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Effect of design of two-wheeled containers on mechanical loading
International Journal of Industrial Ergonomics 31 (2003) 73–86
Effect of design of two-wheeled containers on mechanical loading Idsart Kingmaa,*, P. Paul F.M. Kuijera,b, Marco J.M. Hoozemansb, Jaap H. van Die.ena, Allard J. van der Beekc, Monique H.W. Frings-Dresenb a
Institute of Fundamental and Clinical Human Movement Sciences, Faculty of Human Movement Sciences, Vrije Universiteit, Van der Boechorststraat 9, 1081 BT, Amsterdam, The Netherlands b Coronel Institute for Occupational and Environmental Health, AmCOGG Amsterdam Centre for Research into Health & Health Care, Academic Medical Centre/University of Amsterdam, The Netherlands c Department of Social Medicine, Institute for Research in Extramural Medicine, VU University Medical Centre, Amsterdam, The Netherlands Received 13 March 2002; accepted 20 June 2002
Abstract To prevent low back disorders due to lifting activities, occupational lifting tasks are being replaced by pushing/ pulling tasks, for instance in refuse collection. Design factors of two-wheeled carts, such as centre of mass (COM) and handle location, might have a major impact on joint loading. We varied the (horizontal and vertical) COM and handle location of a two-wheeled container to test their effect on handle forces and joint loading during two-handed steady pushing and pulling. Net torques at the low back, shoulder and elbow were relatively low in the standard container design. Backward displacement of the COM increased low back loading and forward displacement of the COM increased shoulder and elbow loading. A COM displacement in the direction of the axis of the wheels did not have negative effects on joint loading and reduced the forces, needed to tilt the container. A 0:1 m increase of the handle height slightly reduced the required vertical force without adverse effects on joint loading. It is suggested that the design of Dutch two-wheeled containers can be improved by moving the COM of the loaded container in the direction of the axis of the wheels and by slightly raising the height of the handles. Relevance to industry The design of a two-wheeled container, used for refuse collection, might have a major effect on the risk of developing musculoskeletal disorders through mechanical overloading. In this paper the effect of two important design parameters on joint loading is investigated and interpreted in terms of optimising container design. r 2003 Elsevier Science B.V. All rights reserved. Keywords: Pushing and pulling; Two-wheeled container; Joint loading; Design; Centre of mass; handle
1. Introduction *Corresponding author. Tel.: +31-20-4448492; fax: +31-204448529. E-mail address: i [email protected] (I. Kingma).
With growing awareness of the risk of lifting activities for developing musculoskeletal disorders
0169-8141/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 8 1 4 1 ( 0 2 ) 0 0 1 7 6 - 2
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I. Kingma et al. / International Journal of Industrial Ergonomics 31 (2003) 73–86
(Bernard, 1997), tasks that used to involve frequent lifting are being replaced by pushing and pulling tasks. For instance, in refuse collecting, two-wheeled containers have been recommended to replace bags (Frings-Dresen et al., 1995), resulting in a marked reduction of compressive forces at the low back (de Looze et al., 1995; Schibye et al., 1997). However, pushing and pulling are also known to be risk factors for workrelated musculoskeletal disorders. Low back injuries are reported to be caused by pushing and pulling in 9–20% of the cases (Hoozemans et al., 1998). In addition, pushing and pulling tasks seem to increase the risk of developing pain or stiffness in the neck/shoulder region (van der Beek et al., 1993; Hoozemans et al., 1998). In pushing or pulling a fixed bar or a stable (four-wheeled) cart, the direction of the pushing or pulling force can affect the mechanical loading of the shoulder and lumbosacral joint (van der Woude et al., 1995; de Looze et al., 2000). In comparison to pushing or pulling purely horizontally, joint loading can be lower when, simultaneously, one produces a vertical force ðFy Þ that results in a moment effect at the shoulder and lumbosacral joint in opposite direction (Grieve and Pheasant, 1981; Pheasant et al., 1982). An example of such a situation is given in Fig. 1. In contrast to four-wheeled containers, twowheeled containers are unstable during pushing and pulling (see Fig. 2). A subject cannot freely change the Fy that he or she applies at the handles, because this immediately causes a change in the tilt angle of the container. Therefore, two-wheeled containers do not offer the possibility of applying force in a direction that minimises joint loading. Considering this constraint, it might be anticipated that design changes of a two-wheeled container may affect the mechanical loading of different joints during pushing and pulling. Some examples of such design changes are changes in the height or depth of the container, the location of the axis of the wheels or changes of the handle location. Such changes affect the moment arm of the centre of mass (COM) or the moment arm of the hand forces, relative to the axis of the wheels. For two-wheeled trolleys, used to transport gas cylinders, Okunribido and Haslegrave (1999)
d3 d1
Ftotal
Fy
d4
d2
handle
handle
Fx
Fx
(b)
(a)
Fig. 1. Schematic drawing of a participant showing that addition of a vertical reactive force ðFy Þ at the hand in Fig. 1b can reduce loading at the low back and shoulder joint by reducing the moment arm (from d1 and d2 in Fig. a to d3 and d4 in Fig. b).
Fvertical
COM
Ftotal
handle Fhorizontal
Freaction Ffriction-axis Ffriction-ground m.g Fig. 2. Schematic drawing of a participant symmetrically pulling a two-wheeled container. All forces, applying at the container and relevant for joint loading during steady pulling, are indicated in the graph.
reported that the height and the angle of the handle did affect the tilt angle of the trolley. In turn, this tilt angle affected the position of the handles and of the centre of mass (COM) with
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respect to the axis of the wheels, thereby influencing the required forces at the handle and the resulting joint loading. The influence of the COM location of a loaded two-wheeled container on mechanical loading of the joints during pushing and pulling can be illustrated with a diagram showing a subject pushing a container (Fig. 2). Given a specific tilt angle, a forward or backward shift of the COM immediately affects the required Fy due to a change of the moment arm of the COM with respect to the axis of the wheels. It should be noted here, that due to the tilting of two-wheeled containers, not only the forward-backward location but also the height of the COM affects joint loading. For instance, van der Beek et al. (1999) reported a three-fold increase of lumbar compressive forces when the COM height was drastically increased by inserting a tray in a two-wheeled container. To our knowledge, the effect of both the handle location and COM location of the two-wheeled container on handle forces and on joint loading during steady pushing and pulling have hardly been studied. Consequently, the main aim of the current study was to find out how the design of a two-wheeled container, in terms of its COM and handle location, affects handle forces and joint loading during steady, two-handed pushing and pulling. Horizontal (COMx and handlex ) as well as vertical (COMy and handley ) position changes were taken into account. Handle forces and joint loading were quantified in four participants performing a pushing and pulling task using two-wheeled containers with 9 COM locations and 11 handle locations.
2. Methods 2.1. Participants and materials Two standard Dutch refuse containers (Otto, 0:240 m3 ; height 1:00 m; bottom width depth 0:49 0:56 m; wheel diameter 0:2 m) were used for the experiment. The lid was removed from both containers and the handles were replaced by
75
handles attached to 3-D force transducers (AMTI). One container was used to create 9 conditions with different COM locations (Fig. 3a), using foam and concrete blocks. In these 9 conditions a handle location was used that was identical to that of a standard twowheeled container that is used to collect refuse in the Netherlands. The other container was used to create 11 conditions with different handle locations (Fig. 3b) with the aid of aluminium bars, attached to the container. In condition 13 the handle was at the standard location, identical to that which had been used in the COM conditions. Since heavy containers are more likely to cause high joint loading, a container mass was selected that represented a rather (but not extremely) heavy refuse container. The container plus load was intended to be 59:4 kg for the COM as well as the handle conditions. In all conditions, foam and straps were used to prevent slipping of the concrete blocks during the experiment. This caused a slight variation of the actual container mass ðSD ¼ 0:9 kgÞ for the COM conditions. COM locations were measured using a force platform (Kistler). For each of the 20 container conditions COMx was defined as (see Fig. 4a): COMx ¼ dCOM daxis ; where dCOM is the distance from the edge of the forceplate to the centre of pressure measured by the forceplate and daxis is the horizontal distance from the edge of the forceplate to the axis of the container wheels. To prevent slipping of the concrete blocks with the container in horizontal position, the vertical location of the COM (COMy; empty ) was first measured for the containers with force-measurement equipment but without foam and concrete blocks (see Fig. 4b): COMy;empty ¼ dCOM þ daxis : Subsequently, the height of all the concrete blocks and foam was measured while they were placed in the container, and COMy was calculated according to: COMy ¼ ðmempty COMy; empty þ mb1 COMb1 þ ? þ mbn COMbn Þ=ðmempty þ mb1 þ ? þ mbn Þ
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1.2 19
1.2
20
17
1 15 16
0.8
0.8
0.6
0.6
y (m)
y (m)
1
0.4
7
8
4
0.2
2
12 10
13
14 11
0.4
9
5 6
1
0.2
3
0
0
-0.2 (a)
18
0
0.2
-0.2
0.4
x (m)
(b)
0
0.2
0.4
x (m)
Fig. 3. The two-wheeled containers used in the current study with the centre of mass (COM) locations (a) and the handle locations (b) indicated by numbers. The dots in container (b) indicate the (unintended) range of small changes of the COM location due to variation of the location of the handle.
where m is mass, b1 to bn are blocks 1 to n: This was done for all 20 container conditions. Details of the COM and handle locations in the 20 container conditions are given in Table 1. The location of handles plus force transducers caused some unintended variation of the container COM in the handle conditions (Fig. 3b and Table 1). However, this variation was small in comparison to the intentional COM variation in the COM conditions. Although the main interest of this study was the container design rather than participant anthropometry, it was realised that the participant’s body height might have major effects on posture and thus on joint loading. To avoid systematic bias due to body height, we selected one participant with a body height of about the 5th percentile of Dutch males, one participant with a body height of about the 95th percentile of Dutch males, and two participants of intermediate body height (see Table 2 for subject character-
istics). All participants were experienced refuse collectors. 2.2. Experimental procedure For each of the 20 conditions, participants were allowed to practice pushing and pulling a few times before the measurements started, in order to accommodate to the container condition. First, the 9 COM conditions and then the 11 handle conditions were presented in random order. For each condition participants were instructed to grab the handles of the container with both hands and to tilt the container (they were allowed to use a foot if they wanted to), followed by walking backward while pulling the container at a normal constant speed and with the upper body as symmetrical as possible over a distance of about 5 m on a hard, rubber surface. About 1 min later, the participants were asked to tilt the container and walk the trajectory in the forward direction
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Table 2 Descriptive statistics of the participants in this study
y x
COM COMx
COMy
y
daxis forceplate
dCOM
dCOM
Fig. 4. Schematic drawing showing how the horizontal and vertical location of the centre of mass (COM) of the twowheeled containers was measured using a force-platform.
Table 1 Horizontal (COMx ) and vertical (COMy ) centre of mass location and horizontal (handlex ) and vertical (handley ) handle locations for which the effect on mechanical loading was investigated during pushing and pulling of two-wheeled containers
COM 1 2 3 4 5 6 7 8 9 Handle # 10 11 12 13 14 15 16 17 18 19 20
Weight (kg)
Age (yr)
1 2 3 4
1.64 1.72 1.85 1.93
67 65 86 80
28 34 43 32
x
m.g
condition #
Body height (m)
daxis
forceplate m.g
COM
Participant
COMx (m)
COMy (m)
Handlex (m)
Handley (m)
0.138 0.221 0.247 0.115 0.193 0.234 0.090 0.180 0.254
0.120 0.120 0.120 0.230 0.230 0.230 0.360 0.360 0.360
0:174 0:174 0:174 0:174 0:174 0:174 0:174 0:174 0:174
0.900 0.900 0.900 0.900 0.900 0.900 0.900 0.900 0.900
0.170 0.186 0.170 0.179 0.186 0.170 0.179 0.170 0.179 0.170 0.186
0.224 0.224 0.248 0.248 0.248 0.271 0.271 0.296 0.296 0.321 0.321
0:268 0:066 0:270 0:171 0:071 0:269 0:176 0:269 0:170 0:270 0:071
0.840 0.840 0.900 0.900 0.900 1.000 1.000 1.100 1.100 1.200 1.200
Co-ordinates are indicated relative to the axis of the wheels and correspond with locations indicated in Figs. 3 and 5–8.
while symmetrically pushing the container at a constant speed. LED markers were attached to the left and right side of the body at the L5S1 joint according to de Looze et al. (1992), at the acromion, and at the wrist. In addition, markers were attached to the left and right side of the container at the centre of the handles and at 0.4 and 0:6 m above the axis of the wheels. Marker positions were recorded using an opto-electronic system (Optotrak) with an array of three cameras on both sides of the body. Forces and marker positions were sampled in 3 dimensions at 50 Hz: From the 5 m walking trajectory, about the middle 3 m was selected for averaging marker positions and forces over time. In addition, markers on the left and right side of the body and container as well as the left and right handle forces were averaged.
2.3. Biomechanical model Elbow positions in the sagittal plane were calculated from wrist and shoulder markers and known upper arm and forearm length. The markers on the container were used to calculate the average tilt angle during pushing and pulling for each trial. Using this angle, the forces at the handles were transformed from local to global coordinates. Time-averaged sagittal plane marker co-ordinates and forces were used as input for an upper body static 2-D linked segment model. The model consisted of four segments: hands, forearms, upper arms, and trunk plus head. Relative segment masses and segment centre of mass locations were obtained from Plagenhoef et al. (1983). Using standard linked segment mechanics (Elftman, 1939), reactive forces and torques were
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calculated at the elbow ðTe Þ; shoulder ðTs Þ and the lumbosacral ðTL5S1 Þ joints. 2.4. Statistics The experiment consisted of two series of measurements in which a single independent variable was manipulated in a repeated measures design. The independent variables were COM and handle location. This design does not allow analysis of the interaction between these variables. Statistical analyses were performed separately for the two independent variables. The zero hypotheses were that COM and handle location do not affect physical loading. ANOVAs were applied with participant, pushing/pulling activity, and either COM condition or handle condition as independent variables. Interaction effects between pushing/pulling activity, and either the COM condition or the handle condition were taken into account. However, due to limitations in degrees of freedom, interactions with the participant could not be taken into account. The dependent variables were the container tilt angle, the height of the handle of the tilted container during steady
pushing and pulling, Fx and Fy applied at the handles, the torques at the elbow, shoulder and the lumbosacral joint, and upper body segment angles. In addition, ANOVAs with the same independent variables were applied to the peak values of Fx and Fy at the initiation of the tilting of the container. A p-value smaller than 0.05 was considered statistically significant. For all ANOVAs, Bonferroni post-hoc tests were applied to determine whether or not differences between individual conditions were significant.
3. Results ANOVA results showed highly significant effects ðpo0:001Þ of COM condition in 6 out of 12 dependent variables (Table 3) and of handle condition in 8 out of 12 dependent variables (Table 4). For those variables with significant effects of COM or handle condition, post-hoc tests showed significant differences in many adjacent COM or handle locations. Given the large number of possible comparisons and the fact that most
Table 3 Summary of ANOVA results for 4 participants pushing and pulling 9 two-wheeled containers, differing in location of the centre of mass (COM) Independent variables Condition Participant Pull/push Condition pull=push Independent variables Condition Participant pull/push Condition pull=push
DF
Peak initial Fx p
Peak initial Fy p
Fx p
Fy p
Tilt angle p
Tilted handle height p
(8,48) (3,48) (1,48) (8,48)
o0.001 o0.001 0.056 0.751
0.289 o0.001 o0.001 0.951
0.515 0.875 o0.001 0.549
o0.001 o0.001 o0.001 0.914
0.417 o0.001 0.115 0.696
0.273 o0.001 0.094 0.671
DF
Te p
Te p
TL5S1 p
Forearm angle p
Upper arm angle p
Trunk angle p
(8,48) (3,48) (1,48) (8,48)
o0.001 o0.001 0.605 0.048
o0.001 o0.001 o0.001 0.001
o0.001 o0.001 0.008 0.275
0.135 o0.001 0.022 0.218
0.545 o0.001 o0.001 0.975
o0.001 o0.001 o0.001 1.000
Effects on container kinematics and on horizontal ðFx Þ and vertical ðFy Þ forces applied the handles are described in the top block and effects on Te ; Ts and TL5S1 (moments at the elbow, shoulder and lumbosacral joint, respectively) and arm and trunk kinematics are described in the bottom block. Degrees of freedom (DF) in the ANOVAs are indicated in the 2nd column. Significant effects ðpo0:05Þ are indicated by bold numbers.
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Table 4 Summary of ANOVA results for 4 participants pushing and pulling 11 two-wheeled containers, differing in location of the handle Independent variables
DF
Peak initial Fx p
Peak initial Fy p
Fx p
Fy p
Tilt angle p
Tilted handle height p
Condition Participant pull/push Condition pull=push
(10,63) (3,63) (1,63) (10,63)
o0.001 o0.001 0.004 0.182
0.082 0.001 o0.001 0.072
0.073 0.016 o0.001 0.946
o0.001 o0.001 o0.001 0.625
o0.001 o0.001 0.193 0.998
o0.001 o0.001 0.323 0.999
Independent variables
DF
Te p
Te p
TL5S1 p
Forearm angle p
Upper arm angle p
Trunk angle p
Condition Participant pull/push Condition pull=push
(10,63) (3,63) (1,63) (10,63)
o0.001 o0.001 o0.001 0.014
o0.001 o0.001 o0.001 o0.001
0.235 o0.001 0.010 0.986
o0.001 o0.001 0.767 0.978
0.613 o0.001 o0.001 0.971
o0.001 o0.001 o0.001 0.993
Effects on container kinematics and on horizontal ðFx Þ and vertical ðFy Þ forces applied the handles are described in the top block and effects on Te ; Ts and TL5S1 (moments at the elbow, shoulder and lumbosacral joint, respectively) and arm and trunk kinematics are described in the bottom block. Degrees of freedom (DF) in the ANOVAs are indicated in the 2nd column. Significant effects ðpo0:05Þ are indicated by bold numbers.
effects have a rather linear appearance (see Figs. 5–8), we will describe the main tendencies in dependent variables that showed significant effects of COM or handle conditions, and add post-hoc results only in cases where we compare specific conditions (which is in the discussion section). 3.1. Effect of COM and handle location on Fx at the handle Averaged over participants, the peak initial Fx used to tilt the container ranged from 60717 N in the pushing task with the COM located high and close to the axis to 195743 N in the pushing task with the COM located low and far from the axis (Fig. 5d). This force was highly dependent on COMx (Figs. 5a and d) and to a lesser extent on the height and forward-backward position of the handle (Figs. 6a and d). Since COMy hardly affected the peak initial Fx ; it is unlikely that the effect of handley is explained by the concomitant changes in COMy : However, interactions between COMy changes and handley changes cannot completely be excluded here. As might be expected due to the almost constant container mass, steady Fx during pulling and
pushing was not significantly influenced by the location of the COM or the handle (Tables 3 and 4). 3.2. Effect of COM location on tilt angle, forces and joint loading It was expected that, given a certain COM location, participants would manipulate the tilt angle of the container in order to reduce the required Fy and/or to reduce the loading of one or more joints. However, it appeared that the container tilt angle was highly dependent on the participant but not on the COM location (Table 3). Because the COM location had no significant effect on the tilt angle, Fy during pushing and pulling was highly dependent on the COM location of the container (Table 3). More forward (i.e., at a greater distance from the axis and the participant) and lower locations of the COM resulted in negative (downward) changes of Fy (Figs. 5c and f). This effect can be understood by comparing the different COM locations for their resulting horizontal position with respect to the axis of the wheels when the container in Fig. 3a is tilted. The COM location had a large effect on the joint torques (Table 3) and this effect appeared
I. Kingma et al. / International Journal of Industrial Ergonomics 31 (2003) 73–86
80
low
medium
COM & pulling
high
standard
COM & pulling
COM & pulling
40 upward 200
100
150
100
50
30
Fy (N)
tilt angle (deg)
peak Fx (N)
35
25
0
-100
20 -200
0
15 0.1
(a)
0.2 COMx (m)
0.3
0.1
(b)
COM & pushing
0.2 COMx (m)
0.3
0.1
(c)
COM & pushing
0.2 COMx (m)
0.3
COM & pushing
40 upward 200
100
150
100
30
Fy (N)
tilt angle (deg)
peak Fx (N)
35
25
50
20
0
15
0
-100
-200 0.1
(d)
0.2 COMx (m)
0.3
0.1
(e)
0.2 COMx (m)
0.3
0.1
(f)
0.2 COMx (m)
0.3
Fig. 5. The effect of centre of mass (COM) location on the peak tilting horizontal force ðFx ; graphs a and d), on the tilting angle (graphs b and e) and on the vertical force applied at the handle ðFy ; graphs c and f) during steady pulling (graphs a, b and c) and pushing (graphs d, e and f) of a two-wheeled container in a symmetrical way. Error bars indicate one standard deviation. COM heights were 0.12 (low), 0.23 (medium) and 0:36 m (high) above the axis of the wheels (with 0:1 m radius). Horizontal locations on the x-axis equal those of Fig. 3a and Table 1, referring to a container in upright position.
to be systematically related to COMx and COMy (Fig. 7). Interestingly, the effect of COMx and COMy changes on L5S1 joint loading appeared to be opposite to the effect on shoulder and elbow joint loading (Fig. 7). Roughly, these changes can be explained by the changes in Fy ; since, assuming that the shoulders are not in front of the handles, adding a downward force component at the hands must result in a more extending Te and Ts and a less extending TL5S1 : However, with pushing this effect was not consistent at the shoulder joint (Fig. 7e). This might be related to a postural change, since a significant effect of COM condition on the trunk
angle was found (Table 3). While pushing, participants were ‘leaning on the handles’ when large downward forces were required. In this way, the reactive forces at the hand nearly pointed at the shoulder joint, so that only small joint torques occurred. In other words, body mass instead of muscle force was used to produce the downward force by leaning on the handles. 3.3. Effect of handle location on tilt angle, forces and joint loading Contrary to the COM, the handle location significantly affected the tilt angle of the container
I. Kingma et al. / International Journal of Industrial Ergonomics 31 (2003) 73–86
low
normal
high
handle & pulling
higher
highest
*
handle & pulling
81
standard
handle & pulling
40 upward 200
100
150
100
30
Fy (N)
tilt angle (deg)
peak Fx (N)
35
25
50
20
0
15
0
-100
-200
(a)
-0.2 -0.1 handlex (m)
0
-0.2 -0.1 handlex (m)
(b)
handle & pushing
0
-0.2 -0.1 handlex (m)
(c)
handle & pushing
0
handle & pushing
40 upward 200
100
150
100
30
Fy (N)
tilt angle (deg)
peak Fx (N)
35
25
50
20
0
15
0
-100
-200
(d)
-0.2 -0.1 handlex (m)
0
(e)
-0.2 -0.1 handlex (m)
0
(f)
-0.2 -0.1 handlex (m)
0
Fig. 6. The effect of handle location on the horizontal force ðFx ; graphs a and d), on the tilting angle (graphs b and e) and on the vertical force applied at the handle ðFy ; graphs c and f) during steady pulling (graphs a, b and c) and pushing (graphs d, e and f) of a two-wheeled container in a symmetrical way. Error bars indicate one standard deviation. Handle heights were 0.84 (low), 0.90 (normal), 1.00 (high), 1.10 (higher) and 1.20 (highest) above the axis of the wheels (with 0:1 m radius). Horizontal locations on the xaxis equal those of Fig. 3b and Table 1, referring to a container in upright position.
(Table 4; Figs. 6b and e). Higher and more towards the front (at a larger distance from the participant but closer to the axis) locations of the handles were associated with a larger tilt angle. Although interactions with COM location cannot completely be ruled out, this is unlikely to be caused by concomitant COM changes because the COM did not affect the tilt angle (Table 3). The increase in tilt angle only partially compensated for the increased handley ; since there was still a significant effect of the handle location on the handley during pushing and pulling (Table 4). On average, the highest handle locations resulted in a
pushing and pulling height of about 0:18 m higher in comparison with the lowest handle locations. As a result of the increased tilt angle with higher handles, and probably also in part due to the increase of COMy with increasing handley ; the COM of the tilted container was more backward (towards the participant) with respect to the axis of the wheels, resulting in a change of the Fy in less downward or more upward direction (Figs. 6c and f). Consequently, the extending Te (Figs. 8c and f) decreased (in pushing and pulling) and shifted to a flexion Te for the highest handle locations (mainly in pulling). In addition, the extending Ts decreased
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82
low
medium
COM & pulling
high
standard
COM & pulling
COM & pulling
100 extension
extension 40
60
20
20
40
Te (Nm)
80
40
Ts (Nm)
TL5S1 (Nm)
extension
0
0
-20
-20
20 0
0.1
(a)
0.2 COMx (m)
0.3
0.1
(b)
COM & pushing
0.2 COMx (m)
0.3
0.1
(c)
COM & pushing
0.2 COMx (m)
0.3
COM & pushing
100 extension
extension 40
60
20
20
40
Te (Nm)
80
40
Ts (Nm)
TL5S1 (Nm)
extension
0
0
-20
-20
20 0
(d)
0.1
0.2 COMx (m)
0.3
0.1
(e)
0.2 COMx (m)
0.3
0.1
(f)
0.2 COMx (m)
0.3
Fig. 7. The effect of the centre of mass (COM) location on the lumbosacral torque ðTL5S1 ; graphs a and d), shoulder torque ðTs ; graphs b and e) and elbow torque (Te ; graphs c and f) during steady pulling (graphs a, b and c) and pushing (graphs d, e and f) of a twowheeled container in a symmetrical way. Error bars indicate one standard deviation. COM heights were 0.12 (low), 0.23 (medium) and 0:36 m (high and standard) above the axis of the wheels (with 0:1 m radius). Horizontal locations on the x-axis equal those of Fig. 3a and Table 1, referring to a container in upright position.
with forward and upward displacement of the handle during pulling, whereas changes during pushing were marginal (Figs. 8b and e). The decreased extending Te and Ts with higher handley may in part have been caused by the upward COM shift with higher handley : However, the effect of handley was stronger than the effect that would have been expected on the basis of a 0:1 m increase of the COM alone (see Figs. 7b and c). Furthermore, the extending TL5S1 slightly increased from low to high handle locations (Figs. 8a and d). It is uncertain to what extent this is caused by the actual increase of handley alone, or also by the concomitant increase of
COMy : Despite some postural changes (in terms of trunk and forearm angle) due to the changes of handle location, the trend in changes of joint loading (Fig. 8) was in line with the change of the Fy (Figs. 6c and f), i.e. a more upward directed Fy at the handle resulted in a decreased extending Te and Ts and in an increased extending TL5S1 : 3.4. Effects of participant and pushing versus pulling As anticipated, there was a significant effect of participant (most likely due to body height
I. Kingma et al. / International Journal of Industrial Ergonomics 31 (2003) 73–86
low
normal
high
higher
highest
*
handle & pulling
handle & pulling
83
standard
handle & pulling
100 extension
extension 40
60
20
20
40
Te (Nm)
80
40
Ts (Nm)
TL5S1 (Nm)
extension
0
0
-20
-20
20 0
(a)
-0.2 -0.1 handlex (m)
-0.2 -0.1 handlex (m)
0
(b)
0
-0.2 -0.1 handlex (m)
(c)
handle & pushing
handle & pushing
0
handle & pushing
100 extension
extension 40
60
20
20
40
Te (Nm)
80
40
Ts (Nm)
TL5S1 (Nm)
extension
0
0
-20
-20
20 0
(d)
-0.2 -0.1 handlex (m)
0
(e)
-0.2 -0.1 handlex (m)
0
(f)
-0.2 -0.1 handlex (m)
0
Fig. 8. The effect of handle location on the lumbosacral torque ðTL5S1 ; graphs a and d), shoulder torque (Ts ; graphs b and e) and elbow torque (Te ; graphs c and f) during steady pulling (graphs a, b and c) and pushing (graphs d, e and f) of a two-wheeled container in a symmetrical way. Error bars indicate one standard deviation. Handle heights were 0.84 (low), 0.90 (normal and standard), 1.00 (high), 1.10 (higher) and 1.20 (highest) above the axis of the wheels (with 0:1 m radius). Horizontal locations on the x-axis equal those of Fig. 3b and Table 1, referring to a container in upright position.
variations) on most of the dependent variables for the COM conditions (Table 3) and handle conditions (Table 4). Roughly, shorter participants tilted the container more (with consequently more downward directed handle forces and more extending torques in the arms) and taller participants flexed their trunk more (with consequently higher low back loading). Container tilt angles did not differ between pushing and pulling in the COM conditions (Table 3) as well as in the handle conditions (Table 4). As
a consequence of the necessary mechanical equilibrium during steady pushing and pulling, the reversal of sign of Fx resulted in a downward change of Fy at the handle in pushing compared to pulling (Figs. 5c, f, and 6c and f). For the COM as well as the handle conditions, the upper body posture showed some more trunk and shoulder flexion in pushing as compared to pulling, presumably to facilitate downward pushing. In absolute sense, Fy (and thus the total forces at the handle) were greater in pushing as compared to
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pulling in the majority of COM and handle conditions. Interestingly, the reverse was the case for all joint torques in almost all conditions, underscoring the importance of force direction for joint loading.
cluded that the magnitude of the force applied at the handles, is not a good indicator of the mechanical loading of the joints, when the direction of this force as well as the position of the joints are not taken into account. 4.2. Handle location and mechanical loading
4. Discussion 4.1. COM location and mechanical loading Surprisingly, we found little influence of the COM location of the container on the tilt angle. Contrary to expectations, participants hardly adapted the angle of pulling or pushing when the angle of mechanical equilibrium of the container changed. Consequently, Fy ; applied to the handle, and the loading of the joints were highly dependent on the COM location. In contrast to pushing or pulling tasks with a four-wheeled cart or a fixed bar (de Looze et al., 2000), Fy in the current study was imposed by the container tilt angle and therefore it was largely unrelated to the arm orientation. Consequently, some combinations of Fx and Fy resulted in large moment arms with respect to the shoulder joint, so that forces of moderate magnitude could result in high loading of the shoulder joint. Especially the combination of a downward force and a pulling force can result in a total force vector that is nearly perpendicular to the orientation of the arm, causing a large moment arm with respect to the shoulder and elbow joint. This may cause large Ts and Te even when the downward force only has a moderate magnitude. Such a situation occurred when the COM was located low and towards the front (see Figs. 5c, 7b and c). In pushing, a comparable effect was found when the COM was located high and towards the back. In this situation, a combination of pushing and a quite small upward force (Fig. 5f) resulted in large moment arms with respect to the elbow and shoulder joint, with consequently relatively high elbow and shoulder loads (Figs. 7e and f). These examples show that the COM location, which can easily be influenced through the design of the container, is a major determinant of joint loading in pushing and pulling of twowheeled containers. In addition, it can be con-
The finding of a reduction of shoulder joint loading with pulling at increasing handley ; as found in the current study, is in agreement with findings using four-wheeled carts (de Looze et al., 2000), but the magnitude of the effect is larger. One explanation for this finding might be that handley effects may have been amplified by concomitant COMy changes in the current study. Furthermore, Okunribido and Haslegrave (1999) reported, for two-wheeled trolleys, that the handle height did affect the tilt angle of the trolley, but that participants always tilted the trolley at such an angle that one specific grip height was obtained, regardless of other conditions. In the current study a change of handle height also affected the tilt angle, but not to such an extent that equal handle heights were obtained after tilting. Okunribido and Haslegrave (1999) suggested that the high COM of the trolleys that were studied, may have caused their finding of a constant grip height. Comparable to the COM location, handle location affected the external forces and the joint loading not in the same way in the current study. Low handle locations resulted in unfavourable combinations of Fx and Fy : pulling with a low handle resulted in a relatively small downward force (Fig. 6c) but due to a large moment arm of the force in relatively high shoulder loading (Fig. 8b). Compared to the current handle location (handle condition 13), some increase of the height of the handle (handle condition 16) significantly reduced the average Fy ðp ¼ 0:032Þ without adverse effects on joint loading. It should be noted that concomitant COMy changes might have played a role here. However, we do not expect the influence to be profound, since the concomitant COMy change was only 0:02 m: A more than 0:1 m increase of handle height caused short participants to tilt the container quite far, resulting in an increasing joint loading for those subjects.
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4.3. Limitations of the study First, only four participants of different height participated in the experiments. Clearly, this limits generalisation of the current results, since differences in participant body height cannot be separated from other differences between the participants. However, despite the small number of participants, highly significant effects of COM and handle location were found. In addition, those effects were understandable from a mechanical perspective in that joint loading was high for those situations where the resulting pushing or pulling force vector had a large moment arm with respect to those joints. Interaction effects between handle location and COM location were not investigated in the present study. These effects might occur since handle height was found to affect the tilt angle of the container. Also, participants had some time to adapt to every condition but it cannot be excluded that more experience with each condition would affect the body posture during pushing and pulling. Furthermore, refuse collectors often pull twowheeled refuse containers with one hand, walking forward instead of backward. This might influence the effect of handle location on joint loading. For instance, short participants may then be forced to either tilt the container too far or to keep their arm in an extremely extended position. Finally, it should be realised, that other parameters than average joint loading (on which we mainly focussed thus far) are important. Container stability may affect the risk of musculoskeletal injuries, through peak forces necessary to counteract balance disturbances due to an irregular walking surface. In the next section, some inferences on this issue are made, partially using information obtained from the current experiment. 4.4. Can the current design of refuse containers be improved? With homogeneous loading, the COM of the container is normally situated about the midline of the container. Thus, locations 8 and 5 (Fig. 3a) represent COM’s for the current container design, for a completely and an incompletely filled
85
container, respectively. In comparison to those COM locations, a frontal displacement of the COM has negative effects on low back loading ðp ¼ 0:02 and o0:001 for locations 5 and 8, respectively) whereas a backward displacement tended to increase elbow loading during pulling ðp ¼ 0:076 and 0.001 for locations 5 and 8, respectively). It appeared that joint loading is not high for the current design and that none of the COM locations tested, resulted in a substantial reduction of the loading in all three joints (low back, shoulder and elbow). However, improvement of the container design is possible with respect to the COM location. Since, displacement of the COM in the direction of the axis (i.e. downward and backward at the same time, from location 8 to 4 in a fully loaded container, and from location 5 to 1 in a partially filled container, see Fig. 3a) reduces the moment arm of the COM with respect to the axis of the wheels at any tilt angle. The current experiment showed that this displacement reduced the force needed to tilt the container ðp ¼ 0:009 and o0:001 for locations 5 and 8, respectively) and had no negative effects on the loading of any joint ðp ¼ 1:000 in all cases). In daily practice, the stability of the container will improve, since, disturbances of mechanical equilibrium, which constantly occur during pushing and pulling a container on an irregular surface, require less force adaptation to restore the tilt angle. The most obvious way to obtain the proposed COM displacement in the direction of the axis, would be to make the container wider, and at the same time, place the axis more towards the front. It was already mentioned that interaction effects between COM and handle changes were not investigated in the current study. However, as long as the COM changes are along a line towards the axis of the wheels, such interaction effects are not likely to be substantial. Therefore, assuming that the improvement with a 0:1 m increase of the handle height was not caused by the concomitant small COM height increase, the 0:1 m increase of handle height can, most likely, safely be combined with the proposed COM displacement in the direction of the axis. Before we can conclude that design changes as proposed above, will bring about considerable
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improvement, a new design, using the proposed changes, should be tested against the traditional design with more participants, with various container loading conditions and with more realistic working conditions (including one-handed pulling, rotating a container and pulling it up and down the walkway). This study, using 3-D analysis of shoulder (Helm and Veeger, 1996) and low back (Kingma et al., 1996) loading, and including an EMG-assisted translation of net moments to compression and shear forces (van Die.en and Kingma, 1999), is being undertaken and will be published in a separate paper.
Acknowledgements The authors would like to thank the Association for waste and cleansing management (NVRD) and the Association of Dutch waste management companies (VNA) for financially supporting this project. In addition, the refuse collectors and the management team of the Nature and Environment Division of the municipality of Haarlem are acknowledged for their kind co-operation. Furthermore, Tom Kalkman is acknowledged for his assistance during the experiments.
References Bernard, P.B. 1997. Musculoskeletal disorders and workplace factors: a critical review of epidemiological evidence for work-related musculoskeletal disorders of the neck, upper extremity, and low back. National Institute for Occupational Safety and Health: US Department of Health and Human Services (DHHS)., report nr 97-141. de Looze, M.P., Kingma, I., Bussmann, J.B.J., Toussaint, H.M., 1992. Validation of a dynamic linked segment model to calculate joint moments in lifting. Clinical Biomechanics 7, 161–169. de Looze, M.P., Stassen, A., Markslag, A.M.T., Borst, M.J., Wooning, M.M., Toussaint, H.M., 1995. Mechanical loading on the low back in three methods of refuse collecting. Ergonomics 38 (10), 1993–2006. de Looze, M.P., van Greuningen, K., Rebel, J., Kingma, I., Kuijer, P.P.F.M., 2000. Force direction and physical load in dynamic pushing and pulling. Ergonomics 43 (3), 377–390.
Elftman, H., 1939. Forces and energy changes in the leg during walking. American Journal of Physiology 125, 339–356. Frings-Dresen, M.H.W., Kemper, H.C.G., Stassen, A., Markslag, A., de Looze, M.P., Toussaint, H.M., 1995. Guidelines for energetic load in three methods of refuse collecting. Ergonomics 38 (10), 2056–2064. Grieve, D.W., Pheasant, S.T., 1981. Naturally preferred directions for the exertion of maximal manual forces. Ergonomics 24, 685–693. Helm, F.C.T., van der, Veeger, H.E.J., 1996. Quasi-static analysis of muscle forces in the shoulder mechanism during wheelchair propulsion. Journal of Biomechanics 29(1) 39–52. Hoozemans, M.J.M., van der Beek, A.J., Frings-Dresen, M.H.W., van Dijk, F.J., van der Woude, L.H.V., 1998. Pushing and pulling in relation to musculoskeletal disorders: a review of risk factors. Ergonomics 41 (6), 757–781. Kingma, I., de Looze, M.P., Toussaint, H.M., Klijnsma, J.G., Bruijnen, T.B.M., 1996. Validation of a full body 3-D dynamic linked segment model. Human Movement Science 15, 833–860. Okunribido, O.O., Haslegrave, C.M., 1999. Effect of handle design for cylinder trolleys. Applied Ergonomics 30, 407–419. Pheasant, S.T., Grieve, D.W., Rubin, T., Thompson, S.J., 1982. Vector representations of human strength in whole body exertion. Applied Ergonomics 13, 139–144. Plagenhoef, S., Evans, F.G., Abdelnour, T., 1983. Anatomical data for analyzing human motion. Research Quarterly for Exercise and Sport 54 (2), 169–178. Schibye, B., Sgaard, K., Laursen, B., Sjgaard, G., 1997. Mechanical load of the spine during pushing and pulling. Proceedings of the 13th triennial congress of the International Ergonomics Association, Tampere, Finland, pp. 536–537. van der Beek, A.J., Frings-Dresen, M.H.W., Van Dijk, F.J.H., Kemper, H.C.G., Meijman, T.F., 1993. Loading and unloading by lorry drivers and musculoskeletal complaints. International Journal of Industrial Ergonomics 12, 13–23. van der Beek, A.J., Hoozemans, M.J.M., Frings-Dresen, M.H.W., Burdorf, A., 1999. Assessment of exposure to pushing and pulling in epidemiological field studies: an overview of methods, measures and measurement strategies. International Journal of Industrial Ergonomics 24, 417–429. van der Woude, L.H.V., Koningsbruggen, C.M., van Kroes, A.L., Kingma, I., 1995. Effect of push handle height on net moments and forces on the musculoskeletal system during standardized wheelchair pushing tasks. Prosthetics and Orthotics International 19, 188–201. van Die.en, J.H., Kingma, I., 1999. Total trunk muscle force and spinal compression are lower in asymmetric moments as compared to pure extension moments. Journal of Biomechanics 32 (7), 681–687.
Effect of design of two-wheeled containers on mechanical loading Idsart Kingmaa,*, P. Paul F.M. Kuijera,b, Marco J.M. Hoozemansb, Jaap H. van Die.ena, Allard J. van der Beekc, Monique H.W. Frings-Dresenb a
Institute of Fundamental and Clinical Human Movement Sciences, Faculty of Human Movement Sciences, Vrije Universiteit, Van der Boechorststraat 9, 1081 BT, Amsterdam, The Netherlands b Coronel Institute for Occupational and Environmental Health, AmCOGG Amsterdam Centre for Research into Health & Health Care, Academic Medical Centre/University of Amsterdam, The Netherlands c Department of Social Medicine, Institute for Research in Extramural Medicine, VU University Medical Centre, Amsterdam, The Netherlands Received 13 March 2002; accepted 20 June 2002
Abstract To prevent low back disorders due to lifting activities, occupational lifting tasks are being replaced by pushing/ pulling tasks, for instance in refuse collection. Design factors of two-wheeled carts, such as centre of mass (COM) and handle location, might have a major impact on joint loading. We varied the (horizontal and vertical) COM and handle location of a two-wheeled container to test their effect on handle forces and joint loading during two-handed steady pushing and pulling. Net torques at the low back, shoulder and elbow were relatively low in the standard container design. Backward displacement of the COM increased low back loading and forward displacement of the COM increased shoulder and elbow loading. A COM displacement in the direction of the axis of the wheels did not have negative effects on joint loading and reduced the forces, needed to tilt the container. A 0:1 m increase of the handle height slightly reduced the required vertical force without adverse effects on joint loading. It is suggested that the design of Dutch two-wheeled containers can be improved by moving the COM of the loaded container in the direction of the axis of the wheels and by slightly raising the height of the handles. Relevance to industry The design of a two-wheeled container, used for refuse collection, might have a major effect on the risk of developing musculoskeletal disorders through mechanical overloading. In this paper the effect of two important design parameters on joint loading is investigated and interpreted in terms of optimising container design. r 2003 Elsevier Science B.V. All rights reserved. Keywords: Pushing and pulling; Two-wheeled container; Joint loading; Design; Centre of mass; handle
1. Introduction *Corresponding author. Tel.: +31-20-4448492; fax: +31-204448529. E-mail address: i [email protected] (I. Kingma).
With growing awareness of the risk of lifting activities for developing musculoskeletal disorders
0169-8141/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 8 1 4 1 ( 0 2 ) 0 0 1 7 6 - 2
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(Bernard, 1997), tasks that used to involve frequent lifting are being replaced by pushing and pulling tasks. For instance, in refuse collecting, two-wheeled containers have been recommended to replace bags (Frings-Dresen et al., 1995), resulting in a marked reduction of compressive forces at the low back (de Looze et al., 1995; Schibye et al., 1997). However, pushing and pulling are also known to be risk factors for workrelated musculoskeletal disorders. Low back injuries are reported to be caused by pushing and pulling in 9–20% of the cases (Hoozemans et al., 1998). In addition, pushing and pulling tasks seem to increase the risk of developing pain or stiffness in the neck/shoulder region (van der Beek et al., 1993; Hoozemans et al., 1998). In pushing or pulling a fixed bar or a stable (four-wheeled) cart, the direction of the pushing or pulling force can affect the mechanical loading of the shoulder and lumbosacral joint (van der Woude et al., 1995; de Looze et al., 2000). In comparison to pushing or pulling purely horizontally, joint loading can be lower when, simultaneously, one produces a vertical force ðFy Þ that results in a moment effect at the shoulder and lumbosacral joint in opposite direction (Grieve and Pheasant, 1981; Pheasant et al., 1982). An example of such a situation is given in Fig. 1. In contrast to four-wheeled containers, twowheeled containers are unstable during pushing and pulling (see Fig. 2). A subject cannot freely change the Fy that he or she applies at the handles, because this immediately causes a change in the tilt angle of the container. Therefore, two-wheeled containers do not offer the possibility of applying force in a direction that minimises joint loading. Considering this constraint, it might be anticipated that design changes of a two-wheeled container may affect the mechanical loading of different joints during pushing and pulling. Some examples of such design changes are changes in the height or depth of the container, the location of the axis of the wheels or changes of the handle location. Such changes affect the moment arm of the centre of mass (COM) or the moment arm of the hand forces, relative to the axis of the wheels. For two-wheeled trolleys, used to transport gas cylinders, Okunribido and Haslegrave (1999)
d3 d1
Ftotal
Fy
d4
d2
handle
handle
Fx
Fx
(b)
(a)
Fig. 1. Schematic drawing of a participant showing that addition of a vertical reactive force ðFy Þ at the hand in Fig. 1b can reduce loading at the low back and shoulder joint by reducing the moment arm (from d1 and d2 in Fig. a to d3 and d4 in Fig. b).
Fvertical
COM
Ftotal
handle Fhorizontal
Freaction Ffriction-axis Ffriction-ground m.g Fig. 2. Schematic drawing of a participant symmetrically pulling a two-wheeled container. All forces, applying at the container and relevant for joint loading during steady pulling, are indicated in the graph.
reported that the height and the angle of the handle did affect the tilt angle of the trolley. In turn, this tilt angle affected the position of the handles and of the centre of mass (COM) with
I. Kingma et al. / International Journal of Industrial Ergonomics 31 (2003) 73–86
respect to the axis of the wheels, thereby influencing the required forces at the handle and the resulting joint loading. The influence of the COM location of a loaded two-wheeled container on mechanical loading of the joints during pushing and pulling can be illustrated with a diagram showing a subject pushing a container (Fig. 2). Given a specific tilt angle, a forward or backward shift of the COM immediately affects the required Fy due to a change of the moment arm of the COM with respect to the axis of the wheels. It should be noted here, that due to the tilting of two-wheeled containers, not only the forward-backward location but also the height of the COM affects joint loading. For instance, van der Beek et al. (1999) reported a three-fold increase of lumbar compressive forces when the COM height was drastically increased by inserting a tray in a two-wheeled container. To our knowledge, the effect of both the handle location and COM location of the two-wheeled container on handle forces and on joint loading during steady pushing and pulling have hardly been studied. Consequently, the main aim of the current study was to find out how the design of a two-wheeled container, in terms of its COM and handle location, affects handle forces and joint loading during steady, two-handed pushing and pulling. Horizontal (COMx and handlex ) as well as vertical (COMy and handley ) position changes were taken into account. Handle forces and joint loading were quantified in four participants performing a pushing and pulling task using two-wheeled containers with 9 COM locations and 11 handle locations.
2. Methods 2.1. Participants and materials Two standard Dutch refuse containers (Otto, 0:240 m3 ; height 1:00 m; bottom width depth 0:49 0:56 m; wheel diameter 0:2 m) were used for the experiment. The lid was removed from both containers and the handles were replaced by
75
handles attached to 3-D force transducers (AMTI). One container was used to create 9 conditions with different COM locations (Fig. 3a), using foam and concrete blocks. In these 9 conditions a handle location was used that was identical to that of a standard twowheeled container that is used to collect refuse in the Netherlands. The other container was used to create 11 conditions with different handle locations (Fig. 3b) with the aid of aluminium bars, attached to the container. In condition 13 the handle was at the standard location, identical to that which had been used in the COM conditions. Since heavy containers are more likely to cause high joint loading, a container mass was selected that represented a rather (but not extremely) heavy refuse container. The container plus load was intended to be 59:4 kg for the COM as well as the handle conditions. In all conditions, foam and straps were used to prevent slipping of the concrete blocks during the experiment. This caused a slight variation of the actual container mass ðSD ¼ 0:9 kgÞ for the COM conditions. COM locations were measured using a force platform (Kistler). For each of the 20 container conditions COMx was defined as (see Fig. 4a): COMx ¼ dCOM daxis ; where dCOM is the distance from the edge of the forceplate to the centre of pressure measured by the forceplate and daxis is the horizontal distance from the edge of the forceplate to the axis of the container wheels. To prevent slipping of the concrete blocks with the container in horizontal position, the vertical location of the COM (COMy; empty ) was first measured for the containers with force-measurement equipment but without foam and concrete blocks (see Fig. 4b): COMy;empty ¼ dCOM þ daxis : Subsequently, the height of all the concrete blocks and foam was measured while they were placed in the container, and COMy was calculated according to: COMy ¼ ðmempty COMy; empty þ mb1 COMb1 þ ? þ mbn COMbn Þ=ðmempty þ mb1 þ ? þ mbn Þ
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1.2 19
1.2
20
17
1 15 16
0.8
0.8
0.6
0.6
y (m)
y (m)
1
0.4
7
8
4
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2
12 10
13
14 11
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9
5 6
1
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3
0
0
-0.2 (a)
18
0
0.2
-0.2
0.4
x (m)
(b)
0
0.2
0.4
x (m)
Fig. 3. The two-wheeled containers used in the current study with the centre of mass (COM) locations (a) and the handle locations (b) indicated by numbers. The dots in container (b) indicate the (unintended) range of small changes of the COM location due to variation of the location of the handle.
where m is mass, b1 to bn are blocks 1 to n: This was done for all 20 container conditions. Details of the COM and handle locations in the 20 container conditions are given in Table 1. The location of handles plus force transducers caused some unintended variation of the container COM in the handle conditions (Fig. 3b and Table 1). However, this variation was small in comparison to the intentional COM variation in the COM conditions. Although the main interest of this study was the container design rather than participant anthropometry, it was realised that the participant’s body height might have major effects on posture and thus on joint loading. To avoid systematic bias due to body height, we selected one participant with a body height of about the 5th percentile of Dutch males, one participant with a body height of about the 95th percentile of Dutch males, and two participants of intermediate body height (see Table 2 for subject character-
istics). All participants were experienced refuse collectors. 2.2. Experimental procedure For each of the 20 conditions, participants were allowed to practice pushing and pulling a few times before the measurements started, in order to accommodate to the container condition. First, the 9 COM conditions and then the 11 handle conditions were presented in random order. For each condition participants were instructed to grab the handles of the container with both hands and to tilt the container (they were allowed to use a foot if they wanted to), followed by walking backward while pulling the container at a normal constant speed and with the upper body as symmetrical as possible over a distance of about 5 m on a hard, rubber surface. About 1 min later, the participants were asked to tilt the container and walk the trajectory in the forward direction
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Table 2 Descriptive statistics of the participants in this study
y x
COM COMx
COMy
y
daxis forceplate
dCOM
dCOM
Fig. 4. Schematic drawing showing how the horizontal and vertical location of the centre of mass (COM) of the twowheeled containers was measured using a force-platform.
Table 1 Horizontal (COMx ) and vertical (COMy ) centre of mass location and horizontal (handlex ) and vertical (handley ) handle locations for which the effect on mechanical loading was investigated during pushing and pulling of two-wheeled containers
COM 1 2 3 4 5 6 7 8 9 Handle # 10 11 12 13 14 15 16 17 18 19 20
Weight (kg)
Age (yr)
1 2 3 4
1.64 1.72 1.85 1.93
67 65 86 80
28 34 43 32
x
m.g
condition #
Body height (m)
daxis
forceplate m.g
COM
Participant
COMx (m)
COMy (m)
Handlex (m)
Handley (m)
0.138 0.221 0.247 0.115 0.193 0.234 0.090 0.180 0.254
0.120 0.120 0.120 0.230 0.230 0.230 0.360 0.360 0.360
0:174 0:174 0:174 0:174 0:174 0:174 0:174 0:174 0:174
0.900 0.900 0.900 0.900 0.900 0.900 0.900 0.900 0.900
0.170 0.186 0.170 0.179 0.186 0.170 0.179 0.170 0.179 0.170 0.186
0.224 0.224 0.248 0.248 0.248 0.271 0.271 0.296 0.296 0.321 0.321
0:268 0:066 0:270 0:171 0:071 0:269 0:176 0:269 0:170 0:270 0:071
0.840 0.840 0.900 0.900 0.900 1.000 1.000 1.100 1.100 1.200 1.200
Co-ordinates are indicated relative to the axis of the wheels and correspond with locations indicated in Figs. 3 and 5–8.
while symmetrically pushing the container at a constant speed. LED markers were attached to the left and right side of the body at the L5S1 joint according to de Looze et al. (1992), at the acromion, and at the wrist. In addition, markers were attached to the left and right side of the container at the centre of the handles and at 0.4 and 0:6 m above the axis of the wheels. Marker positions were recorded using an opto-electronic system (Optotrak) with an array of three cameras on both sides of the body. Forces and marker positions were sampled in 3 dimensions at 50 Hz: From the 5 m walking trajectory, about the middle 3 m was selected for averaging marker positions and forces over time. In addition, markers on the left and right side of the body and container as well as the left and right handle forces were averaged.
2.3. Biomechanical model Elbow positions in the sagittal plane were calculated from wrist and shoulder markers and known upper arm and forearm length. The markers on the container were used to calculate the average tilt angle during pushing and pulling for each trial. Using this angle, the forces at the handles were transformed from local to global coordinates. Time-averaged sagittal plane marker co-ordinates and forces were used as input for an upper body static 2-D linked segment model. The model consisted of four segments: hands, forearms, upper arms, and trunk plus head. Relative segment masses and segment centre of mass locations were obtained from Plagenhoef et al. (1983). Using standard linked segment mechanics (Elftman, 1939), reactive forces and torques were
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calculated at the elbow ðTe Þ; shoulder ðTs Þ and the lumbosacral ðTL5S1 Þ joints. 2.4. Statistics The experiment consisted of two series of measurements in which a single independent variable was manipulated in a repeated measures design. The independent variables were COM and handle location. This design does not allow analysis of the interaction between these variables. Statistical analyses were performed separately for the two independent variables. The zero hypotheses were that COM and handle location do not affect physical loading. ANOVAs were applied with participant, pushing/pulling activity, and either COM condition or handle condition as independent variables. Interaction effects between pushing/pulling activity, and either the COM condition or the handle condition were taken into account. However, due to limitations in degrees of freedom, interactions with the participant could not be taken into account. The dependent variables were the container tilt angle, the height of the handle of the tilted container during steady
pushing and pulling, Fx and Fy applied at the handles, the torques at the elbow, shoulder and the lumbosacral joint, and upper body segment angles. In addition, ANOVAs with the same independent variables were applied to the peak values of Fx and Fy at the initiation of the tilting of the container. A p-value smaller than 0.05 was considered statistically significant. For all ANOVAs, Bonferroni post-hoc tests were applied to determine whether or not differences between individual conditions were significant.
3. Results ANOVA results showed highly significant effects ðpo0:001Þ of COM condition in 6 out of 12 dependent variables (Table 3) and of handle condition in 8 out of 12 dependent variables (Table 4). For those variables with significant effects of COM or handle condition, post-hoc tests showed significant differences in many adjacent COM or handle locations. Given the large number of possible comparisons and the fact that most
Table 3 Summary of ANOVA results for 4 participants pushing and pulling 9 two-wheeled containers, differing in location of the centre of mass (COM) Independent variables Condition Participant Pull/push Condition pull=push Independent variables Condition Participant pull/push Condition pull=push
DF
Peak initial Fx p
Peak initial Fy p
Fx p
Fy p
Tilt angle p
Tilted handle height p
(8,48) (3,48) (1,48) (8,48)
o0.001 o0.001 0.056 0.751
0.289 o0.001 o0.001 0.951
0.515 0.875 o0.001 0.549
o0.001 o0.001 o0.001 0.914
0.417 o0.001 0.115 0.696
0.273 o0.001 0.094 0.671
DF
Te p
Te p
TL5S1 p
Forearm angle p
Upper arm angle p
Trunk angle p
(8,48) (3,48) (1,48) (8,48)
o0.001 o0.001 0.605 0.048
o0.001 o0.001 o0.001 0.001
o0.001 o0.001 0.008 0.275
0.135 o0.001 0.022 0.218
0.545 o0.001 o0.001 0.975
o0.001 o0.001 o0.001 1.000
Effects on container kinematics and on horizontal ðFx Þ and vertical ðFy Þ forces applied the handles are described in the top block and effects on Te ; Ts and TL5S1 (moments at the elbow, shoulder and lumbosacral joint, respectively) and arm and trunk kinematics are described in the bottom block. Degrees of freedom (DF) in the ANOVAs are indicated in the 2nd column. Significant effects ðpo0:05Þ are indicated by bold numbers.
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Table 4 Summary of ANOVA results for 4 participants pushing and pulling 11 two-wheeled containers, differing in location of the handle Independent variables
DF
Peak initial Fx p
Peak initial Fy p
Fx p
Fy p
Tilt angle p
Tilted handle height p
Condition Participant pull/push Condition pull=push
(10,63) (3,63) (1,63) (10,63)
o0.001 o0.001 0.004 0.182
0.082 0.001 o0.001 0.072
0.073 0.016 o0.001 0.946
o0.001 o0.001 o0.001 0.625
o0.001 o0.001 0.193 0.998
o0.001 o0.001 0.323 0.999
Independent variables
DF
Te p
Te p
TL5S1 p
Forearm angle p
Upper arm angle p
Trunk angle p
Condition Participant pull/push Condition pull=push
(10,63) (3,63) (1,63) (10,63)
o0.001 o0.001 o0.001 0.014
o0.001 o0.001 o0.001 o0.001
0.235 o0.001 0.010 0.986
o0.001 o0.001 0.767 0.978
0.613 o0.001 o0.001 0.971
o0.001 o0.001 o0.001 0.993
Effects on container kinematics and on horizontal ðFx Þ and vertical ðFy Þ forces applied the handles are described in the top block and effects on Te ; Ts and TL5S1 (moments at the elbow, shoulder and lumbosacral joint, respectively) and arm and trunk kinematics are described in the bottom block. Degrees of freedom (DF) in the ANOVAs are indicated in the 2nd column. Significant effects ðpo0:05Þ are indicated by bold numbers.
effects have a rather linear appearance (see Figs. 5–8), we will describe the main tendencies in dependent variables that showed significant effects of COM or handle conditions, and add post-hoc results only in cases where we compare specific conditions (which is in the discussion section). 3.1. Effect of COM and handle location on Fx at the handle Averaged over participants, the peak initial Fx used to tilt the container ranged from 60717 N in the pushing task with the COM located high and close to the axis to 195743 N in the pushing task with the COM located low and far from the axis (Fig. 5d). This force was highly dependent on COMx (Figs. 5a and d) and to a lesser extent on the height and forward-backward position of the handle (Figs. 6a and d). Since COMy hardly affected the peak initial Fx ; it is unlikely that the effect of handley is explained by the concomitant changes in COMy : However, interactions between COMy changes and handley changes cannot completely be excluded here. As might be expected due to the almost constant container mass, steady Fx during pulling and
pushing was not significantly influenced by the location of the COM or the handle (Tables 3 and 4). 3.2. Effect of COM location on tilt angle, forces and joint loading It was expected that, given a certain COM location, participants would manipulate the tilt angle of the container in order to reduce the required Fy and/or to reduce the loading of one or more joints. However, it appeared that the container tilt angle was highly dependent on the participant but not on the COM location (Table 3). Because the COM location had no significant effect on the tilt angle, Fy during pushing and pulling was highly dependent on the COM location of the container (Table 3). More forward (i.e., at a greater distance from the axis and the participant) and lower locations of the COM resulted in negative (downward) changes of Fy (Figs. 5c and f). This effect can be understood by comparing the different COM locations for their resulting horizontal position with respect to the axis of the wheels when the container in Fig. 3a is tilted. The COM location had a large effect on the joint torques (Table 3) and this effect appeared
I. Kingma et al. / International Journal of Industrial Ergonomics 31 (2003) 73–86
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low
medium
COM & pulling
high
standard
COM & pulling
COM & pulling
40 upward 200
100
150
100
50
30
Fy (N)
tilt angle (deg)
peak Fx (N)
35
25
0
-100
20 -200
0
15 0.1
(a)
0.2 COMx (m)
0.3
0.1
(b)
COM & pushing
0.2 COMx (m)
0.3
0.1
(c)
COM & pushing
0.2 COMx (m)
0.3
COM & pushing
40 upward 200
100
150
100
30
Fy (N)
tilt angle (deg)
peak Fx (N)
35
25
50
20
0
15
0
-100
-200 0.1
(d)
0.2 COMx (m)
0.3
0.1
(e)
0.2 COMx (m)
0.3
0.1
(f)
0.2 COMx (m)
0.3
Fig. 5. The effect of centre of mass (COM) location on the peak tilting horizontal force ðFx ; graphs a and d), on the tilting angle (graphs b and e) and on the vertical force applied at the handle ðFy ; graphs c and f) during steady pulling (graphs a, b and c) and pushing (graphs d, e and f) of a two-wheeled container in a symmetrical way. Error bars indicate one standard deviation. COM heights were 0.12 (low), 0.23 (medium) and 0:36 m (high) above the axis of the wheels (with 0:1 m radius). Horizontal locations on the x-axis equal those of Fig. 3a and Table 1, referring to a container in upright position.
to be systematically related to COMx and COMy (Fig. 7). Interestingly, the effect of COMx and COMy changes on L5S1 joint loading appeared to be opposite to the effect on shoulder and elbow joint loading (Fig. 7). Roughly, these changes can be explained by the changes in Fy ; since, assuming that the shoulders are not in front of the handles, adding a downward force component at the hands must result in a more extending Te and Ts and a less extending TL5S1 : However, with pushing this effect was not consistent at the shoulder joint (Fig. 7e). This might be related to a postural change, since a significant effect of COM condition on the trunk
angle was found (Table 3). While pushing, participants were ‘leaning on the handles’ when large downward forces were required. In this way, the reactive forces at the hand nearly pointed at the shoulder joint, so that only small joint torques occurred. In other words, body mass instead of muscle force was used to produce the downward force by leaning on the handles. 3.3. Effect of handle location on tilt angle, forces and joint loading Contrary to the COM, the handle location significantly affected the tilt angle of the container
I. Kingma et al. / International Journal of Industrial Ergonomics 31 (2003) 73–86
low
normal
high
handle & pulling
higher
highest
*
handle & pulling
81
standard
handle & pulling
40 upward 200
100
150
100
30
Fy (N)
tilt angle (deg)
peak Fx (N)
35
25
50
20
0
15
0
-100
-200
(a)
-0.2 -0.1 handlex (m)
0
-0.2 -0.1 handlex (m)
(b)
handle & pushing
0
-0.2 -0.1 handlex (m)
(c)
handle & pushing
0
handle & pushing
40 upward 200
100
150
100
30
Fy (N)
tilt angle (deg)
peak Fx (N)
35
25
50
20
0
15
0
-100
-200
(d)
-0.2 -0.1 handlex (m)
0
(e)
-0.2 -0.1 handlex (m)
0
(f)
-0.2 -0.1 handlex (m)
0
Fig. 6. The effect of handle location on the horizontal force ðFx ; graphs a and d), on the tilting angle (graphs b and e) and on the vertical force applied at the handle ðFy ; graphs c and f) during steady pulling (graphs a, b and c) and pushing (graphs d, e and f) of a two-wheeled container in a symmetrical way. Error bars indicate one standard deviation. Handle heights were 0.84 (low), 0.90 (normal), 1.00 (high), 1.10 (higher) and 1.20 (highest) above the axis of the wheels (with 0:1 m radius). Horizontal locations on the xaxis equal those of Fig. 3b and Table 1, referring to a container in upright position.
(Table 4; Figs. 6b and e). Higher and more towards the front (at a larger distance from the participant but closer to the axis) locations of the handles were associated with a larger tilt angle. Although interactions with COM location cannot completely be ruled out, this is unlikely to be caused by concomitant COM changes because the COM did not affect the tilt angle (Table 3). The increase in tilt angle only partially compensated for the increased handley ; since there was still a significant effect of the handle location on the handley during pushing and pulling (Table 4). On average, the highest handle locations resulted in a
pushing and pulling height of about 0:18 m higher in comparison with the lowest handle locations. As a result of the increased tilt angle with higher handles, and probably also in part due to the increase of COMy with increasing handley ; the COM of the tilted container was more backward (towards the participant) with respect to the axis of the wheels, resulting in a change of the Fy in less downward or more upward direction (Figs. 6c and f). Consequently, the extending Te (Figs. 8c and f) decreased (in pushing and pulling) and shifted to a flexion Te for the highest handle locations (mainly in pulling). In addition, the extending Ts decreased
I. Kingma et al. / International Journal of Industrial Ergonomics 31 (2003) 73–86
82
low
medium
COM & pulling
high
standard
COM & pulling
COM & pulling
100 extension
extension 40
60
20
20
40
Te (Nm)
80
40
Ts (Nm)
TL5S1 (Nm)
extension
0
0
-20
-20
20 0
0.1
(a)
0.2 COMx (m)
0.3
0.1
(b)
COM & pushing
0.2 COMx (m)
0.3
0.1
(c)
COM & pushing
0.2 COMx (m)
0.3
COM & pushing
100 extension
extension 40
60
20
20
40
Te (Nm)
80
40
Ts (Nm)
TL5S1 (Nm)
extension
0
0
-20
-20
20 0
(d)
0.1
0.2 COMx (m)
0.3
0.1
(e)
0.2 COMx (m)
0.3
0.1
(f)
0.2 COMx (m)
0.3
Fig. 7. The effect of the centre of mass (COM) location on the lumbosacral torque ðTL5S1 ; graphs a and d), shoulder torque ðTs ; graphs b and e) and elbow torque (Te ; graphs c and f) during steady pulling (graphs a, b and c) and pushing (graphs d, e and f) of a twowheeled container in a symmetrical way. Error bars indicate one standard deviation. COM heights were 0.12 (low), 0.23 (medium) and 0:36 m (high and standard) above the axis of the wheels (with 0:1 m radius). Horizontal locations on the x-axis equal those of Fig. 3a and Table 1, referring to a container in upright position.
with forward and upward displacement of the handle during pulling, whereas changes during pushing were marginal (Figs. 8b and e). The decreased extending Te and Ts with higher handley may in part have been caused by the upward COM shift with higher handley : However, the effect of handley was stronger than the effect that would have been expected on the basis of a 0:1 m increase of the COM alone (see Figs. 7b and c). Furthermore, the extending TL5S1 slightly increased from low to high handle locations (Figs. 8a and d). It is uncertain to what extent this is caused by the actual increase of handley alone, or also by the concomitant increase of
COMy : Despite some postural changes (in terms of trunk and forearm angle) due to the changes of handle location, the trend in changes of joint loading (Fig. 8) was in line with the change of the Fy (Figs. 6c and f), i.e. a more upward directed Fy at the handle resulted in a decreased extending Te and Ts and in an increased extending TL5S1 : 3.4. Effects of participant and pushing versus pulling As anticipated, there was a significant effect of participant (most likely due to body height
I. Kingma et al. / International Journal of Industrial Ergonomics 31 (2003) 73–86
low
normal
high
higher
highest
*
handle & pulling
handle & pulling
83
standard
handle & pulling
100 extension
extension 40
60
20
20
40
Te (Nm)
80
40
Ts (Nm)
TL5S1 (Nm)
extension
0
0
-20
-20
20 0
(a)
-0.2 -0.1 handlex (m)
-0.2 -0.1 handlex (m)
0
(b)
0
-0.2 -0.1 handlex (m)
(c)
handle & pushing
handle & pushing
0
handle & pushing
100 extension
extension 40
60
20
20
40
Te (Nm)
80
40
Ts (Nm)
TL5S1 (Nm)
extension
0
0
-20
-20
20 0
(d)
-0.2 -0.1 handlex (m)
0
(e)
-0.2 -0.1 handlex (m)
0
(f)
-0.2 -0.1 handlex (m)
0
Fig. 8. The effect of handle location on the lumbosacral torque ðTL5S1 ; graphs a and d), shoulder torque (Ts ; graphs b and e) and elbow torque (Te ; graphs c and f) during steady pulling (graphs a, b and c) and pushing (graphs d, e and f) of a two-wheeled container in a symmetrical way. Error bars indicate one standard deviation. Handle heights were 0.84 (low), 0.90 (normal and standard), 1.00 (high), 1.10 (higher) and 1.20 (highest) above the axis of the wheels (with 0:1 m radius). Horizontal locations on the x-axis equal those of Fig. 3b and Table 1, referring to a container in upright position.
variations) on most of the dependent variables for the COM conditions (Table 3) and handle conditions (Table 4). Roughly, shorter participants tilted the container more (with consequently more downward directed handle forces and more extending torques in the arms) and taller participants flexed their trunk more (with consequently higher low back loading). Container tilt angles did not differ between pushing and pulling in the COM conditions (Table 3) as well as in the handle conditions (Table 4). As
a consequence of the necessary mechanical equilibrium during steady pushing and pulling, the reversal of sign of Fx resulted in a downward change of Fy at the handle in pushing compared to pulling (Figs. 5c, f, and 6c and f). For the COM as well as the handle conditions, the upper body posture showed some more trunk and shoulder flexion in pushing as compared to pulling, presumably to facilitate downward pushing. In absolute sense, Fy (and thus the total forces at the handle) were greater in pushing as compared to
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pulling in the majority of COM and handle conditions. Interestingly, the reverse was the case for all joint torques in almost all conditions, underscoring the importance of force direction for joint loading.
cluded that the magnitude of the force applied at the handles, is not a good indicator of the mechanical loading of the joints, when the direction of this force as well as the position of the joints are not taken into account. 4.2. Handle location and mechanical loading
4. Discussion 4.1. COM location and mechanical loading Surprisingly, we found little influence of the COM location of the container on the tilt angle. Contrary to expectations, participants hardly adapted the angle of pulling or pushing when the angle of mechanical equilibrium of the container changed. Consequently, Fy ; applied to the handle, and the loading of the joints were highly dependent on the COM location. In contrast to pushing or pulling tasks with a four-wheeled cart or a fixed bar (de Looze et al., 2000), Fy in the current study was imposed by the container tilt angle and therefore it was largely unrelated to the arm orientation. Consequently, some combinations of Fx and Fy resulted in large moment arms with respect to the shoulder joint, so that forces of moderate magnitude could result in high loading of the shoulder joint. Especially the combination of a downward force and a pulling force can result in a total force vector that is nearly perpendicular to the orientation of the arm, causing a large moment arm with respect to the shoulder and elbow joint. This may cause large Ts and Te even when the downward force only has a moderate magnitude. Such a situation occurred when the COM was located low and towards the front (see Figs. 5c, 7b and c). In pushing, a comparable effect was found when the COM was located high and towards the back. In this situation, a combination of pushing and a quite small upward force (Fig. 5f) resulted in large moment arms with respect to the elbow and shoulder joint, with consequently relatively high elbow and shoulder loads (Figs. 7e and f). These examples show that the COM location, which can easily be influenced through the design of the container, is a major determinant of joint loading in pushing and pulling of twowheeled containers. In addition, it can be con-
The finding of a reduction of shoulder joint loading with pulling at increasing handley ; as found in the current study, is in agreement with findings using four-wheeled carts (de Looze et al., 2000), but the magnitude of the effect is larger. One explanation for this finding might be that handley effects may have been amplified by concomitant COMy changes in the current study. Furthermore, Okunribido and Haslegrave (1999) reported, for two-wheeled trolleys, that the handle height did affect the tilt angle of the trolley, but that participants always tilted the trolley at such an angle that one specific grip height was obtained, regardless of other conditions. In the current study a change of handle height also affected the tilt angle, but not to such an extent that equal handle heights were obtained after tilting. Okunribido and Haslegrave (1999) suggested that the high COM of the trolleys that were studied, may have caused their finding of a constant grip height. Comparable to the COM location, handle location affected the external forces and the joint loading not in the same way in the current study. Low handle locations resulted in unfavourable combinations of Fx and Fy : pulling with a low handle resulted in a relatively small downward force (Fig. 6c) but due to a large moment arm of the force in relatively high shoulder loading (Fig. 8b). Compared to the current handle location (handle condition 13), some increase of the height of the handle (handle condition 16) significantly reduced the average Fy ðp ¼ 0:032Þ without adverse effects on joint loading. It should be noted that concomitant COMy changes might have played a role here. However, we do not expect the influence to be profound, since the concomitant COMy change was only 0:02 m: A more than 0:1 m increase of handle height caused short participants to tilt the container quite far, resulting in an increasing joint loading for those subjects.
I. Kingma et al. / International Journal of Industrial Ergonomics 31 (2003) 73–86
4.3. Limitations of the study First, only four participants of different height participated in the experiments. Clearly, this limits generalisation of the current results, since differences in participant body height cannot be separated from other differences between the participants. However, despite the small number of participants, highly significant effects of COM and handle location were found. In addition, those effects were understandable from a mechanical perspective in that joint loading was high for those situations where the resulting pushing or pulling force vector had a large moment arm with respect to those joints. Interaction effects between handle location and COM location were not investigated in the present study. These effects might occur since handle height was found to affect the tilt angle of the container. Also, participants had some time to adapt to every condition but it cannot be excluded that more experience with each condition would affect the body posture during pushing and pulling. Furthermore, refuse collectors often pull twowheeled refuse containers with one hand, walking forward instead of backward. This might influence the effect of handle location on joint loading. For instance, short participants may then be forced to either tilt the container too far or to keep their arm in an extremely extended position. Finally, it should be realised, that other parameters than average joint loading (on which we mainly focussed thus far) are important. Container stability may affect the risk of musculoskeletal injuries, through peak forces necessary to counteract balance disturbances due to an irregular walking surface. In the next section, some inferences on this issue are made, partially using information obtained from the current experiment. 4.4. Can the current design of refuse containers be improved? With homogeneous loading, the COM of the container is normally situated about the midline of the container. Thus, locations 8 and 5 (Fig. 3a) represent COM’s for the current container design, for a completely and an incompletely filled
85
container, respectively. In comparison to those COM locations, a frontal displacement of the COM has negative effects on low back loading ðp ¼ 0:02 and o0:001 for locations 5 and 8, respectively) whereas a backward displacement tended to increase elbow loading during pulling ðp ¼ 0:076 and 0.001 for locations 5 and 8, respectively). It appeared that joint loading is not high for the current design and that none of the COM locations tested, resulted in a substantial reduction of the loading in all three joints (low back, shoulder and elbow). However, improvement of the container design is possible with respect to the COM location. Since, displacement of the COM in the direction of the axis (i.e. downward and backward at the same time, from location 8 to 4 in a fully loaded container, and from location 5 to 1 in a partially filled container, see Fig. 3a) reduces the moment arm of the COM with respect to the axis of the wheels at any tilt angle. The current experiment showed that this displacement reduced the force needed to tilt the container ðp ¼ 0:009 and o0:001 for locations 5 and 8, respectively) and had no negative effects on the loading of any joint ðp ¼ 1:000 in all cases). In daily practice, the stability of the container will improve, since, disturbances of mechanical equilibrium, which constantly occur during pushing and pulling a container on an irregular surface, require less force adaptation to restore the tilt angle. The most obvious way to obtain the proposed COM displacement in the direction of the axis, would be to make the container wider, and at the same time, place the axis more towards the front. It was already mentioned that interaction effects between COM and handle changes were not investigated in the current study. However, as long as the COM changes are along a line towards the axis of the wheels, such interaction effects are not likely to be substantial. Therefore, assuming that the improvement with a 0:1 m increase of the handle height was not caused by the concomitant small COM height increase, the 0:1 m increase of handle height can, most likely, safely be combined with the proposed COM displacement in the direction of the axis. Before we can conclude that design changes as proposed above, will bring about considerable
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improvement, a new design, using the proposed changes, should be tested against the traditional design with more participants, with various container loading conditions and with more realistic working conditions (including one-handed pulling, rotating a container and pulling it up and down the walkway). This study, using 3-D analysis of shoulder (Helm and Veeger, 1996) and low back (Kingma et al., 1996) loading, and including an EMG-assisted translation of net moments to compression and shear forces (van Die.en and Kingma, 1999), is being undertaken and will be published in a separate paper.
Acknowledgements The authors would like to thank the Association for waste and cleansing management (NVRD) and the Association of Dutch waste management companies (VNA) for financially supporting this project. In addition, the refuse collectors and the management team of the Nature and Environment Division of the municipality of Haarlem are acknowledged for their kind co-operation. Furthermore, Tom Kalkman is acknowledged for his assistance during the experiments.
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