varying heights were used to minimize trunk #exion when stacking the bricks (Fig. 2). During the packing process, the bricks on the pallet coming from the heating process on a rail were lowered on another pallet in three tiers (120 cm high) for shipment (Fig. 3). Because no footstep was used in this process, vertical locations increased as the bricks were stacked.
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the predictive L5/S1 compression exceeds 4.5 kN. Karwowski et al. (1994) found that the compressive forces on the L5/S1 and the LI are highly and positively correlated at origin with correlation coef"cient of 0.85 and at destination with that of 0.84. They also suggested two sets of regression models describing the relationships between the LI and the compressive forces on the L5/S1 both at the origin and the destination of lift: LI"0.00138386*CF!1.418 at origin LI"0.00170275*CF!0.87125 at destination Fig. 2. Workplace layout of heating process.
The information needed in the multi-task procedure for Job 1 in the heating/packing process is displayed in Table 8. Job 1 is the task where a worker stacks and unloads a brick a minute for 8 h in the heating and packing processes, respectively. As shown in Table 9 CLI of Job 1 is 3.91, indicating that the job is stressful and should be redesigned. Task variables and CLIs of the "ve jobs in the heating/packing processes are exhibited in Table 4, showing that CLIs of all jobs far exceed 1.0. 3.3. LBP incidence and CLI Cha$n and Park (1973) reported that the LBP incidence in repetitive lifting is less than 5% when the predicted compressive force on the L5/S1 is below 2.5 kN, but it increases to over 10% when
where CF represents the compressive force on the L5/S1. In the above equations, LI corresponds to approximately 4.8 when the compressive force on the L5/S1 is 4.5 kN, and thus it can be stated that jobs with LI exceeding 4.8 are unsafe with the LBP incidence rate over 10%, and these jobs need ergonomic intervention. This is also substantiated by the following analysis. Here, we are to analyze the relationship between the composite lifting index (CLI) and the LBP incidence in the manufacturing company under study. CLIs of the LBP-experienced group were shown to be statistically larger than those of the inexperienced (Table 5). Correlation coe$cient between CLI and the status of LBP-experience was found to be 0.41, showing a marginally signi"cant association between the two (p"0.0767). Furthermore, all the workers engaged in jobs with CLI exceeding 5.0 were shown to have experienced LBP. Therefore, it was suggested that those jobs
Fig. 3. Workplace layout of packing process.
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Table 4 Task variables and CLI of the remaining 4 jobs in the heating/packing processes Job No.
Weight of bricks (kg)
Lifting frequenc (lifts/min)
Lifting duration (h)
CLI
1 2 3 4 5
14 52.5 60 60 10
3.3 0.09 0.09 3.3
8 1 8 1 1
3.91 18.91 14.7 13.4 3.7
Table 5 Comparison of CLI between the LBP-experienced and inexperienced workers
Non-LBP Group LBP Group
N
Mean
SD
t-value
p value
8 12
2.21 5.84
0.76 5.39
2.30
0.042
should be redesigned through the administrative or engineering control. This supports Cha$n and Park's suggestion that the LBP incidence increases to over 10% when the compressive forces on the L5/S1 exceeds 4.5 kN (that is, LI becomes approximately 4.8 in our study).
4. Discussion From the NIOSH perspective, the lifting tasks with CLI'1.0 indicate an increased risk for lifting-related low back pain for some fraction of the workforce (Waters et al. 1993b). Therefore, such tasks should be redesigned to achieve a CLI of 1.0 or less. We will discuss a few inexpensive alternatives of redesigning the lifting jobs with CLI'1.0. In Table 7 presenting the procedure of calculating CLI of the forming process, it is seen that the smallest multiplier is 0.43 for the HM, the second 0.63 for the AM, and the third 0.75 for the FM. First of all, the bricks should be positioned closer to the worker in order to redesign Job 1, that is, the horizontal location be reduced from 60 to 30 cm. Secondly, the origin and destination of the lifting
task should be moved closer together to reduce the angle of twist. Pallets located behind the worker should be moved and positioned at the right-hand side of the worker, thereby reducing the asymmetry angle from 100 to 03 for origin and from 130 to 903 for destination. In addition, the employees are given a daily production order for an 8 h work shift, and they normally "nish the job within 3}4 h to have a long break for the rest of the shift. Such work practices result in high lifting frequencies. Therefore, the job should be evenly distributed within 2 h or less work cycles followed by a su$cient recovery period, to keep the lifting frequency less than 0.5 lifts/min. If the workplace is redesigned as recommended above, CLI of Job 1 would change from 2.0 to 0.66, which is safe for most workers. Although Job 12 of the forming process also requires a long horizontal distance and a large asymmetry angle as in Job 1, its CLI was found to be low since the weight of a object lifted is light and the frequency of lifting is low. For the remaining 12 jobs in the forming process, it is recommended that as discussed in Job 1 analysis, horizontal locations of hands should be within 30 cm and that the asymmetry angle should be reduced to 903. Furthermore, the job should be evenly distributed within 2 h or less work cycles followed by a su$cient recovery period. If these recommendations are accepted, all the task variables become "xed except frequency and object weight, which remained eligible for change. Since weight of bricks lifted is not allowed to make a change, the only task variable that remained eligible for change is lifting frequency. Then, we are to "nd out alternative work strategy by investigating a trend of CLIs depending
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Fig. 4. Predicted CLIs of improved lifting tasks.
on the lifting frequency for varying weights of bricks lifted. Illustrated in Fig. 4 are the predicted CLIs for the improved lifting tasks as recommended, depending on the lifting frequency for varying weights of bricks, ranging from 2.5 to 12.5 kg with increment of 2.5 kg. As shown in Fig. 4, objects weighing 2.5 kg should be lifted 3 times or less per minute for CLI to be less than 1.0. If these objects were lifted 4 times or more a minute, CLI becomes in"nite (R) because the frequency multiplier based on the sum of the frequencies for all three tasks is zero. If bricks weighing 7.5 kg were lifted 2 times or less a minute and 10.0 kg bricks were lifted every 2 min or more, these jobs were evaluated to be safe. This implies that Jobs 2, 4, 9, 10, 11, 13 and 14 turn out to be safe in terms of CLI. It was also found from Fig. 4 that the lifting frequency should be kept the lowest (say, every 5 min) for Job 6 with bricks weighing 12.5 kg. Since weight of bricks in Jobs 7 and 8 (25, 40 kg, respectively) exceeds the load constant (23.1 kg) in the revised NIOSH equation, it is impossible to reduce CLI below 1.0. Therefore, these bricks should be instructed to be lifted by two or more workers, or to use mechanical aids. Otherwise, these two jobs should be automated. Table 9 of appendix shows that the smallest multipliers are 0.40}69 for the HM, 0.81 for the AM, and 0.83 for the FM in the heating/packing
processes. As in the forming process, the horizontal location should be reduced to within 25 cm, and work hours for lifting tasks should be evenly distributed within 2 h or less work cycles. If Job 1 is redesigned as recommended, its CLI would be reduced to 1.16, which is moderately safe for most workers. Since weight of bricks handled in Jobs 2, 3 and 4 exceeds the load constant (23.1 kg) used in the revised NIOSH equation, mechanization or automation of the process should be introduced. Taking into account the daily production orders for eight working hours in all the processes, lifting frequency is found to be moderate without showing any signi"cant problem. However, workers tend to "nish their daily assigned production earlier than scheduled so as to take rest during the remaining work hours. Therefore, by educating and training the workers to distribute the work load evenly over 8 h, we can improve the frequency multiplier in the CLI computation. Although the 1991 revised NIOSH guide is admittedly more restrictive of acceptable jobs than the NIOSH guide of 1981 (Karwowski and Brokaw 1992), lifting tasks in these processes turned out to contain risk factors that may cause back injuries. It is also cited as a common problem in all three processes that the bricks handled are located at quite a distance from the center of the body.
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1 1 1
Frequency (lifts/min)
1303 1303 1303 1003 1003 1003 17 7 8 84 74 59 0 75 60 6 6 6 1 2 3
V
67 67 67
A A D H H
V
Destination Origin
60 60 60
Destination Orgin
Fair Fair Fair
Coupling Asymmetric angle Hand location (cm) Object weight (kg) Task no.
Table 6 Task variables of job 1 in the forming process
Vertical distance (cm)
5. Conclusion The questionnaire survey showed that weight of the load signi"cantly in#uenced the incidence of back injuries and that workers who reported to have experienced back injuries were older than those who did not experience them. Judging from the fact that service length of the LBP-experienced appeared to be an average of 5.9 yr, low back injuries occurring in the manufacturing industry can be considered to be a chronic disorder due to cumulative trauma on account of awkward working posture, or heavy weight of load, etc. The current safety-training program was not shown to be e!ective for preventing LBP. Therefore, a study on an e!ective training program including development of safety education materials, education method, etc. is also needed. As seen in the above section, composite lifting indices ranged from 0.86 to 8.8 with an average of 2.73 in the forming process, and from 3.7 to 18.9 with an average of 11.12 in the heating/packing processes. It is found that all the jobs except Job 12 of the forming process are stressful for most workers. Since the objects handled in the heating/packing processes are much heavier and the frequency of lifting/lowering is higher than that in the forming process, the workers in the heating/packing processes are engaged in more
8 8 8
Duration (hr)
Furthermore, severe twisting is required to stack bricks on the pallet behind the worker in the forming process, and the weights of the bricks are excessively heavy for some tasks when compared with the load constant in the revised NIOSH equations. Most of the jobs under study could be redesigned to lessen the biomechanical stress simply by making horizontal locations closer to the worker or by reducing the asymmetric angles. Some jobs in the heating/packing processes, however, should be redesigned to use mechanical aids or to partly include automation of the process. Speci"cally, for the jobs with CLIs exceeding 5.0 (i.e., the compressive force on the L5/S1 is about 4.6 kN), the manager of the company should concentrate his/her e!ort on establishing countermeasures to eliminate the risk factors that may lead to low back injuries.
431
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Table 7 Procedure calculating CLI of job 1 in the forming process Task no.
LC
HM
VM
DM
AM
CM
FIRWL
FM
STRWL
FILI
STLI
New Task No
F
1 2 3
51 51 51
0.43 0.43 0.43
0.96 0.96 0.96
1.0 1.0 0.9
0.63 0.63 0.63
0.95 0.95 0.95
13.6 13.6 12.8
0.75 0.75 0.75
10.2 10.2 9.6
0.97 0.97 1.04
1.30 1.30 1.30
2 3 1
1 1 1
CLI"
STLI # *FILI # *FILI 1 2 3 FILI (1/FM }1/FM ) 2 1,2 1 0.97 (1/.65}1/.75) 1.38 0.20
CLI"
FILI (1/FM }1/FM ) 3 1,2,3 1,2 0.97 (1/.55}1/.65) 0.27
1.85
Table 8 Task variables of job 1 in the heating/packing processess Task no.
Object weight (kg)
Hand location (cm) Origin
1 2 3
14 14 14
Vertical distance (cm) Destination
H
V
H
V
D
66 51 36
175 135 95
39 39 39
27 67 107
46 74 18
Coupling
Frequency (lifts/min)
Duration (hr)
Fair Fair Fair
0.33 0.33 0.33
8 8 8
Asymmetric angle Orgin
Destination
A
A
603 603 603
703 703 703
Table 9 Procedure for calculating CLI of job 1 in the heating/packing process Task no.
LC
HM
VM
DM
AM
CM
FIRWL
FM
STRWL
FILI
STLI
New Task No
F
1 2 3
51 51 51
0.40 0.48 0.69
0.71 0.85 0.96
0.85 0.88 1.0
0.81 0.81 0.81
1.0 1.0 0.95
10.0 14.8 26.0
0.83 0.83 0.83
8.3 12.3 21.6
3.1 2.1 1.2
3.7 2.5 1.4
1 2 3
0.33 0.33 0.33
CLI"
STLI # *FILI # *FILI 1 2 3 FILI (1/FM }1/FM ) 2 1,2 1 2.1 (1/.79}1/.83) 3.7 0.13
CLI"
strenuous jobs. This is also supported by the "nding that more low back injury cases were reported in the packing process than in the forming process in this company. Furthermore, since female workers are also assigned to the packing process, all possible e!orts should be devoted to redesigning jobs in the packing process. In the forming process, severe twisting is unnecessarily required when stacking bricks on the pallet behind the worker,
FILI (1/FM }1/FM ) 3 1,2,3 1,2 1.2 (1/.75}1/.79) 0.08
3.91
which can be eliminated through simple rearrangement of the workplace.
Acknowledgements The research was partially funded by the Korea Science and Engineering Foundation (95-0200-0801-3). The authors gratefully acknowledge the
M.K. Chung, D. Kee / International Journal of Industrial Ergonomics 25 (2000) 423}433
assistance of Young W. Song and Jongho Lim of POSTECH in the collection and analysis of the data presented in this article.
Appendix A Task variables and procedure calculating CLI of Job 1 in forming process and heating/packing processes is shown in Tables 6}9.
References Cha$n, D.B., Park, K.S., 1973. A longitudinal study of low-back pain as associated with occupational weight lifting factors. American Industrial Hygiene Association Journal 34 (12), 513}525. Chung, M.K., Kee, D., Kim, T.B., 1992. Maximum Voluntary Strength Evaluation for Korean Workers. Journal of the Korean Institute of Industrial Engineers 18 (1), 141}153. Cook, T.M., Neumann, D.A., 1987. The e!ects of load placement on the EMG activity of the low back muscle during load carrying by men and women. Ergonomics 30, 1413}1423.
433
Karwowski, W., Brokaw, N., 1992. Implications of the proposed revisions in a draft Revised NIOSH lifting guide (1991) for job redesign: a "eld study. In: Proceedings of the Human Factors and Ergonomics Society 36th Annual Meeting, pp. 659}663. Karwowski, W., Caldwell, M., Gaddie, P., 1994. Relationships between the NIOSH (1991) lifting index, compressive and shear forces on the lumbosacral joint, and low back injury incidence rate based on industrial "eld study. In: Proceedings of the Human Factors and Ergonomics Society 38th Annual Meeting, pp. 654}657. Kee, D., Chung, M.K., 1995. Analysis of symptoms and causes for low back pain reported in the Pohang region. IE Interfaces 8 (4), 145}154. Lee, K.S., Park, H.S., 1995. Assessment of validity of applying the revised NIOSH weight limit of lifts to Korean young male population: psychophysical approach. In: Proceedings of the Human Factors and Ergonomics Society 39th Annual Meeting, Vol. 1, pp. 714}717. NIOSH, 1981. Work Practice Guide for Manual Lifting, DHHS (NIOSH) Publication, Cincinnati. Waters, T., Puts-Anderson, V., Garg, A., 1993a. Applications manual for the revised NIOSH lifting equation. US Department of Health and Human Services, Cincinnati. Waters, T., Puts-Anderson, V., Garg, A., Fine, L., 1993b. Revised NIOSH equation for design and evaluation of manual lifting tasks. Ergonomics 36 (7), 446}749.
Evaluation of lifting tasks frequently performed during "re brick manufacturing processes using NIOSH lifting equations Min K. Chung!,*, Dohyung Kee" !Department of Industrial Engineering, Pohang University of Science and Technology, Hyoja San 31, Pohang 790-784, South Korea "Department of Industrial Engineering, Keimyung University, Taegu 704-701, South Korea Received 1 December 1998; accepted 23 June 1999
Abstract A "re brick manufacturing company with a high prevalence of low back injuries was selected for analysis of lifting tasks using the 1991 revised NIOSH lifting equations. We analyzed several manufacturing processes: forming, heating and packing processes involving frequent lifting and lowering in asymmetric postures. A questionnaire survey showed that weight of the load signi"cantly in#uenced the incidence of back injuries and that workers who reported to have experienced back injuries were older than those who did not experience them. Composite Lifting Indices (CLI) based on the 1991 revised NIOSH lifting equations were calculated for 14 tasks of the forming process and "ve tasks of the heating/packing processes. Calculated CLIs for the tasks ranged from 0.86 to 8.8 (average 2.73) in the forming process and from 3.7 to 18.9 (average 11.12) in the heating/packing processes. The majority of the lifting tasks in this company exceeded the recommended weight limit (RWL). The results suggest that the tasks should be redesigned ergonomically to eliminate the risk factors that may cause low back injuries. Most of the jobs under study could be redesigned to lessen the biomechanical stress simply by making horizontal locations closer to a worker or by reducing the asymmetric angles. Relevance to industry During "re brick manufacturing processes, asymmetric lifting activities are frequently performed in Korea. It is crucial and bene"cial to both management and labor to evaluate such tasks ergonomically based on the 1991 revised NOISH lifting equations for identifying risk factors that may cause musculoskeletal disorders. ( 2000 Elsevier Science B.V. All rights reserved. Keywords: Low back injuries; Asymmetric lifting; Revised NIOSH lifting equations; Lifting index; Recommended weight limit; Fire brick manufacturing
1. Introduction Manual materials handling (MMH) tasks have been one of the most important topics in ergono* Corresponding author. Tel.: #82-562-279-2192; fax: #82562-279-2870. E-mail address: [email protected] (M.K. Chung)
mics, biomechanics and related subjects for the last 40 years. Although today the tasks or processes are being mechanized or automated as the technology has advanced, many tasks are still performed manually in several industrial settings. In fact, approximately one third of all industrial jobs in the US involves some form of MMH such as lifting, lowering, holding, carrying, pushing or pulling (Cook
0169-8141/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 8 1 4 1 ( 9 9 ) 0 0 0 4 1 - 4
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and Neumann 1987; NIOSH 1981). It is also estimated that more tasks are manually performed in Korea, where labor-intensive industries are more prevalent and industrial processes are less automated. Manual handling and lifting are major causes of work-related low back pains (LBP) and impairments (Waters et al. 1993b). Therefore, it is important to design MMH tasks ergonomically and safely from an economic and ethical point of view so that industry-related accidents including LBP can be prevented. The National Institute for Occupational Safety and Health (NIOSH) developed an equation in 1981 and revised it in 1991 to assist safety and health practitioners in evaluating lifting demands (NIOSH 1981; Waters et al. 1993b). These lifting guidelines are being widely used by occupational health practitioners because they provide an empirical method for evaluating manual lifting tasks. In this study, we surveyed the potential hazards of manual lifting performed in one of manufacturing industries in Korea, and proposed improved methods to overcome the potential risks found from a job analysis. Based on a statistical survey of compensated low back injury cases reported in the Pohang region, a "re brick manufacturing company with a high prevalence of low back injuries was selected for the study (Kee and Chung 1995). We administered a questionnaire survey to workers performing the jobs selected for this study, and evaluated the lifting indices for the lifting tasks based on the 1991 revised NIOSH lifting equations. The company manufactures "re bricks used in the blast furnace of the iron and steel industry. Eighty-eight occupational injury cases were reported in this company from 1985 to 1993, and 28% (25 cases) of the reported cases were low back injuries. Most of the manufacturing processes are performed by typical MMH activities such as lifting and carrying. The weights of the manufactured "re bricks ranged from 1.2 to 90 kg: 60% of the products are 7}8 kg and 20% are 10}15 kg. MMH tasks are largely employed in the three "re brick production processes: forming, heating and packing. Most of the low back pain complaints were reported by the workers performing these tasks.
2. Questionnaire survey To examine the status of low back injuries in an industrial setting, a questionnaire survey was conducted taking into account the following information: (1) employee's personal information including gender, age, anthropometric data, somatotype, department, type of shift; (2) job-related information including size and weight of objects handled, job description, handling frequency, working hour and posture, trunk twisting; and (3) LBP-related information including LBP experience, injury date, place of origin, pain description, causes of LBP, e!ect of safety training program. The safety personnel of the participating company conducted a questionnaire survey. Questionnaires were distributed to 60 workers engaged in the forming, heating and packing processes, and 43 workers responded (response rate: 71.7%). From these respondents, we analyzed the data from 37 workers excluding 6 workers who did not answer the questionnaire item of whether or not LBP was experienced. The LBP group and the non-LBP group were compared and analyzed according to several questionnaire items. 2.1. LBP classixed by department Twenty-six out of 37 workers responded &yes' to the question of whether or not LBP was experienced, indicating that they had indeed experienced pain in the lower back for at least three consecutive days. Among them, 12 workers belonged to the packing process group, 11 workers were in the forming process group, and the remaining 3 workers in the heating process group. 2.2. LBP classixed by gender The twenty-six workers experiencing LBP consisted of 20 males and 6 females. All of the females who experienced LBP worked only in the packing process, and half of the LBP-experienced workers were shown to be female.
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425
Comparing the 26 LBP-experienced workers with the remaining 11 non-LBP workers (Table 1), we found that the mean age of the former group was older than the latter by about 4 yr though the di!erence was not statistically signi"cant (p"0.426). Anthropometric data including stature and body weight showed no signi"cant di!erence between the two groups (p"0.248 and p"0.752, respectively). However, the mean weight of the bricks handled by the former was signi"cantly larger than that of the latter by 15 kg or more (p"0.009).
had mild pain initially and felt more painful later, and "ve had general back discomfort. It is postulated that many LBPs might not be reported for compensation claims because, in Korea, LBP was recognized as an occupational injury only when it had occurred at work. The average service length of the LBP-experienced workers was 5.9 yr with a standard error of 2.9 yr, and its maximum value was 11 yr. This indicates that low back injuries in the manufacturing industry are mainly chronic due to repetitive trauma on the lumbosacral disc. This agrees with the comment of the safety sta! in the company that workers engaged in lifting longer than 5 yr are more prone to low back injuries.
2.4. Task type and working posture
2.6. Age and location during LBP experience
We found that occurrence of LBP was not significantly a!ected by the tasks of lifting, carrying, pushing, pulling or lowering, nor by working postures such as standing, seating, kneeling, squatting and stooping. On the other hand, 21 out of 28 workers (78.5%) performing tasks that require trunk twisting were reported to have experienced LBP, while 5 out of 9 workers (55.6%) performing tasks that do not require trunk twisting were reported to have experienced LBP. However, s2-test revealed that the trunk twisting did not show a statistically signi"cant e!ect on the occurrence of LBP (p"0.267).
Twenty one out of the 26 workers (80.7%) experiencing LBP were in an age group of 25}44 years old (Table 2). This concurs with the previous study that the percentage of back injury compensation was highest in the 30s, second in the 20s, and third in the 40s in the Pohang region of Korea (Kee and Chung 1995). 24 out of 26 workers (92%) in the LBP group answered that they experienced LBP at work, and the remaining two stated that they experienced LBP at home.
2.3. Anthropometric data and weight of object handled
2.5. Level of low back pain and length of service When workers "rst experienced LBP, 14 workers (54%) reported to have felt pain suddenly, three
2.7. Training E!ects of safety training, exercise for preventing LBP and use of mechanical aids, etc. did not show any signi"cant di!erence between the LBP-experienced and the inexperienced. Although the safety training program was administered every two
Table 1 Age, anthropometric data, and weight of bricks lifted for the LBP group and the non-LBP group!
LBP Group Non-LBP Group
No. of cases
Age
Stature (cm)
Body weight (kg)
Weight of bricks lifted (kg)
26
38.6 (7.4)
167.8 (6.5)
62.9 (7.2)
28.1 (24.0)
11
34.9 (8.6)
170.2 (5.3)
63.5 (4.2)
13.3 (9.7)
!The value in the parenthesis represents standard deviation.
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Table 2 Age distribution of LBP group Age group
20}24
25}34
35}34
45}54
Over 55
Number in LBP Group
2 (7.7%)
9 (34.6%)
12 (46.1%)
3 (11.5%)
0 (0.0%)
months in the company, the program was not shown to be e!ective for preventing LBP in the industrial sites.
performs several tasks, the multi-task procedure was used when calculating LI. 3.1. Forming process
3. Ergonomic evaluation of 5re brick manufacturing processes In this study, 14 jobs in the forming process and "ve jobs in the heating/packing processes were selected for analysis based on the 1991 revised NIOSH equations. The jobs under study were reported to have high lifting frequency and long duration according to the questionnaire survey. The heating and packing processes were analyzed together since the task variables in the two processes are similar. The only di!erence between the two processes is that objects are lifted in the heating process while they are lowered in the packing process. Since the NIOSH lifting equations were developed based on American workers, it may not be appropriate to directly apply the equations to Korean workers with di!erent anthropometric characteristics and physical capacities. It was reported in previous studies that arm and leg strengths of Korean workers are greater than those of American workers although their torso strength is much less (Chung et al. 1992). The maximum acceptable weight of loads (MAWL) obtained by the young healthy male Korean students were reported to be about the same as American workers (Lee and Park 1995). Such "ndings suggest that the young, healthy, male Korean population can be well protected by the NIOSH lifting equation. Because of the potential inaccuracies that can occur when task variables are averaged for multitask assessments, NIOSH provided two procedures for calculating LI: a single-task and a multi-task procedure. Since a worker in the selected company
The shape of "re bricks is formed during the forming process. In this process, a worker manually "lls up a press mold with raw materials reduced to powder from ore or stone and actuates the press. Then he moves the formed "re bricks to the worktable, and moves them again onto the pallet in three tiers (Fig. 1). Twisting and bending are necessary when picking up and putting down the "re bricks on the pallet. Since the worker is free to take one or two steps in the workplace to get closer to the object on the pallet, only one layer in depth from the front of the pallet is analyzed. Because the job consists of more than one distinct task and the task variables often change, the multi-task lifting analysis procedure with three tasks should be used. Each job in the process is divided into three tasks representing the three tiers of bricks. Since bricks are continuously stacked at the rate of 3/min, it was assumed in the multitask lifting analysis that one brick was stacked per tier, thereby each task frequency becoming 1 lift/min/tier. The information needed in the multi-task procedure is presented in Table 6 (appendix), where the weight of bricks lifted, lifting frequency and duration vary according to jobs being performed, and the remaining variables are constant. The procedure for calculating the composite lifting index (CLI) of Job 1 was summarized in Table 7, where CLI was shown to be 1.85, indicating the job to be stressful for a majority of the workers. Table 7 shows that the frequency-independent lifting index (FILI) value of Task 3 exceeds 1.0, indicating that this task is stressful on a biomechanical
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427
Fig. 1. Workplace layout of forming process.
Table 3 Task variables and CLI of the remaining 13 jobs in the forming process Job No.
Weight of bricks (kg)
Lifting frequency (lifts/min)
1 2 3 4 5 6 7 8 9 10 11 12 13 14
6.0 10.0 12.0 9.0 10.0 12.0 25.0 40.0 7.1 8.6 9.0 3.8 4.0 6.0
1.0 0.17 0.17 0.17 0.2 0.3 0.14 0.04 0.14 0.14 0.06 0.2 0.7 0.3
basis, and that all tasks are stressful if they are performed individually, i.e., all single task lifting index (STLI) values are above 1.0. Table 3 exhibits task variables and CLIs of the 14 jobs in the forming process, which ranged from 0.86 to 8.8 with an average of 3.02. Therefore, most of the jobs except Job 12 (CLI"0.86) were found to be stressful, which could lead to low back pain and other musculoskeletal injuries. 3.2. Heating and packing processes During the heating process, the formed "re bricks were treated under high temperature to
Lifting duration (h)
8h
CLI
1.85 2.3 2.8 2.1 2.3 3.0 5.7 8.8 1.7 2.0 2.0 0.86 1.2 1.5
enhance their strength. The bricks moved onto a palletized cart were lowered and stacked again on another pallet, which is used in the heating process. The bricks were laid up to 120 cm when stacked in 3 tiers, and then three types of footstep with varying heights were used to minimize trunk #exion when stacking the bricks (Fig. 2). During the packing process, the bricks on the pallet coming from the heating process on a rail were lowered on another pallet in three tiers (120 cm high) for shipment (Fig. 3). Because no footstep was used in this process, vertical locations increased as the bricks were stacked.
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the predictive L5/S1 compression exceeds 4.5 kN. Karwowski et al. (1994) found that the compressive forces on the L5/S1 and the LI are highly and positively correlated at origin with correlation coef"cient of 0.85 and at destination with that of 0.84. They also suggested two sets of regression models describing the relationships between the LI and the compressive forces on the L5/S1 both at the origin and the destination of lift: LI"0.00138386*CF!1.418 at origin LI"0.00170275*CF!0.87125 at destination Fig. 2. Workplace layout of heating process.
The information needed in the multi-task procedure for Job 1 in the heating/packing process is displayed in Table 8. Job 1 is the task where a worker stacks and unloads a brick a minute for 8 h in the heating and packing processes, respectively. As shown in Table 9 CLI of Job 1 is 3.91, indicating that the job is stressful and should be redesigned. Task variables and CLIs of the "ve jobs in the heating/packing processes are exhibited in Table 4, showing that CLIs of all jobs far exceed 1.0. 3.3. LBP incidence and CLI Cha$n and Park (1973) reported that the LBP incidence in repetitive lifting is less than 5% when the predicted compressive force on the L5/S1 is below 2.5 kN, but it increases to over 10% when
where CF represents the compressive force on the L5/S1. In the above equations, LI corresponds to approximately 4.8 when the compressive force on the L5/S1 is 4.5 kN, and thus it can be stated that jobs with LI exceeding 4.8 are unsafe with the LBP incidence rate over 10%, and these jobs need ergonomic intervention. This is also substantiated by the following analysis. Here, we are to analyze the relationship between the composite lifting index (CLI) and the LBP incidence in the manufacturing company under study. CLIs of the LBP-experienced group were shown to be statistically larger than those of the inexperienced (Table 5). Correlation coe$cient between CLI and the status of LBP-experience was found to be 0.41, showing a marginally signi"cant association between the two (p"0.0767). Furthermore, all the workers engaged in jobs with CLI exceeding 5.0 were shown to have experienced LBP. Therefore, it was suggested that those jobs
Fig. 3. Workplace layout of packing process.
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429
Table 4 Task variables and CLI of the remaining 4 jobs in the heating/packing processes Job No.
Weight of bricks (kg)
Lifting frequenc (lifts/min)
Lifting duration (h)
CLI
1 2 3 4 5
14 52.5 60 60 10
3.3 0.09 0.09 3.3
8 1 8 1 1
3.91 18.91 14.7 13.4 3.7
Table 5 Comparison of CLI between the LBP-experienced and inexperienced workers
Non-LBP Group LBP Group
N
Mean
SD
t-value
p value
8 12
2.21 5.84
0.76 5.39
2.30
0.042
should be redesigned through the administrative or engineering control. This supports Cha$n and Park's suggestion that the LBP incidence increases to over 10% when the compressive forces on the L5/S1 exceeds 4.5 kN (that is, LI becomes approximately 4.8 in our study).
4. Discussion From the NIOSH perspective, the lifting tasks with CLI'1.0 indicate an increased risk for lifting-related low back pain for some fraction of the workforce (Waters et al. 1993b). Therefore, such tasks should be redesigned to achieve a CLI of 1.0 or less. We will discuss a few inexpensive alternatives of redesigning the lifting jobs with CLI'1.0. In Table 7 presenting the procedure of calculating CLI of the forming process, it is seen that the smallest multiplier is 0.43 for the HM, the second 0.63 for the AM, and the third 0.75 for the FM. First of all, the bricks should be positioned closer to the worker in order to redesign Job 1, that is, the horizontal location be reduced from 60 to 30 cm. Secondly, the origin and destination of the lifting
task should be moved closer together to reduce the angle of twist. Pallets located behind the worker should be moved and positioned at the right-hand side of the worker, thereby reducing the asymmetry angle from 100 to 03 for origin and from 130 to 903 for destination. In addition, the employees are given a daily production order for an 8 h work shift, and they normally "nish the job within 3}4 h to have a long break for the rest of the shift. Such work practices result in high lifting frequencies. Therefore, the job should be evenly distributed within 2 h or less work cycles followed by a su$cient recovery period, to keep the lifting frequency less than 0.5 lifts/min. If the workplace is redesigned as recommended above, CLI of Job 1 would change from 2.0 to 0.66, which is safe for most workers. Although Job 12 of the forming process also requires a long horizontal distance and a large asymmetry angle as in Job 1, its CLI was found to be low since the weight of a object lifted is light and the frequency of lifting is low. For the remaining 12 jobs in the forming process, it is recommended that as discussed in Job 1 analysis, horizontal locations of hands should be within 30 cm and that the asymmetry angle should be reduced to 903. Furthermore, the job should be evenly distributed within 2 h or less work cycles followed by a su$cient recovery period. If these recommendations are accepted, all the task variables become "xed except frequency and object weight, which remained eligible for change. Since weight of bricks lifted is not allowed to make a change, the only task variable that remained eligible for change is lifting frequency. Then, we are to "nd out alternative work strategy by investigating a trend of CLIs depending
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Fig. 4. Predicted CLIs of improved lifting tasks.
on the lifting frequency for varying weights of bricks lifted. Illustrated in Fig. 4 are the predicted CLIs for the improved lifting tasks as recommended, depending on the lifting frequency for varying weights of bricks, ranging from 2.5 to 12.5 kg with increment of 2.5 kg. As shown in Fig. 4, objects weighing 2.5 kg should be lifted 3 times or less per minute for CLI to be less than 1.0. If these objects were lifted 4 times or more a minute, CLI becomes in"nite (R) because the frequency multiplier based on the sum of the frequencies for all three tasks is zero. If bricks weighing 7.5 kg were lifted 2 times or less a minute and 10.0 kg bricks were lifted every 2 min or more, these jobs were evaluated to be safe. This implies that Jobs 2, 4, 9, 10, 11, 13 and 14 turn out to be safe in terms of CLI. It was also found from Fig. 4 that the lifting frequency should be kept the lowest (say, every 5 min) for Job 6 with bricks weighing 12.5 kg. Since weight of bricks in Jobs 7 and 8 (25, 40 kg, respectively) exceeds the load constant (23.1 kg) in the revised NIOSH equation, it is impossible to reduce CLI below 1.0. Therefore, these bricks should be instructed to be lifted by two or more workers, or to use mechanical aids. Otherwise, these two jobs should be automated. Table 9 of appendix shows that the smallest multipliers are 0.40}69 for the HM, 0.81 for the AM, and 0.83 for the FM in the heating/packing
processes. As in the forming process, the horizontal location should be reduced to within 25 cm, and work hours for lifting tasks should be evenly distributed within 2 h or less work cycles. If Job 1 is redesigned as recommended, its CLI would be reduced to 1.16, which is moderately safe for most workers. Since weight of bricks handled in Jobs 2, 3 and 4 exceeds the load constant (23.1 kg) used in the revised NIOSH equation, mechanization or automation of the process should be introduced. Taking into account the daily production orders for eight working hours in all the processes, lifting frequency is found to be moderate without showing any signi"cant problem. However, workers tend to "nish their daily assigned production earlier than scheduled so as to take rest during the remaining work hours. Therefore, by educating and training the workers to distribute the work load evenly over 8 h, we can improve the frequency multiplier in the CLI computation. Although the 1991 revised NIOSH guide is admittedly more restrictive of acceptable jobs than the NIOSH guide of 1981 (Karwowski and Brokaw 1992), lifting tasks in these processes turned out to contain risk factors that may cause back injuries. It is also cited as a common problem in all three processes that the bricks handled are located at quite a distance from the center of the body.
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1 1 1
Frequency (lifts/min)
1303 1303 1303 1003 1003 1003 17 7 8 84 74 59 0 75 60 6 6 6 1 2 3
V
67 67 67
A A D H H
V
Destination Origin
60 60 60
Destination Orgin
Fair Fair Fair
Coupling Asymmetric angle Hand location (cm) Object weight (kg) Task no.
Table 6 Task variables of job 1 in the forming process
Vertical distance (cm)
5. Conclusion The questionnaire survey showed that weight of the load signi"cantly in#uenced the incidence of back injuries and that workers who reported to have experienced back injuries were older than those who did not experience them. Judging from the fact that service length of the LBP-experienced appeared to be an average of 5.9 yr, low back injuries occurring in the manufacturing industry can be considered to be a chronic disorder due to cumulative trauma on account of awkward working posture, or heavy weight of load, etc. The current safety-training program was not shown to be e!ective for preventing LBP. Therefore, a study on an e!ective training program including development of safety education materials, education method, etc. is also needed. As seen in the above section, composite lifting indices ranged from 0.86 to 8.8 with an average of 2.73 in the forming process, and from 3.7 to 18.9 with an average of 11.12 in the heating/packing processes. It is found that all the jobs except Job 12 of the forming process are stressful for most workers. Since the objects handled in the heating/packing processes are much heavier and the frequency of lifting/lowering is higher than that in the forming process, the workers in the heating/packing processes are engaged in more
8 8 8
Duration (hr)
Furthermore, severe twisting is required to stack bricks on the pallet behind the worker in the forming process, and the weights of the bricks are excessively heavy for some tasks when compared with the load constant in the revised NIOSH equations. Most of the jobs under study could be redesigned to lessen the biomechanical stress simply by making horizontal locations closer to the worker or by reducing the asymmetric angles. Some jobs in the heating/packing processes, however, should be redesigned to use mechanical aids or to partly include automation of the process. Speci"cally, for the jobs with CLIs exceeding 5.0 (i.e., the compressive force on the L5/S1 is about 4.6 kN), the manager of the company should concentrate his/her e!ort on establishing countermeasures to eliminate the risk factors that may lead to low back injuries.
431
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Table 7 Procedure calculating CLI of job 1 in the forming process Task no.
LC
HM
VM
DM
AM
CM
FIRWL
FM
STRWL
FILI
STLI
New Task No
F
1 2 3
51 51 51
0.43 0.43 0.43
0.96 0.96 0.96
1.0 1.0 0.9
0.63 0.63 0.63
0.95 0.95 0.95
13.6 13.6 12.8
0.75 0.75 0.75
10.2 10.2 9.6
0.97 0.97 1.04
1.30 1.30 1.30
2 3 1
1 1 1
CLI"
STLI # *FILI # *FILI 1 2 3 FILI (1/FM }1/FM ) 2 1,2 1 0.97 (1/.65}1/.75) 1.38 0.20
CLI"
FILI (1/FM }1/FM ) 3 1,2,3 1,2 0.97 (1/.55}1/.65) 0.27
1.85
Table 8 Task variables of job 1 in the heating/packing processess Task no.
Object weight (kg)
Hand location (cm) Origin
1 2 3
14 14 14
Vertical distance (cm) Destination
H
V
H
V
D
66 51 36
175 135 95
39 39 39
27 67 107
46 74 18
Coupling
Frequency (lifts/min)
Duration (hr)
Fair Fair Fair
0.33 0.33 0.33
8 8 8
Asymmetric angle Orgin
Destination
A
A
603 603 603
703 703 703
Table 9 Procedure for calculating CLI of job 1 in the heating/packing process Task no.
LC
HM
VM
DM
AM
CM
FIRWL
FM
STRWL
FILI
STLI
New Task No
F
1 2 3
51 51 51
0.40 0.48 0.69
0.71 0.85 0.96
0.85 0.88 1.0
0.81 0.81 0.81
1.0 1.0 0.95
10.0 14.8 26.0
0.83 0.83 0.83
8.3 12.3 21.6
3.1 2.1 1.2
3.7 2.5 1.4
1 2 3
0.33 0.33 0.33
CLI"
STLI # *FILI # *FILI 1 2 3 FILI (1/FM }1/FM ) 2 1,2 1 2.1 (1/.79}1/.83) 3.7 0.13
CLI"
strenuous jobs. This is also supported by the "nding that more low back injury cases were reported in the packing process than in the forming process in this company. Furthermore, since female workers are also assigned to the packing process, all possible e!orts should be devoted to redesigning jobs in the packing process. In the forming process, severe twisting is unnecessarily required when stacking bricks on the pallet behind the worker,
FILI (1/FM }1/FM ) 3 1,2,3 1,2 1.2 (1/.75}1/.79) 0.08
3.91
which can be eliminated through simple rearrangement of the workplace.
Acknowledgements The research was partially funded by the Korea Science and Engineering Foundation (95-0200-0801-3). The authors gratefully acknowledge the
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assistance of Young W. Song and Jongho Lim of POSTECH in the collection and analysis of the data presented in this article.
Appendix A Task variables and procedure calculating CLI of Job 1 in forming process and heating/packing processes is shown in Tables 6}9.
References Cha$n, D.B., Park, K.S., 1973. A longitudinal study of low-back pain as associated with occupational weight lifting factors. American Industrial Hygiene Association Journal 34 (12), 513}525. Chung, M.K., Kee, D., Kim, T.B., 1992. Maximum Voluntary Strength Evaluation for Korean Workers. Journal of the Korean Institute of Industrial Engineers 18 (1), 141}153. Cook, T.M., Neumann, D.A., 1987. The e!ects of load placement on the EMG activity of the low back muscle during load carrying by men and women. Ergonomics 30, 1413}1423.
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Karwowski, W., Brokaw, N., 1992. Implications of the proposed revisions in a draft Revised NIOSH lifting guide (1991) for job redesign: a "eld study. In: Proceedings of the Human Factors and Ergonomics Society 36th Annual Meeting, pp. 659}663. Karwowski, W., Caldwell, M., Gaddie, P., 1994. Relationships between the NIOSH (1991) lifting index, compressive and shear forces on the lumbosacral joint, and low back injury incidence rate based on industrial "eld study. In: Proceedings of the Human Factors and Ergonomics Society 38th Annual Meeting, pp. 654}657. Kee, D., Chung, M.K., 1995. Analysis of symptoms and causes for low back pain reported in the Pohang region. IE Interfaces 8 (4), 145}154. Lee, K.S., Park, H.S., 1995. Assessment of validity of applying the revised NIOSH weight limit of lifts to Korean young male population: psychophysical approach. In: Proceedings of the Human Factors and Ergonomics Society 39th Annual Meeting, Vol. 1, pp. 714}717. NIOSH, 1981. Work Practice Guide for Manual Lifting, DHHS (NIOSH) Publication, Cincinnati. Waters, T., Puts-Anderson, V., Garg, A., 1993a. Applications manual for the revised NIOSH lifting equation. US Department of Health and Human Services, Cincinnati. Waters, T., Puts-Anderson, V., Garg, A., Fine, L., 1993b. Revised NIOSH equation for design and evaluation of manual lifting tasks. Ergonomics 36 (7), 446}749.