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Maximum acceptable repetitive lifting workload by Chinese subjects a

ZUXIANG ZHU & ZHIJUN ZHANG

a

a

Department of Psychology , University of Hangzhou , Zhejiang, People's Republic of China Published online: 11 Mar 2008.

To cite this article: ZUXIANG ZHU & ZHIJUN ZHANG (1990) Maximum acceptable repetitive lifting workload by Chinese subjects, Ergonomics, 33:7, 875-884, DOI: 10.1080/00140139008925295 To link to this article: http://dx.doi.org/10.1080/00140139008925295

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ERGONOMICS, 1990, VOL. 33, No.7, 875-884

Maximum acceptable repetitive lifting workload by Chinese subjects ZUXIANG ZHU and ZHIJUN ZHANG Department of Psychology, University of Hangzhou, Zhejiang, People's Republic of China

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Keywords: Lifting; Maximum acceptable workload; Psychophysics; Physiological responses; Anthropometry.

This study used psychophysical methods to determine the acceptable mean maximum liftingworkload for eight Chinese young male subjects,and examined the effects of lifting technique (including freestyle, stoop and squat), lifting frequency (including 2, 3, 4, 5 and 6lifts/min) and physical characteristics on the maximum acceptable workload. The results are described as follows: (1) The maximum acceptable weightsselectedby subjects varied from 11·34 to 18·33 kg with changes in lifting technique and frequency. These data were lower than those previously obtained; (2) The upper limit of physiological tolerance over an 8 h workday was also generally lower than previously suggested. However, this upper limit varied with changes in lifting technique and frequency, and in some circumstances it was the same as or even higher than previous limit; (3) Lifting efficiency was affected significantly by technique and frequency. The rank order of efficiency for three lifting techniques were freestyle, stoop and squat. Efficiency was greatest when liftingfrequencywas between 5 and 6lifts/min; and (4)The correlations between the maximum acceptable workloads selectedby subjects and anthropometric sizeswere significant, but those between maximum acceptable workload and isometric strength were not.

I. Introduction Efficiency and safety are two opposing variables which should be traded off when setting workload standards (Ayoub et al. 1979, Schilling 1981). When the level of workload increases, working efficiency also increases, but worker safety declines (Chaffin and Park 1973, Snook 1983, 1985, Snook et al. 1978). Researchers have devised the term 'maximum acceptable workload', or MAL to refer to this situation (Datta et al. 1979, Garg and Ayoub 1980). The maximum acceptable workload represents the maximum level of physical activity which can be sustained by an individual in an 8-h working day in physiologically stable state which will not cause fatigue or discomfort (Datta et al. 1979). Lifting tasks are common activities in industry (Mital 1983, McCormick and Sanders 1982). The maximum weight which can be lifted safely by an individual (maximum acceptable weight, MAW) reflects his/her lifting capacity. There are many factors which affect the maximum acceptable weight, such as lifting technique and frequency (Ayoub et al. 1980 b, Garg and Saxena 1980). Studies oflifting capacity have been conducted in Europe and the USA for many years. However, the results were often inconsistent, which could be due to inconsistencies in experimental control. Also, differences in physique and stature among subjects have significant effects on lifting capacity (Ayoub et al. 1980 a, Garg and Ayoub 1980, Garg and Saxena 1979, Legg and Myles 1981, Snook 1978, Snook and Ciriello 1974). In our study, we employed

Zuxiang Zhu and Zhijun Zhang

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psychophysical methods to determine the maximum acceptable workload of young, healthy Chinese men, and examined the effectsoflifting technique, lifting frequency and physical characteristics on these results.

2. Method Eight young male volunteers were used as subjects. They were judged to be in good physical health, having never had any cardiovascular problems. The main physical characteristics of the subjects are shown in table I. All subjects were asked to perform lifting exercises for two days prior to the experiment. There were two independent variables in our study. These were lifting technique and frequency. The lifting techniques were: (I) freestyle-the technique which the subject felt to be the most suitable for himself; (2) stoop-lifting with bent-back and straight-knees; and (3) squat-the technique subjects used to lift with straight-back and bent-knees. The lifting frequencies included 2, 3, 4, 5 and 6liftsjmin. The following psychophysical method was employed in the study. The subjects were required to lift a tote box from floor to working desk and down again repeatedly using a prespecified lifting technique. The lifting frequency was regulated by an electronic metronome. During lifting, the subjects were instructed to adjust the weight of the tote box by adding or subtracting the number of sandbags inside it to the maximum amount they could lift without strain or discomfort. The subjects were encouraged to make weight adjustments. The instructions given to the subjects were similar to those used by Garg and Saxena (1979)in their study, i.e.,'Imagine you are on piece work, getting paid for the amount of work you do, but working a normal8-hour shift that allows you to go home without strain or discomfort and not becoming tired, weakened, over-heated, or out of breath'. The duration of the weight-adjustment part of each lifting trial was set at 20 min. The maximum acceptable weight was determined by weighing the tote box at the end of each trial. Expired air was collected in a Douglas Bag during the last 2 min of each trial. The content of expired air was recorded with a RCD-211 oxygen analyser. Heart rate was also recorded at the end of each trial. The tote box used in the study measured 40 x 28 x 24cm 3 with two solid handles attached to each side. The initial weight of the tote box randomly varied between 6 and 25 kg. The working desk was 72 ern in height. The air temperature was approx. 20°C during the whole study. 3. Results 3.1. Maximum acceptable weight and maximum acceptable workload The mean maximum acceptable weights of all eight subjects are shown in table 2. It can be noted from table 2 that the maximum acceptable weight declined markedly while the lifting technique varied from freestyle to stoop to squat, or while the lifting frequency increased from 2 to 6liftsjmin. When subjects lifted with a frequency of 21iftsjmin in Table I.

Mean s.d.

Main physical characteristics of the subjects (n = 8).

Age (years)

Body weight (kg)

Stature (em)

Body surface area (rn')

PWC 17 0 (kgm/min)

VO,max (rnl/rnin)

20·25 2·10

54·74 2·68

165·19 5·37

1·58 0·06

1026·00 91·39

2·55 0-14

877

Maximum acceptable repetitive lifting load

freestyle, the weight acceptable to them was the greatest (18'33 kg). When they lifted with a frequency of 6 lifts/min in the squat style, the weight acceptable to them was the smallest (11'34 kg). An analysis of variance demonstrated that both lifting technique and frequency had significant effects on the maximum acceptable weight (p, j'ilj\'il3i\-to

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