Job rotation as a factor in reducing physical workload ...

Ergonomics

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Job rotation as a factor in reducing physical workload at a refuse collecting department P. PAUL F. M. KUIJER , BART VISSER & HAN C. G. KEMPER To cite this article: P. PAUL F. M. KUIJER , BART VISSER & HAN C. G. KEMPER (1999) Job rotation as a factor in reducing physical workload at a refuse collecting department, Ergonomics, 42:9, 1167-1178, DOI: 10.1080/001401399185054 To link to this article: http://dx.doi.org/10.1080/001401399185054

Published online: 10 Nov 2010.

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Date: 06 November 2015, At: 01:24

E RGO N OM ICS , 1999,

VOL .

42,

NO.

9, 1167 ± 117 8

Job rotation as a factor in reducing physical workload at a refuse collecting department P. P AU L F . M . K UIJER ² *, B ART V ISSER ² and H AN C . G. K EM PER ³

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² ERGOcare, Faculty of Human Movement Sciences and ³ Institute of Research in Extramural Medicine (EMGO), Faculty of Medicine, Vrije Universiteit Amsterdam, Van der Boechorststraat 9, 1081 BT Amsterdam, The Netherlands Keywords: Job design; Job rotation; Physical workload; Refuse collecting; Manual materials handling. The eŒect of job rotation on the physical workload was investigated for male employees working at a refuse collecting department. Before the introduction of job rotation, an employee worked as a street sweeper, as a refuse collector or as a driver. After the introduction of job rotation, every employee was allowed to alternate between two of the three possible jobs during the day, i.e. refuse collecting / street sweeping, refuse collecting /driving or street sweeping /driving. Two non-rotation groups (i.e. refuse collectors and street sweepers) and two rotation groups (i.e. refuse collectors/street sweepers and street sweepers /drivers) were mutually compared. The physical workload was determined by measuring the perceived load, energetic load and postural load during a full working day. Job rotation resulted in a signi® cant decrease of the perceived load and energetic load and a slight decrease of the postural load. The results indicate that the total amount of work performed by means of job rotation resulted in an overall reduced physical workload of the employees of the refuse collecting department.

1. Introduction M anual materials handling is considered to be a major cau se of musculoskeletal disorders, sickness, disability and high costs (Badger 1981 , M ital et al. 1993). A great number of variables (and their interactions) in¯ uences the risk of these health hazards. Although there is a general agreem ent that variables such as the weight of handled objects or the time during which the tasks are performed are of primary importance, there is rarely an eŒort made to establish the relationship between laboratory and ® eld studies concerning physical workload and the onset of musculoskeletal disorders. Therefore, in order to prevent the onset of musculoskeletal disorders, it is probab ly most eŒective to try to aŒect several variables sim ultaneously. Possible preventive m easures can be divided into ® ve categories: (1) engineering modi® cations (e.g. workstations), (2) organization of work (e.g. work / rest schedules), (3) personal protective equipment (e.g. clothing), (4) training (e.g. work methods) and (5) administrative control (e.g. employee selection). The starting point of an ergonomics intervention, to reduce the risk of musculoskeletal disorders, should always be to strive for a redesign of the workplace. There are, however, jobs that do not have a speci® c workplace layout, e.g. refuse collecting. The redesign is mainly limited by the way the refuse is collected. Recent studies show that the use of mini-containers is less physically demanding for the refuse collectors than the use of *Author for correspondence. 0014± 0139 / 99 $12.00

Ó

1999 T aylor & Francis Ltd

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polythene bags (Frings-Dresen et al. 1995a , de Looze et al. 1995). U nfortunately, in a historic city such as Am sterdam , the use of mini-containers is not possible for a large part of the city due to architectural aesthetics and the number of inhabitants 2 per km . Therefore, to reduce the physical workload of collecting polythene bags, it is probably most eŒective to try to change the organization of the work by introducing job rotation. Job rotation can be de® ned as `regular, alternating between diŒerent jobs within an organization on basis of a scheme or spontaneously on basis of a personal appointm ent’ . Although job rotation is often used to enhance the skills of employees an d to reduce monotony in daily work, it can also be used as a means to alternate between diŒerent types of mechanical loads or to alternate between high and low energetic loads of diŒerent jobs. Especially jobs with a dynam ic type of work and great diŒerences in muscular activities should be able to bene® t from this principle (Jonsson 1998). Although job rotation is often advocated, only a few studies report its possible eŒects. In a study on the physical workload of long-line bank ® shing, Rodahl and Vokac (1977) found that the crew has resorted to their own job rotation system. Although the study was not designed to evaluate the eŒect of job rotation, it was concluded that contrary to general belief, bank ® shing need not be unsuitable for older ® shermen, provided that an eŒective system for job rotation is provided. Henderson (1992) developed a rotation scheme for poultry processing. Every job was rated on a scale ranging from low physical stress to unacceptably high physical stress. These last jobs were not to be performed and were given a high priority for ergonomic redesign. The guidelines for the job rotation scheme were that no back-toback high stress positions were to be performed after each other and that each highstress job was preceded and followed by a low-stress job. Although no precise data were availa ble, observations indicated that the number of musculoskeletal com plaints had decreased. In a study on the design of check-out systems, Hinnen et al. (1992) found that job rotation had a very bene® cial impact on the prevalence of musculoskeletal disorders in cashier work with scanners. The objective of this study is to investigate the eŒect of job rotation on the physical workload of employees working at a refuse collecting department. 2. M ethod 2.1. Job rotation Before the introduction of job rotation in a refuse collecting department, an employee would start as a street sweeper, after a few years could become a refuse collector, and could eventually become a driver of a dustcart or a sweeping m achine. After the introduction of job rotation, every employee was allowed to perform all three jobs. Job rotation was introduced in the following manner. To reduce the physical workload, rotation between jobs during the day was suggested. As most work was perform ed in the morning, employees rotated between jobs after the ® rst break in the morning and after the lunch break. D uring the day, employees alternated between two of the three possible rotation combinations, e.g. between refuse collecting and street sweeping, between refuse collecting and driving a vehicle, or between street sweeping and driving a vehicle. 2.2. Participants The sample in this study consisted of male employees of a refuse collecting department. Sixteen employees voluntarily participated in this study. To increase

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the number of employees in each group, four of the six possible jobs were studied. Eight employees worked according to a non-rotation scheme. The two most physical demanding jobs were selected: four men worked as refuse collectors (group RR) and four men worked as street sweepers (group SS). The other eight employees worked according to a rotation schem e. The most and least physically demanding jobs were selected: four men worked as refuse collectors / street sweepers (group RS) and four m en worked as street sweepers / drivers (group SD). Two employees of each group of four would start the day with one job; the other two employees would start the day with the other job, e.g. two em ployees would start the day with refuse collecting, then alternate to street sweeping and ® nally ® nish the day with refuse collecting. The other two employees would start the day with street sweeping, then alternate to refuse collecting and ® nally ® nish the day with street sweeping. The physical workload of the group of employees working according to a nonrotation scheme was compared with that of the group working according to a rotation scheme. The em ployees in each group were matched on age and did not diŒer in height, body weight, sum of skinfolds, maximum oxygen uptake and diŒerence between maximum heart rate and heart rate during rest. The diŒerent variables were determined during a test in a laboratory (Kemper et al. 1990) (table 1).

Table 1. Mean, standard deviation (SD) and range of age, height, body weight, sum of skinfolds, maximum oxygen uptake (VO 2 m a x ) and diŒerence between maximum heart rate and heart rate during rest (Hf m a x Ð Hfr es t ) of the refuse collectors (RR), street sweepers (SS), refuse collectors /street sweepers (RS) and street sweepers /drivers (SD). RR

SS

RS

SD

Age (years) SD Range

34 6 29 ± 42

34 7 28 ± 42

33 6 26 ± 40

35 15 26 ± 58

ns

Height (m) SD Range

1.78 0.02 1.76 ± 1.81

1.73 0.05 1.76 ± 1.87

1.77 0.04 1.68 ± 1.84

1.73 0.03 1.68 ± 1.76

ns

Weight (kg) SD Range

75.5 4.2 72.1 ± 81.6

76.3 13.7 63.7 ± 91.6

76.9 8.0 66.0 ± 85.4

70.9 7.6 65.8 ± 82.0

ns

Skinfold (cm) SD Range

10.9 1.6 9.7 ± 13.2

9.2 3.8 4.3 ± 12.6

16.7 6.2 10.7 ± 22.0

8.3 3.7 5.7 ± 13.6

ns

44.0 4.0 40.0 ± 49.2

44.2 9.2 36.4 ± 54.3

42.3 9.3 31.5 ± 52.4

49.6 11.4 38.0 ± 59.7

ns

107 19 89 ± 123

116 20 93 ± 134

107 17 87 ± 123

ns

VO 2 m a x (ml kg SD Range

± 1

± 1

min )

± 1

Hf m a x ± Hfr es t (beats min ) 109 SD 13 Range 98 ± 127 ns, Not signi® cant.

Signi® cance

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2.3. Physical workload The physical workload was determined by measuring the perceived load, energetic load and postural load during a full working day. Perceived load was de® ned as the mean rate of the perceived fatigue and the mean rate of the perceived exertion during a working day. Every hour, the employee rated the perceived fatigue (Borg and Borg 1987) and the perceived exertion (M eijman et al. 1986). Energetic load was de® ned as the mean heart rate during the working day, represented as a percentage of the heart rate reserve (AÊ strand and Rodahl 1986). This was done to prevent possible biases due to age diŒerences between employees. The heart rate was continuously registered by means of a sm all computerized recorder (Sporttester PE 3000 ; Polar Electro, Finland). Postural load was de® ned as the time during which ¯ exion of the trunk was > 45 8 and elevation of one or two of the upper arm(s) was > 60 8 . Only tw o variables and a total of ® ve categories within variables were observed to increase the validity and reliability of the observations of the posture (Beek et al. 1992 , De Looze et al. 1994, Frings-Dresen et al. 1995b). The position of the trunk and the upper arms were recorded by means of TRAC (Task Recording and Analysis on Com puter) on a multi-moment basis during the tasks of refuse collecting and street sweeping. TRAC was originally developed at the Robens Institute of Health and Safety (University of Surrey, Guildford, U K) (Ridd et al. 1989 ) and adapted by the Coronel Institute and ERGO care (Frings-Dresen and Kuijer 1995). Every 15 s, at an audible cue, an observation was made and recorded on a pocket computer (Psion Organiser II; Psion, UK (Beek et al. 1992). 2.4. Confoundin g variables M easurem ents were carried out during the norm al daily routine of the employees, thereby several confounding variables might in¯ uence the results. For this reason, a second observer performed a task analysis on a real-time basis. Possible confounding variables, such as the duration of the tasks and the number of objects handled, were registered by means of TR AC during each full working day. The following variables and categories within variables were observed: task (e.g. refuse collecting, street sweeping or pausing), activity (e.g. walking, sweeping or shovelling), load handled (e.g. polythene bag, broom or shovel) and number of bags lifted per lift (e.g. 1, 2 or 5) (Frings-Dresen et al. 1995a) . 2.5. Data analysis M ean perceived load, mean energetic load and mean postural load were calculated for each employee and consecutively for each group over a full working day. The testing of a diŒerence between the groups regarding the physical workload and the confounding variables was done by using an ANOVA and subsequently with a posthoc F-test. Testing of a diŒerence between two groups was performed with an unpaired t-test. The eŒect of the diŒerent confounding variables on the physical workload was tested with a stepwise multiple regression analysis. The confounding variables included in the stepwise multiple regression analysis are presented in table 2. Statistical analysis was performed with the software package StatView SE+ G (Abacus Inc.). p < 0.05 was considered statistically signi® cant.

Job rotation and workload in refuse collecting

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Table 2.

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Possible confounding variables included in the stepwise multiple regression analysis to explain the physical workload.

Variables

Categories within variables

Task (min)

refuse collecting, street sweeping, driving, pausing, non-working time, other tasks, length of working day

Activities (min)

carrying, sweeping

Objects handled

total number of bags collected during the day, number of bags collected per min during refuse collecting

Workload during refuse collecting and street sweeping

% VO 2 m a x , % HRR

3. Results 3.1. Physical workload In table 3, the results of the four groups are summarized. Job rotation has an eŒect on the perceived fatigue (RPF) and the perceived exertion (RPE) during the working day (F = 7.48, d.f. = 3,12, p = 0.004; F = 9.3, d.f. = 3,12, p = 0.002). The group of refuse collectors rate the perceived fatigue and the perceived exertion higher than the other groups during a working day. The RPF and RPE of the other three groups do not diŒer signi® cantly during a working day. Job rotation has an eŒect on the energetic load (F = 6.47, d.f. = 3,12, p = 0.008). The heart rate reserve (% HRR ) of the group of refuse collectors is signi® cantly higher than that of the other groups during a working day, while % HR R of the other three groups do not diŒer signi® cantly during a working day. Job rotation has an eŒect on the time during which ¯ exion of the trunk is > 45 8 (F = 37.03, d.f. = 3,12; p = 0.0001) . Flexion of the trunk of > 45 8 occurs signi® cantly m ore often for the group of refuse collectors than for the other three groups. Flexion of the trunk of > 45 8 also occurs more often for the group of refuse collectors /street sweepers than for the groups of street sweepers and street sweepers / drivers. The last two groups do not diŒer signi® cantly. Job rotation has no eŒect on the time during which elevation of an upper arm is > 60 8 . 3.2. Confou nding variables M ean, standard deviation (SD) and range of the possible confounding variables of the four groups are summarized in tables 4 and 5. The group of refuse collectors perform the task `refuse collecting’ and the activity `carrying’ during a longer period than the group of refuse collectors /street sweepers (t = 4.41, p = 0.002; t = 3.87, p = 0.004). The group of refuse collectors lift more than twice the am ount of bags during a working day than the group of refuse collectors /street sweepers (t = 6.37, p < 0.001). Job rotation has no eŒect on the period during which street sweeping takes place. Job rotation has an eŒect on the period that employees drive (F = 7.27, d.f. = 3,12, p = 0.005). The group of refuse collectors and the group of refuse

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Table 3. Mean, standard deviation (SD) and range of the rated perceived fatigue (RPF), rated perceived exertion (RPE), percentage of `heart rate reserve’ (% HRR), time during which ¯ exion of the trunk was > 45 8 and elevation of one or two of the upper arm(s) was > 60 8 of the refuse collectors (RR), street sweepers (SS), refuse collectors/street sweepers (RS) and street sweepers /drivers (SD) during a full working day. RR

SS

RS

SD

Signi® cance

RPF SD Range

2.0 0.9 1.1 ± 3.0

0.8 0.3 0.6 ± 1.2

0.3 0.1 0.2 ± 0.4

0.6 0.6 0.0 ± 1.4

RR> SS,RS, SD

RPE SD Range

54.2 22.4 23.8 ± 77.5

27.7 6.0 20.6 ± 33.1

11.1 4.0 5.7 ± 14.4

19.5 7.6 11.3 ± 27.5

RR> SS, RS, SD

% HRR SD Range

36.4 7.5 27.4 ± 43.9

22.6 5.0 16.6 ± 28.9

23.8 4.5 17.9 ± 28.0

1.55 9.2 6.0 ± 28.1

RR> SS, RS, SD

Trunk> 45 8 (min) SD Range

44.6 6.8 38.9 ± 52.5

5.0 6.4 0.5 ± 14.4

22.0 8.3 11.9 ± 30.5

2.6 2.7 0.4 ± 6.4

RR> SS, RS, SD RS> SS, SD

Arm(s)> 60 8 (min) SD Range

9.5 3.7 5.0 ± 12.8

24.4 6.8 19.6 ± 34.3

14.4 9.2 5.1 ± 25.9

16.4 7.5 8.8 ± 25.8

ns

ns, Not signi® cant.

collectors / street sweepers drive during a shorter period than the group of street sweepers and the group of street sweepers / drivers. Job rotation has an eŒect on the non-working time. The period performing nonwork-related tasks is less for the group working according to a non-rotation scheme than for the group working according to a rotation scheme (F = 3.66, d.f. = 3,12, p = 0.047). Job rotation also has an eŒect on the length of a working day (F = 7.27, d.f. = 3,12, p = 0.005). The working day of the group of refuse collectors is shorter than of the other three groups. The other confounding variables to not diŒer signi® cantly between the four groups. 3.3. EŒect of confounding variables on physical workload The results of the stepwise multiple regression analysis of the physical workload during refuse collecting and street sweeping are presented in table 6. The physical workload during refuse collecting can be explained by only a few variables. The results show that the rated perceived fatigue and the rated perceived exertion can be explained for respectively 91% (F = 58.5, p < 0.001) and 87% (F = 40.3, p < 0.001 ) by the variable `time carrying bags’ . The energetic load during refuse collecting can for 76% be explained by the variables `time trunk > 45 8 , and `length of the working day’ (F = 7.9, p < 0.05). The postural load of the trunk during refuse collecting can for 81% be explained by the variable `num ber of bags handled’ (F = 58.5, p < 0.05).

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Table 4. Mean, standard deviation (SD) and range of the possible confounding variables used in the stepwise regression analysis of the perceived load, energetic load and postural load. RR

SS

Refuse collecting (min) SD Range

172.6

±

Street sweeping (min) SD Range

±

RS 71.0

39.0 127.6 ± 222.8

SD ±

RR> RS

24.5 36.5 ± 89.0 153.2

88.6

99.2

38.3 111.9 ± 204.2

52.6 36.6 ± 155.6

34.3 68.5 ± 140.6

Driving (min) SD Range

90.5 153.1 14.1 39.0 72.3 ± 106.0 121.6 ± 207.1

75.8 177.7 39.0 44.8 34.9 ± 125.5 111.5 ± 210.7

Pausing (min) SD Range

72.1 18.6 50.8 ± 96.3

76.5 34.2 28.7 ± 104.0

Non-working time (min) SD Range

10.6

41.6

10.8 1.2 ± 22.1

52.7 11.2 ± 120.1

78.2 58.1 ± 228.6

20.0 63.1 ± 102.6

Other tasks (min) SD Range

18.6 10.3 7.9 ± 31.0

10.1 6.2 5.6 ± 19.2

33.7 34.7 1.5 ± 82.4

13.7 6.2 7.1 ± 21.6

Working day (min) SD Range

364.5

Signi® cance

108.5 39.2 73.9 ± 160.2

466.4

113.3

459.0

95.0 40.6 40.0 ± 134.6 85.6

471.3

ns

RR < SS SD RS< SS SD

ns

RR < RS SD

ns

RR < SS RS SD

18.6 8.3 13.0 10.4 341.6 ± 381.3 459.7 ± 478.0 440.3 ± 470.2 456.8 ± 479.7

ns, Not signi® cant.

Neither the stepwise multiple regression analysis of the postural load of the arm / shoulder nor the stepwise m ultiple regression analysis of the physical workload during street sweeping resulted in an unam biguous explanation. 4. Discussion 4.1. Job rotation The aim of the introduction of job rotation was to divide equally the physically demanding work of refuse collecting am ong the employees during the day. The results show that this target has not been fully accomplished. For instance, the total number of bags collected by the non-rotation group is more than twice as much as that of the group of refuse collectors /street sweepers. This result is probably not due to a lesser number of bags placed on the street during those days. A m ore plausible explanation is the lack of ® ne tuning between the two groups of refuse collectors / street sweepers. During the m easurement period each day, only one employee was observed. The colleague with whom he rotates during the day was not

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Table 5. Mean, standard deviation (SD) and range of the possible confounding variables used in the stepwise regression analysis of the perceived load, energetic load and postural load. RR

SS

RS

SD

Signi® cance

Total number of bags SD Range

1556

±

572

±

RR> RS

Number of bags per min SD Range

9.2

±

ns

Carrying (min) SD Range

85.4 22.0 60.2 ± 113.6

±

37.2 11.7 20.1 ± 46.2

±

RR> RS

Sweeping (min) SD Range

±

97.6 21.9 76.4 ± 128.2

58.3 35.4 25.4 ± 107.4

56.8 18.4 40.8 ± 77.7

ns

% VO 2 m a x refuse collecting SD Range

61.5

±

61.0

±

ns

% VO 2 m a x sweeping SD Range

±

ns

% HRR refuse collecting SD Range

60.9

% HRR sweeping SD Range

±

229 1256 ± 1779

208 292 ± 783 ±

8.0

1.2 7.5 ± 10.3

0.6 7.4 ± 8.8

11.8 45.1 ± 72.4

13.5 47.8 ± 78.2 41.5

38.4

35.6

13.8 30.3 ± 57.0

8.5 30.8 ± 57.0

13.6 20.4 ± 51.0

±

59.3

±

ns

33.0 11.8 17.5 ± 45.4

ns

12.3 43.4 ± 70.6

14.2 43.4 ± 75.4 41.3 20.7 17.6 ± 68.0

37.0 6.1 30.3 ± 43.7

ns, Not signi® cant.

simultaneously observed. The standard deviation and the range of the variables `non-working tim e’ and `total number of bags collected’ of the group of refuse collectors / street sweepers compared with the group of refuse collectors indicate that the non-observed refuse collectors / street sweepers might have collected m ore bags than they should. This result ham pers the drawing of conclusions. Despite this fact, a few comments can still be made on the eŒect of job rotation on the physical workload. Refuse collecting is the most physically demanding task due to the high energetic and postural load. Job rotation seem s to have no eŒect on the intensity with which this task is performed. The number of bags collected per m inute, the percentage of

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Job rotation and workload in refuse collecting Table 6.

2

Explained variance (r ) of the perceived load, energetic load and postural load during refuse collecting and street sweeping.

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Refuse collecting

Street sweeping

Variable

r

Rated perceived fatigue (RPF)

carrying

Rated perceived exertion (RPE)

2

Variable

r

0.91

±

±

carrying

0.87

±

±

Percentage heart rate reserve (% HRR)

trunk> 45 8 working day

0.76

±

±

Postural load of the trunk (time trunk > 45 8 [min])

number of bags

0.81

±

±

Postural load of the upper arms (time arm(s) > 60 8 [min])

±

±

±

±

2

± , No unambiguous explanation.

the heart rate reserve and the percentage of the VO 2 m a x during refuse collecting do not diŒer between the group of refuse collectors and the group of refuse collectors / street sweepers. The time during which the bags are collected does diŒer between the two groups. This is due to job rotation and the lesser number of bags collected by the group of refuse collectors / street sweepers. Therefore, it is not surprising that the physical workload is the highest in the group of refuse collectors. However, one result is very remarkable and points out the positive eŒect of job rotation on diminishing the physical workload. W hen comparing the physical workload of the group of refuse collectors / street sweepers with the groups of street sweepers and street sweepers / drivers, only the postural load of the trunk diŒers. The rated perceived fatigue, rated perceived exertion, energetic load and postural load of the arm / shoulder do not diŒer between both groups. Two possible explanations can be formulated. First, the change in tasks led to diŒerent postures and activities and, therefore, to a more heterogeneous physical workload. Lifting and throwing of bags is probably more strenuous for the lower back than sweeping with a broom . On the other hand, sweeping might be more strenuous for the shoulder and upper arm due to the prolonged static workload. Furthermore, van DieeÈ n and Oude Vrielink (1994) showed that job rotation might change more than only the temporal aspects of work. W hile not changing the (peak) intensity of the work, job rotation may have a positive eŒect on the energy storage in the segments. Owing to the viscoelastic behaviour of biological structures, van DieeÈ n and Oude Vrielink (1994) found a decrease in energy storage up to 24% . Therefore, it is not unreasonable to assum e that the same am ount of work perform ed while working according to a rotation scheme might result in an overall lower physical workload of the employees compared with the sam e am ount of work performed according to a non-rotation scheme. M oreover, the relationship between work output and fatigue might not be linear but exponential (Kilbom 1995).

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Above a certain am ount of work, fatigue rises more quickly. Therefore, job rotation between more and less strenuous work might result in preventing the crossing of a `fatigue threshold’ , the point where a more or less linear increase of the worktask / workload relationship starts to increase exponentially. Secondly, job rotation results in a more com plete job. The employee is enabled to perform a larger part in the process of refuse collecting. Several studies report that the perceived load depends on more than only the physical workload (Borg 1978). Other relevant factors are `task aversion’ and `motivation’ . The enrichm ent of the job makes the job less monotonous and therefore might decrease task aversion and increase the motivation of the employee. This increase of intrinsic job satisfaction probably results in a reduction of the perceived load. 4.2. Guidelines Frings-Dresen et al. (1995c) have formulated guidelines for the physical workload of refuse collectors in The Netherlands. The guidelines are based on the energetic load and the assumption that an overall energetic load of < 30% VO 2 m ax is acceptable for an 8-h working day. The guidelines are formulated in terms of the acceptable duration of the period of refuse collecting and the am ount of collected refuse. The most stringent limit is form ulated for refuse collecting of polythene bags in a city by refuse collectors > 39 years of age. The maximal acceptable period of refuse collecting and the maximal acceptable am ount of collected refuse for 90% (P 10 limit) of the refuse collectors > 39 years of age are respectively 1.7 h and 4000 kg. The group working according to a rotation schem e does not exceed the P 1 0 limit. The mean period of refuse collecting and the mean am ount of collected refuse are respectively 1.2 h and 4004 kg (an average bag of refuse weighs around 7 kg; Kemper et al. 1990). The group working according to a non-rotation scheme does exceed the P 10 limit. The mean period of refuse collecting and the mean am ount of collected refuse are respectively 2.8 h and 10892 kg. Although these results suggest that no further action is required to reduce the physical workload in the group working according to a rotation scheme, two rem arks have to be m ade. First of all, the guidelines by Frings-D resen et al. (1995c) are based on an energetic criterion because there is no generally accepted biom echanical criterion for an 8-h working day. De Looze et al. (1995) quanti® ed the biom echanical load on the back during the lifting and throwing of polythene bags. Average peak compression forces ranged from 3341 to 5179 N. These com pression forces exceed the NIOSH criterion for lumbar disc com pression of 3400 N (W aters et al. 1993). Therefore, manually collecting polythene bags in the conventional way remains a risky activity. Second, the energetic acceptable peak load of 50% VO 2 m a x for 1 h (Kem per et al. 1990 ) is exceeded by both groups working according to a rotation and a nonrotation scheme. In this study, the mean energetic load during refuse collecting in both groups is about 60% VO 2 m a x for > 1 h. A further reduction of the physical workload in Amsterdam can be achieved by looking for new ways of refuse collecting, e.g. by applying the M ETRO-system (citizens drop their refuse in dustbins on the street that have large depots underground. The refuse from the depots is then removed mechanically by special dustcarts). Another possible solution is improving the present way of working, e.g. by creating refuse collecting points in every street to which citizens brings their polythene bags. By designing these

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collecting points in such a manner that the dustcart can approach them easily, and by heightening the collecting points, the period during which bags are carried and when ¯ exion of the trunk occurs might be reduced and thus reducing the energetic and biomechanical workload. Acknowledgements This study was supported ® nancially by the municipality of Amsterdam , in particular by the `de Pijp’ district.

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