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Differences in lifting strength profiles between experienced workers and novices at various exertion heights
International Journal of Industrial Ergonomics 41 (2011) 53e58
Contents lists available at ScienceDirect
International Journal of Industrial Ergonomics journal homepage: www.elsevier.com/locate/ergon
Differences in lifting strength profiles between experienced workers and novices at various exertion heights Yi-Lang Chen a, *, Yu-Chi Lee b, Chia-Jung Chen a a b
Department of Industrial Engineering and Management, Mingchi University of Technology, Taipei, Taiwan Department of Industrial Engineering and Engineering Management, National Tsing Hua University, Hsinchu, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 September 2009 Received in revised form 29 October 2010 Accepted 4 November 2010 Available online 30 November 2010
This study compared lifting strength patterns between experienced workers and novices at various exertion heights. Twenty-one experienced workers and 21 novices were recruited to determine their static-lifting strength under various heights (10e150 cm in increments of 10 cm) using two exertion methods (vertically upward lifting, VUL, and toward body lifting, TBL). Testing posture was also recorded by a motion analysis system for comparing with the corresponding strength values. Results showed that strength during VUL were much higher than strength during TBL at 15 height positions (p < 0.001). Strength in all 30 task combinations showed no difference between the two groups except for the VUL at heights of 100e120 cm. On the average, strength exerted by novices during VUL was 4.57e7.61 kg lower than that of workers while lifting at 100e120 cm heights (all p < 0.05). The strength during TBL consistently decreased with increased heights of lifting. When workers performed VUL, the strength surprisingly remained nearly unchanged throughout the heights of interest. The postures adopted by workers during VUL were also highly differentiated from novices while performing near-floor positions, but the strength was equivalent to each other. This study demonstrated that the static-lifting strength of novices were significantly lower than those of experienced workers while upward lifting near the participant’s elbow height. It was concluded that workers tend to adopt a safer (i.e., more flexed knees) and more skillful technique than novices to generate forces, resulting in lower spinal loads during both methods of lifting. Relevance to Industry: Many studies have established the human strength data based on student participants who do not have experience in manual materials handling. The present findings clearly suggest that lifting strength data collected on novices (e.g., on students) should be carefully applied in the task (re)design at the workplace as their strength profiles and the postures adopted during lifting differ from workers. Ó 2010 Elsevier B.V. All rights reserved.
Keywords: Static-lifting strength Upward lifting Toward body lifting Motion analysis Posture recording
1. Introduction Many studies that established the human strength data for various occupational tasks were usually derived from student participants (Yates et al., 1980; Kumar, 1991; Lee, 2004; Herring and Hallbeck, 2007; Knapik and Marras, 2009; Kim et al., 2010). These studies conducted on student participants were on the basis of: ease of recruitment, greater reliability in collecting data, lower cost in recruiting, and their availability for a long-term study (Mital, 1987). However, it is essential to identify the suitability of an individual’s functional capabilities before directly applying the
* Corresponding author. E-mail address: [email protected] (Y.-L. Chen). 0169-8141/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ergon.2010.11.006
findings conducted on a student population to an entire industrial population. Based on the lifting-related literature, a lifting technique of highly skilled workers substantially differs from that of novices (Authier et al., 1996; Gagnon et al., 1996; Hodder et al., 2010). Field studies have also shown that workers use methods other than the recommended ones (Kuorinka et al., 1994; Baril-Gingras and Lortie, 1995), and lack a consensus on the best methods for lifting (Authier and Lortie, 1993). For a given lifting task, in general, experienced workers employ lower biomechanical spinal loads (Marras et al., 2006), lesser back-muscle activity (Keir and MacDonell, 2004; Hodder et al., 2010), more psychophysically accepted weights (Mital, 1987), and higher subjective discomfort thresholds (Parakkat et al., 2007) than novices. On the contrary, some investigations suggest similar patterns of Maximum acceptable weight of lift (MAWL) for industrial and non-industrial workers in manual
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Y.-L. Chen et al. / International Journal of Industrial Ergonomics 41 (2011) 53e58
lifting activities (Mital and Manivasagan, 1983; Garg and Saxena, 1979). Such a discrepancy may attribute to the diverse experimental settings utilized these studies. One of the factors affecting lifting strength is exertion height (Yates et al., 1980; Mital et al., 1993; Lee, 2004). This is extremely realistic in the service warehouse stores as the storage spaces have to be economized and maximized. Investigations on lifting strength are usually confined to some specific height levels (e.g., floor, knuckle, and shoulder). Height of lifting a workload, in addition to the posture adopted during a lift also influenced human strengths (Yates et al., 1980; Lee, 2004), but also in posture strategy (Chen, 2000). Vertically upward lifting (VUL) has been accepted as the standard method in previous strength measurement studies (Ayoub et al., 1978; Chaffin, 1975; Kumar, 1991, Lee, 2004). However, another human strength measurement that usually adopted is toward body lifting (TBL). In a study conducted by Garg et al. (1983), their participants pulled the load toward the body while lifting psychophysically determined maximum weights. Lifting at an angle resulted in a decrease in moment at the upper arms and spinal L5/S1 joints and an increase in moment at the knees and ankles. Lee and Chen (1996) requested the participant to pull on the handle toward the body and found that the lifting strength can be a better predictor for the MAWL than other isometric strength. Therefore, identification of strength differences generated by different exertion directions deserves further investigation. Due to the presence of varied lifting heights and postures adopted in actual working conditions, more exhaustive strength data is needed for ergonomic considerations at the workplace. Every week, in a majority large-scale warehouse stores (e.g., the hypermarket) with high employee turnover rates recruit novice workers or part-timers in Taiwan (Chen, 2008). To our knowledge, no comparative studies have been found in the literature that systemically assessed the simulated lifting strengths of workers and novices under a full range of working heights. Therefore, it is becoming imperative to develop training programs and suitable shelf designs for lifting-related tasks by understanding the differences in lifting strengths between workers and novices. This study, therefore, consists of exertion heights and lifting methods to differentiate the lifting strengths of experienced workers from novices. The results from the present study would be important in improving the shelf design and direct the novice workers to a safer and more effective lifting technique. We hypothesized that lifting strategies adopted by experienced workers and novices would be different because of workers’ lifting experience. The differences in lifting postures among participants would therefore generate different lifting strength profiles under a full range of working heights. 2. Methods
data and isometric strengths (Digital Dynamometer, T.K.K., Tokyo, Japan) using the protocol of Chaffin (1975) were measured (Table 1). 2.2. Experimental apparatus Each participant’s static-lifting strength was measured using a Static-Lifting Strength Tester (SLST), as illustrated in Fig. 1. The SLST, with an incremental height setting consisted of: a standing platform; a steel frame with 20 positioning holes (ranging from 10 cm to 200 cm in increments of 5 cm); a sliding height stopper along the frame; and a 55 cm handle bar (diameter: 3.5 cm) attached to the stopper. The force applied on the handle bar was measured by a load cell (Jackson Strength Evaluation System, JSES Model 32628, U.S.A.) that connected to the handle. Strength values at 60 Hz were transferred to a digital readout unit by an AeD converter. The AeD converter was calibrated prior to the strength testing against known weights. 2.3. Body posture recording Lifting postures of a random sample (8 workers and 8 students) were videotaped for the study. Six adhesive reflective markers were attached to the participant’s wrist, elbow, shoulder, hip, knee, and ankle joints on the right side (Fig. 1). In the study, the joint angles were defined as the inter-joint angles between the two adjacent segments. For example, the knee angle was the included angle between the line formed from the knee to the ankle and the line formed from the knee to the hip (Fig. 2). The digital video camera (Qualisys MacReflex, Sweden) set up at a distance of 5 m from participants, was used to record the positions of each joint marker when participants performed the strength test. The camera heights were adjusted to align the participant’s hip positions orthogonally to eliminate, if any, the distortion errors resulting from the varied testing heights. An image processing software (Qualisys MacReflex, Sweden) was then used to calculate the relevant joint angles. 2.4. Experimental procedure All participants were familiarized with the experimental procedures, and stretched themselves at least 10 min before the data was collected. Fifteen lifting heights, ranging from 10 to 150 cm and spaced 10 cm apart, were set up on the SLST. There were two exertion methods performed by each participant at all
Table 1 Data of the experienced workers and novices (21 of each) in this study. Variablesa
2.1. Participants Twenty-one experienced workers [age in mean (SD): 23.1 (3.0) years] and 21 novices [23.3 (2.4) years], with no prior history of any musculoskeletal disorder, volunteered for this study. This study was approved by Chang Gung Memorial Hospital’s (Taiwan) Ethics Committee. Workers, with at least two-years of experience in manual lifting, were chosen as experienced participants. They were selected from a large hypermarket, and their duties consisted mainly in varied replenishing tasks onto the shelves. Novice participants (with no manual materials handling experience) were recruited from university students. All participants were paid and fully informed of the testing procedure, and full consent was obtained. During the experiment, the participants were asked to wear light clothing and rubber gym shoes. Their anthropometric
Age (years) Stature (cm) Body weight (kg) Acromial height (cm) Elbow height (cm) Knuckle height (cm) Hip height (cm) Knee height (cm) Hand length (cm) Hand width (cm) Isometric strengths (kg) Composite strength Back strength Shoulder strength Arm strength a
Workers
Novices
Mean
S.D.
Mean
S.D.
22.8 174.9 67.4 144.5 110.1 70.4 87.3 48.0 16.7 8.4
2.9 6.5 11.0 6.5 5.1 4.4 5.7 2.5 0.8 0.7
23.4 172.8 66.1 143.0 109.0 69.7 89.2 48.0 16.5 8.3
2.3 3.5 6.7 3.2 3.0 2.4 2.8 2.3 0.7 0.4
91.0 76.9 43.5 31.4
18.8 20.2 10.5 8.1
84.3 67.3 46.2 33.4
15.6 19.5 14.0 7.2
No difference was found between two groups in all variables, all p > 0.05.
Y.-L. Chen et al. / International Journal of Industrial Ergonomics 41 (2011) 53e58
55
Height positioning device
Load cell
A-D converter
Fig. 1. The device of static-lifting strength and schematic testing posture in the study.
lifting heights. One exertion method was VUL and the other was TBL. They were randomly requested to perform maximum voluntary exertion at a specific exertion height and method in free style but with sagittally symmetrical lifting postures. All participants were requested to perform the maximum lifting strengths at least three times for each task combination. The strength-testing procedure was performed according to the methodology used by Chaffin (1975) and Ayoub et al. (1978). A period of 5 s was chosen as sufficient time to build up and maintain a constant level of exertion, while being brief enough to be endured without perceiving any muscular fatigue. A minimum rest period of 2 min was required between successive trials. The horizontalprojection distance between the tip of the shoes of the participants and the handle bar was set at 15 cm. As was observed in a recent field study (Chen, 2008), this distance took the most advantageous testing posture for participants to apply the force for a full range of heights. Moreover, the horizontal distance was maintained at about half of the width of the box that was used extensively in the Taiwanese hypermarkets. Each strength measurement was repeated until three readings were obtained, which were consistent within a range of 10%, as suggested by Chaffin (1975). As a result, a total of 3780 strength
Shoulder
Hip
Elbow
Knee
15 cm Fig. 2. The definition of the body joint angles while testing in this study.
values (42 participants 2 exertion methods 15 heights 3 repetitions) were determined. 2.5. Statistical analysis The experiment’s data was analyzed using SPSS 10.0 with a significance level of 0.05. While determining the lifting strength, a nested design was used for strength data analysis. Each participant was considered a block. We analyzed the strength data using a two-way analysis of variance (ANOVA) and used Duncan’s multiple-range test for post-hoc comparisons. Differences in participants’ anthropometry, isometric strength of various body parts (Table 1), and strength values for VUL and TBL methods were examined by a t-test. 3. Results 3.1. Differences in strength between workers and novices In this study, there were no significant differences between the two groups with respect to their demographics, anthropometry, and strength measurements. The lifting height influenced the force exerted during TBL among workers (F(14,300) ¼ 42.92, p < 0.001) and novices (F(14,300) ¼ 47.70, p < 0.001). The height of lifting also influenced the forces exerted during VUL in novices (F(14,300) ¼ 6.71, p < 0.001). It was noteworthy that the forces exerted during VUL in workers were not affected by the lifting height (F(14,300) ¼ 0.32, p > 0.05). Post-hoc comparisons demonstrated that the forces exerted during VUL among workers were much higher than that of during TBL method (31.67 kg:11.67 kg); as were the forces among novices (30.45 kg : 10.33 kg). Different strength profiles under various exertion heights are illustrated in Fig. 3. As aforementioned, the VUL forces were always higher than the TBL, regardless of working experience. There were no differences between forces generated during TBL among workers and novices at 15 height levels. However, the VUL strength profiles exhibited an inconsistent trend for the two groups at heights of 100, 110, and 120 cm, with differences of 7.28 kg (t(0.025,40) ¼ 3.699, p < 0.01), 7.61 kg (t(0.025,40) ¼ 3.778, p < 0.01), and 4.57 kg (t(0.025,40) ¼ 2.029, p < 0.05), respectively.
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40
Lifting strengths (kg)
35 30 25
Novices_VUL
20
Novices_TBL 15
Workers_VUL
10
Workers_TBL
5 0 10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
Exertion heights (cm) Fig. 3. Lifting strength profiles under various exertion heights and methods.
3.2. Posture analyses During VUL, lifting postures while exerting maximum strength indicated that there were higher knee, shoulder, and elbow angles at lower height levels (e.g., 10e30 cm) in novices than that for workers’ (Fig. 4). From the figure, it can also be seen that workers employed relatively consistent patterns of joint angles than did the novices, for whichever VUL or TBL that was performed. Fig. 5 further shows the posture for the VUL at a height of 10 cm by one representative participant from each group. In contrast to the TBL method, different postures were adopted between the two groups for the VUL method, however the strength values remained unchanged (Fig. 3). 4. Discussion This study was the first to investigate the static-lifting strength associated with a full range of lifting heights between experienced workers and novices. The results provide an indication that there exists a discrepancy in strength during VUL between the two participant groups at the heights from 100 to 120 cm. Besides this result, similar trends in strength with increased heights were exhibited by VUL and TBL, regardless of working experience. These findings imply that application of lifting strength collected on novices (i.e., students in our study) should be applied cautiously at the workplace as their strength profiles and the postures adopted during lifting differ from workers. That is, a novice worker should be trained to adopt a safer posture at near-floor lifts and a more skillful technique when lifting from about elbow height. Comparative studies have been investigated on the lifting capability of individuals with respect to their work experience. Mital (1987) compared the workers’ MAWL to novices and found that significant differences existed in an 8-h MAWL between the two groups, but no differences in their physiques and isometric strengths was reported. Results from this study corroborated with Mital’s study. The procedure outlined by Chaffin (1975) was followed for the strength measurement in this study, as was adopted by Mital (1987). As shown in Table 1, the measured anthropometric data and isometric strengths of novices were statistically identical to those of workers. However, the lifting strengths were substantially different in VUL between the two groups when the exertion heights were placed at 100e120 cm (p < 0.05). Some investigators
have suggested that the response patterns of MAWL determination of workers and novices to task variables in lifting activities are similar (Garg and Saxena, 1979; Mital and Manivasagan, 1983). Their results may not be completely true as compared to our study, especially for some specific task conditions. VUL strength profiles among novices were in agreement with the result of Lee (2004). Lee examined the lifting strengths of twelve university student participants at 10, 45, 75, 105 and 140 cm, and found the lowest strength to be at the exertion height of 105 cm. However, the author did not provide the discrepancy in strength at 140 versus at 105 cm. As illustrated by the VUL postures in Fig. 4, at about 110 cm height of exertion, location of the shoulder of novices was at its minimum in extension whereas the hip reached its maximum extension. Since novices may apply force only by their upper arms’ muscular strength, this posture can partially explain the lower strength values obtained at 105 cm. It is well-recognized that arm and shoulder strengths are dominant while lifting from higher positions, especially above waist height (Yates et al., 1980). We can therefore attribute the aforementioned phenomenon to a relatively disadvantageous position in the upper extremities, i.e., at about the participant’s elbow height (data of 109.0 cm in Table 1). When the height of lifting exceeded this position, the shoulders of novices began to extend, and then gradually changed to a more beneficial posture for generating maximum forces. With the present findings, it is difficult to understand why the dramatic decrease obtained in VUL forces among novices was not observed among the experienced workers. It should be noted that the strength values of worker’s shoulders and arms were similar to that of novices (Table 1). Furthermore, no pronounced difference in VUL posture strategies at shoulders and arms was also observed at the corresponding heights between the two groups (Fig. 4). However, Keir and MacDonell (2004) found that their experienced patient handlers demonstrated approximately two times greater neck and shoulder muscle activities than the novices, combined with lower back muscle activities. They attributed this biomechanical effect as a learned behavior of the experienced handlers to protect the spine by distributing the load to the shoulder. Recently, Hodder et al. (2010) also found that the experienced nurses had a smaller range of spine motion than novices in addition to greater muscle activation in trapezius and deltoid regions, which may imply a load reduction strategy for the back. Irrespective of the
Y.-L. Chen et al. / International Journal of Industrial Ergonomics 41 (2011) 53e58
57
Fig. 4. Comparison of the testing postures while participants performed the VUL and TBL.
lifting height, it might be that a better working strategy was developed by our worker participants from their daily shelf works exposure resulting in their VUL profiles to be nearly constant, irrespective of the lifting height. In other words, the relative weakness in upper extremities’ strength compared to lower extremities’ of novices at higher levels during VUL may be offset by the worker’s lifting experiences for satisfying the job demands. During the TBL method, similar strength profiles and no significant differences were found between the novices and the workers (Fig. 3). In general, lifting strengths during TBL consistently decreased with increased heights. It is worthwhile to point out that lifting strengths toward the body were much lower than during VUL. One possible reason is that in this experiment, the participants were required to voluntarily exert their maximum exertions without losing their balance. Unlike the VUL method that completely counteracted with the ground during exertion, the TBL method generates resistance between the ground, and the participant’s shoes. This mechanism results in a lower strength measure because more force for body stabilization is needed during lifting,
and was particularly true as the lifting height increased from 10 to 150 cm. Work experience influenced the adopted posture while lifting, especially in the near-floor VUL strength values. Fig. 5 illustrates the comparison of the testing postures between worker and novice participants at a height of 10 cm. For the VUL test, workers chose a more knee-flexion strategy, whereas the novices tended to choose a more stooped one. It is generally accepted that a stooped lifting posture, which can provide higher lifting strengths, would impose more compressive forces on the spinal L5/S1 disc. From the figure, we also can observe a larger horizontal distance (i.e., moment arm) from the handle to the participants’ lower back in novices than that of in workers. This would lead the workers to be more safety conscious than would the novices. The result did not corroborate with the findings of Gagnon et al. (1996). These authors found that the expert handler reduced knee-flexion more than the novice to conserve more energy. However, only the static-lifting strengths examined in our study were not similar to Gagnon et al’s dynamic lifting activity. In the present study, instructions about the testing
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Y.-L. Chen et al. / International Journal of Industrial Ergonomics 41 (2011) 53e58
Novice Worker
15 cm
15 cm
VUL
TBL
Fig. 5. Comparison of the schematic postures between the experienced workers and novices while performing static lifting at a height of 10 cm.
posture were not given to participants. Any difference in postures between the two groups in this study therefore can be reasonably attributed to the influence of work experience in lifting loads. As shown in Fig. 4, the workers adopted a more consistent body posture for both VUL and TBL tests. In contrast, the novices adopted postures depending on the exertion method or height of lift. Irrespective of these postural differences, there was no difference in strength values between workers and novices while performing VUL at lower heights. Based on the present findings, we conclude that the workers, experienced in realistic lifting tasks, would choose a safer and more skillful technique than the novices in generating lifting strength profiles. 5. Conclusion This study demonstrates that the static-lifting strengths of novices were significantly lower than those of experienced workers while upward lifting near the participant’s elbow height (i.e., 100e120 cm). Although similar strength values were reported between workers and novices, a safer technique was chosen by workers at near-floor upward lifting. For both groups, strength exerted toward the body was significantly lower than that of during upward lifting. The results of this study provide an inference that experienced workers may have learned a protective strategy and a more effective lifting technique to satisfy their daily job demand. Furthermore, the results also indicated that the lifting strength data collected on novices (e.g., student participants) should be used judiciously for whichever task variable is to be taken into consideration. Acknowledgement This study was supported by funding from the National Science Council, ROC, grant no. NSC95-2221-E-131-012-MY2. This funding is gratefully acknowledged. References Authier, M., Lortie, M., 1993. Assessment of factors considered to be important in handling tasks by expert handlers. International Journal of Industrial Ergonomics 11, 331e340. Authier, M., Lortie, M., Gagnon, M., 1996. Manual handling techniques: comparing novices and experts. International Journal of Industrial Ergonomics 17, 419e429. Ayoub, M.M., Bethea, N.J., Deivanayagam, S., Asfour, S.S., Bakken, G.M., Liles, D., Mital, A., Sherif, M., 1978. Determination and Modeling of a Lifting Capacity, Final Report, DHEW. NIOSH, Cincinnati.
Baril-Gingras, G., Lortie, M., 1995. The handling of objects other than boxes: univariate analysis of handling techniques in a large transport company. Ergonomics 38, 905e925. Chaffin, D.B., 1975. Ergonomics guide for the assessment of human static strength. American Industrial Hygiene Association Journal 36, 505e511. Chen, Y.L., 2000. Optimal lifting techniques adopted by Chinese men when determining their maximum acceptable weight of lifting. American Industrial Hygiene Association Journal 61, 642e648. Chen, C.J., 2008 Investigation of Musculoskeletal Disorders For the Warehouse Workers in Taiwan, Master Dissertation, Mingchi University of Technology, Taipei, Taiwan. Gagnon, M., Plamondon, A., Gravel, D., Lortie, M., 1996. Knee movement strategies differentiate expert from novice workers in asymmetrical manual materials handling. Journal of Biomechanics 29, 1445e1453. Garg, A., Saxena, U., 1979. Effects of lifting frequency and technique on physical fatigue with special reference to psychophysical methodology and metabolic rate. American Industrial Hygiene Association Journal 49, 894e903. Garg, A., Sharma, D., Chaffin, D.B., Schmidler, J.M., 1983. Biomechanical stresses as related to motion trajectory of lifting. Human Factors 25, 527e539. Herring, S., Hallbeck, S., 2007. The effects of distance and height on maximal isometric push and pull strength with reference to manual transmission truck drivers. International Journal of Industrial Ergonomics 37, 685e696. Hodder, J.N., MacKinnon, S.N., Ralhan, A., Keir, P.J., 2010. Effects of training and experience on patient transfer biomechanics. International Journal of Industrial Ergonomics 40, 282e288. Keir, P.J., MacDonell, C.W., 2004. Muscle activity during patient transfers: a preliminary study on the influence of lift assists and experience. Ergonomics 47, 296e306. Kim, S., Lockhart, T., Nam, C.S., 2010. Leg strength comparison between younger and middle-age adults. International Journal of Industrial Ergonomics 40, 315e320. Knapik, G.G., Marras, W.S., 2009. Spinal loading at different lumbar levels during pushing and pulling. Ergonomics 52, 60e70. Kumar, S., 1991. Arm lift strength in work space. Applied Ergonomics 22, 317e328. Kuorinka, I., Lortie, M., Gautreau, M., 1994. Manual handling in warehouses: the illusion of correct working postures. Ergonomics 37, 655e661. Lee, Y.H., Chen, Y.L., 1996. An isometric predictor for maximal acceptable weight of lift for Chinese men. Human Factors 38, 646e653. Lee, T.H., 2004. Static lifting strengths at different exertion heights. International Journal of Industrial Ergonomics 34, 263e269. Marras, W.S., Parakkat, J., Chany, A.M., Yang, G., Burr, D., Lavender, S.A., 2006. Spine loading as a function of lifting frequency, exposure duration, and work experience. Clinical Biomechanics 21, 345e352. Mital, A., Manivasagan, I., 1983. Maximum acceptable weight of lift as a function of material density, center of gravity location, hand preference, and frequency. Human Factors 25, 33e42. Mital, A., Garg, A., Karwowski, W., Kumar, S., Smith, J.L., Ayoub, M.M., 1993. Status in human strength research and application. IIE Transactions 25, 57e69. Mital, A., 1987. Patterns of differences between the maximum weights of lift acceptable to experienced and inexperienced materials handlers. Ergonomics 30, 1137e1147. Parakkat, J., Yang, G., Chany, A.M., Burr, D., Marras, W.S., 2007. The influence of lift frequency, lift duration and work experience on discomfort reporting. Ergonomics 50, 396e409. Yates, J.W., Kamon, E., Rodgers, S.H., Champney, P.C., 1980. Static lifting strength and maximal isometric voluntary contractions of back, arm and shoulder muscles. Ergonomics 23, 37e47.
Contents lists available at ScienceDirect
International Journal of Industrial Ergonomics journal homepage: www.elsevier.com/locate/ergon
Differences in lifting strength profiles between experienced workers and novices at various exertion heights Yi-Lang Chen a, *, Yu-Chi Lee b, Chia-Jung Chen a a b
Department of Industrial Engineering and Management, Mingchi University of Technology, Taipei, Taiwan Department of Industrial Engineering and Engineering Management, National Tsing Hua University, Hsinchu, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 September 2009 Received in revised form 29 October 2010 Accepted 4 November 2010 Available online 30 November 2010
This study compared lifting strength patterns between experienced workers and novices at various exertion heights. Twenty-one experienced workers and 21 novices were recruited to determine their static-lifting strength under various heights (10e150 cm in increments of 10 cm) using two exertion methods (vertically upward lifting, VUL, and toward body lifting, TBL). Testing posture was also recorded by a motion analysis system for comparing with the corresponding strength values. Results showed that strength during VUL were much higher than strength during TBL at 15 height positions (p < 0.001). Strength in all 30 task combinations showed no difference between the two groups except for the VUL at heights of 100e120 cm. On the average, strength exerted by novices during VUL was 4.57e7.61 kg lower than that of workers while lifting at 100e120 cm heights (all p < 0.05). The strength during TBL consistently decreased with increased heights of lifting. When workers performed VUL, the strength surprisingly remained nearly unchanged throughout the heights of interest. The postures adopted by workers during VUL were also highly differentiated from novices while performing near-floor positions, but the strength was equivalent to each other. This study demonstrated that the static-lifting strength of novices were significantly lower than those of experienced workers while upward lifting near the participant’s elbow height. It was concluded that workers tend to adopt a safer (i.e., more flexed knees) and more skillful technique than novices to generate forces, resulting in lower spinal loads during both methods of lifting. Relevance to Industry: Many studies have established the human strength data based on student participants who do not have experience in manual materials handling. The present findings clearly suggest that lifting strength data collected on novices (e.g., on students) should be carefully applied in the task (re)design at the workplace as their strength profiles and the postures adopted during lifting differ from workers. Ó 2010 Elsevier B.V. All rights reserved.
Keywords: Static-lifting strength Upward lifting Toward body lifting Motion analysis Posture recording
1. Introduction Many studies that established the human strength data for various occupational tasks were usually derived from student participants (Yates et al., 1980; Kumar, 1991; Lee, 2004; Herring and Hallbeck, 2007; Knapik and Marras, 2009; Kim et al., 2010). These studies conducted on student participants were on the basis of: ease of recruitment, greater reliability in collecting data, lower cost in recruiting, and their availability for a long-term study (Mital, 1987). However, it is essential to identify the suitability of an individual’s functional capabilities before directly applying the
* Corresponding author. E-mail address: [email protected] (Y.-L. Chen). 0169-8141/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ergon.2010.11.006
findings conducted on a student population to an entire industrial population. Based on the lifting-related literature, a lifting technique of highly skilled workers substantially differs from that of novices (Authier et al., 1996; Gagnon et al., 1996; Hodder et al., 2010). Field studies have also shown that workers use methods other than the recommended ones (Kuorinka et al., 1994; Baril-Gingras and Lortie, 1995), and lack a consensus on the best methods for lifting (Authier and Lortie, 1993). For a given lifting task, in general, experienced workers employ lower biomechanical spinal loads (Marras et al., 2006), lesser back-muscle activity (Keir and MacDonell, 2004; Hodder et al., 2010), more psychophysically accepted weights (Mital, 1987), and higher subjective discomfort thresholds (Parakkat et al., 2007) than novices. On the contrary, some investigations suggest similar patterns of Maximum acceptable weight of lift (MAWL) for industrial and non-industrial workers in manual
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lifting activities (Mital and Manivasagan, 1983; Garg and Saxena, 1979). Such a discrepancy may attribute to the diverse experimental settings utilized these studies. One of the factors affecting lifting strength is exertion height (Yates et al., 1980; Mital et al., 1993; Lee, 2004). This is extremely realistic in the service warehouse stores as the storage spaces have to be economized and maximized. Investigations on lifting strength are usually confined to some specific height levels (e.g., floor, knuckle, and shoulder). Height of lifting a workload, in addition to the posture adopted during a lift also influenced human strengths (Yates et al., 1980; Lee, 2004), but also in posture strategy (Chen, 2000). Vertically upward lifting (VUL) has been accepted as the standard method in previous strength measurement studies (Ayoub et al., 1978; Chaffin, 1975; Kumar, 1991, Lee, 2004). However, another human strength measurement that usually adopted is toward body lifting (TBL). In a study conducted by Garg et al. (1983), their participants pulled the load toward the body while lifting psychophysically determined maximum weights. Lifting at an angle resulted in a decrease in moment at the upper arms and spinal L5/S1 joints and an increase in moment at the knees and ankles. Lee and Chen (1996) requested the participant to pull on the handle toward the body and found that the lifting strength can be a better predictor for the MAWL than other isometric strength. Therefore, identification of strength differences generated by different exertion directions deserves further investigation. Due to the presence of varied lifting heights and postures adopted in actual working conditions, more exhaustive strength data is needed for ergonomic considerations at the workplace. Every week, in a majority large-scale warehouse stores (e.g., the hypermarket) with high employee turnover rates recruit novice workers or part-timers in Taiwan (Chen, 2008). To our knowledge, no comparative studies have been found in the literature that systemically assessed the simulated lifting strengths of workers and novices under a full range of working heights. Therefore, it is becoming imperative to develop training programs and suitable shelf designs for lifting-related tasks by understanding the differences in lifting strengths between workers and novices. This study, therefore, consists of exertion heights and lifting methods to differentiate the lifting strengths of experienced workers from novices. The results from the present study would be important in improving the shelf design and direct the novice workers to a safer and more effective lifting technique. We hypothesized that lifting strategies adopted by experienced workers and novices would be different because of workers’ lifting experience. The differences in lifting postures among participants would therefore generate different lifting strength profiles under a full range of working heights. 2. Methods
data and isometric strengths (Digital Dynamometer, T.K.K., Tokyo, Japan) using the protocol of Chaffin (1975) were measured (Table 1). 2.2. Experimental apparatus Each participant’s static-lifting strength was measured using a Static-Lifting Strength Tester (SLST), as illustrated in Fig. 1. The SLST, with an incremental height setting consisted of: a standing platform; a steel frame with 20 positioning holes (ranging from 10 cm to 200 cm in increments of 5 cm); a sliding height stopper along the frame; and a 55 cm handle bar (diameter: 3.5 cm) attached to the stopper. The force applied on the handle bar was measured by a load cell (Jackson Strength Evaluation System, JSES Model 32628, U.S.A.) that connected to the handle. Strength values at 60 Hz were transferred to a digital readout unit by an AeD converter. The AeD converter was calibrated prior to the strength testing against known weights. 2.3. Body posture recording Lifting postures of a random sample (8 workers and 8 students) were videotaped for the study. Six adhesive reflective markers were attached to the participant’s wrist, elbow, shoulder, hip, knee, and ankle joints on the right side (Fig. 1). In the study, the joint angles were defined as the inter-joint angles between the two adjacent segments. For example, the knee angle was the included angle between the line formed from the knee to the ankle and the line formed from the knee to the hip (Fig. 2). The digital video camera (Qualisys MacReflex, Sweden) set up at a distance of 5 m from participants, was used to record the positions of each joint marker when participants performed the strength test. The camera heights were adjusted to align the participant’s hip positions orthogonally to eliminate, if any, the distortion errors resulting from the varied testing heights. An image processing software (Qualisys MacReflex, Sweden) was then used to calculate the relevant joint angles. 2.4. Experimental procedure All participants were familiarized with the experimental procedures, and stretched themselves at least 10 min before the data was collected. Fifteen lifting heights, ranging from 10 to 150 cm and spaced 10 cm apart, were set up on the SLST. There were two exertion methods performed by each participant at all
Table 1 Data of the experienced workers and novices (21 of each) in this study. Variablesa
2.1. Participants Twenty-one experienced workers [age in mean (SD): 23.1 (3.0) years] and 21 novices [23.3 (2.4) years], with no prior history of any musculoskeletal disorder, volunteered for this study. This study was approved by Chang Gung Memorial Hospital’s (Taiwan) Ethics Committee. Workers, with at least two-years of experience in manual lifting, were chosen as experienced participants. They were selected from a large hypermarket, and their duties consisted mainly in varied replenishing tasks onto the shelves. Novice participants (with no manual materials handling experience) were recruited from university students. All participants were paid and fully informed of the testing procedure, and full consent was obtained. During the experiment, the participants were asked to wear light clothing and rubber gym shoes. Their anthropometric
Age (years) Stature (cm) Body weight (kg) Acromial height (cm) Elbow height (cm) Knuckle height (cm) Hip height (cm) Knee height (cm) Hand length (cm) Hand width (cm) Isometric strengths (kg) Composite strength Back strength Shoulder strength Arm strength a
Workers
Novices
Mean
S.D.
Mean
S.D.
22.8 174.9 67.4 144.5 110.1 70.4 87.3 48.0 16.7 8.4
2.9 6.5 11.0 6.5 5.1 4.4 5.7 2.5 0.8 0.7
23.4 172.8 66.1 143.0 109.0 69.7 89.2 48.0 16.5 8.3
2.3 3.5 6.7 3.2 3.0 2.4 2.8 2.3 0.7 0.4
91.0 76.9 43.5 31.4
18.8 20.2 10.5 8.1
84.3 67.3 46.2 33.4
15.6 19.5 14.0 7.2
No difference was found between two groups in all variables, all p > 0.05.
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Height positioning device
Load cell
A-D converter
Fig. 1. The device of static-lifting strength and schematic testing posture in the study.
lifting heights. One exertion method was VUL and the other was TBL. They were randomly requested to perform maximum voluntary exertion at a specific exertion height and method in free style but with sagittally symmetrical lifting postures. All participants were requested to perform the maximum lifting strengths at least three times for each task combination. The strength-testing procedure was performed according to the methodology used by Chaffin (1975) and Ayoub et al. (1978). A period of 5 s was chosen as sufficient time to build up and maintain a constant level of exertion, while being brief enough to be endured without perceiving any muscular fatigue. A minimum rest period of 2 min was required between successive trials. The horizontalprojection distance between the tip of the shoes of the participants and the handle bar was set at 15 cm. As was observed in a recent field study (Chen, 2008), this distance took the most advantageous testing posture for participants to apply the force for a full range of heights. Moreover, the horizontal distance was maintained at about half of the width of the box that was used extensively in the Taiwanese hypermarkets. Each strength measurement was repeated until three readings were obtained, which were consistent within a range of 10%, as suggested by Chaffin (1975). As a result, a total of 3780 strength
Shoulder
Hip
Elbow
Knee
15 cm Fig. 2. The definition of the body joint angles while testing in this study.
values (42 participants 2 exertion methods 15 heights 3 repetitions) were determined. 2.5. Statistical analysis The experiment’s data was analyzed using SPSS 10.0 with a significance level of 0.05. While determining the lifting strength, a nested design was used for strength data analysis. Each participant was considered a block. We analyzed the strength data using a two-way analysis of variance (ANOVA) and used Duncan’s multiple-range test for post-hoc comparisons. Differences in participants’ anthropometry, isometric strength of various body parts (Table 1), and strength values for VUL and TBL methods were examined by a t-test. 3. Results 3.1. Differences in strength between workers and novices In this study, there were no significant differences between the two groups with respect to their demographics, anthropometry, and strength measurements. The lifting height influenced the force exerted during TBL among workers (F(14,300) ¼ 42.92, p < 0.001) and novices (F(14,300) ¼ 47.70, p < 0.001). The height of lifting also influenced the forces exerted during VUL in novices (F(14,300) ¼ 6.71, p < 0.001). It was noteworthy that the forces exerted during VUL in workers were not affected by the lifting height (F(14,300) ¼ 0.32, p > 0.05). Post-hoc comparisons demonstrated that the forces exerted during VUL among workers were much higher than that of during TBL method (31.67 kg:11.67 kg); as were the forces among novices (30.45 kg : 10.33 kg). Different strength profiles under various exertion heights are illustrated in Fig. 3. As aforementioned, the VUL forces were always higher than the TBL, regardless of working experience. There were no differences between forces generated during TBL among workers and novices at 15 height levels. However, the VUL strength profiles exhibited an inconsistent trend for the two groups at heights of 100, 110, and 120 cm, with differences of 7.28 kg (t(0.025,40) ¼ 3.699, p < 0.01), 7.61 kg (t(0.025,40) ¼ 3.778, p < 0.01), and 4.57 kg (t(0.025,40) ¼ 2.029, p < 0.05), respectively.
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40
Lifting strengths (kg)
35 30 25
Novices_VUL
20
Novices_TBL 15
Workers_VUL
10
Workers_TBL
5 0 10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
Exertion heights (cm) Fig. 3. Lifting strength profiles under various exertion heights and methods.
3.2. Posture analyses During VUL, lifting postures while exerting maximum strength indicated that there were higher knee, shoulder, and elbow angles at lower height levels (e.g., 10e30 cm) in novices than that for workers’ (Fig. 4). From the figure, it can also be seen that workers employed relatively consistent patterns of joint angles than did the novices, for whichever VUL or TBL that was performed. Fig. 5 further shows the posture for the VUL at a height of 10 cm by one representative participant from each group. In contrast to the TBL method, different postures were adopted between the two groups for the VUL method, however the strength values remained unchanged (Fig. 3). 4. Discussion This study was the first to investigate the static-lifting strength associated with a full range of lifting heights between experienced workers and novices. The results provide an indication that there exists a discrepancy in strength during VUL between the two participant groups at the heights from 100 to 120 cm. Besides this result, similar trends in strength with increased heights were exhibited by VUL and TBL, regardless of working experience. These findings imply that application of lifting strength collected on novices (i.e., students in our study) should be applied cautiously at the workplace as their strength profiles and the postures adopted during lifting differ from workers. That is, a novice worker should be trained to adopt a safer posture at near-floor lifts and a more skillful technique when lifting from about elbow height. Comparative studies have been investigated on the lifting capability of individuals with respect to their work experience. Mital (1987) compared the workers’ MAWL to novices and found that significant differences existed in an 8-h MAWL between the two groups, but no differences in their physiques and isometric strengths was reported. Results from this study corroborated with Mital’s study. The procedure outlined by Chaffin (1975) was followed for the strength measurement in this study, as was adopted by Mital (1987). As shown in Table 1, the measured anthropometric data and isometric strengths of novices were statistically identical to those of workers. However, the lifting strengths were substantially different in VUL between the two groups when the exertion heights were placed at 100e120 cm (p < 0.05). Some investigators
have suggested that the response patterns of MAWL determination of workers and novices to task variables in lifting activities are similar (Garg and Saxena, 1979; Mital and Manivasagan, 1983). Their results may not be completely true as compared to our study, especially for some specific task conditions. VUL strength profiles among novices were in agreement with the result of Lee (2004). Lee examined the lifting strengths of twelve university student participants at 10, 45, 75, 105 and 140 cm, and found the lowest strength to be at the exertion height of 105 cm. However, the author did not provide the discrepancy in strength at 140 versus at 105 cm. As illustrated by the VUL postures in Fig. 4, at about 110 cm height of exertion, location of the shoulder of novices was at its minimum in extension whereas the hip reached its maximum extension. Since novices may apply force only by their upper arms’ muscular strength, this posture can partially explain the lower strength values obtained at 105 cm. It is well-recognized that arm and shoulder strengths are dominant while lifting from higher positions, especially above waist height (Yates et al., 1980). We can therefore attribute the aforementioned phenomenon to a relatively disadvantageous position in the upper extremities, i.e., at about the participant’s elbow height (data of 109.0 cm in Table 1). When the height of lifting exceeded this position, the shoulders of novices began to extend, and then gradually changed to a more beneficial posture for generating maximum forces. With the present findings, it is difficult to understand why the dramatic decrease obtained in VUL forces among novices was not observed among the experienced workers. It should be noted that the strength values of worker’s shoulders and arms were similar to that of novices (Table 1). Furthermore, no pronounced difference in VUL posture strategies at shoulders and arms was also observed at the corresponding heights between the two groups (Fig. 4). However, Keir and MacDonell (2004) found that their experienced patient handlers demonstrated approximately two times greater neck and shoulder muscle activities than the novices, combined with lower back muscle activities. They attributed this biomechanical effect as a learned behavior of the experienced handlers to protect the spine by distributing the load to the shoulder. Recently, Hodder et al. (2010) also found that the experienced nurses had a smaller range of spine motion than novices in addition to greater muscle activation in trapezius and deltoid regions, which may imply a load reduction strategy for the back. Irrespective of the
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Fig. 4. Comparison of the testing postures while participants performed the VUL and TBL.
lifting height, it might be that a better working strategy was developed by our worker participants from their daily shelf works exposure resulting in their VUL profiles to be nearly constant, irrespective of the lifting height. In other words, the relative weakness in upper extremities’ strength compared to lower extremities’ of novices at higher levels during VUL may be offset by the worker’s lifting experiences for satisfying the job demands. During the TBL method, similar strength profiles and no significant differences were found between the novices and the workers (Fig. 3). In general, lifting strengths during TBL consistently decreased with increased heights. It is worthwhile to point out that lifting strengths toward the body were much lower than during VUL. One possible reason is that in this experiment, the participants were required to voluntarily exert their maximum exertions without losing their balance. Unlike the VUL method that completely counteracted with the ground during exertion, the TBL method generates resistance between the ground, and the participant’s shoes. This mechanism results in a lower strength measure because more force for body stabilization is needed during lifting,
and was particularly true as the lifting height increased from 10 to 150 cm. Work experience influenced the adopted posture while lifting, especially in the near-floor VUL strength values. Fig. 5 illustrates the comparison of the testing postures between worker and novice participants at a height of 10 cm. For the VUL test, workers chose a more knee-flexion strategy, whereas the novices tended to choose a more stooped one. It is generally accepted that a stooped lifting posture, which can provide higher lifting strengths, would impose more compressive forces on the spinal L5/S1 disc. From the figure, we also can observe a larger horizontal distance (i.e., moment arm) from the handle to the participants’ lower back in novices than that of in workers. This would lead the workers to be more safety conscious than would the novices. The result did not corroborate with the findings of Gagnon et al. (1996). These authors found that the expert handler reduced knee-flexion more than the novice to conserve more energy. However, only the static-lifting strengths examined in our study were not similar to Gagnon et al’s dynamic lifting activity. In the present study, instructions about the testing
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Novice Worker
15 cm
15 cm
VUL
TBL
Fig. 5. Comparison of the schematic postures between the experienced workers and novices while performing static lifting at a height of 10 cm.
posture were not given to participants. Any difference in postures between the two groups in this study therefore can be reasonably attributed to the influence of work experience in lifting loads. As shown in Fig. 4, the workers adopted a more consistent body posture for both VUL and TBL tests. In contrast, the novices adopted postures depending on the exertion method or height of lift. Irrespective of these postural differences, there was no difference in strength values between workers and novices while performing VUL at lower heights. Based on the present findings, we conclude that the workers, experienced in realistic lifting tasks, would choose a safer and more skillful technique than the novices in generating lifting strength profiles. 5. Conclusion This study demonstrates that the static-lifting strengths of novices were significantly lower than those of experienced workers while upward lifting near the participant’s elbow height (i.e., 100e120 cm). Although similar strength values were reported between workers and novices, a safer technique was chosen by workers at near-floor upward lifting. For both groups, strength exerted toward the body was significantly lower than that of during upward lifting. The results of this study provide an inference that experienced workers may have learned a protective strategy and a more effective lifting technique to satisfy their daily job demand. Furthermore, the results also indicated that the lifting strength data collected on novices (e.g., student participants) should be used judiciously for whichever task variable is to be taken into consideration. Acknowledgement This study was supported by funding from the National Science Council, ROC, grant no. NSC95-2221-E-131-012-MY2. This funding is gratefully acknowledged. References Authier, M., Lortie, M., 1993. Assessment of factors considered to be important in handling tasks by expert handlers. International Journal of Industrial Ergonomics 11, 331e340. Authier, M., Lortie, M., Gagnon, M., 1996. Manual handling techniques: comparing novices and experts. International Journal of Industrial Ergonomics 17, 419e429. Ayoub, M.M., Bethea, N.J., Deivanayagam, S., Asfour, S.S., Bakken, G.M., Liles, D., Mital, A., Sherif, M., 1978. Determination and Modeling of a Lifting Capacity, Final Report, DHEW. NIOSH, Cincinnati.
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