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Physiological workload evaluation of screw driving tasks in automobile assembly jobs
International Journal of Industrial Ergonomics 28 (2001) 181–188
Physiological workload evaluation of screw driving tasks in automobile assembly jobs Min K. Chung*, Inseok Lee, Yoon S. Yeo Division of Mechanical and Industrial Engineering, Pohang University of Science and Technology, Hyoja San 31, Pohang 790-784, South Korea Received 15 March 1999; accepted 16 September 1999
Abstract For ergonomic intervention of manual materials handling (MMH) tasks, biomechanical, physiological, and psychophysical approaches have been studied. In the automobile assembly jobs, however, very few studies on physiological stresses have been carried out, whereas many studies have been reported in biomechanical and postural stresses. In this study, physiological workloads were evaluated during screw driving tasks in the automobile assembly line. The effects of load and work postures on heart rate, oxygen consumption and subjective ratings (5-point scale) during a screw driving task were studied using nine male subjects, with a within-subject design. Heart rate and subjective ratings of all nine subjects and oxygen consumption of five subjects were evaluated during a simulated screw driving task with varying loads (8.2 kg, 16.4 kg), leg postures (standing and squatting) and trunk postures (forward flexion, forward flexion with lateral bending and twisting). The effect of load, leg and trunk postures were found to be significant on heart rate, oxygen consumption and subjective rating. Heart rate and subjective ratings showed a high correlation for all the combined conditions. For screw driving tasks, it is recommended to redesign work methods in order to avoid squat sitting, lateral bending and twisting. Relevance to industry Automobile assembly job is one of the most labor-intensive industries and workers often perform their tasks in poor working postures repetitively due to constrained work places. This study was conducted to evaluate the physiological workload of screw driving tasks in awkward working postures, such as laterally bent and twisted trunk and squat sitting. Ergonomic guidelines for such assembly tasks were recommended based on the results of a laboratory experiment. r 2001 Elsevier Science B.V. All rights reserved. Keywords: Posture; Physiological workload; Automobile assembly job; Heart rate; Oxygen consumption; Subjective discomfort rating
1. Introduction Musculoskeletal disorders such as low back pain are the most common work-related injuries in *Corresponding author. Tel.: +82-562-279-2192; fax: +82562-279-2870. E-mail address: [email protected] (M.K. Chung).
manual materials handling (MMH) tasks. It has been reported in many studies that such diseases are mainly caused from either over-exertions or repetitive/prolonged poor working postures during performing tasks (Gallagher et al., 1988; Grandjean, 1988; Ayoub and Mital, 1989; Chaffin and Andersson, 1991; Genaidy et al., 1994). Since working posture is determined by the interaction
0169-8141/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 8 1 4 1 ( 0 1 ) 0 0 0 3 1 - 2
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of many factors in the workplace such as features of work station layout, handtool design characteristics, work methods, and anthropometric characteristics (Keyserling, 1986), it has more difficulties in control than weights of the load or over-exertion problem. Particularly in many industries like automobile assembly or underground mining, workers are forced to take awkward working postures due to the restricted working environment, and endure stresses to their musculoskeletal system (Gallagher et al., 1988). Gallagher et al. (1988) showed decreases of psychophysical lifting capacity as well as increases of the metabolic stress and internal load on the spine in the kneeling posture compared to the stooped posture, which are frequently taken during underground mining tasks. Many of the manual tasks in the manufacturing industry, which is a major part of modern industries in developing countries, still require workers to carry out their jobs in improper postures and even involve manual handling of heavy loads. Automobile manufacturing is one of the major industries that require labor-intensive jobs among the modern manufacturing industries. From a brief work surveillance of automobile assembly lines, it was observed that the most prevalent tasks in automobile assembly lines were carrying parts of the automobile from storage site near the line to the assembly line and driving screws with an air tool to assemble them. During screw driving tasks, many workers must adopt awkward working postures regardless of their preference, because working postures are mainly affected by the height of the conveyor, type of the automobile, position of the assembled parts, methods of assembly and type of air tool, and so on. When a certain task requires a worker to take an awkward working posture such as a bent and twisted trunk, he will adopt the posture more than 400 times during an 8 h shift, which can be very harmful to the worker’s musculoskeletal system. The workplace and work balancing should be redesigned taking into account these effects. The most prevalent awkward working postures during screw driving tasks in the automobile assembly line are squatting and forward flexion with lateral bending and twisting in trunk. These
postures with long duration and high repetition are believed to have harmful effects on workers. In a laboratory experiment, it was shown that postural workload of squatting posture is affected by the height of support and duration, and it was recommended that a proper height of the stool should be used in performing prolonged task in a squat sitting posture (Lee, 1997). In the automobile assembly line, however, many workers were not able to use the stool when they adopted a squat sitting posture since they have to move quickly to the next task. Therefore, it is presumed that there is a high risk of incurring musculoskeletal disorders in the back and the knee due to repetitive squatting working postures. For the trunk postures, it is shown that sustained static postures of the trunk such as prolonged sitting or forward bending result in an increased risk of low back pain (Magora, 1970; Kelsey and Hardy, 1975). There are also many studies citing trunk flexion, lateral bending, and twisting as factors in the development of back pain (Chaffin, 1973; Anderson et al., 1977; Schultz et al., 1982). In this study, the effect of weight of load and the posture of trunk and leg on the worker’s physiological stresses were evaluated by a laboratory experiment simulating the screw driving tasks. As workers in the automobile assembly job are required to perform the tasks repetitively during their shift, the physiological method is used in the evaluation of job stresses rather than biomechanical approaches. In fact, screw driving tasks hardly involve handling of heavy objects.
2. Materials and methods Nine healthy male students participated in a laboratory experiment. The mean (s.d.) age, height and weight of the subjects were 23.8 (2.0) years, 172.8 (4.8) cm and 63.9 (6.2) kg, respectively. Table 1 shows the subjects’ anthropometric information. They were paid volunteers who had never experienced any musculoskeletal disorders. The independent variables used in the experiment were weight of load, trunk posture and leg posture. These three variables (factors) were considered to have a strong influence on the
M.K. Chung et al. / International Journal of Industrial Ergonomics 28 (2001) 181–188 Table 1 Subjects’ anthropometric information
Age (years) Height (cm) Weight (kg)
Mean (S.D.)
Max
Min
23.8 (2.0) 172.8 (4.8) 63.9 (6.2)
27.0 181.0 77.0
21.0 169.0 58.0
Table 2 Independent variables Independent variable
Level
Weight of load
Light load: 8.2 kg Heavy load: 16.4 kg Standing (STD) Squatting (SQT) Forward flexion (FF) Forward flexion & lateral bending & twisting (FF/LBT)
Leg posture Trunk posture
physiological workload of the automobile assembly tasks. We designed each factor at two levels as presented in Table 2. The weight of load was varied at 8.2 kg (light) and 16.4 kg (heavy), which were determined based on the average weights of a front bumper and of a front seat, respectively. They were typical objects representing the heavy parts which workers handled manually on the automobile assembly line. The levels of trunk and leg postures were selected as the postures frequently assumed by workers during automobile assembly tasks. Two levels of trunk posture were forward flexion (FF) and forward flexion with lateral bending and twisting (FF/LBT). The trunk angle of forward flexion was 451 and the angle of lateral bending and twisting was over 201. Two levels of leg posture were squatting and standing. The knees were fully flexed in the squatting posture while they were not flexed in the standing posture. In the laboratory experiment, the subjects assumed four different working postures: forward flexed trunk in standing (FF+STD), forward flexed trunk in squatting (FF+SQT), forward flexed, laterally bent and twisted trunk in standing (FF/LBT+STD), and forward flexed, laterally bent and twisted trunk in squatting (FF/ LBT+SQT). Fig. 1 exhibits the four working
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postures. Each subject performed the same task with two weight conditions (light and heavy) in each posture, since the experiment was withinsubject, 23 full factorial design. Heart rate (HR) and oxygen consumption (V’O2 ) were measured while subjects simulated the assembly tasks for 25 min, and the data for the last 5 min were analyzed. We used Metamax (Cortex, Germany) in measuring HR and V’O2 : The subject was also asked to rate his perceived exertion on a 5-point scale at the end of every minute for the last 5 min. The 5-point scale rating of perceived exertion (RPE) was a modified RPE, which was designed to decrease the difficulty of the general population in discriminating the finer details of Borg’s 10-point scale RPE (Borg, 1982; Varghese et al., 1994). The scale has verbal description at each level: very light (1), light (2), moderately heavy (3), heavy (4), and very heavy (5). The simulated automobile assembly tasks consisted of carrying objects and screw driving with an air tool. The layout of the simulated workplace was displayed in Fig. 2. First, the subject carried an object from the first table (‘‘Table 1’’ in Fig. 2, 71 cm height) to the second table (‘‘Table 2’’ in Fig. 2, 30 cm height), and he brought the air tool and bolts from ‘‘Table 1’’ to the bolt assembly panel, which was prepared for the experiment. It was designed so that the height and the angle of the assembly panel could be easily adjusted. Then he assumed the specified postures of leg and trunk, and assembled four bolts to the assembly panel. After finishing the assembly, he walked back to ‘‘Table 1’’, and laid the air tool on the table. The cycle time was fixed to 30 s, and the subject was allowed to rest in the chair next to ‘‘Table 1’’ after finishing the task within the cycle time.
3. Results The data for heart rate, oxygen consumption and subjective ratings were analyzed through an analysis of variance (ANOVA). The results of the analysis are shown in Tables 3–5 and Figs. 3–5. The ANOVA for heart rate showed statistically significant effects of weight of load ( p ¼ 0:0244)
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Fig. 1. Working postures in the experiment.
and of trunk posture ( p ¼ 0:0157). Leg posture and any interaction effects exhibited no significant effect on heart rate. Fig. 3 depicts the mean heart rate under each condition. The average heart rate in the heavy load condition was 98.46 beats/ min, which was significantly higher than that in the light load condition (94.96 beats/min). When the trunk was laterally bent and twisted, the heart rate increased significantly as compared to the trunk posture without such motion (98.76 vs. 94.66 beats/min). Although squatting posture showed higher mean heart rate than standing posture, there was no statistically significant difference in the mean heart rate between them.
The results of the ANOVA for oxygen consumption (V’O2 ) were shown in Table 4 and Fig. 4. From the analysis of variance for V’O2 ; leg posture showed a statistically siginificant effect ( p ¼ 0:0020), and the weight of load effect was also marginally significant ( p ¼ 0:0757). No other factors showed significant effects on oxygen consumption (Table 4). The mean V’O2 in standing (0.65 l/min) increased by as much as 0.92 l/min in the squatting posture. As seen in Fig. 4, it also increased in the heavy load condition (0.89 l/min) as compared to that in the light load condtion (0.68 l/min). The results of the ANOVA for subjective discomfort rating showed that all main effects
M.K. Chung et al. / International Journal of Industrial Ergonomics 28 (2001) 181–188
Fig. 2. Layout of the workspace for the experiment.
and some interaction effects were significant ( po0:05; see Table 5). Fig. 5 showed that the subjects rated higher discomfort in the heavy load condition, squatting posture, and forward flexion posture with lateral bending and twisting of trunk. The mean subjective discomfort ratings for heavy load, forward flexed and laterally bent and twisted trunk, and squatting were around 3.5 in 5.0 scale, whereas they were around 2.7 for light
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load, no lateral bending and twisting of trunk, and standing. It has been known that heart rate has a good linear relationship with oxygen consumption and subjective rating (Rodahl, 1989; Chaffin and Andersson, 1991). In this study, we have investigated these relationships with the data of the experiment, which was mainly focused on the postural effects. As seen in Fig. 6, a simple, linear regression analysis showed a fairly good linear relationship between the heart rate and subjective discomfort rating (r2 ¼ 0:773). In contrast to the linear relationship between heart rate and discomfort rating, a poor linear relationship was found between heart rate and oxygen consumption (Fig. 7).
4. Discussion From the results of the subjective discomfort rating, it is confirmed that the physiological cost of the screw driving tasks significantly increased for
Table 3 ANOVA results of heart rate Source
DF
Sum of squares
Mean square
F value
Pr>F
Weights of load (W) Trunk posture (T) Leg posture (L) WT WL TL WTL
1 1 1 1 1 1 1
2128.88 2814.08 301.89 18.05 326.53 3.62 18.07
2128.88 2814.08 301.89 18.05 326.53 3.62 18.07
7.65 9.33 1.30 0.08 0.81 0.03 0.28
0.0244a 0.0157a 0.2871 0.7825 0.3934 0.8728 0.6136
a
Statistically significant at a ¼ 0:05:
Table 4 ANOVA results of oxygen consumption Source
DF
Sum of squares
Mean square
F value
Pr>F
Weights of load (W) Trunk posture (T) Leg posture (L) WT WL TL WTL
1 1 1 1 1 1 1
4.72 0.003 7.36 0.69 0.90 0.08 0.08
4.72 0.003 7.36 0.69 0.90 0.08 0.08
5.68 0.01 51.50 1.58 0.76 0.68 0.33
0.0757a 0.9264 0.0020b 0.2778 0.4335 0.4557 0.5971
a b
Statistically significant at a ¼ 0:1: Statistically significant at a ¼ 0:05:
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Table 5 ANOVA results of subjective rating Source
DF
Sum of squares
Mean square
F value
Pr>F
Weights of load (W) Trunk posture (T) Leg posture (L) WT WL TL WTL
1 1 1 1 1 1 1
55.93 53.44 44.73 2.22 6.43 0.08 3.31
55.93 53.44 44.73 2.22 6.43 0.08 3.31
41.79 54.26 25.43 4.97 14.20 0.27 5.46
0.0002a 0.0001a 0.0010a 0.0564b 0.0055a 0.6197 0.0476a
a b
Statistically significant at a ¼ 0:05: Statistically significant at a ¼ 0:1:
Fig. 3. Mean heart rate (*po0:05).
Fig. 4. Mean oxygen comsumption (*po0:05; **po0:1).
the heavy load conditions, squatting posture, and forward flexed trunk posture with lateral bending and twisting. However, for the heart rate and oxygen consumption, not all the factors showed
significant effects. Such a different finding between discomfort rating and heart rate/oxygen consumption is perhaps explained by the psychophysical aspects of the subjective discomfort rating. Sub-
M.K. Chung et al. / International Journal of Industrial Ergonomics 28 (2001) 181–188
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Fig. 5. Mean subjective discomfort rating (*po0:05).
Fig. 7. Relationship between heart rate and oxygen consumption. Fig. 6. Relationship between heart rate and subjective discomfort rating.
jective discomfort rating is a psychophysical measurement that reflects not only physiological stress but also all sources of physical stresses including biomechanical load and poor working postures. Since heart rate and oxygen consumption are workload assessing measurements solely based on the physiological load, their results tend to refer to more specific stresses, whereas the subjective discomfort rating would reflect the combined stresses. Although heart rate and V’O2 showed an increased level for the heavy weight of load, these measures showed a different result for the poor leg and trunk postures. It is very interesting to note that an increase in heart rate was not significant in a squatting posture, while increase in V’O2 was significant and the subjects expressed higher dis-
comfort ratings in such a posture. It is also noteworthy that no significant increase in V’O2 was found for the laterally bent and twisted trunk posture. This is perhaps due to the constrained inhalation and exhalation of the worker, which was caused by the twisted trunk postures. It is postulated that constrained inhalation and exhalation led to less oxygen consumption measurement than the physiological cost actually required in that posture. According to Snook and Irvine (1969), the limiting heart rate and oxygen consumption for the 8 h job, especially for the physical tasks such as screw driving tasks that are mainly performed using the arms, was 99 beats/min and 1.0 l/min, respectively. In this study, the average heart rates for the heavy load condition and for lateral bending and twisting in trunk showed over 99 beats/min, and the
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average oxygen consumption for the heavy load condition and for squatting posture also showed over 1.0 l/min. Such findings indicate that automobile assembly tasks in those conditions need administrative and engineering controls.
5. Conclusion In this study, a laboratory experiment was carried out to investigate the effects of weight of load, trunk posture and leg posture on workers’ physiological workload during screw driving and carrying tasks that were frequently performed in the automobile assembly line. During the simulated automobile assembly tasks, heart rate, oxygen consumption and subjective discomfort rating were evaluated. Although no statistically significant increase was observed in heart rate for the poor leg posture, all three physiological measures, heart rate, oxygen consumption and subjective discomfort rating, showed a significant increase for the heavy load and for the laterally bent and twisted trunk posture. From the results it was confirmed that heavy load and lateral bending and twisting postures are very harmful to workers from a physiological as well as a biomechanical perspective. Therefore, in the automobile assembly line, the height of the conveyor line and the orientation of workplaces should be redesigned to avoid squatting postures and lateral bending and twisting postures, in addition to reducing the weight of the objects handled.
References Anderson, B., Ortengren, R., Herberts, P., 1977. Quantitative electromyographic studies of back muscle activity related
posture and loading. Orthopedic Clinics of North America 8, 85–96. Ayoub, M.M., Mital, A., 1989. Manual Materials Handling. Taylor & Francis, London. Borg, G., 1982. Psychological basis of physical exertion. Medicine and Science in Sports and Exercise 14, 377–381. Chaffin, D., 1973. Localized muscle fatigueFdefinition and measurement. Journal of Occupational Medecine 15, 346–354. Chaffin, D.B., Andersson, G.B.J., 1991. Occupational Biomechanics, 2nd Edition. Wiley, New York. Gallagher, S., Marras, W.S., Bobick, T.G., 1988. Lifting in stooped and kneeling postures: effects on lifting capacity, metabolic costs, and electromyography of eight trunk muscles. International Journal of Industrial Ergonomics 3, 65–76. Genaidy, A.M., Al-shedi, A.A., Karwowski, W., 1994. Postural stress analysis in industry. Applied Ergonomics 25 (2), 77–87. Grandjean, E., 1988. Fitting the Task to the Man, 4th ed. Taylor & Francis, London. Kelsey, J., Hardy, R., 1975. Driving of motor vehicles as a risk factor for acute herniated lumbar intervertebral disk. American Journal of Epidemiology 102, 63–73. Keyserling, W.M., 1986. A computer-aided system to evaluate postural stress in the workplace. American Industrial Hygiene Association Journal 47, 641–649. Lee, I., 1997. Workload evaluation of squat sitting postures. Unpublished Thesis of Master. Pohang University of Science and Technology (in Korean). Magora, A., 1970. Investigation of the relation between low back pain and occupation, Part II. Industrial Medicine Surgery 39, 28–34. Rodahl, K., 1989. The Physiology of Work. Taylor & Francis, London. Schultz, A., Anderson, G., Ortengren, A., Nachemson, A., Haderspeck, K., 1982. Loads on the lumbar spine: validation of a biomechanical analysis: measurements of interdiscal pressures and myoelectric signals. Journal of Bone and Joint Surgery 64A, 713–720. Snook, S.H., Irvine, C.H., 1969. Phychophysical studies of physiological fatigue criteria. Human Factors 11, 291–299. Varghese, M.A., Saha, P.N., Atreya, N., 1994. A rapid appraisal of occupational workload from a modified scale of perceived exertion. Ergonomics 37 (3), 485–491.
Physiological workload evaluation of screw driving tasks in automobile assembly jobs Min K. Chung*, Inseok Lee, Yoon S. Yeo Division of Mechanical and Industrial Engineering, Pohang University of Science and Technology, Hyoja San 31, Pohang 790-784, South Korea Received 15 March 1999; accepted 16 September 1999
Abstract For ergonomic intervention of manual materials handling (MMH) tasks, biomechanical, physiological, and psychophysical approaches have been studied. In the automobile assembly jobs, however, very few studies on physiological stresses have been carried out, whereas many studies have been reported in biomechanical and postural stresses. In this study, physiological workloads were evaluated during screw driving tasks in the automobile assembly line. The effects of load and work postures on heart rate, oxygen consumption and subjective ratings (5-point scale) during a screw driving task were studied using nine male subjects, with a within-subject design. Heart rate and subjective ratings of all nine subjects and oxygen consumption of five subjects were evaluated during a simulated screw driving task with varying loads (8.2 kg, 16.4 kg), leg postures (standing and squatting) and trunk postures (forward flexion, forward flexion with lateral bending and twisting). The effect of load, leg and trunk postures were found to be significant on heart rate, oxygen consumption and subjective rating. Heart rate and subjective ratings showed a high correlation for all the combined conditions. For screw driving tasks, it is recommended to redesign work methods in order to avoid squat sitting, lateral bending and twisting. Relevance to industry Automobile assembly job is one of the most labor-intensive industries and workers often perform their tasks in poor working postures repetitively due to constrained work places. This study was conducted to evaluate the physiological workload of screw driving tasks in awkward working postures, such as laterally bent and twisted trunk and squat sitting. Ergonomic guidelines for such assembly tasks were recommended based on the results of a laboratory experiment. r 2001 Elsevier Science B.V. All rights reserved. Keywords: Posture; Physiological workload; Automobile assembly job; Heart rate; Oxygen consumption; Subjective discomfort rating
1. Introduction Musculoskeletal disorders such as low back pain are the most common work-related injuries in *Corresponding author. Tel.: +82-562-279-2192; fax: +82562-279-2870. E-mail address: [email protected] (M.K. Chung).
manual materials handling (MMH) tasks. It has been reported in many studies that such diseases are mainly caused from either over-exertions or repetitive/prolonged poor working postures during performing tasks (Gallagher et al., 1988; Grandjean, 1988; Ayoub and Mital, 1989; Chaffin and Andersson, 1991; Genaidy et al., 1994). Since working posture is determined by the interaction
0169-8141/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 8 1 4 1 ( 0 1 ) 0 0 0 3 1 - 2
182
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of many factors in the workplace such as features of work station layout, handtool design characteristics, work methods, and anthropometric characteristics (Keyserling, 1986), it has more difficulties in control than weights of the load or over-exertion problem. Particularly in many industries like automobile assembly or underground mining, workers are forced to take awkward working postures due to the restricted working environment, and endure stresses to their musculoskeletal system (Gallagher et al., 1988). Gallagher et al. (1988) showed decreases of psychophysical lifting capacity as well as increases of the metabolic stress and internal load on the spine in the kneeling posture compared to the stooped posture, which are frequently taken during underground mining tasks. Many of the manual tasks in the manufacturing industry, which is a major part of modern industries in developing countries, still require workers to carry out their jobs in improper postures and even involve manual handling of heavy loads. Automobile manufacturing is one of the major industries that require labor-intensive jobs among the modern manufacturing industries. From a brief work surveillance of automobile assembly lines, it was observed that the most prevalent tasks in automobile assembly lines were carrying parts of the automobile from storage site near the line to the assembly line and driving screws with an air tool to assemble them. During screw driving tasks, many workers must adopt awkward working postures regardless of their preference, because working postures are mainly affected by the height of the conveyor, type of the automobile, position of the assembled parts, methods of assembly and type of air tool, and so on. When a certain task requires a worker to take an awkward working posture such as a bent and twisted trunk, he will adopt the posture more than 400 times during an 8 h shift, which can be very harmful to the worker’s musculoskeletal system. The workplace and work balancing should be redesigned taking into account these effects. The most prevalent awkward working postures during screw driving tasks in the automobile assembly line are squatting and forward flexion with lateral bending and twisting in trunk. These
postures with long duration and high repetition are believed to have harmful effects on workers. In a laboratory experiment, it was shown that postural workload of squatting posture is affected by the height of support and duration, and it was recommended that a proper height of the stool should be used in performing prolonged task in a squat sitting posture (Lee, 1997). In the automobile assembly line, however, many workers were not able to use the stool when they adopted a squat sitting posture since they have to move quickly to the next task. Therefore, it is presumed that there is a high risk of incurring musculoskeletal disorders in the back and the knee due to repetitive squatting working postures. For the trunk postures, it is shown that sustained static postures of the trunk such as prolonged sitting or forward bending result in an increased risk of low back pain (Magora, 1970; Kelsey and Hardy, 1975). There are also many studies citing trunk flexion, lateral bending, and twisting as factors in the development of back pain (Chaffin, 1973; Anderson et al., 1977; Schultz et al., 1982). In this study, the effect of weight of load and the posture of trunk and leg on the worker’s physiological stresses were evaluated by a laboratory experiment simulating the screw driving tasks. As workers in the automobile assembly job are required to perform the tasks repetitively during their shift, the physiological method is used in the evaluation of job stresses rather than biomechanical approaches. In fact, screw driving tasks hardly involve handling of heavy objects.
2. Materials and methods Nine healthy male students participated in a laboratory experiment. The mean (s.d.) age, height and weight of the subjects were 23.8 (2.0) years, 172.8 (4.8) cm and 63.9 (6.2) kg, respectively. Table 1 shows the subjects’ anthropometric information. They were paid volunteers who had never experienced any musculoskeletal disorders. The independent variables used in the experiment were weight of load, trunk posture and leg posture. These three variables (factors) were considered to have a strong influence on the
M.K. Chung et al. / International Journal of Industrial Ergonomics 28 (2001) 181–188 Table 1 Subjects’ anthropometric information
Age (years) Height (cm) Weight (kg)
Mean (S.D.)
Max
Min
23.8 (2.0) 172.8 (4.8) 63.9 (6.2)
27.0 181.0 77.0
21.0 169.0 58.0
Table 2 Independent variables Independent variable
Level
Weight of load
Light load: 8.2 kg Heavy load: 16.4 kg Standing (STD) Squatting (SQT) Forward flexion (FF) Forward flexion & lateral bending & twisting (FF/LBT)
Leg posture Trunk posture
physiological workload of the automobile assembly tasks. We designed each factor at two levels as presented in Table 2. The weight of load was varied at 8.2 kg (light) and 16.4 kg (heavy), which were determined based on the average weights of a front bumper and of a front seat, respectively. They were typical objects representing the heavy parts which workers handled manually on the automobile assembly line. The levels of trunk and leg postures were selected as the postures frequently assumed by workers during automobile assembly tasks. Two levels of trunk posture were forward flexion (FF) and forward flexion with lateral bending and twisting (FF/LBT). The trunk angle of forward flexion was 451 and the angle of lateral bending and twisting was over 201. Two levels of leg posture were squatting and standing. The knees were fully flexed in the squatting posture while they were not flexed in the standing posture. In the laboratory experiment, the subjects assumed four different working postures: forward flexed trunk in standing (FF+STD), forward flexed trunk in squatting (FF+SQT), forward flexed, laterally bent and twisted trunk in standing (FF/LBT+STD), and forward flexed, laterally bent and twisted trunk in squatting (FF/ LBT+SQT). Fig. 1 exhibits the four working
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postures. Each subject performed the same task with two weight conditions (light and heavy) in each posture, since the experiment was withinsubject, 23 full factorial design. Heart rate (HR) and oxygen consumption (V’O2 ) were measured while subjects simulated the assembly tasks for 25 min, and the data for the last 5 min were analyzed. We used Metamax (Cortex, Germany) in measuring HR and V’O2 : The subject was also asked to rate his perceived exertion on a 5-point scale at the end of every minute for the last 5 min. The 5-point scale rating of perceived exertion (RPE) was a modified RPE, which was designed to decrease the difficulty of the general population in discriminating the finer details of Borg’s 10-point scale RPE (Borg, 1982; Varghese et al., 1994). The scale has verbal description at each level: very light (1), light (2), moderately heavy (3), heavy (4), and very heavy (5). The simulated automobile assembly tasks consisted of carrying objects and screw driving with an air tool. The layout of the simulated workplace was displayed in Fig. 2. First, the subject carried an object from the first table (‘‘Table 1’’ in Fig. 2, 71 cm height) to the second table (‘‘Table 2’’ in Fig. 2, 30 cm height), and he brought the air tool and bolts from ‘‘Table 1’’ to the bolt assembly panel, which was prepared for the experiment. It was designed so that the height and the angle of the assembly panel could be easily adjusted. Then he assumed the specified postures of leg and trunk, and assembled four bolts to the assembly panel. After finishing the assembly, he walked back to ‘‘Table 1’’, and laid the air tool on the table. The cycle time was fixed to 30 s, and the subject was allowed to rest in the chair next to ‘‘Table 1’’ after finishing the task within the cycle time.
3. Results The data for heart rate, oxygen consumption and subjective ratings were analyzed through an analysis of variance (ANOVA). The results of the analysis are shown in Tables 3–5 and Figs. 3–5. The ANOVA for heart rate showed statistically significant effects of weight of load ( p ¼ 0:0244)
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Fig. 1. Working postures in the experiment.
and of trunk posture ( p ¼ 0:0157). Leg posture and any interaction effects exhibited no significant effect on heart rate. Fig. 3 depicts the mean heart rate under each condition. The average heart rate in the heavy load condition was 98.46 beats/ min, which was significantly higher than that in the light load condition (94.96 beats/min). When the trunk was laterally bent and twisted, the heart rate increased significantly as compared to the trunk posture without such motion (98.76 vs. 94.66 beats/min). Although squatting posture showed higher mean heart rate than standing posture, there was no statistically significant difference in the mean heart rate between them.
The results of the ANOVA for oxygen consumption (V’O2 ) were shown in Table 4 and Fig. 4. From the analysis of variance for V’O2 ; leg posture showed a statistically siginificant effect ( p ¼ 0:0020), and the weight of load effect was also marginally significant ( p ¼ 0:0757). No other factors showed significant effects on oxygen consumption (Table 4). The mean V’O2 in standing (0.65 l/min) increased by as much as 0.92 l/min in the squatting posture. As seen in Fig. 4, it also increased in the heavy load condition (0.89 l/min) as compared to that in the light load condtion (0.68 l/min). The results of the ANOVA for subjective discomfort rating showed that all main effects
M.K. Chung et al. / International Journal of Industrial Ergonomics 28 (2001) 181–188
Fig. 2. Layout of the workspace for the experiment.
and some interaction effects were significant ( po0:05; see Table 5). Fig. 5 showed that the subjects rated higher discomfort in the heavy load condition, squatting posture, and forward flexion posture with lateral bending and twisting of trunk. The mean subjective discomfort ratings for heavy load, forward flexed and laterally bent and twisted trunk, and squatting were around 3.5 in 5.0 scale, whereas they were around 2.7 for light
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load, no lateral bending and twisting of trunk, and standing. It has been known that heart rate has a good linear relationship with oxygen consumption and subjective rating (Rodahl, 1989; Chaffin and Andersson, 1991). In this study, we have investigated these relationships with the data of the experiment, which was mainly focused on the postural effects. As seen in Fig. 6, a simple, linear regression analysis showed a fairly good linear relationship between the heart rate and subjective discomfort rating (r2 ¼ 0:773). In contrast to the linear relationship between heart rate and discomfort rating, a poor linear relationship was found between heart rate and oxygen consumption (Fig. 7).
4. Discussion From the results of the subjective discomfort rating, it is confirmed that the physiological cost of the screw driving tasks significantly increased for
Table 3 ANOVA results of heart rate Source
DF
Sum of squares
Mean square
F value
Pr>F
Weights of load (W) Trunk posture (T) Leg posture (L) WT WL TL WTL
1 1 1 1 1 1 1
2128.88 2814.08 301.89 18.05 326.53 3.62 18.07
2128.88 2814.08 301.89 18.05 326.53 3.62 18.07
7.65 9.33 1.30 0.08 0.81 0.03 0.28
0.0244a 0.0157a 0.2871 0.7825 0.3934 0.8728 0.6136
a
Statistically significant at a ¼ 0:05:
Table 4 ANOVA results of oxygen consumption Source
DF
Sum of squares
Mean square
F value
Pr>F
Weights of load (W) Trunk posture (T) Leg posture (L) WT WL TL WTL
1 1 1 1 1 1 1
4.72 0.003 7.36 0.69 0.90 0.08 0.08
4.72 0.003 7.36 0.69 0.90 0.08 0.08
5.68 0.01 51.50 1.58 0.76 0.68 0.33
0.0757a 0.9264 0.0020b 0.2778 0.4335 0.4557 0.5971
a b
Statistically significant at a ¼ 0:1: Statistically significant at a ¼ 0:05:
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Table 5 ANOVA results of subjective rating Source
DF
Sum of squares
Mean square
F value
Pr>F
Weights of load (W) Trunk posture (T) Leg posture (L) WT WL TL WTL
1 1 1 1 1 1 1
55.93 53.44 44.73 2.22 6.43 0.08 3.31
55.93 53.44 44.73 2.22 6.43 0.08 3.31
41.79 54.26 25.43 4.97 14.20 0.27 5.46
0.0002a 0.0001a 0.0010a 0.0564b 0.0055a 0.6197 0.0476a
a b
Statistically significant at a ¼ 0:05: Statistically significant at a ¼ 0:1:
Fig. 3. Mean heart rate (*po0:05).
Fig. 4. Mean oxygen comsumption (*po0:05; **po0:1).
the heavy load conditions, squatting posture, and forward flexed trunk posture with lateral bending and twisting. However, for the heart rate and oxygen consumption, not all the factors showed
significant effects. Such a different finding between discomfort rating and heart rate/oxygen consumption is perhaps explained by the psychophysical aspects of the subjective discomfort rating. Sub-
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Fig. 5. Mean subjective discomfort rating (*po0:05).
Fig. 7. Relationship between heart rate and oxygen consumption. Fig. 6. Relationship between heart rate and subjective discomfort rating.
jective discomfort rating is a psychophysical measurement that reflects not only physiological stress but also all sources of physical stresses including biomechanical load and poor working postures. Since heart rate and oxygen consumption are workload assessing measurements solely based on the physiological load, their results tend to refer to more specific stresses, whereas the subjective discomfort rating would reflect the combined stresses. Although heart rate and V’O2 showed an increased level for the heavy weight of load, these measures showed a different result for the poor leg and trunk postures. It is very interesting to note that an increase in heart rate was not significant in a squatting posture, while increase in V’O2 was significant and the subjects expressed higher dis-
comfort ratings in such a posture. It is also noteworthy that no significant increase in V’O2 was found for the laterally bent and twisted trunk posture. This is perhaps due to the constrained inhalation and exhalation of the worker, which was caused by the twisted trunk postures. It is postulated that constrained inhalation and exhalation led to less oxygen consumption measurement than the physiological cost actually required in that posture. According to Snook and Irvine (1969), the limiting heart rate and oxygen consumption for the 8 h job, especially for the physical tasks such as screw driving tasks that are mainly performed using the arms, was 99 beats/min and 1.0 l/min, respectively. In this study, the average heart rates for the heavy load condition and for lateral bending and twisting in trunk showed over 99 beats/min, and the
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average oxygen consumption for the heavy load condition and for squatting posture also showed over 1.0 l/min. Such findings indicate that automobile assembly tasks in those conditions need administrative and engineering controls.
5. Conclusion In this study, a laboratory experiment was carried out to investigate the effects of weight of load, trunk posture and leg posture on workers’ physiological workload during screw driving and carrying tasks that were frequently performed in the automobile assembly line. During the simulated automobile assembly tasks, heart rate, oxygen consumption and subjective discomfort rating were evaluated. Although no statistically significant increase was observed in heart rate for the poor leg posture, all three physiological measures, heart rate, oxygen consumption and subjective discomfort rating, showed a significant increase for the heavy load and for the laterally bent and twisted trunk posture. From the results it was confirmed that heavy load and lateral bending and twisting postures are very harmful to workers from a physiological as well as a biomechanical perspective. Therefore, in the automobile assembly line, the height of the conveyor line and the orientation of workplaces should be redesigned to avoid squatting postures and lateral bending and twisting postures, in addition to reducing the weight of the objects handled.
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