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Psychophysical models for manual lifting tasks
Applied Ergonomics 1990, 21.4,295-303
Psychophysical models for manual lifting tasks A.M. Genaidy*, S.S. Asfour t, A. Mital**, and S.M. Waly t *Western Michigan University, Kalamazoo,Michigan, USA, tUniversity of Miami, Coral Gables, Florida, USA, **University of Cincinnati, Cincinnati, Ohio, USA. This paper provides two models for males and females to assessthe psychophysical maximum acceptable weight of lift. The weight guidelines generated by the models are a function of lifting frequency, height of lift, sagittal or asymmetrical lifting, task duration, container size in the sagittal plane, presence or absence of container couplings, and percentage of the working population. The developed models were generated from a knowledge base available in the published literature. A computer program was written in BASIC to assist the user in determining the safe load that could be handled by a specified working population. Model validation showed that the models developed predict the maximum acceptable weight of lift with a reasonable degree of accuracy. A comparison between the National Institute for Occupational Safety and Health recommendations and those made on the basis of the models developed in this paper is also presented.
Keywords: Lifting, psyehophysical assessment, human performance, models
Introduction The psychophysical criterion is one design approach that can be used in setting guidelines for manual lifting tasks with an aim to eliminate and/or minimise the number and severity of back injuries (Liles et al, 1984). In the psychophysical approach, individuals are instructed to adjust, by adding or subtracting, the amount of load moved to achieve a weight they are willing to sustain over a given period of time. In the literature, this adjusted weight is referred to as the maximum acceptable weight of lift (MAWL). Based on a recent literature survey (Genaidy et al, 1988), it was found that most psychophysical capacity models do not account for the effects of many important parameters such as task duration, asymmetrical lifting activities, percentage of the working population, and container couplings. If capacity models are to apply to realistic jobs, it is important that these parameters be considered. The purpose of this work was to provide MAWL recommendations for lifting tasks through the development of models that take into account, among other factors, the effects of the aforementioned variables. The proposed models are based on the latest published research in manual materials handling. The secondary objective of this work was to compare the proposed manual lifting recommendations with the lifting guidelines published by the National Institute for Occupational Safety and Health (NIOSH, 1981).
Review of psychophysical studies Three major psychophysical studies were conducted by Ayoub et al (1978), Snook (1978) and Mital (1983, 1984a,
0003-6870/90/04 0295-09 $03.00 ©1990 Butterworth-Heinemann Ltd
1984b) on industrial workers. A summary of the experimental methods and procedures of these studies is given in Table 1. Ayoub et al (1978) collected data on MAWL based on 73 male and 73 female industrial workers. Each subject was given 2 0 - 2 5 min to estimate MAWL for an 8-h work day. Snook (1978) integrated the results of his earlier work in the form of tables that could aid in the design of manual lifting tasks. The MAWL results were reported for various percentiles of the worker population. A normal distribution was assumed for this purpose. Task duration was set to 40 min in all experiments. The data collected by Snook was based on 43 male and 15 female industrial workers. Mital (1983) questioned the validity of the psychophysical approach in determining MAWL for an 8-h work day based on trial periods of less than 1 h. Experiments aimed at determining the maximum acceptable weight of lift for 8 h and 12 h were conducted. Five male and five female industrial workers participated in the experiments. Each subject was given 20 min to estimate MAWL for an 8-h work day. On the following day, he/she was asked to lift the amount of load determined on the previous day for 12 h. They were permitted to make any necessary adjustment during this period. The results indicated that MAWL decreased at a rate of 3.4% per hour for males, and 2% per hour for females. Thirty-seven male and 37 female industrial workers participated in another set of experiments by Mital (1984a, 1984b). Each subject estimated MAWL for an 8-h work shift over a period of 2 0 - 2 5 min. They were then given an
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Table I: Summary of variables and experimental design procedures used in psychophysical studies i
Source
Variables
Experimental design used
Ayoub eta/(1978)
Frequency of lift (2, 4, 6, 8 times/min)
Incomplete randomised block design where each subject performed a total of nine combinations out of 72 possible treatments
Height of lift (floor-to-knuckle, floor-to-shoulder, floor-to-reach, knuckle-to-shoulder, knuckle-to-reach, shoulder-to-reach) Box dimension in sagittal plane (30.48, 45.72, 60"96 cm) Gender (73 male and 73 female industrial workers) Snook (1978)
Frequency of lift (1,4.29, 6.67, 12 times/min)
Factorial design
Height of lift (floor-to-knuckle, knuckle-to-shoulder, sh ou Ider-to-reach ) Vertical distance of lift (25, 51,76 cm) Box dimension in sagittal plane (36, 49, 75 cm) Gender (43 male and 15 female industrial workers)
Mital (1984b, 1984c)
Frequency of lift (1,4, 8, 12 times/rain) Height of lift (floor-to-knuckle, knuckle-to-shoulder, shoulder-to-reach)
Incomplete randomised block design where each subject performed a total of nine combinations out of 36 possible treatments
Box dimension in sagittal plane (30"48, 45-72, 60"96 cm) Gender (37 male and 37 female industrial workers)
additional 20 min to estimate MAWL for a 12-h work shift. All data collected on the 74 subjects were adjusted by the multipliers developed from Mital's previous study (1983). Since MAWL decreased as a function of time, the database developed by Ayoub et al (1978) is appropriate only for estimating an individual's MAWL for a lifting duration of less than 1 h. Similarly, the data obtained by Snook (1978) on industrial workers are appropriate only for estimating an individual's MAWL for a lifting duration of less than 1 h.
Model development In this work, the data collected by Ayoub et al (1978), Snook (1978) and Mital (1984a, 1984b) on industrial workers were integrated to form the MAWL knowledge base. Based on this knowledge base, two regression models were developed. These models are intended to estimate MAWL for male or for female industrial workers. This separation was necessary in order to improve the predictive ability of the models, instead of combining the two models into one and incorporating gender as an independent variable in the model. The inputs to the models are: vertical range of lift, frequency of lift, box dimension in the sagittal plane, popu-
296
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tation percentage, task duration, presence or absence of handles, and sagittal or asymmetrical lifting condition. In this study, the vertical range of lift is defined as the vertical travel distance in cm between the origin and destination of lift. The frequency of lift is the average number of lifts on the job per minute. The box dimension in the sagittal plane is the width of the box away from the body. The population percentage is based on the criterion of MAWL; for example, 90 indicates that 90 out of 100 will be able to handle the load on the job. Task duration is the amount of time in hours spent while lifting loads. Sagittal lifting involves lifting loads in front of the body without twisting the upper torso, while asymmetrical lifting includes lifting loads to the sides of the body or while twisting the upper torso. A total of 2760 observations was used in developing the models. The following concepts were used in the development of the models reported in this study: 1. To account for the effects of task duration, the MAWL database of Ayoub et al (1978), Snook (1978), and Mital (1984a, 1984b) were adjusted by the multipliers provided by Mital (1983).
Table 2 : M a x i m u m acceptable weight of lift models
.
I
Sex
R2
Model
Male
0.8323
MAW
= [36.19824 - 0.68647"HL - 0.79578*FREO - 0.02843"BS - 0"18687"PP] * [1.0142 - 0 . 0 3 4 * T I M E ] * [1 - 0 . 0 6 7 5 " T L - O . 0 7 2 * H A N D ]
Female
0-8596
MAW
= [21.04973-0.58098"HL - 0.40147*FREO - 0.02497"BS - 0.08236"PP] * [1.0081 - O . 0 1 9 4 * T I M E ] * [ 1 - 0 . 0 6 7 5 " TL - O . 0 7 2 * H A N D ]
Note: = M a x i m u m acceptable w e i g h t of l i f t (kg) = Height o f l i f t (floor-to-knuckle = 1, floor-toshoulder = 2, floor-to-reach = 3, knuckle-toshoulder = 4, knuckle-to-reach = 5, shoulder-toreach = 6) F R E Q = Frequency of lift (times/min) BS = Box dimension in the sagittal plane (cm) PP = Percentage of w o r k i n g p o p u l a t i o n (e g, 95) T I M E = Working time (hours) TL = Type of lift (sagittal = 0, asymmetrical = 1 ) H A N D = Handles (no handles = 0, handles = 1 ) MAW HL
2. The studies by Ayoub et al (1978), Snook (1978) and Mital (1984a, 1984b) assumed that a lifting task consists of moving a box fitted with handles. However, Garg and Saxena (1980) demonstrated that the absence of handles on the container to be lifted reduces MAWL on the average by 7"2%. Thus, in order to account for the effect of a container with no handles, the MAWL data were reduced by a multiplier of 7"2%.
The lifting height range was def'med by the origin and destination of lift. For starting and termination points other than the knuckle, shoulder and reach heights, the procedure described in Table 3 was used. Table 3 requires the knowledge of knuckle, shoulder and reach heights. In the original work of Ayoub et al (1978) only the knowledge of knuckle height was requiredt shoulder height was approximated by adding 51 cm to the knuckle height. Table 3 incorporates additional information to account for various anthropometric data of the working population. . A single task may require lifting in different ranges. This is determined using the following procedure: a. Determine the minimum and maximum points of origin of lift with respect to the floor level in cm. b. Determine the minimum and maximum points of destination of lift with respect to the floor level in cm.
c. Determine from Table 3 the range of assignment that the following points belong to: (i) minimum point of origin of lift and minimum point of destination of lift; (ii) minimum point of origin of lift and maximum point of destination of lift; (iii) maximum point of origin of lift and minimum point of destination of lift; (iv) maximum point of origin of lift and maximum point of destination of lift. Disregard the combinations that do not belong to the range assignments in Table 3. The following example will illustrate the abovementioned procedure: Suppose that K = 76 cm, S = 127 c m , R = 178 cm. Thus, (K/2) = 38 cm, (K+S)/2 = 101-5 cm, (S+R)/2 = 152.5 cm. These values will be the basis for the origin and destination of lift for Table 3 as follows: Origin of lift
Destination of lift
Range assignment
0 t o 38
0 to 101"5 101.5 to 152"5 152"5 and above
floor-knuckle floor-shoulder floor-reach
38 t o 76
38 t o 101 "5 101 "5 to 152.5 152"5 and above
floor-knuckle knuckle-shoulder knuckle-reach
4. A normal distribution was assumed to compute MAWL for various percentages of the working population.
76 to 101.5
76 to 152"5 152.5 and above
knuckle-shoulder knuckle-reach
5. In instances where the MAWL was available under identical experimental conditions (such as the same levels of task variables), the lower MAWL value was used.
101 "5 to 127
101 '5 t o 152"5 152"5 and above
knuckle-shoulder shoulder-reach
127 and above
127 and above
shoulder-reach
3. In the studies by Ayoub et al (1978), Snook (1978) and Mital (1984a, 1984b), a lifting task was assumed to be performed in the sagittal plane without twisting. Studies by Asfour et al (1984), Garg and Badger (1986), and Mital and Fard (1986) showed that asymmetrical lifting reduces MAWL by 5% to 8.5%. Thus, to account for asymmetrical lifting, the MAWL data were reduced by 6-75%.
6. Since MAWL is determined for a given percentage of the population, the knuckle, shoulder and reach heights of this population were used. The following model, based on data reported by Kroemer et al (1986), was developed to predict these height values (R 2 = 0"9964): H V = [35-30985 + 64-14671"HL - l l - 7 1 * S E X - 0-18187*PP] where: HI/" HL SEX PP
= height value (cm), = height level (knuckle = 1, shoulder = 2, reach = 3). = sex (male = I, female = 2), = percentage of the working population (e g, 95).
Assume the following: (i) origin of lift: minimum point = 20 cm above floor level, maximum point = 90 cm abovefloor level; (ii) destination of lift: minimum point = 70 cm, maximum point = 190 cm. By looking at the above results, one can arrive at the following range assignment: floor-knuckle, floor-reach, knuckle-reach. It should be noted that the 90 to 70 height range was disregarded since it does not belong to any of the range assignments. Once the lifting ranges required by each lifting task are recognised, one can determine the lifting capacity for each
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Table 3: Origin and destination of lift (Modified from A y o u b e t a l (1978)
Origin of lift (cm)
Destinationof lift (cm)
Range assignment
0 to K/2
0 to (K + S)/2 (K + S)/2 to (S + R)/2 (S + R)/2 and above
Floor-knuckle Floor-shoulder Floor--reach
K/2 to K
K/2 to ( K + S)/2 (K + S)/2 to (S + R)/2 (S + R)/2 and above
Floor-knuckle Knuckle-shoulder Knuckle--reach
K to (K + S)/2
K to (S + R)/2 (S + R)/2 and above
Knuckle-shoulder Knuckle-reach
(K + S)/2 to S
(K + S)/2 to (S + R)/2 (S + R)/2 and above
Knuckle-shoulder Shoulder--reach
S and above
S and above
Shoulder-reach
Note: K = Knuckle height S = Shoulder height R = Reach height
of the required lifting ranges. The lowest MAWL obtained will be considered as the recommended value.
Model limitations
(2) height 172 + 5 cm, and (3) weight 72 -+ 13 kg. Frequency of lift was the independent variable used in this study. The levels of the frequency of lift were 4, 6, 8 and 10 times/min. The height of lift and box dimension in the sagittal plane were limited to the floor-to-knuckle and 38 cm, respectively. MAWL was determined for a task duration of 30 rain. The correlation coefficient between the observed and predicted MAWL was 0-844, and the coefficient of variation was 19"83%. These findings suggest that the models can predict MAWL with a reasonable degree of accuracy.
The models devloped in this study are limited to the following ranges of independent variables:
Comparison of lifting guidelines
A computer program (Appendix 1) was written in VAX-BASIC to determine the lifting ranges of lift and the corresponding MAWL. Appendix 2 shows a run of the computer program for two case studies. This computer program should provide practitioners with considerable help in arriving at MAWL.
1. Height range of lift: floor-to-knuckle, floor-to-shoulder, floor-to-reach, knuckle-to-shoulder, knuckle.to-reach, and shoulder-to-reach. 3. Box dimension in the sagittal plane: 31-61 cm.
The lifting guidelines developed by NIOSH (1981) are based on the concept of action limit (AL) and maximum permissible limit (MPL). Over 99% of men can lift loads described by AL while only 25% of men are considered capable of performing above MPL. AL and MPL are computed as follows:
4. Population percentage: 1-99%.
AL(kg)
2. Frequency of lift: 1-12 times/min.
6. Type of lifting: 0 if sagittal and 1 if asymmetrical.
= 40 (15//-/) (1-0.004 V-75) (0.7 + 7.5/D) (1-F/Fmax), MPL (kg) = 3 AL,
7. Handles: presence and absence of handles.
where,
5. Task duration: 1A-12 hours.
H
= horizontal location (cm), forward of mid-point between ankles at origin of lift,
V
--- vertical location (cm) at origin of lift,
D
= vertical travel distance (cm) between origin and destination of lift,
F
= average frequency of lift (lifts/min),
Fma x
= maximum.frequency which can be sustained.
Model validation The model developed in this study for male subjects was validated using MAWL values reported in the literature. The data obtained by Asfour et al (1985) and Garg and Badger (1986) were employed for this purpose. Results showed that Pearson correlation coefficients between observed and predicted MAWL were 0"808 and 0-879, respectively. Moreover, the coefficients of variation were 20-06% and 12.63%, respectively. An experiment was also conducted to further validate the models. Ten male subjects, experienced in manual handling tasks, participated in the experiment. The physical characteristics of the subjects were: (1) age 23 -+ 2 years,
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Fig. 1 shows a comparison between the 99% population limit based on the male model and the AL defined by the NIOSH guide for the floor-to-knuckle height of lift (H = 25 cm, V= 25 cm, D = 48 cm, Fma x = 12 and task duration = 8 hours). Similarly, Fig. 2 gives a comparison between the 25% population limit based on the male model
20
differences between the proposed lifting recommendations on the basis of the models developed in this study and recommendations from Snook (1978), Ayoub et al (1978) and NIOSH (1981). It should be noted that the NIOSH guide provides recommendations on MAWL of lift for either 1 h or 8 h. Although the task duration was cited in the guide as an important task parameter, the effect of lifting duration was not considered on the battery of design criteria of lifting tasks.
18 16
~
a
~
6
The proposed guidelines are partially supported by Garg (1987), who reported that the mean values for MAWL are significantly greater than the corresponding MPL values. According to our findings, this may be true only for highfrequency lifts.
2 0
0
I0 Frequency of lift {times/rain) • NIOSH
Fig. 1
--
• Present study
Action limit and 99% population~:imit as a function of frequency of lift
and the M P L for the floor.to-knuclde'height o f lift. Both
comparisons show that, while the NIOSH limits yield higher load values in the frequency range 1 - 6 times/min, an opposite trend is observed in the frequency range 6 - 1 2 times/min. For the present psychophysical models, the ratio between the 25% and 99% population limits is not 3 to 1. Instead, the ratio increased with an increase in the frequency of lift, from a value of 1.85 at a frequency of 1 time/min to a value of 3.05 at a frequency of 12 times/min. Discussion
Traditionally, the psychophysical approach has been used in setting lifting limits for an 8-h work day based on trial periods of less than 1 h. Recent research, however, has disproved this hypothesis. This could partly explain the 50
2. Determine employee weight handling capability on the basis of the models developed in this study. 3. Compute job stress index (JSI) which is defined as the ratio of job requirements (weight handled on the job) and employee capability. If JSI is greater than I, apply engineering and/or administrative controls to reduce job requirements within employee capability. If JSI is equal to or less than 1, conduct an epidemiological study to ensure that injury statistics (e g, incidence rate and number of lost work days on the job) are substantially reduced due to the application of ergonomics-based interventions in the workplace.
Asfour, S.S., Ayoub, M.M., and Genaidy, A.M. 1984, J Human Ergology, 13, 3-14. A psychophysical study of the effect of task variables on manual lifting and lowering tasks. Asfour, S.S., Genaidy, A.M., Khalfl, T.M., and Gre¢o, E.C. 1985, A combined approach for the determination of lifting capacity. In: Eberts, R., and Eberts, C.G. (Eds), Trends in ergonomics/human factors II. Elsevier Science Publishers, Amsterdam, 609-615.
"O
30
2o
o E
< ~ 10
10
Frequency of lift (times/min) •
Fig. 2
1. Determine job requirements in terms of weight handled on the job, frequency, height range of lift as well as other variables.
References
40
~ E
The combination of latest research and additional task parameters used in constructing the models developed in this study make them more realistic and useful to practitioners of safety and health. Since a single task may require lifting in different ranges, a procedure is needed to account for these height ranges. Such a procedure is described in this paper. Moreover, the following strategy could be used in evaluating the severity of lifting tasks performed in industrial settings:
NIOSH
•
Present study
Maximum permissible l i m i t and 25% population limit as a function of frequency of lift
Ayoub, M.M., Bethea, N.J., Deivanayagam, S., Asfour, S.S., Bakken, G.M., Liles, D., Mital, A., and Sherif, M. 1978, Determination and modeling of lifting capacity. DHEW (NIOSH), Grant No 5RO1OH00545-02, Texas Tech University, Lubbock, Texas (September). Garg, A,, 1987, A psychophsyical, physiological and biomechanical evaluation of NIOSH guidelines for manual handling. In: Asfour, S.S. (Ed). Trends in ergonomics/ human factors IV. Elsevier Science Publishers, Amsterdam, 925-931. Garg, A., and Badger, D. 1986, Ergonomics, 29,879-892. Maximum acceptable weights and maximum voluntary isometric strengths for asymmetric lifting.
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Garg, A., and Saxena, U. 1980, Human Factors, 22,487 496. Container characteristics and maximum acceptable weight of lift.
Mital, A. 1984a, Ergonomics, 27(11), 1115-1126. Maximum weights of lift acceptable to male and female industrial workers for extended work shifts.
Genaidy, A.M., Asfour, S.S., Mital, A., and Tritar, A. 1988, Human Factors, 30, 319-337. Psychophysical capacity
Mital, A. 1984b, Ergonomics, 27(11), 1127 1138. Comprehensive maximum acceptable weight of lift database for regular 8-hour workshifts.
modeling in frequent manual materials handling activities.
Kroemer, K.H.E., Kroemer, H.J., and Kroemer-Elbert, K. 1986, Engineering physiology. Physiologic bases of human factors/ergonomics. Elsevier Science Publishers, Amsterdam,
Liles, D.H., Deivanayagam, S., Ayoub, M.M., and Mahajan, P. 1984, Human Factors, 2 6 , 6 8 3 - 6 9 3 . A job severity index
Mital, A., and Fard, H. 1986, Ergonomics, 29, 1263-.1272. Psychophysical and physiological responses to lifting symmetrical and asymmetrical loads symmetrically and asymmetrically.
National Institute for Occupational Safety and Health.
for the evaluation and control of lifting injury.
1981, Work practices guide for manual lifting. DHHS (NIOSH), Pub No 81-122, Cincinnati, Ohio (March).
Mital, A. 1983, Human Factors, 2 5 , 4 8 5 - 4 9 0 . The psychophysical approach in manual lifting: A verification study.
Snook, S.H. 1978, Ergonomics, 21,963-985. The design of manual handling tasks.
APPENDIX 1
Program listing 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 310 320 330 340 350 360 370 A 380 390 400 410 420 430 440 450 460 470 480 490 500 510 520 530
300
PRINT PRINT PRINT PRINT PRINT PRINT PRINT PRINT PRINT PRINT PRINT PRINT PRINT INPUT PRINT PRINT PRINT INPUT PRINT INPUT PRINT INPUT PRINT INPUT PRINT INPUT PRINT INPUT LET K LET S LET R PRINT PRINT INPUT PRINT PRINT INPUT PRINT PR INT INPUT PRINT PRINT INPUT PRINT
** ** ** ** ** ** **
THE M A I N O B J E C T I V E OF THIS SOFTWARE IS TO D E T E R M I N E THE M A X I M U M A C C E P T A B L E W E I G H T FOR F R E Q U E N T L I F T I N G TASKS G I V E N THE PERCENTAGE OF W O R K I N G P O P U L A T I O N ( 1 - 9 9 % ) , F R E Q U E N C Y OF L I F T ( 1 - 1 2 T I M E S / M I N . ) , C O N T A I N E R SIZE IN THE S A G I T T A L P L A N E ( 3 1 - 6 1 CM.), SEX, L I F T I N G D U R A T I O N ( 0 . 5 - 1 2 . 0 HRS.), C O N T A I N E R H A N D L E S , A N G L E S OF TWIST OF THE B O D Y A N D H E I G H T RANGE OF L I F T
** ** ** ** ** ** **
' ENTER < 1 > FOR M A L E A N D < 2 > FOR F E M A L E ' ; Sl ' ENTER DESIRED PERCENTAGE OF W O R K I N G P O P U L A T I O N ' ' (E.G. 95 I N D I C A T E S T H A T 95 O U T OF 100 W O R K E R S W l L L ' ' BE A B L E TO H A N D L E THE W E I G H T ON THE JOB: ' ; P ' ENTER A V E R A G E C O N T A I N E R SIZE A W A Y FROM THE B O D Y IN CM : ' ; B ' ENTER L I F T I N G D U R A T I O N IN HOURS: ' ; T
' ENTER A V E R A G E N U M B E R OF L I F T S PER M I N U T E : ' ; F ' ENTER < 0 > FOR C O N T A I N E R WITH NO H A N D L E S A N D < 1 > OTHERWISE' ; H ' ENTER < 0 > IF NO TWISTING I N V O L V E D IN L I F T I N G A N D < 1 > OTHERWISE' ; = 99.45656 - 11.71 * S l - .18187"P = K + 64.14671 = S + 64.14671 ' ENTER M I N I M U M P O I N T OF O R I G I N OF L I F T WITH RESPECT' ' TO THE F L O O R L E V E L : ' ; A • ENTER M A X I M U M P O I N T O F O R I G I N OF L I F T WITH RESPECT' 'TOTHEFLOOR LEVEL:';B • ENTER M I N I M U M P O I N T OF D E S T I N A T I O N OF L I F T WITH RESPECT' 'TOTHE FLOOR LEVEL: ' ;C ' ENTER M A X I M U M P O I N T OF D E S T I N A T I O N OF L I F T WITH RESPECT • ' TO THE F L O O R L E V E L : ' ; D
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Appendix 1 - continued 540 55O 560 570 58O 590 600 610 620 630 640 650 660 670 68O 690 700 710 720 730 740 750 760 770 78O 790 80O 810 82O 83O 84O 850 860 870 880 89O 900 910 920 930 940 950 960 970 980 990 1000 1010 1020 1030 1040 1050 1060 1070 1080 1090 1100 1110 1120 1130 1140 1150 1160 1170 1180 1190 1200 1210
KAT=1000 IF AB > = C THEN GO TO 560 IF AB > = D THEN GO TO 590 Z=AB GOSU B 640 IF B > = C THEN GO TO 600 IF B > = D THEN GO TO 1340 Z=B GOSUB 640 GO TO 1260 IF Z < = K/2 AND Z >= 0 THEN GOSUB 710 IFZ<=K ANDZ>K/2 THEN GOSUB 820 IF Z < = (K+S)/2 AND Z > K THEN GOSUB 930 IF Z < = S A N D Z > (K+S)/2 THEN GOSUB 1040 IF Z > S THEN GOSUB 1130 RETURN ! IF C < (K+S)/2 AND C >=0 THEN FL = 1 IF C < (K+S)/2 AND C >= (K+S)/2 THEN FL = 2 IF C >= (S+R)/2 THEN FL = 3 GOSUB 1210 T IF D < (K+S)/2 AND D > = 0 THEN FL = 1 IF D < (S+R)/2 AND D >= (K+S)/2 THEN FL = 2 IF D > = (S+R)/2 THEN FL = 3 GOSUB 1210 ! RETURN IF C < (K+S)/2 AND C > = K/2 THEN FL = 1 IF C < (S+R)/2 AND C > = (K+S)/2 THEN FL = 4 IF C > = (S+R)/2 THEN FL = 5 GOSUB 1210 ! IF D < (K+S)/2 AND D > = K/2 THEN FL = 1 IF D < (S+R)/2 AND D > = (K+S)/2 THEN FL = 4 IF D > = (S+R)/2 THEN FL = 5 GOSUB 1210 RETURN IFC< (S+R)/2ANDC> =KTHEN FL=4 IF C > = (R+S)/2 THEN FL = 5 GOSUB 1210 ! IF D < (K+S)/2 AND D > = K THEN FL = 4 IF D > = (K+S)/2 THEN FL = 5 GOSUB 1210 ! RETURN ! IF C < (S+R)/2 AND C > = (K+S)/2 THEN FL = 4 IF C > = (S+R)/2 THEN FL = 6 GOSUB 1210 ! IF D < (S+R)/2 AND D > = (K+S)/2 THEN FL = 4 IF D > = (S+R)/2 THEN FL = 6 GOSUB 1210 ! RETURN IF C > = S THEN FL = 6 GOSUB 1210 ! IF D > = S T H E N F L = 6 GOSUB 1210 ! RETURN ! IF $1 = 1 THEN M1 = (36.19824--0.68647*FL-O.79578*F-0.02843*B-0.18687*P) M2 = (1.0142-.034"T) * (1--0.0675*A--0.072*H) M = MI*M2
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Appendix 1 - continued
1220 1230 1240 1250 1260 1270 1280 1290 1300 1310 1320 1330
E LSE M3 = (21:04973-0.58098" F L-0.40147 * F - 0 . 0 2 4 9 7 " B-0.08236 * P) M4 = (1.0081-0.0194"T) * ( 1 - 0 . 0 6 7 5 * A - 0 . 0 7 2 * H ) M = M3*M4 END IF LET J1 = M IF J1 <= K A T THEN LET K A T = J1 RETURN ! PRINT ' THE COMPUTED MAW IS ' ; KAT;' KG' PRINT PRINT ' IF YOU WANT TO RERUN THE PROGRAM, PLEASE ENTER' INPUT ' < 1 > FOR YES AND < 2 > O T H E R W l S E ' , V A R IF V A R = I THEN GOTO 220 PRINT PRINT PRINT' * * * * * * * * * * * * * * * * * * * * * * * END OF P R O G R A M * * * * * * * * * * * * * * * * * * * * * * * * *
1340
END
APPENDIX 2
Program output * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * ~ * * * * * * * * * *
** ** ** ** ** ** **
*
THE M A I N OBJECTIVE OF THIS SOFTWARE IS TO DETERMINE THE M A X I M U M ACCEPTABLE WEIGHT FOR FREQUENT LIFTING TASKS GIVEN THE PERCENTAGE OF WORKING POPULATION (1-99%), FREQUENCY OF LIFT ( 1 - 1 2 TIMES/MIN.), CONTAINER SIZE IN THE S A G I T T A L PLANE (31-61 CM.), SEX, LIFTING D U R A T I O N (0.5-12.0 HRS.), CONTAINER HANDLES, ANGLE OF TWIST OF THE BODY AND HEIGHT RANGE OF LIFT * * * * * ~ * * * * * * ~ * * * * * * * * * * * * * * * * * * * * ~ * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
* * * * * * ~ * ~ * * * * * * * * ~ * * ~ f * ~ * * * * * *
~ * * * * * * * * * * * * * * * * * * * * * * * * * * * @ * *
*
**
** ** ** ** ** ** * * *
* * * * * * * * ~ * * * * * * * * *
ENTER < 1 > FOR MALE AND < 2 > FOR FEMALE ? 1 ENTER DESIRED PERCENTAGE OF WORKING POPULATION (E.G. 95 INDICATES T H A T 95 OUT OF 100 WORKERS WILL BE ABLE TO HANDLE THE WEIGHT ON THE JOB: ? 10 ENTER A V E R A G E CONTAINER SIZE A W A Y FROM THE BODY IN CM : ? 38 ENTER LIFTING DURATION IN HOURS: ? 5 ENTER A V E R A G E NUMBER OF LIFTS PER MINUTE: ? 4 ENTER < 0 > FOR CONTAINER WITH NO HANDLES AND < 1 > OTHERWISE ? 0 ENTER < 0 > IF NO TWISTING INVOLVED IN LIFTING AND < t > OTHERWISE ? 1 ENTER M I N I M U M POINT OF ORIGIN OF LIFT WITH RESPECT TO THE FLOOR LEVEL: ? 10 ENTER M A X I M U M POINT OF ORIGIN OF LIFT WITH RESPECT TO THE FLOOR LEVEL: ? 40 ENTER M I N I M U M POINT OF DESTINATION OF LIFT WITH RESPECT TO THE FLOOR LEVEL: ? 100 ENTER M A X I M U M POINT OF DESTINATION OF L I F T W l T H RESPECT TO THE FLOOR LEVEL: ? 150 THE COMPUTED MAW IS 7.85679 KG IF YOU WANT TO RERUN THE PROGRAM, PLEASE ENTER < 1 > FOR YES AND < 2 > OTHERWISE ?1 ENTER < 1 > FOR MALE AND < 2 > FOR FEMALE ? 1 ENTER DESIRED PERCENTAGE OF WORKING POPULATION (E.G. 96 INDICATES T H A T 95 OUT OF 100 WORKERS WILL BE ABLE TO HANDLE THE WEIGHT ON THE JOB: ? 5
302
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Appendix 2 - continued ENTER AVERAGE CONTAINER SIZE AWAY FROM THE BODY IN CM : ? 20 ENTER LIFTING DURATION IN HOURS: ? 3 ENTER AVERAGE NUMBER OF LIFTS PER MINUTE: ? 15 ENTER < 0 > FOR CONTAINER WITH NO HANDLES AND < 1 > OTHERWISE ? 1 ENTER < 0 > IF NO TWISTING INVOLVED IN LIFTING AND < 1 > OTHERWISE ? 1 ENTER MINIMUM POINT OF ORIGIN OF LIFT WITH RESPECT TO THE FLOOR LEVEL: ?0 ENTER MAXIMUM POINT OF ORIGIN OF LIFT WITH RESPECT TO THE FLOOR LEVEL: ? 40 ENTER MINIMUM POINT OF DESTINATION OF LIFTWlTH RESPECT TO THE FLOOR LEVEL: ? 30 ENTER MAXIMUM POINT OF DESTINATION OF LIFT WITH RESPECT TO THE FLOOR LEVEL: ? 100 THE COMPUTED MAW IS 18.2032 KG IF YOU WANT TO RERUN THE PROGRAM, PLEASE ENTER < 1 > FOR YES AND < 2 > OTHERWISE ?2 *******************************
END OF P R O G R A M * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
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December 1990
303
Psychophysical models for manual lifting tasks A.M. Genaidy*, S.S. Asfour t, A. Mital**, and S.M. Waly t *Western Michigan University, Kalamazoo,Michigan, USA, tUniversity of Miami, Coral Gables, Florida, USA, **University of Cincinnati, Cincinnati, Ohio, USA. This paper provides two models for males and females to assessthe psychophysical maximum acceptable weight of lift. The weight guidelines generated by the models are a function of lifting frequency, height of lift, sagittal or asymmetrical lifting, task duration, container size in the sagittal plane, presence or absence of container couplings, and percentage of the working population. The developed models were generated from a knowledge base available in the published literature. A computer program was written in BASIC to assist the user in determining the safe load that could be handled by a specified working population. Model validation showed that the models developed predict the maximum acceptable weight of lift with a reasonable degree of accuracy. A comparison between the National Institute for Occupational Safety and Health recommendations and those made on the basis of the models developed in this paper is also presented.
Keywords: Lifting, psyehophysical assessment, human performance, models
Introduction The psychophysical criterion is one design approach that can be used in setting guidelines for manual lifting tasks with an aim to eliminate and/or minimise the number and severity of back injuries (Liles et al, 1984). In the psychophysical approach, individuals are instructed to adjust, by adding or subtracting, the amount of load moved to achieve a weight they are willing to sustain over a given period of time. In the literature, this adjusted weight is referred to as the maximum acceptable weight of lift (MAWL). Based on a recent literature survey (Genaidy et al, 1988), it was found that most psychophysical capacity models do not account for the effects of many important parameters such as task duration, asymmetrical lifting activities, percentage of the working population, and container couplings. If capacity models are to apply to realistic jobs, it is important that these parameters be considered. The purpose of this work was to provide MAWL recommendations for lifting tasks through the development of models that take into account, among other factors, the effects of the aforementioned variables. The proposed models are based on the latest published research in manual materials handling. The secondary objective of this work was to compare the proposed manual lifting recommendations with the lifting guidelines published by the National Institute for Occupational Safety and Health (NIOSH, 1981).
Review of psychophysical studies Three major psychophysical studies were conducted by Ayoub et al (1978), Snook (1978) and Mital (1983, 1984a,
0003-6870/90/04 0295-09 $03.00 ©1990 Butterworth-Heinemann Ltd
1984b) on industrial workers. A summary of the experimental methods and procedures of these studies is given in Table 1. Ayoub et al (1978) collected data on MAWL based on 73 male and 73 female industrial workers. Each subject was given 2 0 - 2 5 min to estimate MAWL for an 8-h work day. Snook (1978) integrated the results of his earlier work in the form of tables that could aid in the design of manual lifting tasks. The MAWL results were reported for various percentiles of the worker population. A normal distribution was assumed for this purpose. Task duration was set to 40 min in all experiments. The data collected by Snook was based on 43 male and 15 female industrial workers. Mital (1983) questioned the validity of the psychophysical approach in determining MAWL for an 8-h work day based on trial periods of less than 1 h. Experiments aimed at determining the maximum acceptable weight of lift for 8 h and 12 h were conducted. Five male and five female industrial workers participated in the experiments. Each subject was given 20 min to estimate MAWL for an 8-h work day. On the following day, he/she was asked to lift the amount of load determined on the previous day for 12 h. They were permitted to make any necessary adjustment during this period. The results indicated that MAWL decreased at a rate of 3.4% per hour for males, and 2% per hour for females. Thirty-seven male and 37 female industrial workers participated in another set of experiments by Mital (1984a, 1984b). Each subject estimated MAWL for an 8-h work shift over a period of 2 0 - 2 5 min. They were then given an
Applied Ergonomics
December1990
295
Table I: Summary of variables and experimental design procedures used in psychophysical studies i
Source
Variables
Experimental design used
Ayoub eta/(1978)
Frequency of lift (2, 4, 6, 8 times/min)
Incomplete randomised block design where each subject performed a total of nine combinations out of 72 possible treatments
Height of lift (floor-to-knuckle, floor-to-shoulder, floor-to-reach, knuckle-to-shoulder, knuckle-to-reach, shoulder-to-reach) Box dimension in sagittal plane (30.48, 45.72, 60"96 cm) Gender (73 male and 73 female industrial workers) Snook (1978)
Frequency of lift (1,4.29, 6.67, 12 times/min)
Factorial design
Height of lift (floor-to-knuckle, knuckle-to-shoulder, sh ou Ider-to-reach ) Vertical distance of lift (25, 51,76 cm) Box dimension in sagittal plane (36, 49, 75 cm) Gender (43 male and 15 female industrial workers)
Mital (1984b, 1984c)
Frequency of lift (1,4, 8, 12 times/rain) Height of lift (floor-to-knuckle, knuckle-to-shoulder, shoulder-to-reach)
Incomplete randomised block design where each subject performed a total of nine combinations out of 36 possible treatments
Box dimension in sagittal plane (30"48, 45-72, 60"96 cm) Gender (37 male and 37 female industrial workers)
additional 20 min to estimate MAWL for a 12-h work shift. All data collected on the 74 subjects were adjusted by the multipliers developed from Mital's previous study (1983). Since MAWL decreased as a function of time, the database developed by Ayoub et al (1978) is appropriate only for estimating an individual's MAWL for a lifting duration of less than 1 h. Similarly, the data obtained by Snook (1978) on industrial workers are appropriate only for estimating an individual's MAWL for a lifting duration of less than 1 h.
Model development In this work, the data collected by Ayoub et al (1978), Snook (1978) and Mital (1984a, 1984b) on industrial workers were integrated to form the MAWL knowledge base. Based on this knowledge base, two regression models were developed. These models are intended to estimate MAWL for male or for female industrial workers. This separation was necessary in order to improve the predictive ability of the models, instead of combining the two models into one and incorporating gender as an independent variable in the model. The inputs to the models are: vertical range of lift, frequency of lift, box dimension in the sagittal plane, popu-
296
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tation percentage, task duration, presence or absence of handles, and sagittal or asymmetrical lifting condition. In this study, the vertical range of lift is defined as the vertical travel distance in cm between the origin and destination of lift. The frequency of lift is the average number of lifts on the job per minute. The box dimension in the sagittal plane is the width of the box away from the body. The population percentage is based on the criterion of MAWL; for example, 90 indicates that 90 out of 100 will be able to handle the load on the job. Task duration is the amount of time in hours spent while lifting loads. Sagittal lifting involves lifting loads in front of the body without twisting the upper torso, while asymmetrical lifting includes lifting loads to the sides of the body or while twisting the upper torso. A total of 2760 observations was used in developing the models. The following concepts were used in the development of the models reported in this study: 1. To account for the effects of task duration, the MAWL database of Ayoub et al (1978), Snook (1978), and Mital (1984a, 1984b) were adjusted by the multipliers provided by Mital (1983).
Table 2 : M a x i m u m acceptable weight of lift models
.
I
Sex
R2
Model
Male
0.8323
MAW
= [36.19824 - 0.68647"HL - 0.79578*FREO - 0.02843"BS - 0"18687"PP] * [1.0142 - 0 . 0 3 4 * T I M E ] * [1 - 0 . 0 6 7 5 " T L - O . 0 7 2 * H A N D ]
Female
0-8596
MAW
= [21.04973-0.58098"HL - 0.40147*FREO - 0.02497"BS - 0.08236"PP] * [1.0081 - O . 0 1 9 4 * T I M E ] * [ 1 - 0 . 0 6 7 5 " TL - O . 0 7 2 * H A N D ]
Note: = M a x i m u m acceptable w e i g h t of l i f t (kg) = Height o f l i f t (floor-to-knuckle = 1, floor-toshoulder = 2, floor-to-reach = 3, knuckle-toshoulder = 4, knuckle-to-reach = 5, shoulder-toreach = 6) F R E Q = Frequency of lift (times/min) BS = Box dimension in the sagittal plane (cm) PP = Percentage of w o r k i n g p o p u l a t i o n (e g, 95) T I M E = Working time (hours) TL = Type of lift (sagittal = 0, asymmetrical = 1 ) H A N D = Handles (no handles = 0, handles = 1 ) MAW HL
2. The studies by Ayoub et al (1978), Snook (1978) and Mital (1984a, 1984b) assumed that a lifting task consists of moving a box fitted with handles. However, Garg and Saxena (1980) demonstrated that the absence of handles on the container to be lifted reduces MAWL on the average by 7"2%. Thus, in order to account for the effect of a container with no handles, the MAWL data were reduced by a multiplier of 7"2%.
The lifting height range was def'med by the origin and destination of lift. For starting and termination points other than the knuckle, shoulder and reach heights, the procedure described in Table 3 was used. Table 3 requires the knowledge of knuckle, shoulder and reach heights. In the original work of Ayoub et al (1978) only the knowledge of knuckle height was requiredt shoulder height was approximated by adding 51 cm to the knuckle height. Table 3 incorporates additional information to account for various anthropometric data of the working population. . A single task may require lifting in different ranges. This is determined using the following procedure: a. Determine the minimum and maximum points of origin of lift with respect to the floor level in cm. b. Determine the minimum and maximum points of destination of lift with respect to the floor level in cm.
c. Determine from Table 3 the range of assignment that the following points belong to: (i) minimum point of origin of lift and minimum point of destination of lift; (ii) minimum point of origin of lift and maximum point of destination of lift; (iii) maximum point of origin of lift and minimum point of destination of lift; (iv) maximum point of origin of lift and maximum point of destination of lift. Disregard the combinations that do not belong to the range assignments in Table 3. The following example will illustrate the abovementioned procedure: Suppose that K = 76 cm, S = 127 c m , R = 178 cm. Thus, (K/2) = 38 cm, (K+S)/2 = 101-5 cm, (S+R)/2 = 152.5 cm. These values will be the basis for the origin and destination of lift for Table 3 as follows: Origin of lift
Destination of lift
Range assignment
0 t o 38
0 to 101"5 101.5 to 152"5 152"5 and above
floor-knuckle floor-shoulder floor-reach
38 t o 76
38 t o 101 "5 101 "5 to 152.5 152"5 and above
floor-knuckle knuckle-shoulder knuckle-reach
4. A normal distribution was assumed to compute MAWL for various percentages of the working population.
76 to 101.5
76 to 152"5 152.5 and above
knuckle-shoulder knuckle-reach
5. In instances where the MAWL was available under identical experimental conditions (such as the same levels of task variables), the lower MAWL value was used.
101 "5 to 127
101 '5 t o 152"5 152"5 and above
knuckle-shoulder shoulder-reach
127 and above
127 and above
shoulder-reach
3. In the studies by Ayoub et al (1978), Snook (1978) and Mital (1984a, 1984b), a lifting task was assumed to be performed in the sagittal plane without twisting. Studies by Asfour et al (1984), Garg and Badger (1986), and Mital and Fard (1986) showed that asymmetrical lifting reduces MAWL by 5% to 8.5%. Thus, to account for asymmetrical lifting, the MAWL data were reduced by 6-75%.
6. Since MAWL is determined for a given percentage of the population, the knuckle, shoulder and reach heights of this population were used. The following model, based on data reported by Kroemer et al (1986), was developed to predict these height values (R 2 = 0"9964): H V = [35-30985 + 64-14671"HL - l l - 7 1 * S E X - 0-18187*PP] where: HI/" HL SEX PP
= height value (cm), = height level (knuckle = 1, shoulder = 2, reach = 3). = sex (male = I, female = 2), = percentage of the working population (e g, 95).
Assume the following: (i) origin of lift: minimum point = 20 cm above floor level, maximum point = 90 cm abovefloor level; (ii) destination of lift: minimum point = 70 cm, maximum point = 190 cm. By looking at the above results, one can arrive at the following range assignment: floor-knuckle, floor-reach, knuckle-reach. It should be noted that the 90 to 70 height range was disregarded since it does not belong to any of the range assignments. Once the lifting ranges required by each lifting task are recognised, one can determine the lifting capacity for each
Applied Ergonomics
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297
Table 3: Origin and destination of lift (Modified from A y o u b e t a l (1978)
Origin of lift (cm)
Destinationof lift (cm)
Range assignment
0 to K/2
0 to (K + S)/2 (K + S)/2 to (S + R)/2 (S + R)/2 and above
Floor-knuckle Floor-shoulder Floor--reach
K/2 to K
K/2 to ( K + S)/2 (K + S)/2 to (S + R)/2 (S + R)/2 and above
Floor-knuckle Knuckle-shoulder Knuckle--reach
K to (K + S)/2
K to (S + R)/2 (S + R)/2 and above
Knuckle-shoulder Knuckle-reach
(K + S)/2 to S
(K + S)/2 to (S + R)/2 (S + R)/2 and above
Knuckle-shoulder Shoulder--reach
S and above
S and above
Shoulder-reach
Note: K = Knuckle height S = Shoulder height R = Reach height
of the required lifting ranges. The lowest MAWL obtained will be considered as the recommended value.
Model limitations
(2) height 172 + 5 cm, and (3) weight 72 -+ 13 kg. Frequency of lift was the independent variable used in this study. The levels of the frequency of lift were 4, 6, 8 and 10 times/min. The height of lift and box dimension in the sagittal plane were limited to the floor-to-knuckle and 38 cm, respectively. MAWL was determined for a task duration of 30 rain. The correlation coefficient between the observed and predicted MAWL was 0-844, and the coefficient of variation was 19"83%. These findings suggest that the models can predict MAWL with a reasonable degree of accuracy.
The models devloped in this study are limited to the following ranges of independent variables:
Comparison of lifting guidelines
A computer program (Appendix 1) was written in VAX-BASIC to determine the lifting ranges of lift and the corresponding MAWL. Appendix 2 shows a run of the computer program for two case studies. This computer program should provide practitioners with considerable help in arriving at MAWL.
1. Height range of lift: floor-to-knuckle, floor-to-shoulder, floor-to-reach, knuckle-to-shoulder, knuckle.to-reach, and shoulder-to-reach. 3. Box dimension in the sagittal plane: 31-61 cm.
The lifting guidelines developed by NIOSH (1981) are based on the concept of action limit (AL) and maximum permissible limit (MPL). Over 99% of men can lift loads described by AL while only 25% of men are considered capable of performing above MPL. AL and MPL are computed as follows:
4. Population percentage: 1-99%.
AL(kg)
2. Frequency of lift: 1-12 times/min.
6. Type of lifting: 0 if sagittal and 1 if asymmetrical.
= 40 (15//-/) (1-0.004 V-75) (0.7 + 7.5/D) (1-F/Fmax), MPL (kg) = 3 AL,
7. Handles: presence and absence of handles.
where,
5. Task duration: 1A-12 hours.
H
= horizontal location (cm), forward of mid-point between ankles at origin of lift,
V
--- vertical location (cm) at origin of lift,
D
= vertical travel distance (cm) between origin and destination of lift,
F
= average frequency of lift (lifts/min),
Fma x
= maximum.frequency which can be sustained.
Model validation The model developed in this study for male subjects was validated using MAWL values reported in the literature. The data obtained by Asfour et al (1985) and Garg and Badger (1986) were employed for this purpose. Results showed that Pearson correlation coefficients between observed and predicted MAWL were 0"808 and 0-879, respectively. Moreover, the coefficients of variation were 20-06% and 12.63%, respectively. An experiment was also conducted to further validate the models. Ten male subjects, experienced in manual handling tasks, participated in the experiment. The physical characteristics of the subjects were: (1) age 23 -+ 2 years,
298
Applied Ergonomics
December 1990
Fig. 1 shows a comparison between the 99% population limit based on the male model and the AL defined by the NIOSH guide for the floor-to-knuckle height of lift (H = 25 cm, V= 25 cm, D = 48 cm, Fma x = 12 and task duration = 8 hours). Similarly, Fig. 2 gives a comparison between the 25% population limit based on the male model
20
differences between the proposed lifting recommendations on the basis of the models developed in this study and recommendations from Snook (1978), Ayoub et al (1978) and NIOSH (1981). It should be noted that the NIOSH guide provides recommendations on MAWL of lift for either 1 h or 8 h. Although the task duration was cited in the guide as an important task parameter, the effect of lifting duration was not considered on the battery of design criteria of lifting tasks.
18 16
~
a
~
6
The proposed guidelines are partially supported by Garg (1987), who reported that the mean values for MAWL are significantly greater than the corresponding MPL values. According to our findings, this may be true only for highfrequency lifts.
2 0
0
I0 Frequency of lift {times/rain) • NIOSH
Fig. 1
--
• Present study
Action limit and 99% population~:imit as a function of frequency of lift
and the M P L for the floor.to-knuclde'height o f lift. Both
comparisons show that, while the NIOSH limits yield higher load values in the frequency range 1 - 6 times/min, an opposite trend is observed in the frequency range 6 - 1 2 times/min. For the present psychophysical models, the ratio between the 25% and 99% population limits is not 3 to 1. Instead, the ratio increased with an increase in the frequency of lift, from a value of 1.85 at a frequency of 1 time/min to a value of 3.05 at a frequency of 12 times/min. Discussion
Traditionally, the psychophysical approach has been used in setting lifting limits for an 8-h work day based on trial periods of less than 1 h. Recent research, however, has disproved this hypothesis. This could partly explain the 50
2. Determine employee weight handling capability on the basis of the models developed in this study. 3. Compute job stress index (JSI) which is defined as the ratio of job requirements (weight handled on the job) and employee capability. If JSI is greater than I, apply engineering and/or administrative controls to reduce job requirements within employee capability. If JSI is equal to or less than 1, conduct an epidemiological study to ensure that injury statistics (e g, incidence rate and number of lost work days on the job) are substantially reduced due to the application of ergonomics-based interventions in the workplace.
Asfour, S.S., Ayoub, M.M., and Genaidy, A.M. 1984, J Human Ergology, 13, 3-14. A psychophysical study of the effect of task variables on manual lifting and lowering tasks. Asfour, S.S., Genaidy, A.M., Khalfl, T.M., and Gre¢o, E.C. 1985, A combined approach for the determination of lifting capacity. In: Eberts, R., and Eberts, C.G. (Eds), Trends in ergonomics/human factors II. Elsevier Science Publishers, Amsterdam, 609-615.
"O
30
2o
o E
< ~ 10
10
Frequency of lift (times/min) •
Fig. 2
1. Determine job requirements in terms of weight handled on the job, frequency, height range of lift as well as other variables.
References
40
~ E
The combination of latest research and additional task parameters used in constructing the models developed in this study make them more realistic and useful to practitioners of safety and health. Since a single task may require lifting in different ranges, a procedure is needed to account for these height ranges. Such a procedure is described in this paper. Moreover, the following strategy could be used in evaluating the severity of lifting tasks performed in industrial settings:
NIOSH
•
Present study
Maximum permissible l i m i t and 25% population limit as a function of frequency of lift
Ayoub, M.M., Bethea, N.J., Deivanayagam, S., Asfour, S.S., Bakken, G.M., Liles, D., Mital, A., and Sherif, M. 1978, Determination and modeling of lifting capacity. DHEW (NIOSH), Grant No 5RO1OH00545-02, Texas Tech University, Lubbock, Texas (September). Garg, A,, 1987, A psychophsyical, physiological and biomechanical evaluation of NIOSH guidelines for manual handling. In: Asfour, S.S. (Ed). Trends in ergonomics/ human factors IV. Elsevier Science Publishers, Amsterdam, 925-931. Garg, A., and Badger, D. 1986, Ergonomics, 29,879-892. Maximum acceptable weights and maximum voluntary isometric strengths for asymmetric lifting.
Applied Ergonomics
December 1990
299
Garg, A., and Saxena, U. 1980, Human Factors, 22,487 496. Container characteristics and maximum acceptable weight of lift.
Mital, A. 1984a, Ergonomics, 27(11), 1115-1126. Maximum weights of lift acceptable to male and female industrial workers for extended work shifts.
Genaidy, A.M., Asfour, S.S., Mital, A., and Tritar, A. 1988, Human Factors, 30, 319-337. Psychophysical capacity
Mital, A. 1984b, Ergonomics, 27(11), 1127 1138. Comprehensive maximum acceptable weight of lift database for regular 8-hour workshifts.
modeling in frequent manual materials handling activities.
Kroemer, K.H.E., Kroemer, H.J., and Kroemer-Elbert, K. 1986, Engineering physiology. Physiologic bases of human factors/ergonomics. Elsevier Science Publishers, Amsterdam,
Liles, D.H., Deivanayagam, S., Ayoub, M.M., and Mahajan, P. 1984, Human Factors, 2 6 , 6 8 3 - 6 9 3 . A job severity index
Mital, A., and Fard, H. 1986, Ergonomics, 29, 1263-.1272. Psychophysical and physiological responses to lifting symmetrical and asymmetrical loads symmetrically and asymmetrically.
National Institute for Occupational Safety and Health.
for the evaluation and control of lifting injury.
1981, Work practices guide for manual lifting. DHHS (NIOSH), Pub No 81-122, Cincinnati, Ohio (March).
Mital, A. 1983, Human Factors, 2 5 , 4 8 5 - 4 9 0 . The psychophysical approach in manual lifting: A verification study.
Snook, S.H. 1978, Ergonomics, 21,963-985. The design of manual handling tasks.
APPENDIX 1
Program listing 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 310 320 330 340 350 360 370 A 380 390 400 410 420 430 440 450 460 470 480 490 500 510 520 530
300
PRINT PRINT PRINT PRINT PRINT PRINT PRINT PRINT PRINT PRINT PRINT PRINT PRINT INPUT PRINT PRINT PRINT INPUT PRINT INPUT PRINT INPUT PRINT INPUT PRINT INPUT PRINT INPUT LET K LET S LET R PRINT PRINT INPUT PRINT PRINT INPUT PRINT PR INT INPUT PRINT PRINT INPUT PRINT
** ** ** ** ** ** **
THE M A I N O B J E C T I V E OF THIS SOFTWARE IS TO D E T E R M I N E THE M A X I M U M A C C E P T A B L E W E I G H T FOR F R E Q U E N T L I F T I N G TASKS G I V E N THE PERCENTAGE OF W O R K I N G P O P U L A T I O N ( 1 - 9 9 % ) , F R E Q U E N C Y OF L I F T ( 1 - 1 2 T I M E S / M I N . ) , C O N T A I N E R SIZE IN THE S A G I T T A L P L A N E ( 3 1 - 6 1 CM.), SEX, L I F T I N G D U R A T I O N ( 0 . 5 - 1 2 . 0 HRS.), C O N T A I N E R H A N D L E S , A N G L E S OF TWIST OF THE B O D Y A N D H E I G H T RANGE OF L I F T
** ** ** ** ** ** **
' ENTER < 1 > FOR M A L E A N D < 2 > FOR F E M A L E ' ; Sl ' ENTER DESIRED PERCENTAGE OF W O R K I N G P O P U L A T I O N ' ' (E.G. 95 I N D I C A T E S T H A T 95 O U T OF 100 W O R K E R S W l L L ' ' BE A B L E TO H A N D L E THE W E I G H T ON THE JOB: ' ; P ' ENTER A V E R A G E C O N T A I N E R SIZE A W A Y FROM THE B O D Y IN CM : ' ; B ' ENTER L I F T I N G D U R A T I O N IN HOURS: ' ; T
' ENTER A V E R A G E N U M B E R OF L I F T S PER M I N U T E : ' ; F ' ENTER < 0 > FOR C O N T A I N E R WITH NO H A N D L E S A N D < 1 > OTHERWISE' ; H ' ENTER < 0 > IF NO TWISTING I N V O L V E D IN L I F T I N G A N D < 1 > OTHERWISE' ; = 99.45656 - 11.71 * S l - .18187"P = K + 64.14671 = S + 64.14671 ' ENTER M I N I M U M P O I N T OF O R I G I N OF L I F T WITH RESPECT' ' TO THE F L O O R L E V E L : ' ; A • ENTER M A X I M U M P O I N T O F O R I G I N OF L I F T WITH RESPECT' 'TOTHEFLOOR LEVEL:';B • ENTER M I N I M U M P O I N T OF D E S T I N A T I O N OF L I F T WITH RESPECT' 'TOTHE FLOOR LEVEL: ' ;C ' ENTER M A X I M U M P O I N T OF D E S T I N A T I O N OF L I F T WITH RESPECT • ' TO THE F L O O R L E V E L : ' ; D
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Appendix 1 - continued 540 55O 560 570 58O 590 600 610 620 630 640 650 660 670 68O 690 700 710 720 730 740 750 760 770 78O 790 80O 810 82O 83O 84O 850 860 870 880 89O 900 910 920 930 940 950 960 970 980 990 1000 1010 1020 1030 1040 1050 1060 1070 1080 1090 1100 1110 1120 1130 1140 1150 1160 1170 1180 1190 1200 1210
KAT=1000 IF AB > = C THEN GO TO 560 IF AB > = D THEN GO TO 590 Z=AB GOSU B 640 IF B > = C THEN GO TO 600 IF B > = D THEN GO TO 1340 Z=B GOSUB 640 GO TO 1260 IF Z < = K/2 AND Z >= 0 THEN GOSUB 710 IFZ<=K ANDZ>K/2 THEN GOSUB 820 IF Z < = (K+S)/2 AND Z > K THEN GOSUB 930 IF Z < = S A N D Z > (K+S)/2 THEN GOSUB 1040 IF Z > S THEN GOSUB 1130 RETURN ! IF C < (K+S)/2 AND C >=0 THEN FL = 1 IF C < (K+S)/2 AND C >= (K+S)/2 THEN FL = 2 IF C >= (S+R)/2 THEN FL = 3 GOSUB 1210 T IF D < (K+S)/2 AND D > = 0 THEN FL = 1 IF D < (S+R)/2 AND D >= (K+S)/2 THEN FL = 2 IF D > = (S+R)/2 THEN FL = 3 GOSUB 1210 ! RETURN IF C < (K+S)/2 AND C > = K/2 THEN FL = 1 IF C < (S+R)/2 AND C > = (K+S)/2 THEN FL = 4 IF C > = (S+R)/2 THEN FL = 5 GOSUB 1210 ! IF D < (K+S)/2 AND D > = K/2 THEN FL = 1 IF D < (S+R)/2 AND D > = (K+S)/2 THEN FL = 4 IF D > = (S+R)/2 THEN FL = 5 GOSUB 1210 RETURN IFC< (S+R)/2ANDC> =KTHEN FL=4 IF C > = (R+S)/2 THEN FL = 5 GOSUB 1210 ! IF D < (K+S)/2 AND D > = K THEN FL = 4 IF D > = (K+S)/2 THEN FL = 5 GOSUB 1210 ! RETURN ! IF C < (S+R)/2 AND C > = (K+S)/2 THEN FL = 4 IF C > = (S+R)/2 THEN FL = 6 GOSUB 1210 ! IF D < (S+R)/2 AND D > = (K+S)/2 THEN FL = 4 IF D > = (S+R)/2 THEN FL = 6 GOSUB 1210 ! RETURN IF C > = S THEN FL = 6 GOSUB 1210 ! IF D > = S T H E N F L = 6 GOSUB 1210 ! RETURN ! IF $1 = 1 THEN M1 = (36.19824--0.68647*FL-O.79578*F-0.02843*B-0.18687*P) M2 = (1.0142-.034"T) * (1--0.0675*A--0.072*H) M = MI*M2
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Appendix 1 - continued
1220 1230 1240 1250 1260 1270 1280 1290 1300 1310 1320 1330
E LSE M3 = (21:04973-0.58098" F L-0.40147 * F - 0 . 0 2 4 9 7 " B-0.08236 * P) M4 = (1.0081-0.0194"T) * ( 1 - 0 . 0 6 7 5 * A - 0 . 0 7 2 * H ) M = M3*M4 END IF LET J1 = M IF J1 <= K A T THEN LET K A T = J1 RETURN ! PRINT ' THE COMPUTED MAW IS ' ; KAT;' KG' PRINT PRINT ' IF YOU WANT TO RERUN THE PROGRAM, PLEASE ENTER' INPUT ' < 1 > FOR YES AND < 2 > O T H E R W l S E ' , V A R IF V A R = I THEN GOTO 220 PRINT PRINT PRINT' * * * * * * * * * * * * * * * * * * * * * * * END OF P R O G R A M * * * * * * * * * * * * * * * * * * * * * * * * *
1340
END
APPENDIX 2
Program output * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * ~ * * * * * * * * * *
** ** ** ** ** ** **
*
THE M A I N OBJECTIVE OF THIS SOFTWARE IS TO DETERMINE THE M A X I M U M ACCEPTABLE WEIGHT FOR FREQUENT LIFTING TASKS GIVEN THE PERCENTAGE OF WORKING POPULATION (1-99%), FREQUENCY OF LIFT ( 1 - 1 2 TIMES/MIN.), CONTAINER SIZE IN THE S A G I T T A L PLANE (31-61 CM.), SEX, LIFTING D U R A T I O N (0.5-12.0 HRS.), CONTAINER HANDLES, ANGLE OF TWIST OF THE BODY AND HEIGHT RANGE OF LIFT * * * * * ~ * * * * * * ~ * * * * * * * * * * * * * * * * * * * * ~ * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
* * * * * * ~ * ~ * * * * * * * * ~ * * ~ f * ~ * * * * * *
~ * * * * * * * * * * * * * * * * * * * * * * * * * * * @ * *
*
**
** ** ** ** ** ** * * *
* * * * * * * * ~ * * * * * * * * *
ENTER < 1 > FOR MALE AND < 2 > FOR FEMALE ? 1 ENTER DESIRED PERCENTAGE OF WORKING POPULATION (E.G. 95 INDICATES T H A T 95 OUT OF 100 WORKERS WILL BE ABLE TO HANDLE THE WEIGHT ON THE JOB: ? 10 ENTER A V E R A G E CONTAINER SIZE A W A Y FROM THE BODY IN CM : ? 38 ENTER LIFTING DURATION IN HOURS: ? 5 ENTER A V E R A G E NUMBER OF LIFTS PER MINUTE: ? 4 ENTER < 0 > FOR CONTAINER WITH NO HANDLES AND < 1 > OTHERWISE ? 0 ENTER < 0 > IF NO TWISTING INVOLVED IN LIFTING AND < t > OTHERWISE ? 1 ENTER M I N I M U M POINT OF ORIGIN OF LIFT WITH RESPECT TO THE FLOOR LEVEL: ? 10 ENTER M A X I M U M POINT OF ORIGIN OF LIFT WITH RESPECT TO THE FLOOR LEVEL: ? 40 ENTER M I N I M U M POINT OF DESTINATION OF LIFT WITH RESPECT TO THE FLOOR LEVEL: ? 100 ENTER M A X I M U M POINT OF DESTINATION OF L I F T W l T H RESPECT TO THE FLOOR LEVEL: ? 150 THE COMPUTED MAW IS 7.85679 KG IF YOU WANT TO RERUN THE PROGRAM, PLEASE ENTER < 1 > FOR YES AND < 2 > OTHERWISE ?1 ENTER < 1 > FOR MALE AND < 2 > FOR FEMALE ? 1 ENTER DESIRED PERCENTAGE OF WORKING POPULATION (E.G. 96 INDICATES T H A T 95 OUT OF 100 WORKERS WILL BE ABLE TO HANDLE THE WEIGHT ON THE JOB: ? 5
302
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Appendix 2 - continued ENTER AVERAGE CONTAINER SIZE AWAY FROM THE BODY IN CM : ? 20 ENTER LIFTING DURATION IN HOURS: ? 3 ENTER AVERAGE NUMBER OF LIFTS PER MINUTE: ? 15 ENTER < 0 > FOR CONTAINER WITH NO HANDLES AND < 1 > OTHERWISE ? 1 ENTER < 0 > IF NO TWISTING INVOLVED IN LIFTING AND < 1 > OTHERWISE ? 1 ENTER MINIMUM POINT OF ORIGIN OF LIFT WITH RESPECT TO THE FLOOR LEVEL: ?0 ENTER MAXIMUM POINT OF ORIGIN OF LIFT WITH RESPECT TO THE FLOOR LEVEL: ? 40 ENTER MINIMUM POINT OF DESTINATION OF LIFTWlTH RESPECT TO THE FLOOR LEVEL: ? 30 ENTER MAXIMUM POINT OF DESTINATION OF LIFT WITH RESPECT TO THE FLOOR LEVEL: ? 100 THE COMPUTED MAW IS 18.2032 KG IF YOU WANT TO RERUN THE PROGRAM, PLEASE ENTER < 1 > FOR YES AND < 2 > OTHERWISE ?2 *******************************
END OF P R O G R A M * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
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