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Effects of the adjustment period on psychophysically determined maximum acceptable weight of lift and the physiological cost
International Journal of Industrial Ergonomics 31 (2003) 287–294
Effects of the adjustment period on psychophysically determined maximum acceptable weight of lift and the physiological cost Swei-Pi Wu*, Jing-Ping Chen Ergonomics Research Laboratory, Department of Industrial Management, Huafan University, 1 Hua Fan Road, Shihtin Hsiang, Taipei Hsien 223, Taiwan, ROC Received 11 March 2002; received in revised form 6 May 2002; accepted 31 October 2002
Abstract The purpose of this study was to investigate the influence of adjustment periods (20, 30, 40, and 50 min) on the maximum acceptable weights of lift (MAWL) and the resulting responses (heart rate and rating of perceived exertion) on participants during lifting a container from the floor to knuckle height at various frequencies (1, 2, 4, and 6 lifts/min). A total of 6 males were recruited as the participants to perform 16 different combinations of lifting tasks. The results show that: (1) The adjustment period had a significant effect on the MAWL, and the MAWL decreased significantly as adjustment period increased. However, the effect of the adjustment period on the heart rate was not significant. In addition, the effect of the adjustment period on the rating of perceived exertion was significant. The ratings of perceived exertion (RPE) value increased as the adjustment period increased; (2) Even though the lifting frequency significantly affected the maximum acceptable weights, the lifting frequency had no significant effect on the percentage of decrease in MAWL from the 20-min adjustment period values. The participants lifted 3%, 11% and 11% less MAWL when the lifting task was performed at 30, 40, and 50 min adjustment periods, respectively. Relevance industry At the current time, there are not only extensive databases on lifting and lowering tasks, but also various multipliers and correction factors to extend the range of applicability for this data. Though many researchers have investigated the validity and reliability of the psychophysical data generated in short trials for 8-h or more of work, a very important issue concerning the effect of the adjustment period of psychophysical approach on the lifting capacity has been overlooked and not been studied. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Psychophysics; Adjustment periods; Manual materials handling; Maximum acceptable weight of lift
1. Introduction *Corresponding author. Tel.: +886-2-266-32-102; fax: +886-2266-31-119. E-mail address: [email protected] (S.-P. Wu).
The psychophysical approach, proposed by Snook and Irvine (1967), has been utilized extensively to investigate human capacity in
0169-8141/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-8141(02)00218-4
288
S.-P. Wu, J.-P. Chen / International Journal of Industrial Ergonomics 31 (2003) 287–294
manual material handling tasks (MMH) for over three decades. As mentioned by Gamberale et al. (1987), this approach aimed to quantify human lifting capacity based on the subjective perception of exertion, under the assumption that workers are able to determine with a reasonable accuracy the highest acceptable workload they could sustain over an 8-h workday. According to Snook et al. (1970), the experimental procedures required the participants to imagine working on an incentive basis, as hard as they could (lifting as much as they could), without straining themselves or becoming unusually tired, weak, overheated, or out-ofbreath. Snook et al. (1970) proposed using their experimental procedures to determine the participant’s preference for the weight to be lifted under given task conditions, based on monitoring his/her own feelings of exertion or fatigue, and to call such subjectively chosen weights the maximum acceptable weights of lift (MAWL). In addition, Snook (1978) referred to this method as a combination of adjustment and tracking, that allows a worker to ‘‘integrate the sensory inputs into one meaningful response’’. In general, the use of a psychophysical approach in the study of MMH tasks requires the participants to begin with a random weight choice (either a very heavy or very light load) and to adjust the load such that repetitive lifting or handling is acceptable. The adjustment period lasts approximately 20–40 min, and the final weight at the end of this adjustment period is considered the maximum weight that the participant can lift comfortably for 8-h. It is expected that the weight selected by a participant at the end of the 20– 40 min adjustment period will not change if the participant continues lifting for 8-h. Many investigations have found that the psychophysical approach appears to produce reliable results for MMH tasks characterized by low and moderate frequencies (Legg and Myles, 1981; Mital and Manivasagan, 1983; Aghazadeh and Ayoub, 1985; Karwowski and Yates, 1984, 1986; Ciriello et al., 1990; Fernandez et al., 1991). At the current time, there are not only extensive databases on lifting and lowering tasks (Snook and Ciriello, 1991), but also various multipliers and correction factors (Mital et al. 1993) to extend the
range of applicability for this data. Though many researchers have investigated the validity and reliability of the psychophysical data generated in short trials for 8-h or more of work, a very important issue concerning the adjustment period for the psychophysical approach has been overlooked and not been studied. After reviewing the previous studies, a significant difference was found among the adjustment periods. According to Snook (1978), a 40-min adjustment period was provided to allow participants to monitor their own feelings and adjust the object weight. However, many researchers (Legg and Myles, 1981; Mital, 1983, 1984; Mital and Manivasagan, 1983; Karwowski and Yates, 1986; Mital and Aghazadeh, 1987; Zhu and Zhang, 1990; Ostrom et al., 1990; Fernandez et al., 1991; Chen et al., 1992) had participants determine MAWL within shorter periods (such as 15, 20, 25, or 30 min). Seldom have researchers (Garg and Saxena, 1982; Garg and Beller, 1994) used a longer period (such as 45, 50, or 60 min) for the adjustment period. In general, previous studies have identified many factors affecting the perceived subjective response. These factors can be classified as worker characteristics, object, task, and environment. However, the variation of the adjustment period may also be a factor influencing the maximum acceptable weight of lift. The purpose of this study was therefore to evaluate the effect of the psychophysical approach adjustment period on the maximum acceptable weights of lift and the resulting responses of the participants.
2. Methods 2.1. Participants Six healthy participants with no history of musculoskeletal problems were recruited from the male student population at the University of Huafan. They were at least one year of materials handling experience in psychophysical experiment and compensated for their participation in the study. After participants were familiarized with the experimental procedures, the database for each
S.-P. Wu, J.-P. Chen / International Journal of Industrial Ergonomics 31 (2003) 287–294 Table 1 Summary of anthropometric and strength measurements for the male participants Items
Mean
S.D.
Range
Age (years) Weight (kg) Height (cm) Acromial height (cm) Iliac crest height (cm) Knuckle height (cm) Abdominal depth (cm) Isometric strength (kg) Arm Stooped back Composite Shoulder Leg HR max (beats/min)
21.3 63.6 168.9 138.8 96.2 72.9 13.6
1.64 9.19 6.40 6.65 4.91 3.58 2.36
19–24 49.0–76.0 160.5–175.0 131.0–147.5 89.8–103.8 69.5–78.5 10.4–16.7
24.8 57.2 84.8 35.8 79.8 198.0
5.10 18.70 11.30 8.30 19.60 5.70
20.4–34.3 31.6–78.5 68.8–100.1 26.6–44.9 61.3–117.1 192–204
participant’s physical condition was collected including the anthropometric measurements, the strength tests and the heart rate (HR max) tests. The procedures were described as Wu and Chen (2001). The participants were instructed to refrain from eating, smoking, consumption of alcoholic beverages or carbonated liquids for at least two hours prior to the data collection session. The participants were also instructed to avoid taking part in any strenuous physical activity prior to the experiment and to maintain their normal sleeping patterns. Table 1 lists the physical characteristics of the participants. 2.2. Experimental design A two-factor randomized complete block design, with the participants as blocks, was used to investigate the effects of the adjustment period on the psychophysically determined maximum acceptable weights of lift (MAWL), heart rate, and ratings of perceived exertion (RPE) from the floor to knuckle height. The factors were the adjustment period with four levels (20, 30, 40 and 50 min) and lifting frequency with four levels (1, 2, 4, and 6 lifts/min). The adjustment period and lifting frequency were considered fixed factors and the participants a random factor. The room temperature was maintained at 22B241C; the relative
289
humidity was 45–55%. Weights were lifted in a box 48 cm wide (frontal plane), 36 cm deep (sagittal plane), 22 cm high, 15 cm thick and fitted with handles (5 cm 150 cm opening 5 cm below the top edge). Lifting was performed using a freestyle technique. Each participant performed lifting for all 16 tasks. The order of presentation of conditions was completely randomized. As a result, a total of 96 experimental lifting trials (6 participants 4 adjustment periods 4 frequencies) were performed. The participants wore work shoes and comfortable clothing during the experiment. 2.3. Participant training The participants were given a six-day training and conditioning program designed to improve their strength and endurance for perform the lifting tasks and to familiarize them fully with the psychophysical weight adjustment method. The daily training program included two parts: (1) a 30-min series of exercises designed to improve basic strength and flexibility, and (2) practice in using the psychophysical approach to set the loads lifted under the experimental conditions. In the load adjustment part of the daily training, participants were given four 15-min periods to simulate different lifting tasks at four different frequencies. The four lifting tasks were performed in a random order with the participants resting between each condition until their heart rate returned to their resting levels. In addition, in order to ensure that the participants understood the meaning of simulating the task for 8 h, on training day 5 and 6, two different experimental conditions were randomly chosen and performed by the participants. 2.4. Experimental procedure Following the anthropometric measurements, strength tests and participant training, participants were asked to perform lifting tasks from the floor to knuckle height. The instructions given to the participants were essentially the same as those used by Snook et al. (1970) and Snook and Ciriello (1974). The experimental procedure was essentially
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the same as that used in Ciriello et al. (1993). The participants asked to adjust the weight until it represented the maximum the amount that they could handle for 8-h without straining themselves or becoming unusually tired, weakened, overheated, or out of breath. During the lifting tasks participants varied the weight in the wooden boxes by adding or subtracting lead shots. In an attempt to minimize visual cues, the boxes contained false bottoms that could hold up to 15 kg. Participants were aware of the false bottoms but never knew how much weight they contained. The amount of weights in the false bottoms was randomly varied. Each participant was allowed to make as many weight adjustments as he felt necessary in order to arrive at the weight he felt he could lift continuously for an 8-h period at the specified lifting trial. A metronome was used to pace each participant. As soon as the participants heard the sound of the metronome, they lifted the box then waited for the next sound. No incentives or emotional appeals were applied to minimize the emotional influence. The entire adjustment period took 20, 30, 40, or 50 min in a random order. The participants’ heart rate was continuously monitored during the last 10-min and the mean heart rate for the last 10-min was used for analysis. Each participant was also asked to rate the perceived exertion (RPE) in the wrists, arms, shoulders, legs, back and whole body at the end of each lifting task (Borg, 1962). On any given day, data for only one lifting combination were collected for each participant.
3. Results 3.1. Interaction effects The results of the ANOVA analysis (Table 2) showed that the adjustment period and frequency have not significant interaction effects on the MAWL and heard rate (p > 0:05). This result shows that the main effect of adjustment period on MAWL is independent of the main effect of lifting frequency, and without considering the combined effect of these two factors for different task. However the interaction effect of adjustment
period and frequency on overall RPE is significant (po0:05). This was due to the RPE values of the 2 lifts/min being smaller than that for 1 lift/min during 50 min adjustment period.
3.2. Effect of adjustment period As can be seen in Table 2, the adjustment period had a significant effect on the maximum acceptable weights of lift (po0:01). The Duncan multiple range test (Table 3) also showed a significant difference (po0:05) in MAWL among the four adjustment periods. It was found that the MAWL for the 20-min adjustment period was significantly larger than that for the 40 and 50-min adjustment periods, but not significantly different from the
Table 2 Summary of the analysis of variance with MAWL, heart rate and overall RPE as the dependent variables
Participants Adjustment period Frequency Adjustment period Frequency
MAWL
Heart rate
Overall RPE
**
**
**
**
ns
**
**
**
**
ns
ns
*
*
Significant at 0.05. Significant at 0.01. ns=Non-significant. **
Table 3 Overall means and Duncan results for MAWL, heart rate and RPE Variables MAWL (kg) Heart rate (beats/min) Overall RPE Adjustment period 20 24.47 30 23.85 40 21.77 50 21.76
(min) A A B B
117.22 122.00 119.53 120.85
A A A A
12.77 13.18 13.20 13.60
A B B C
Lifting frequency (lifts/min) 1 27.20 A 103.05 2 24.29 B 109.14 4 21.62 C 128.44 6 18.75 D 138.96
A B C D
12.68 12.96 13.29 13.89
A A B C
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30
160
MAWL (kg)
25
1 lifts/min
20
2 lifs/min
15
4 lifts/min 6 lifts/min
10
Heart rate (beats/min)
35
5
291
140 1 lifts/min 120
2 lifts/min
100
4 lifts/min 6 lifts/min
80 60
0 20
30
40
40
50
20
Adjustment period (min)
30-min adjustment period. However, the MAWL for the 40 and 50-min adjustment periods did not differ significantly from one another (p > 0:05). Fig. 1 shows the effect of the adjustment period on MAWL. Even though there was a decrease in the maximum acceptable weights of lift, however, the overall mean for the heart rate was not significantly changed with adjustment period (p > 0:05). Fig. 2 shows the effect of the adjustment period on the overall men heart rate while lifting for the different frequencies. The effect of the adjustment period on RPE for the whole body was statistically significant (po0:01). The subsequent Duncan test indicated that the RPE for the 20-min adjustment period was significantly smaller (12.77) than those for the 30, 40, and 50-min adjustment periods. However, the 30 and 40-min adjustment periods did not differ significantly from one another, and the RPE for the 50-min adjustment period was the largest (13.60). Fig. 3 shows the effect of the adjustment period on RPE and the difference in RPE for various body parts while lifting under the different adjustment periods. 3.3. Effect of lifting frequency As expected, the maximum acceptable weights of lift were significantly influenced by the lifting frequency (po0:01). The Duncan multiple range test indicated that the MAWLs for all frequencies were different from one another (po0:05). Even though there was a decrease in the maximum acceptable weights of lift, the heart rate
50
Fig. 2. Effect of the adjustment period and frequency on heart rate. 16.00 14.00
RPE
Fig. 1. Effect of the adjustment period and frequency on MAWL.
30 40 Adjustment period (min)
12.00
wrists
10.00
arms shoulders
8.00
back 6.00
legs
4.00
whole
2.00 0.00 20
30
40
50
Adjustment period (min)
Fig. 3. Effect of the adjustment period on RPE.
increased with an increase in the lifting frequency. The analysis of variance results also showed that the lifting frequency had a highly significant effect on the heart rate (po0:01). The Duncan test also showed that the heart rates for all frequencies were significantly different from one another (po0:05). The effect of the lifting frequency on the rating of perceived exertion for the whole body was also statistically significant (po0:01). The Duncan test also showed significant differences (po0:05) among the four lifting frequencies. In general, the participants rated their perceived stress level as increasing with lifting frequency, and the most stressed body parts were the legs and back.
4. Discussion 4.1. Comparison with past studies This study confirms the result of previous studies (Ciriello and Snook, 1983; Ciriello et al,
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Table 4 Decrement (%) in maximum acceptable weights of lift with lifting frequency at various adjustment periods based on the present and past study Lifting frequency (lifts/min) 1 2 4 6
20 min
40 min
Present study
Chen et al. (1992)
Wu and Hsu (1993)
Present study
Ciriello and Snook (1983)
Ciriello et al. (1990)
100 87 77 70
100 91 78 &
100 & 78 70
100 90 81 70
100 & & 66n
100 & 84 &
&Experimental data not collected. n Lifting frequency is 6.7 lifts/min.
1990; Chen et al. 1992; Wu and Hsu, 1993), that the maximum acceptable weights of lift is significantly affected by the lifting frequency. Direct comparison of MAWL values from present and past studies is difficult due to differences in experimental conditions. However, for the purpose of analysis and comparisons, the MAWL was expressed as a percentage. Weight lifted at a frequency of one lift per min was considered as the 100% level. Acceptable weights of lift at other lifting frequency levels were expressed as percentage of the weight lifted at these datum levels. Table 4 shows the decline in MAWL for various lifting frequencies relative to 20 and 40 min adjustment periods. As shown in Table 4, the decrement in MAWL with lifting frequency for both 20 and 40 min adjustment periods was about the same as frequency increased from 1 to 6 lifts/min (70%). In addition, the decrement in MAWL with frequency was in complete agreement between the present study and previous studies. This confirms the external validity of this study. 4.2. Adjustment period vs. psychophysics As mentioned earlier, the main purpose of this study was to examine the effects of the adjustment period on psychophysical weight estimated and the resulting heart rate responses and the participants’ perceived stress level. This study revealed that the maximum acceptable weights of lift and the rating of perceived exertion were significantly affected by
Table 5 Maximum acceptable weights of lift expressed as a percentage of the corresponding values for the 20-min adjustment period Adjustment period (min) 20 30 40 50
Frequency (lift/min) 1
2
4
100 98.64 86.88 85.45
100 101.90 90.45 88.95
100 97.70 92.30 94.30
6
Average
100 100 90.03 97.07 86.47 89.02 88.09 89.20
the adjustment period. Even though the MAWL decreased with an increase in the adjustment period, the overall RPE increased as the adjustment period increased. In addition, the effect of adjustment time on the heart rate was not significant. In order to explore the differences in the MAWL between the various lifting frequencies, the MAWL values were expressed as a percentage. Table 5 lists the MAWL as a percentage of the respective weights relative to the 20-min adjustment period for all four lifting frequencies. These data were obtained by dividing the weight for a given lifting frequency and adjustment period by the weight for the same lifting frequency relative to the 20-min adjustment period and multiplying it by 100. As can be seen, there was a consistent decrease in the percentages of MAWL with an increase in the adjustment period. On average, the participants lifted 3%, 11% and 11% less MAWL when the lifting tasks were
S.-P. Wu, J.-P. Chen / International Journal of Industrial Ergonomics 31 (2003) 287–294
performed at 30, 40 and 50 min adjustment periods, respectively. Further analysis indicated that when the adjustment period increased from 20 to 30-min, the MAWL percentages decreased by approximately 3%. A further decrease of 8% was observed when the adjustment period increased from 30 to 40-min. However, when the adjustment period increased from 40 to 50-min, the MAWL percentage remained unchanged. In other words, when the adjustment period was greater than 40-min, the change in trend of decrease in MAWL was flat. Therefore, using a 40-min adjustment period for the psychophysical approach is needed. This finding was consistent with the adjustment period used by Snook (1978). Noticeably, when the adjustment period was lower than 40-min, the MAWL decided by the participants was overestimated. Therefore, further revision is needed. When the MAWL expressed as a percentage of the corresponding values for the 40-min adjustment period used by Snook (1978). The overestimated percentage of MAWL for the 20 and 30-min adjustment periods were approximately 12.4% and 9.5%, respectively. In general, the psychophysical approach has been utilized extensively to collect the data of maximum acceptable weights of handling, and the validity had been investigated by many studies. However, the adjustment period is a key factor of psychophysics and has been shown to directly affect the maximum acceptable weights determined by the subjects. Therefore, the effects of adjustment period should be considered when using the approach. Of course, further study is needed due to the use of smaller participants.
293
adjustment period on the MAWL and the resulting responses from the participants during lifting a container from floor to knuckle height for an 8-h workday. The results obtained and the comparison of these results with those from previous studies lead to the following conclusions. First, the results showed that the maximum acceptable weights were affected significantly by the adjustment period. The MAWL decreased with an increase in the adjustment time. However, the physiological costs demonstrated no significant difference among the four adjustment periods. In addition, the effect of the adjustment period on the rating of perceived exertion was significant. Second, the results revealed that the lifting frequency had no significant effect on the percentage of decrease in the maximum acceptable weight of lift from the 20-min adjustment period values. The participants lifted 3%, 11% and 11% less MAWL when the adjustment period was 30, 40 and 50-min, respectively. Third, based on the 40-min adjustment period used by Snook (1978), this study showed that the MAWLs were overestimated by about 9.5 and 12.4% when the adjustment periods were decreased to 30, and 20 min, respectively.
Acknowledgements This work was supported by the National Science Research Council, Taiwan, ROC. Project No. NSC 89-2213-E-211-017.
References 5. Conclusions For approximately three decades, researchers have utilized the psychophysical approach to determine MAWLs for MMH tasks. Early studies by Stoves Snook and his colleagues provided 40 min periods for participants to monitor their feelings and adjust the object weight. However, many researchers used shorter periods for participants to determine MAWLs. The purpose of this study was to investigate the influence of the
Aghazadeh, F., Ayoub, M.M., 1985. A comparison of dynamicand static-strength models for prediction of lifting capacity. Ergonomics 28 (10), 1409–1417. Borg, G., 1962. Physical Performance and Perceived Exertion. Gleerup, Lund. Chen, F., Aghazadeh, F., Lee, K.S., 1992. Prediction of the maximum acceptable weight of symmetrical and asymmetrical lift using direct estimation method. Ergonomics 35, 755–768. Ciriello, V.M., Snook, S.H., 1983. A study of size, distance height and frequency effects on manual handling tasks. Human Factors 25, 473–484.
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Ciriello, V.M., Snook, S.H., Blick, A.C., Wilkinson, P.L., 1990. The effects duration on psychophysically determined maximum acceptable weights and forces. Ergonomics 33, 187–200. Ciriello, V.M., Snook, S.H., Hughes, G.J., 1993. Further studies of psychophysically determined maximum acceptable weights and forces. Human Factors 35, 175–186. Fernandez, J.E., Ayoub, M.M., Smith, J.L., 1991. Psychophysical lifting capacity over extended periods. Ergonomics 34, 23–32. Garg, A., Beller, D., 1994. A comparison of isokinetic lifting strength with static strength and maximum acceptable weight with special reference to speed of lifting. Ergonomics 37 (8), 1363–1374. Garg, A., Saxena, U., 1982. Maximum frequency acceptable to female workers for one-hand lifts in the horizontal plane. Ergonomics 25 (9), 839–853. Gamberale, F., Ljungberg, A.S., Annwall, A., Kilbom, A., 1987. An experimental evaluation of psychophysical criteria for repetitive lifting work. Applied Ergonomics 18, 311–321. Karwowski, W., Yates, J.W., 1984. The effects of time in the psychophysical study of the maximum acceptable amounts of liquid lifted by females. In: Proceedings of the Human Factors Society’s 28th Annual Meeting, San Antonio, Texas, pp. 586–590. Karwowski, W., Yates, J.W., 1986. Reliability of the psychophysical approach to manual lifting of liquids by females. Ergonomics 29, 237–248. Legg, S.J., Myles, W.S., 1981. Maximum acceptable repetitive lifting workload for an 8 hour work-day using psychophysical and subjective rating methods. Ergonomics 24, 907–916. Mital, A., 1983. The psychophysical approach in manual lifting—a verification study. Human Factors 25, 485–491.
Mital, A., 1984. Maximum weights of lift acceptable to male and female industrial workers for extended work shifts. Ergonomics 27 (11), 1115–1126. Mital, A., Aghazadeh, F., 1987. Psychophysical lifting capabilities for overreach heights. Ergonomics 30 (6), 901–909. Mital, A., Manivasagan, I., 1983. Subjective estimates of onehanded carrying tasks. Applied Ergonomics 14, 265–269. Mital, A., Nicholson, A.S., Ayoub, M.M., 1993. A Guide to Manual Materials Handling. Taylor & Francis, London. Ostrom, L.T., Smith, J.L., Ayoub, M.M., 1990. The effect of rigid container shape on maximum acceptable weight of lift. Proceedings of the Human Factors Society 34th Annual Meeting, Santa Monica, CA. Snook, S.H., 1978. The design of manual handling tasks. Ergonomics 21, 963–985. Snook, S.H., Ciriello, V.M., 1974. Maximum weights and workloads acceptable to female workers. Journal of Occupational Medicine 16 (8), 527–534. Snook, S.H., Ciriello, V.M., 1991. The design of manual handling tasks: revised table of maximum acceptable weight and forces. Ergonomics 34 (9), 1197–1213. Snook, S.H., Irvine, C.H., 1967. Maximum acceptable weights of lift. American Industrial Hygiene Association Journal 28, 322–329. Snook, S.H., Irvine, C.H., Bass, S.F., 1970. Maximum weights and work loads acceptable to male industrial workers. American Industrial Hygiene Association Journal 31, 579–586. Wu, S.P., Chen, C.C., 2001. Psychophysical determination of load carrying capacity for a 1-h work period by Chinese males. Ergonomics 44 (11), 1008–1023. Wu, S.P., Hsu, S.H., 1993. Psychophysical modeling of lifting capacity using strength variables for Chinese males. Applied Ergonomics 24 (3), 251–257. Zhu, Z., Zhang, Z., 1990. Maximum acceptable repetitive lifting workload by Chinese subjects. Ergonomics 33 (7), 875–884.
Effects of the adjustment period on psychophysically determined maximum acceptable weight of lift and the physiological cost Swei-Pi Wu*, Jing-Ping Chen Ergonomics Research Laboratory, Department of Industrial Management, Huafan University, 1 Hua Fan Road, Shihtin Hsiang, Taipei Hsien 223, Taiwan, ROC Received 11 March 2002; received in revised form 6 May 2002; accepted 31 October 2002
Abstract The purpose of this study was to investigate the influence of adjustment periods (20, 30, 40, and 50 min) on the maximum acceptable weights of lift (MAWL) and the resulting responses (heart rate and rating of perceived exertion) on participants during lifting a container from the floor to knuckle height at various frequencies (1, 2, 4, and 6 lifts/min). A total of 6 males were recruited as the participants to perform 16 different combinations of lifting tasks. The results show that: (1) The adjustment period had a significant effect on the MAWL, and the MAWL decreased significantly as adjustment period increased. However, the effect of the adjustment period on the heart rate was not significant. In addition, the effect of the adjustment period on the rating of perceived exertion was significant. The ratings of perceived exertion (RPE) value increased as the adjustment period increased; (2) Even though the lifting frequency significantly affected the maximum acceptable weights, the lifting frequency had no significant effect on the percentage of decrease in MAWL from the 20-min adjustment period values. The participants lifted 3%, 11% and 11% less MAWL when the lifting task was performed at 30, 40, and 50 min adjustment periods, respectively. Relevance industry At the current time, there are not only extensive databases on lifting and lowering tasks, but also various multipliers and correction factors to extend the range of applicability for this data. Though many researchers have investigated the validity and reliability of the psychophysical data generated in short trials for 8-h or more of work, a very important issue concerning the effect of the adjustment period of psychophysical approach on the lifting capacity has been overlooked and not been studied. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Psychophysics; Adjustment periods; Manual materials handling; Maximum acceptable weight of lift
1. Introduction *Corresponding author. Tel.: +886-2-266-32-102; fax: +886-2266-31-119. E-mail address: [email protected] (S.-P. Wu).
The psychophysical approach, proposed by Snook and Irvine (1967), has been utilized extensively to investigate human capacity in
0169-8141/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-8141(02)00218-4
288
S.-P. Wu, J.-P. Chen / International Journal of Industrial Ergonomics 31 (2003) 287–294
manual material handling tasks (MMH) for over three decades. As mentioned by Gamberale et al. (1987), this approach aimed to quantify human lifting capacity based on the subjective perception of exertion, under the assumption that workers are able to determine with a reasonable accuracy the highest acceptable workload they could sustain over an 8-h workday. According to Snook et al. (1970), the experimental procedures required the participants to imagine working on an incentive basis, as hard as they could (lifting as much as they could), without straining themselves or becoming unusually tired, weak, overheated, or out-ofbreath. Snook et al. (1970) proposed using their experimental procedures to determine the participant’s preference for the weight to be lifted under given task conditions, based on monitoring his/her own feelings of exertion or fatigue, and to call such subjectively chosen weights the maximum acceptable weights of lift (MAWL). In addition, Snook (1978) referred to this method as a combination of adjustment and tracking, that allows a worker to ‘‘integrate the sensory inputs into one meaningful response’’. In general, the use of a psychophysical approach in the study of MMH tasks requires the participants to begin with a random weight choice (either a very heavy or very light load) and to adjust the load such that repetitive lifting or handling is acceptable. The adjustment period lasts approximately 20–40 min, and the final weight at the end of this adjustment period is considered the maximum weight that the participant can lift comfortably for 8-h. It is expected that the weight selected by a participant at the end of the 20– 40 min adjustment period will not change if the participant continues lifting for 8-h. Many investigations have found that the psychophysical approach appears to produce reliable results for MMH tasks characterized by low and moderate frequencies (Legg and Myles, 1981; Mital and Manivasagan, 1983; Aghazadeh and Ayoub, 1985; Karwowski and Yates, 1984, 1986; Ciriello et al., 1990; Fernandez et al., 1991). At the current time, there are not only extensive databases on lifting and lowering tasks (Snook and Ciriello, 1991), but also various multipliers and correction factors (Mital et al. 1993) to extend the
range of applicability for this data. Though many researchers have investigated the validity and reliability of the psychophysical data generated in short trials for 8-h or more of work, a very important issue concerning the adjustment period for the psychophysical approach has been overlooked and not been studied. After reviewing the previous studies, a significant difference was found among the adjustment periods. According to Snook (1978), a 40-min adjustment period was provided to allow participants to monitor their own feelings and adjust the object weight. However, many researchers (Legg and Myles, 1981; Mital, 1983, 1984; Mital and Manivasagan, 1983; Karwowski and Yates, 1986; Mital and Aghazadeh, 1987; Zhu and Zhang, 1990; Ostrom et al., 1990; Fernandez et al., 1991; Chen et al., 1992) had participants determine MAWL within shorter periods (such as 15, 20, 25, or 30 min). Seldom have researchers (Garg and Saxena, 1982; Garg and Beller, 1994) used a longer period (such as 45, 50, or 60 min) for the adjustment period. In general, previous studies have identified many factors affecting the perceived subjective response. These factors can be classified as worker characteristics, object, task, and environment. However, the variation of the adjustment period may also be a factor influencing the maximum acceptable weight of lift. The purpose of this study was therefore to evaluate the effect of the psychophysical approach adjustment period on the maximum acceptable weights of lift and the resulting responses of the participants.
2. Methods 2.1. Participants Six healthy participants with no history of musculoskeletal problems were recruited from the male student population at the University of Huafan. They were at least one year of materials handling experience in psychophysical experiment and compensated for their participation in the study. After participants were familiarized with the experimental procedures, the database for each
S.-P. Wu, J.-P. Chen / International Journal of Industrial Ergonomics 31 (2003) 287–294 Table 1 Summary of anthropometric and strength measurements for the male participants Items
Mean
S.D.
Range
Age (years) Weight (kg) Height (cm) Acromial height (cm) Iliac crest height (cm) Knuckle height (cm) Abdominal depth (cm) Isometric strength (kg) Arm Stooped back Composite Shoulder Leg HR max (beats/min)
21.3 63.6 168.9 138.8 96.2 72.9 13.6
1.64 9.19 6.40 6.65 4.91 3.58 2.36
19–24 49.0–76.0 160.5–175.0 131.0–147.5 89.8–103.8 69.5–78.5 10.4–16.7
24.8 57.2 84.8 35.8 79.8 198.0
5.10 18.70 11.30 8.30 19.60 5.70
20.4–34.3 31.6–78.5 68.8–100.1 26.6–44.9 61.3–117.1 192–204
participant’s physical condition was collected including the anthropometric measurements, the strength tests and the heart rate (HR max) tests. The procedures were described as Wu and Chen (2001). The participants were instructed to refrain from eating, smoking, consumption of alcoholic beverages or carbonated liquids for at least two hours prior to the data collection session. The participants were also instructed to avoid taking part in any strenuous physical activity prior to the experiment and to maintain their normal sleeping patterns. Table 1 lists the physical characteristics of the participants. 2.2. Experimental design A two-factor randomized complete block design, with the participants as blocks, was used to investigate the effects of the adjustment period on the psychophysically determined maximum acceptable weights of lift (MAWL), heart rate, and ratings of perceived exertion (RPE) from the floor to knuckle height. The factors were the adjustment period with four levels (20, 30, 40 and 50 min) and lifting frequency with four levels (1, 2, 4, and 6 lifts/min). The adjustment period and lifting frequency were considered fixed factors and the participants a random factor. The room temperature was maintained at 22B241C; the relative
289
humidity was 45–55%. Weights were lifted in a box 48 cm wide (frontal plane), 36 cm deep (sagittal plane), 22 cm high, 15 cm thick and fitted with handles (5 cm 150 cm opening 5 cm below the top edge). Lifting was performed using a freestyle technique. Each participant performed lifting for all 16 tasks. The order of presentation of conditions was completely randomized. As a result, a total of 96 experimental lifting trials (6 participants 4 adjustment periods 4 frequencies) were performed. The participants wore work shoes and comfortable clothing during the experiment. 2.3. Participant training The participants were given a six-day training and conditioning program designed to improve their strength and endurance for perform the lifting tasks and to familiarize them fully with the psychophysical weight adjustment method. The daily training program included two parts: (1) a 30-min series of exercises designed to improve basic strength and flexibility, and (2) practice in using the psychophysical approach to set the loads lifted under the experimental conditions. In the load adjustment part of the daily training, participants were given four 15-min periods to simulate different lifting tasks at four different frequencies. The four lifting tasks were performed in a random order with the participants resting between each condition until their heart rate returned to their resting levels. In addition, in order to ensure that the participants understood the meaning of simulating the task for 8 h, on training day 5 and 6, two different experimental conditions were randomly chosen and performed by the participants. 2.4. Experimental procedure Following the anthropometric measurements, strength tests and participant training, participants were asked to perform lifting tasks from the floor to knuckle height. The instructions given to the participants were essentially the same as those used by Snook et al. (1970) and Snook and Ciriello (1974). The experimental procedure was essentially
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the same as that used in Ciriello et al. (1993). The participants asked to adjust the weight until it represented the maximum the amount that they could handle for 8-h without straining themselves or becoming unusually tired, weakened, overheated, or out of breath. During the lifting tasks participants varied the weight in the wooden boxes by adding or subtracting lead shots. In an attempt to minimize visual cues, the boxes contained false bottoms that could hold up to 15 kg. Participants were aware of the false bottoms but never knew how much weight they contained. The amount of weights in the false bottoms was randomly varied. Each participant was allowed to make as many weight adjustments as he felt necessary in order to arrive at the weight he felt he could lift continuously for an 8-h period at the specified lifting trial. A metronome was used to pace each participant. As soon as the participants heard the sound of the metronome, they lifted the box then waited for the next sound. No incentives or emotional appeals were applied to minimize the emotional influence. The entire adjustment period took 20, 30, 40, or 50 min in a random order. The participants’ heart rate was continuously monitored during the last 10-min and the mean heart rate for the last 10-min was used for analysis. Each participant was also asked to rate the perceived exertion (RPE) in the wrists, arms, shoulders, legs, back and whole body at the end of each lifting task (Borg, 1962). On any given day, data for only one lifting combination were collected for each participant.
3. Results 3.1. Interaction effects The results of the ANOVA analysis (Table 2) showed that the adjustment period and frequency have not significant interaction effects on the MAWL and heard rate (p > 0:05). This result shows that the main effect of adjustment period on MAWL is independent of the main effect of lifting frequency, and without considering the combined effect of these two factors for different task. However the interaction effect of adjustment
period and frequency on overall RPE is significant (po0:05). This was due to the RPE values of the 2 lifts/min being smaller than that for 1 lift/min during 50 min adjustment period.
3.2. Effect of adjustment period As can be seen in Table 2, the adjustment period had a significant effect on the maximum acceptable weights of lift (po0:01). The Duncan multiple range test (Table 3) also showed a significant difference (po0:05) in MAWL among the four adjustment periods. It was found that the MAWL for the 20-min adjustment period was significantly larger than that for the 40 and 50-min adjustment periods, but not significantly different from the
Table 2 Summary of the analysis of variance with MAWL, heart rate and overall RPE as the dependent variables
Participants Adjustment period Frequency Adjustment period Frequency
MAWL
Heart rate
Overall RPE
**
**
**
**
ns
**
**
**
**
ns
ns
*
*
Significant at 0.05. Significant at 0.01. ns=Non-significant. **
Table 3 Overall means and Duncan results for MAWL, heart rate and RPE Variables MAWL (kg) Heart rate (beats/min) Overall RPE Adjustment period 20 24.47 30 23.85 40 21.77 50 21.76
(min) A A B B
117.22 122.00 119.53 120.85
A A A A
12.77 13.18 13.20 13.60
A B B C
Lifting frequency (lifts/min) 1 27.20 A 103.05 2 24.29 B 109.14 4 21.62 C 128.44 6 18.75 D 138.96
A B C D
12.68 12.96 13.29 13.89
A A B C
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30
160
MAWL (kg)
25
1 lifts/min
20
2 lifs/min
15
4 lifts/min 6 lifts/min
10
Heart rate (beats/min)
35
5
291
140 1 lifts/min 120
2 lifts/min
100
4 lifts/min 6 lifts/min
80 60
0 20
30
40
40
50
20
Adjustment period (min)
30-min adjustment period. However, the MAWL for the 40 and 50-min adjustment periods did not differ significantly from one another (p > 0:05). Fig. 1 shows the effect of the adjustment period on MAWL. Even though there was a decrease in the maximum acceptable weights of lift, however, the overall mean for the heart rate was not significantly changed with adjustment period (p > 0:05). Fig. 2 shows the effect of the adjustment period on the overall men heart rate while lifting for the different frequencies. The effect of the adjustment period on RPE for the whole body was statistically significant (po0:01). The subsequent Duncan test indicated that the RPE for the 20-min adjustment period was significantly smaller (12.77) than those for the 30, 40, and 50-min adjustment periods. However, the 30 and 40-min adjustment periods did not differ significantly from one another, and the RPE for the 50-min adjustment period was the largest (13.60). Fig. 3 shows the effect of the adjustment period on RPE and the difference in RPE for various body parts while lifting under the different adjustment periods. 3.3. Effect of lifting frequency As expected, the maximum acceptable weights of lift were significantly influenced by the lifting frequency (po0:01). The Duncan multiple range test indicated that the MAWLs for all frequencies were different from one another (po0:05). Even though there was a decrease in the maximum acceptable weights of lift, the heart rate
50
Fig. 2. Effect of the adjustment period and frequency on heart rate. 16.00 14.00
RPE
Fig. 1. Effect of the adjustment period and frequency on MAWL.
30 40 Adjustment period (min)
12.00
wrists
10.00
arms shoulders
8.00
back 6.00
legs
4.00
whole
2.00 0.00 20
30
40
50
Adjustment period (min)
Fig. 3. Effect of the adjustment period on RPE.
increased with an increase in the lifting frequency. The analysis of variance results also showed that the lifting frequency had a highly significant effect on the heart rate (po0:01). The Duncan test also showed that the heart rates for all frequencies were significantly different from one another (po0:05). The effect of the lifting frequency on the rating of perceived exertion for the whole body was also statistically significant (po0:01). The Duncan test also showed significant differences (po0:05) among the four lifting frequencies. In general, the participants rated their perceived stress level as increasing with lifting frequency, and the most stressed body parts were the legs and back.
4. Discussion 4.1. Comparison with past studies This study confirms the result of previous studies (Ciriello and Snook, 1983; Ciriello et al,
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Table 4 Decrement (%) in maximum acceptable weights of lift with lifting frequency at various adjustment periods based on the present and past study Lifting frequency (lifts/min) 1 2 4 6
20 min
40 min
Present study
Chen et al. (1992)
Wu and Hsu (1993)
Present study
Ciriello and Snook (1983)
Ciriello et al. (1990)
100 87 77 70
100 91 78 &
100 & 78 70
100 90 81 70
100 & & 66n
100 & 84 &
&Experimental data not collected. n Lifting frequency is 6.7 lifts/min.
1990; Chen et al. 1992; Wu and Hsu, 1993), that the maximum acceptable weights of lift is significantly affected by the lifting frequency. Direct comparison of MAWL values from present and past studies is difficult due to differences in experimental conditions. However, for the purpose of analysis and comparisons, the MAWL was expressed as a percentage. Weight lifted at a frequency of one lift per min was considered as the 100% level. Acceptable weights of lift at other lifting frequency levels were expressed as percentage of the weight lifted at these datum levels. Table 4 shows the decline in MAWL for various lifting frequencies relative to 20 and 40 min adjustment periods. As shown in Table 4, the decrement in MAWL with lifting frequency for both 20 and 40 min adjustment periods was about the same as frequency increased from 1 to 6 lifts/min (70%). In addition, the decrement in MAWL with frequency was in complete agreement between the present study and previous studies. This confirms the external validity of this study. 4.2. Adjustment period vs. psychophysics As mentioned earlier, the main purpose of this study was to examine the effects of the adjustment period on psychophysical weight estimated and the resulting heart rate responses and the participants’ perceived stress level. This study revealed that the maximum acceptable weights of lift and the rating of perceived exertion were significantly affected by
Table 5 Maximum acceptable weights of lift expressed as a percentage of the corresponding values for the 20-min adjustment period Adjustment period (min) 20 30 40 50
Frequency (lift/min) 1
2
4
100 98.64 86.88 85.45
100 101.90 90.45 88.95
100 97.70 92.30 94.30
6
Average
100 100 90.03 97.07 86.47 89.02 88.09 89.20
the adjustment period. Even though the MAWL decreased with an increase in the adjustment period, the overall RPE increased as the adjustment period increased. In addition, the effect of adjustment time on the heart rate was not significant. In order to explore the differences in the MAWL between the various lifting frequencies, the MAWL values were expressed as a percentage. Table 5 lists the MAWL as a percentage of the respective weights relative to the 20-min adjustment period for all four lifting frequencies. These data were obtained by dividing the weight for a given lifting frequency and adjustment period by the weight for the same lifting frequency relative to the 20-min adjustment period and multiplying it by 100. As can be seen, there was a consistent decrease in the percentages of MAWL with an increase in the adjustment period. On average, the participants lifted 3%, 11% and 11% less MAWL when the lifting tasks were
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performed at 30, 40 and 50 min adjustment periods, respectively. Further analysis indicated that when the adjustment period increased from 20 to 30-min, the MAWL percentages decreased by approximately 3%. A further decrease of 8% was observed when the adjustment period increased from 30 to 40-min. However, when the adjustment period increased from 40 to 50-min, the MAWL percentage remained unchanged. In other words, when the adjustment period was greater than 40-min, the change in trend of decrease in MAWL was flat. Therefore, using a 40-min adjustment period for the psychophysical approach is needed. This finding was consistent with the adjustment period used by Snook (1978). Noticeably, when the adjustment period was lower than 40-min, the MAWL decided by the participants was overestimated. Therefore, further revision is needed. When the MAWL expressed as a percentage of the corresponding values for the 40-min adjustment period used by Snook (1978). The overestimated percentage of MAWL for the 20 and 30-min adjustment periods were approximately 12.4% and 9.5%, respectively. In general, the psychophysical approach has been utilized extensively to collect the data of maximum acceptable weights of handling, and the validity had been investigated by many studies. However, the adjustment period is a key factor of psychophysics and has been shown to directly affect the maximum acceptable weights determined by the subjects. Therefore, the effects of adjustment period should be considered when using the approach. Of course, further study is needed due to the use of smaller participants.
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adjustment period on the MAWL and the resulting responses from the participants during lifting a container from floor to knuckle height for an 8-h workday. The results obtained and the comparison of these results with those from previous studies lead to the following conclusions. First, the results showed that the maximum acceptable weights were affected significantly by the adjustment period. The MAWL decreased with an increase in the adjustment time. However, the physiological costs demonstrated no significant difference among the four adjustment periods. In addition, the effect of the adjustment period on the rating of perceived exertion was significant. Second, the results revealed that the lifting frequency had no significant effect on the percentage of decrease in the maximum acceptable weight of lift from the 20-min adjustment period values. The participants lifted 3%, 11% and 11% less MAWL when the adjustment period was 30, 40 and 50-min, respectively. Third, based on the 40-min adjustment period used by Snook (1978), this study showed that the MAWLs were overestimated by about 9.5 and 12.4% when the adjustment periods were decreased to 30, and 20 min, respectively.
Acknowledgements This work was supported by the National Science Research Council, Taiwan, ROC. Project No. NSC 89-2213-E-211-017.
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