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Working height, block mass and one- vs. two-handed block handling: thecontribution to low back and shoulder loading during masonry workG. S. Faber a; I. Kingma a; P. P. F. M. Kuijer b; H. F. van der Molen bc; M. J. M. Hoozemans ad; M. H. W.Frings-Dresen b; J. H. van Dieën a

a Research Institute MOVE, Faculty of Human Movement Sciences, VU University Amsterdam, TheNetherlands b Academic Medical Center, Universiteit van Amsterdam, Coronel Institute of OccupationalHealth, Amsterdam, The Netherlands c Arbouw, Harderwijk, the Netherlands d EXPres, Faculty of HumanMovement Sciences, VU University Amsterdam, The Netherlands

Online Publication Date: 01 September 2009

To cite this Article Faber, G. S., Kingma, I., Kuijer, P. P. F. M., van der Molen, H. F., Hoozemans, M. J. M., Frings-Dresen, M. H. W.and van Dieën, J. H.(2009)'Working height, block mass and one- vs. two-handed block handling: the contribution to low back andshoulder loading during masonry work',Ergonomics,52:9,1104 — 1118

To link to this Article: DOI: 10.1080/00140130902915947

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Working height, block mass and one- vs. two-handed block handling: the contribution to low back

and shoulder loading during masonry work

G.S. Fabera, I. Kingmaa*, P.P.F.M. Kuijerb, H.F. van der Molenb,c, M.J.M. Hoozemansa,d,M.H.W. Frings-Dresenb and J.H. van Dieena

aResearch Institute MOVE, Faculty of Human Movement Sciences, VU University Amsterdam, The Netherlands;bAcademic Medical Center, Universiteit van Amsterdam, Coronel Institute of Occupational Health, Amsterdam,The Netherlands; cArbouw, Harderwijk, the Netherlands; dEXPres, Faculty of Human Movement Sciences,

VU University Amsterdam, The Netherlands

The goal of this study was to compare the effects of the task variables block mass, working height andone- vs. two-handed block handling on low back and shoulder loading during masonry work. In a mock-up of amasonry work site, nine masonry workers performed one- and two-handed block-lifting and block-placing tasks atvarying heights (ranging from floor to shoulder level) with blocks of varying mass (ranging from 6 to 16 kg).Kinematics and ground reaction forces were measured and used in a 3-D linked segment model to calculate low backand shoulder loading. Increasing lifting height appeared to be the most effective way to reduce low back loading.However, working at shoulder level resulted in relatively high shoulder loading. Therefore, it was recommended toorganise masonry work in such a way that blocks are handled with the hands at about iliac crest height as much aspossible.

Keywords: low back loading; shoulder loading; construction work; lifting

1. Introduction

Musculoskeletal disorders are a major medical as wellas an economical problem in Western industrialisedcountries. In the Netherlands, musculoskeletal dis-orders were the second most expensive disease groupfor healthcare in 1994 (6% of the total healthcareexpenses) (Meerding et al. 1998). Low back pain andshoulder pain were the most frequent self-reportedmusculoskeletal disorders in the Netherlands in 1998(point prevalence of 26.9% and 20.9%, respectively)(Picavet and Schouten 2003).

Epidemiology has shown that physically highdemanding tasks performed during work (e.g. heavylifting, working above shoulder level, repetitive move-ments, whole body vibrations) is associated with lowback pain (Norman et al. 1998, Granata and Marras1999, Hoogendoorn et al. 1999, Kuiper et al. 1999,Lotters et al. 2003, Kuiper et al. 2005) and shoulder pain(Grieco et al. 1998, van der Windt et al. 2000, Mirandaet al. 2001, 2008, Pope et al. 2001, Cassou et al. 2002,Harkness et al. 2003, Grooten et al. 2007, Palmer andSmedley 2007). In construction workers, low back andshoulder pain are very common (Holmstrom andEngholm 2003), which could be related to the physicallyhigh demanding tasks they perform during work.

Hildebrandt (1995) reported that Dutch constructionworkers had the highest prevalence for having ‘backpain quite often’ of all occupational groups and thatthis prevalence was significantly higher than theprevalence in the total work population (39% vs. 27%).Holmstrom et al. (1992) found a dose–responserelationship in Swedish construction workers betweenworking above shoulder level and shoulder pain andLatza et al. (2000) found, in German constructionworkers, that lifting heavy sandstone building blocks(7–10 kg) for 2 h or more was associated with a higherprevalence of low back pain.

In the Netherlands, even heavier blocks (up to16 kg) are frequently handled in the constructionindustry. In a field study (van der Molen et al. 2007),it was observed that masons spent on average about2.5 hours/day handling these blocks. Usually, thework situation was such that the masons liftedblocks from a pallet and transported them overseveral metres to a site where a wall wasconstructed. Blocks were generally lifted from apallet at heights between floor and hip level. Blockplacing occurred from floor to above shoulder level.About half of the blocks were handled with onehand (van der Molen et al. 2007).

*Corresponding author. Email: i.kingma@fbw.vu.nl

Ergonomics

Vol. 52, No. 9, September 2009, 1104–1118

ISSN 0014-0139 print/ISSN 1366-5847 online

� 2009 Taylor & Francis

DOI: 10.1080/00140130902915947

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To derive directions for ergonomic improvementsof the work situation of masons, insight into determi-nants of musculoskeletal loading during the mostfrequently performed work task is necessary. There-fore, the goal of the present study was to compare theeffects of the task variables block mass, working heightand one- vs. two-handed block handling on low backand shoulder loading during masonry work. To thisend, experienced masonry workers performed one- andtwo-handed block-lifting and block-placing tasks atvarying heights with blocks of varying mass. Tosimulate the actual working condition, all tasks wereperformed in a mock-up of a masonry work site.

2. Methods

2.1. Subjects

Nine healthy male masons participated in the experi-ment after signing informed consent. They had aminimum experience on the job of 6 months and allwere right-handed. Their average (SD) age, height andbody mass were 36 (14) years, 184 (8) cm, and 89 (14)kg, respectively.

2.2. Building blocks

Four sandstone building blocks (Figure 1) were used inthe experiment. The masses were 6 kg, 11 kg, 14 kg

and 16 kg, respectively, and the dimensions (width 6depth 6 height) were 30 6 12 6 10, 30 6 15 614.5, 44 6 10 6 19.5 and 30 6 21.5 6 14.5 cm,respectively. The 11-, 14- and 16-kg blocks are usedfrequently in the construction industry in theNetherlands (van der Molen et al. 2007). Based on theNational Institute for Occupational Safety and Health(NIOSH) equation (Waters et al. 1993), the 6-kg blockwas previously recommended as an upper health limitfor the mass of blocks manually handled by masons(Arbouw 2002).

2.3. Mock-up simulation of work tasks

The protocol had been approved by the local ethicscommittee and consisted of the block-lifting and block-placing tasks most frequently observed in the field(van der Molen et al. 2007). Because of the limitedmeasurement volume of the optoelectric system, block-lifting and block-placing tasks were measured inseparate experimental conditions. An overview of allexperimental conditions is presented in Table 1.

2.3.1. Lifting blocks from the pallet

In the block-lifting tasks, subjects lifted a buildingblock from a pallet, turned around, took a few stepsand placed the block on a table. The block-lifting taskwas executed from two initial heights (in separate trials)representing the top and bottom layer of a standardpallet with building blocks. For lifting from the bottomlayer, only one layer of building blocks was placed onthe pallet, which had a height of 10 cm (Figure 2a). Forlifting from the top layer, three layers of building blockswere stacked on a pallet, which was elevated such thatthe top of the third layer was at a height of 90 cm(Figure 2b). To give the participants the opportunity touse the depalletising method that they normally use intheir daily work, (i.e. either to lift blocks from the toplayer only or to alternate with lifting blocks from thesecond layer), they were allowed to also lift the buildingblocks from the layer beneath the top layer of the pallet.

Table 1. Measurement scheme: Measured conditions are indicated by an x.

Lifting blocks from pallet Placing blocks in the wall

Block handling height Bottom layer Top layer Floor levelIliac crest

level Shoulder level

Number of hands two one two one two one two one two one6 kg block x x x x x x x x x11 kg block x x x x x14 kg block x x x x x x x x x16 kg block x x x x x

Note: Data of conditions indicated by a bold x have also been used in a previous study (Faber et al. 2007).

Figure 1. Building blocks used in the study (from left toright, the 6 kg, 11 kg, 14 kg and 16 kg blocks).

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Only the lifts from the top layer were analysed.Participants were instructed to keep on liftingbuilding blocks from the pallet until they felt that thenext block was so far from the edge that they wouldnormally step on the pallet or walk around it. In orderto restrict the amount of lifts performed in theexperiment, participants were instructed to lift thebuilding blocks from the right side of the pallet only.

2.3.2. Placing blocks in the wall

In the block-placing tasks, subjects grasped a buildingblock from the table, turned around, took a few stepsand placed the building block on top of a simulatedwall. Three wall heights were used, i.e. floor level, iliaccrest level and shoulder level height (Figure 2c,d,e).When working at floor level, masons were instructed toplace the blocks from a standing position because this

is the way the building blocks used in the present studyare usually handled at the worksite (Hopmans et al.2007). For each wall height, two blocks had to beplaced in the wall (next to each other), but only thesecond block was analysed.

2.3.3. One- and two-handed block handling

For block-lifting as well as block-placing conditions,blocks were handled with one hand and with twohands in separate trials. In the two-handed lifts, blockswere generally grasped with the thumbs in the holes ontop of the building block. In the one-handed lifts,blocks were generally grasped with the dominant, righthand at the centre of the block. The 16- and 11-kgblocks were not lifted one-handed because they weretoo large to be grasped in this way. To limit theamount of time spent on changing the mock-up work

Figure 2. Block lifting and placing tasks: lifting blocks from the bottom layer (a) and the top layer (b) of thepallet. Placing blocks at floor (c), iliac crest (d) and shoulder level (e).

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situation in between trials, the order in which theconditions were performed was not randomised butsystematically varied over subjects. The experimentalconditions were divided into three main groups: liftingblocks from the top layer of the pallet; lifting blocksfrom the bottom layer of the pallet; placing blocks inthe wall. The order in which these main groups wereperformed was systematically varied over subjects.Subsequently, within these main groups, experimentalconditions were systematically varied over subjects, ina hierarchical way for block-placing height, followedby block type and one- vs. two-handed lifting.

2.4. Measurements and data analysis

2.4.1. Moments at the L5/S1 and shoulder joints

Using anthropometrical data, kinematics of (body)segments and external forces as input, a dynamic 3-Dlinked segment model (LSM), described and validated(by comparing a top-down to a bottom-up calculationof net moments) by Kingma et al. (1996), was employedto estimate net moments and reaction forces at the L5/S1 joint and at the right shoulder joint. Anthropome-trical data (mass, position of the centre of mass andmoments of inertia) of all measured body segment werecalculated using regression equations reported byMcConville and Churchill (1980). 3-D kinematics ofthe body segments were measured with clusters of threeOptotrak markers (Northern Digital, Waterloo ON,Canada) at a sample rate of 50 samples/s (see Figure 2).Ground reaction forces were measured using a custom-made 1 m 6 1 m force plate at a sample rate of 200samples/s. Before kinematics and force plate data wereused as inputs to the LSM, they were low-pass filteredwith a cut-off frequency of 10 Hz.

A bottom-up version of the LSM was used todetermine the 3-D net moments and reaction forces atthe L5/S1 joint. This model used the kinematics of thesegments below the L5/S1 joint (pelvis, upper legs andlower legs plus feet) and the ground reaction forces asinput. Net moments were projected on the localcoordinate system of the pelvis (see section 2.3.3 forthe definition of this local coordinate system) and thenet reaction forces were projected on the sacralendplate, which is rotated 418 forwards with regardto the global horizontal when a subject is standing inan upright posture (Kuntz et al. 2007).

Net moments at the right shoulder joint wereestimated by a top-down version of the LSM, whichused kinematics of the right arm (upper arm, lowerarm and hand) as input. External forces exerted at thebuilding block were not directly measured, because thiswould interfere with the natural lifting behaviour ofthe masons. To account for the effect of the weight and

acceleration of the building block on the shoulder netmoment, the mass of the block (half of the mass in thetwo-handed lifts) was added to the mass of the hand atthe centre of the hand palm. As the hands do not fullycarry the load of the block when it is in contact withthe pallet or the wall, shoulder moments were onlycalculated for time instants at which it was reasonableto assume that there was no contact. For block lifting,it was assumed that contact was lost starting at theinstant of the peak net total L5/S1 moment (Kjellberget al. 1998). For block placing, it was assumed thatthere was no contact until the net total L5/S1 momentreached a peak, while the hand was within 10 cmdistance from the wall.

2.4.2. Joint contact forces at the L5/S1and shoulder joint

The compression and forward shear forces producedby the trunk muscles at the L5/S1 joint (L5/S1Cmuscle

and L5/S1Smuscle, respectively) were estimated usingthe L5/S1 net total moment (L5/S1Mtotal) and regressionequations reported by van Dieen and Kingma (2005):

L5=S1Cmuscle ¼ L5=S1Mtotal � 17:07þ 434 R ¼ 0:902ð Þ

L5=S1Smuscle ¼ L5=S1Mtotal � 2:85þ 180 R ¼ 0:699ð Þ

These equations were developed using net totalmoments (estimated with the present LSM) andcompression and forward shear forces at the L5/S1joint (estimated with a detailed 3-D electromyographic(EMG)-driven trunk model) for a large variety oftasks, including symmetric and asymmetric lifting,sitting and pulling. Total compression force andtotal forward shear force at the L5/S1 joint wereobtained by adding the L5/S1 compression andforward shear components of the net reaction force(estimated by the LSM) to L5/S1Cmuscle and

L5/S1Smuscle,respectively.

Muscle plus net reaction compression forces at theshoulder joint (shoulderCtotal) were directly estimatedusing the net total shoulder moment (shoulderMtotal) anda regression equation reported by Praagman et al.(2000):

shoulderCtotal ¼ shoulderMtotal � 35:3þ 8:12 R ¼ 0:96ð Þ

This regression equation was developed using nettotal moments and total compression forces at theshoulder joint, estimated with the Delft ShoulderModel (van der Helm et al. 1992) over 25 arm postures(including anteflexion, abduction and horizontalanteflexion).

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2.4.3. Lumbar angles

Because flexion and rotation of the trunk have beenshown to be risk factors for low back pain (Hoogen-doorn et al. 2000), 3-D lumbar angles were calculatedduring the experiment. The local axes of trunk and pelviswere defined during a reference measurement in whichthe participant was asked to stand in an upright posture.The directions of the local axis (with regard to theparticipant) were as follows: positive X-axis to the front,positive Y-axis to the left and positive Z-axis upward.3-D lumbar angles (Euler angles) were calculated bydecomposing the trunk orientation, expressed in thepelvis coordinate system, in the order Y – X – Z.

2.5. Statistics

For the statistical analysis, the peak values of the timeseries of the L5/S1 compression and forward shearforce and of the shoulder total moment and compres-sion force were determined for all conditions. Notethat one peak per lifting condition rather than onepeak per lift was calculated. For lifting from a pallet,this peak usually occurred when lifting the last block.Furthermore, at the instant of peak L5/S1 compressionforce, the following variables were analysed: theabsolute 3-D components of the L5/S1 net moment(L5/S1 extension, lateral flexion and torsion moment)and the absolute 3-D components of the lumbar angle(lumbar flexion, lateral flexion and twist angle).

The effect of block mass, working height and thenumber of hands used on the above-mentionedvariables was quantified separately for the block-liftingand block-placing tasks using generalised estimatedequations (GEE) regression analyses (Liang and Zeger1993). A GEE regression analysis considers themeasurements within participants as repeated mea-surements and accounts for this dependency. Forlifting blocks from the pallet, the following regressionequation was used:

Outcome variable block lifting ¼Constant A þ

A1 � block mass (kg) þA2 � number of hands used (2 hands ¼ 0,

1 hand ¼ 1) þA3 � reducing block lifting height from top to

bottom layer level(top layer ¼ 0, bottom layer ¼ 1)

For placing blocks in the wall the followingequation was used:

Outcome variable block placing ¼Constant B þ

B1 � block mass (kg) þB2 � number of hands used (2 hands ¼ 0,

1 hand ¼ 1) þB3 � reducing block placing height from iliac crest

to floor level (iliac crest ¼ 0, floor ¼ 1) þB4 � increasing block placing height from iliac crestto shoulder level (iliac crest ¼ 0, shoulder ¼ 1)

where constants A and B are the regressionconstants and A1–A3 and B1–B4 are the regressioncoefficients for the block-lifting and -placing tasks,respectively. Regression coefficients of all the two-wayinteractions possible in this experimental setup (A4–A6 and B5–B8 for block lifting and placing,respectively) were also calculated but, for the sake ofclarity, not given in the equation. Each of theinteraction terms was screened separately forsignificance. Only significant interaction terms wereadded to the models. If multiple significantinteraction terms were found, only those thatremained significant after incorporating allsignificant interactions, were incorporated in the finalmodels.

3. Results

Figures 3, 4 and 5 show, for all experimentalconditions, the joint contact forces at the L5/S1 andshoulder joint, the moments at the L5/S1 and shoulderjoint, and the lumbar angles, respectively. Results ofthe GEE regression analysis for lifting blocks from thepallet and for placing blocks in the wall are presentedin Tables 2 and 3, respectively.

3.1. Lifting blocks from the pallet

3.1.1. Joint contact forces at the L5/S1 and shoulderjoints

The L5/S1 and shoulder loads were significantlyaffected by all three independent variables but not bythe interactions of those. When block mass wasincreased, L5/S1 compression forces, L5/S1 forwardshear forces and the shoulder compression forcesincreased by 91 (SE 9), 19 (SE 2) and 39 (SE 6) N/kg,respectively. Furthermore, when block height wasreduced from top layer to bottom layer level, L5/S1compression and forward shear forces increased by794 (SE 201) and 410 (SE 49) N, respectively, whileshoulder compression forces reduced by 421 (SE 42)N. In comparison with two-handed lifting, one-handed lifting resulted in 469 N (SE 79) highershoulder compression forces but in 395 N (SE 63)lower L5/S1 compression forces and in 130 N (SE 17)lower L5/S1 forward shear forces (see Figure 3, leftside).

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3.1.2. Moments at the L5/S1 and shoulder joints

Because the shoulder total moment was the onlyvariable used to estimate shoulder compression force,the relative effects of the independent variables on theshoulder total moment were almost identical to theeffects on the shoulder compression force. Also forthe L5/S1 extension moments, the effects were verysimilar to the effects found on the L5/S1 compressionforce. When blocks were lifted one-handed instead oftwo-handed, the L5/S1 lateral flexion moments in-creased with 21 (SE 5) Nm and, when lifting height wasreduced from top layer to bottom layer level, L5/S1lateral flexion moments reduced by 30 (SE 7) Nm. Forthe L5/S1 torsion moments, a significant effect ofblock-lifting height and of the interaction betweenlifting height and the number of hands used was found.

These effects indicated that the L5/S1 torsionmoment was not affected by the number of hands usedwhen lifting from the top layer of the pallet, but that,when lifting from the bottom layer of the pallet, one-handed lifting resulted in smaller L5/S1 torsionmoments than two-handed lifting (see Figure 4, leftside).

3.1.3. Lumbar angles

The lumbar flexion angle was mainly affected by theblock lifting height: lumbar flexion angle was largerwhen lifting blocks from the bottom layer than whenlifting blocks from the top layer. Furthermore, a maineffect of number of hands used was found, but thiseffect was counteracted by the interaction withblock mass, resulting in negligible difference between

Figure 3. Peak L5/S1 compression force, peak L5/S1 forward shear force and peak shoulder compression force for allexperimental conditions. Error bars indicate 1 SEM.

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one- and two-handed lifting (see Figure 5). Lumbarlateral flexion angle was affected by number of handsused and the interaction of the number of hands usedwith block mass and block-lifting height. These effectsindicated that, especially when lifting lighter blocksfrom the bottom layer of the pallet, one-handed liftingresulted in higher lumbar lateral flexion angles thantwo-handed lifting. For the lumbar torsion angle, the

only significant regression coefficient was theinteraction between number of hands used andblock-lifting height. This interaction indicated that,compared to two-handed lifting, one-handed liftingtended to decrease lumbar torsion angles when liftingfrom the top layer of the pallet and tended to increaselumbar torsion angles when lifting from the bottomlayer (see Figure 5, left side).

Figure 4. L5/S1 extension, lateral flexion and torsion moments at the instant of peak L5/S1 compression force (PCF)and the peak shoulder total moment. Error bars indicate 1 SEM.

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3.2. Placing blocks in the wall

3.2.1. Joint contact forces at the L5/S1 and shoulderjoints

For placing the blocks in the wall, results were in linewith the results for lifting blocks from the pallet: the L5/S1 compression and forward shear forces were affectedby all the independent variables but not by theinteractions between these variables. When block masswas increased, L5/S1 compression and forward shearforces increased by 89 (SE 6) and 20 (SE 3) N/kg,respectively, and when blocks were lifted with one handinstead of two hands, they decreased by 195 (SE 50) and52 (SE 14) N. Furthermore, when block placing heightwas reduced from iliac crest to floor level, L5/S1compression and forward shear forces increased by1473 (SE 173) and 658 (SE 31) N, respectively, and when

block placing height was increased from iliac crest toshoulder level they decreased by 162 (SE 70) and 52 (SE17) N, respectively (see Figure 3, right side).

For the shoulder compression forces, threesignificant main effects and three significantinteractions were found. Most noticeable were theeffects of working height: higher shoulder compressionforces were found when placing blocks at iliac crestlevel than when placing blocks at floor level and theshoulder compression forces increased further whenplacing blocks at shoulder level. Furthermore, withincreasing block mass, shoulder compression forcesincreased and the magnitude of this effect was affectedby the number of hands used and by reducing block-placing height from iliac crest to floor level. For thenumber of hands used, no main effect was found, butthere were significant interactions with block mass and

Figure 5. Lumbar flexion, lateral flexion and torsion angles at the instant of peak L5/S1 compression force (PCF).Error bars indicate 1 SEM.

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reducing block-placing height from iliac crest to floorlevel. The most substantial difference between one- andtwo-handed block placing was found when placing14 kg blocks at iliac crest level: one-handed liftingresulted in this condition in about 600 N highershoulder compression forces than two-handed lifting.

3.2.2. Moments at the L5/S1 and shoulder joints

The effects for the L5/S1 extension moment werecomparable to those found for the compression force.The L5/S1 lateral flexion moment increased with anincreased block mass and the L5/S1 torsion momentdecreased with increasing block-placing height fromiliac crest to shoulder height, but these effects weresmall. As mentioned before, because the shouldertotal moment was the only variable used to estimateshoulder compression force, the relative effects of theindependent variables on the shoulder total momentwere almost identical to the effects on the shouldercompression force (see Figure 4, right side).

3.2.3. Lumbar angles

Lumbar flexion angles were mainly affected bydecreasing the block-placing height from iliac crest tofloor level. Asymmetric lumbar angles were small inthe block-placing task; with the exception of only one

small interaction effect, no significant effects werefound (see Figure 5 right side).

4. Discussion

The present study was designed to compare the effectson low back and shoulder loading between the taskvariables block mass, working height and one- vs. two-handed block handling in masonry work. Based uponthe results of an observational study (van der Molenet al. 2007), a mock-up was built to allow realisticsimulation of the most common and strenuousworking situations. Block mass, working height andblock-handling method (one- or two-handed lifting)were varied.

In accordance with previous reports (Hartmannand Fleischer 2005), masons participating in thepresent study, were frequently subjected to low backloads in excess of the 3400 N compression force limitproposed for the general population by NIOSH(Waters et al. 1993). Even the limit of 5500 N proposedby Jager et al. (1991) for 25-year-old men was reachedin the present study when masons handled blocks nearground level. For shoulder loading, guidelines are notwell established. Equations published by Chaffin et al.(1991) suggest that about 95% of the male populationis capable of generating about 45 Nm anteflexionmoment at the shoulder joints in 908 arm elevation. In

Table 2. Results of the generalised estimated equation analyses for lifting blocks from the pallet for all dependentvariables, quantifying the effects of all independent variables (A1–A3) and their significant interactions (A4–A6).

1 2 3 16 2 16 3 26 3

ConstantA

Blockmass(kg)

Number ofhands used(two ¼ 0,one ¼ 1)

Reducingblock liftingheight from

top tobottom layerlevel (toplayer ¼ 0,bottom

layer ¼ 1)A1 A2 A3 A4 A5 A6

L5/S1 compression force (N) 3494 (238) 91 (9) 7395 (63) 794 (201)L5/S1 forward shear force (N) 1069 (66) 19 (2) 7130 (17) 410 (49)Shoulder compression force (N) 557 (74) 39 (6) 469 (79) 7421 (42)

L5/S1 extension moment (Nm) 150 (13) 4.6 (0.7) 734 (4) 70 (11)L5/S1 lateral flexion moment (Nm) 60 (5) 0.6 (0.6) 21 (5) 730 (7)L5/S1 torsion moment (Nm) 10 (5) 0.39 (0.32) 0.01(1.83) 9.0 (4.4) 713 (5.0)Shoulder total moment (Nm) 16 (2) 1.1 (0.16) 13 (2) 712 (1)

Lumbar flexion angle (8) 21 (3) 70.2 (0.1) 74.8 (2.1) 24 (3) 0.35 (0.14)Lumbar lateral flexion angle (8) 5.7 (0.9) 70.06 (0.04) 4.3 (1.4) 2.1 (1.3) 70.27 (0.13) 2.2 (0.9)Lumbar torsion angle (8) 5.2 (1.2) 70.04 (0.06) 72.50 (1.3) 70.47 (1.78) 8.3 (2.5)

Note: Actual mean (SE) values of these regression coefficients and of the regression constants are presented. The constants representsthe predicted values of the outcome measures for lifting a 0 kg block with two-hands from the top layer of the pallet.

Significant effects (p 5 0.05) are indicated in bold.

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Table3.

Resultsofthegeneralisedestimatedequationanalysesforplacingblocksin

thewallforalldependentvariables,quantifyingtheeff

ectsofallindependentvariables

(B1–B4)andtheirsignificantinteractions(B5–B8).

12

34

16

216

316

426

3

Block

mass

(kg)

Number

of

handsused

(two¼

0,

one¼

1)

Reducingblock

placingheight

from

iliaccrest

tofloorlevel

(iliaccrest¼

0,

floor¼

1)

Increasing

block

placing

heightfrom

iliaccrestto

shoulder

level

(iliaccrest¼

0,

shoulder¼

1)

ConstantB

B1

B2

B3

B4

B5

B6

B7

B8

L5/S1compressionforce(N

)2428(205)

89(6)

7195(50)

1473(173)

7162(70)

L5/S1forw

ard

shearforce(N

)680(50)

20(3)

752(14)

658(31)

752(17)

Shoulder

compressionforce(N

)499(79)

65(4)

101(70)

7356(79)

184(45)

33(13)

723(4)

7278(62)

L5/S1extensionmoment(N

m)

88(10)

4.6

(0.5)

714(3)

108(10)

712(3)

L5/S1lateralflexionmoment(N

m)

20(7)

0.82(0.24)

4.2

(3.0)

3.5

(6.0)

1.3

(4.6)

L5/S1torsionmoment(N

m)

11(6)

0.10(0.27)

70.58(1.30)

2.3

(3.3)

73.8

(1.8)

Shoulder

totalmoment(N

m)

14(2)

1.8

(0.1)

2.8

(2.0)

710(2)

5.2

(1.3)

0.93(0.37)

70.67(0.10)

77.9

(1.8)

Lumbarflexionangle

(8)

5.3

(1.3)

0.18(0.09)

1.49(0.91)

41(5)

70.64(0.76)

70.57(0.19)

73.8

(1.4)

Lumbarlateralflexionangle

(8)

2.6

(1.17)

0.05(0.06)

70.34(0.76)

1.65(1.8)

70.42(0.60)

2.1

(0.85)

Lumbartorsionangle

(8)

6.1

(2.1)

70.03(0.08)

70.62(0.60)

71.2

(1.3)

71.9

(1.3)

Note:Actualmean(SE)values

oftheseregressioncoeffi

cientsandoftheregressionconstantsare

presented.Theconstantsrepresentthepredictedvalues

oftheoutcomemeasuresforplacinga0kg

block

withtw

o-handsin

thewallatiliaccrestlevel.

Significanteff

ects

(p5

0.05)are

indicatedin

bold.

Ergonomics 1113

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the present study, this limit was exceeded whenhandling heavy building blocks at high workingheights. It should be noticed that the equationsmentioned above do not consider cumulative loadingand, therefore, they probably overestimate the capacityin repetitive tasks.

The above-mentioned results stress the importanceof reduction of the loads on the low back and theshoulder during masonry work. To find the mosteffective way to obtain low back and shoulder loadreduction, the relative contribution of the investigatedtask variables is discussed below.

4.1. Effects of block mass and working height

Reducing load mass and increasing initial block height(from top layer to bottom layer level) reduced lowback loading during the lifting tasks. This has beenshown before many times in more standardised liftingconditions (Buseck et al. 1988, Potvin et al. 1992,Schipplein et al. 1995, Fathallah et al. 1997, Lavenderet al. 1999, Davis and Marras 2000, Ferguson et al.2002, Kingma et al. 2004, 2006, Hoozemans et al.2008). When comparing the effects of working heightand block mass, it can be noted that, for lifting blocksfrom the pallet, increasing lifting height from bottomto top layer, resulted in a reduction of the L5/S1compression forces that was comparable to a blockmass reduction of 10 kg (23% and 26% of theregression constant, respectively). For L5/S1 forwardshear forces, the effect of increasing lifting height waseven substantially larger than for 10 kg block massreduction (38% vs. 17%). Moreover, for the block-placing task, the difference in effects of lifting height(increasing block-placing height from ground to iliaccrest level) and 10 kg block mass reduction was evenmore pronounced (for the L5/S1 compression forces61% vs. 37%, respectively; for the L5/S1 forward shearforces 97% vs. 29%, respectively). A reason that theeffect of working height was larger for block placingthan for block lifting is probably that the height of thetop layer was somewhat lower than optimal (with thehands at 90 cm, about 20 cm below iliac crest level forthe masons participating in the present study). Anotherreason might be that the horizontal distance of theblock relative to the lower back was larger when liftingfrom the pallet (Faber et al. 2007) than when placingblocks in the wall.

Besides reducing back and shoulder loading,another advantage of optimizing working height isthat it strongly reduces lumbar flexion, which has alsobeen suggested to be a separate risk factor fordeveloping low back pain (Hoogendoorn et al. 2000).Furthermore, it should be noted that reducing blockmass by 10 kg would either have a strong effect on

productivity or would drastically increase the numberof blocks that is handled over a day. The latter mightin fact lead to an increase of the cumulative load on thelow back (de Looze et al. 1996). Cumulative loadinghas been shown to be a separate risk factor for thedevelopment of low back pain (Norman et al. 1998).Participants furthermore indicated that they wouldprobably lift two blocks at once, when working with6 kg blocks. Considering the above, it is likely thatoptimising working height is more effective in reducingthe risk of developing low back pain than reduction ofblock mass. In fact, it has already been shown inintervention studies that optimising working heightreduces the incidence of low back pain (Marras et al.2000, van der Molen et al. 2004, Luijsterburg et al.2005).

For shoulder loading, the effect of working heightwas found to be opposite to the effect of workingheight on low back loading. Shoulder loading(shoulder total moments and compression forces) wassmallest when working close to the floor and largestwhen working above shoulder height. The latter isconsistent with literature reporting a dose–responserelationship in Swedish construction workers betweenthe amount of time working above shoulder height andshoulder pain (Holmstrom et al. 1992, Engholm andHolmstrom 2005). Furthermore, the relative effect ofblock mass was higher for shoulder loading than forlow back loading. For two-handed block handling,shoulder loading approached shoulder muscle capacitylimits for single lifts when working with heavy blocksat iliac crest or shoulder level. Note that blocks aregenerally grasped at their top so that, when placing ablock at iliac crest level, peak hand height may havebeen close to shoulder height (see Figure 2d).Therefore, it might be better to perform block placingwith the hands at about iliac crest height rather thanwith the bottom of the building blocks at iliac crestheight.

Considering the effects of working height on backand shoulder loading discussed above, optimalworking height is probably with the hands at aboutiliac crest height. If this optimal working height couldbe attained, peak L5/S1 compression forces woulddecrease considerably from about 5500 N, to about4000 N (see Figure 3), corresponding to the loadinglimit proposed by Jager et al. (1991) for 25-year-oldand 40-year-old men, respectively. It should bementioned that these limits might be too low forphysically active (Porter et al. 1989) and, at least in thepresent population, relatively tall masons(Ebbesen et al. 1999, Rector et al. 2009). On theother hand, these limits do probably overestimatestrength in cumulative loading (Brinckmann et al.1988).

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4.2. One-handed vs. two-handed block handling

In the present study, subjects performed the one-handed block-lifting and -placing tasks with the righthand and generally lifted the blocks from the right,homolateral, side of the body. In line with previousstudies on one-handed lifting from the homolateralside of the body (Marras and Davis 1998, Kingma andvan Dieen 2004), the present study found that one-handed lifting resulted in slightly lower extensionmoments and compression forces than two-handedlifting. The same was found for block placing. Anexplanation for this could be that, during one-handedblock handling, the arm that was not used to handlethe block served as counterweight, resulting in lowernet extension moments and compression forces at theL5/S1 joint. A possible disadvantage of one-handedlifting is that it could result in a larger asymmetry thantwo-handed lifting (Marras and Davis 1998, Kingmaand van Dieen 2004), which is associated with a higherrisk for the development of low back pain (Lavenderet al. 2007) and the occurrence of intervertebral discherniation (Kelsey et al. 1984). In the present study,one-handed lifting resulted in larger lateral flexionmoments at the L5/S1 joint. Furthermore, when liftingblocks from the bottom layer of the pallet, one-handedlifting resulted in larger lumbar torsion angles thantwo-handed lifting. However, in the same liftingcondition, L5/S1 torsion moments were smaller duringone-handed lifting, probably because subjects, at leastin part, turned around prior to lifting the block. Due toincreased lumbar torsion angles with reduced L5/S1torsion moments when lifting blocks from the bottomlayer of the pallet, the resultant effect of asymmetry inone-handed vs. two-handed lifting and placing taskson the risk of developing low back pain is unclear.

A more obvious disadvantage of one-handed blockhandling is that it caused a substantial increase inshoulder loading, especially when lifting from the toplayer of the pallet and when placing heavy blocks atiliac crest level (one-handed block handling aboveshoulder level was not performed during theexperiment).

4.3. Practical implications: weighting back vs.shoulder loading

Based on the discussions in sections 4.1 and 4.2, theresearchers recommend designing the workplace ofmasons such that work can be performed with thehands at about iliac crest height level as much aspossible because working at lower levels stronglyincreases low back loading and working at higherlevels results in shoulder loading that approachedshoulder muscle capacity limits.

Back loads could be reduced by using lifting tables(mechanically adjustable in height) to increase liftingheight (Marras et al. 2000) when lifting blocks from apallet. This intervention does not prevent block placingnear floor level so mechanical aids may be needed toassist in block placing in this situation. Alternatively, achange of block-handling technique might help toreduce back loading when placing blocks near floorlevel. For example, masons could start by gathering anumber of blocks together, adopt a kneeling postureand begin laying the first courses of blocks. However,while this will probably reduce low back loading duringblock placing, blocks still need to be placed near floorlevel from a standing posture during the blockgathering. Another possibility to reduce low backloading is by working in a squatted instead of stoopedposture (van Dieen et al. 1999, Kingma et al. 2004,2006), but it is unlikely that masons will adopt asquatting posture very often, because of the higherenergetic loading involved (Welbergen et al. 1991).Furthermore, prolonged squatting and kneeling duringwork has been associated with an increased risk for kneeosteoarthritis (Cooper et al. 1994, Coggon et al. 2000).

Shoulder loads could be reduced by avoidingworking above iliac crest height by using adjustablescaffolds to stand on when placing the blocks in thewall. With regard to the one-handed lifts, it was foundthat the one-handed lifts resulted in slightly lower lowback loading than two-handed lifting but that itresulted in higher shoulder loading, especially at higherworking heights. Therefore, it is recommended to onlywork one-handed when handling blocks near the floor.This is consistent with the fact that the masons whoparticipated in the present study reported that innormal working conditions they only employ the one-handed technique, when lifting from low locations(near floor level).

4.4. Limitations

Some limitations of this study should be mentioned.First, spinal compression forces were estimated basedon net reaction forces plus net moments in combinationwith a set of simple regression equations. A number ofprevious studies used an EMG-driven trunk musclemodel rather than the present set of simple regressionequations in order to estimate compression forces fromnet moments. Such an approach would have been moreaccurate in the present study as well, in that co-contraction would have been included in a more specificway. However, because of the aim to closely simulateworking conditions, the disadvantages of additionalEMG measurements in terms of preparation time andthe participant being ‘packed with equipment’ weredeemed larger than the advantages for the present

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study. Furthermore, measurement of forces at thehand(s) was not possible in the present approach.Therefore, shoulder loading could only be calculatedduring the time that it could be deduced that the blockswere not in contact with pallet or wall. Furthermore,taking the hand centre as the point of application ofhand forces is a simplification that will have causedsome error. Finally, it should be mentioned that one-handed lifting probably would have resulted in smallerlow back loading if the free arm would have been usedas a support (Kingma and van Dieen 2004), e.g. byleaning on other blocks or by leaning on a knee. It isunclear whether this technique is used in practice butsubjects were not allowed to use it in this experiment.

5. Conclusions

The results of the present mock-up study suggest thatmasons are frequently subjected to back and shoulderloads in excess of current health limits. Furthermore,increasing lifting height appeared to be the mosteffective way to reduce low back loading. However,working with the hands much higher than iliac crestheight should be avoided because it leads to highshoulder loading. Therefore, it is recommended toorganise masonry work in such a way that blocks arehandled with the hands at about iliac crest height asmuch as possible. Finally, one-handed block handlingat other locations than at near floor level resulted inrelatively high shoulder loads and, therefore, should beavoided in these conditions.

Acknowledgements

The authors would like to thank Robin Grouwstra forassistance in the data collection, the construction workerswho participated in the experiment, Calduran for supplying thebuilding blocks and employers’ and employee organisations inthe construction industry for financial support.

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