article info abstract

Article history:

Received 28 July 2014

Received in revised form

24 January 2015

Accepted 6 March 2015

Available online

Keywords:

Low back biomechanics

Fatigue failure

Bag stacking

Palletizing

Mining

Many mining commodities are packaged and shipped using bags. Small bags are typically loaded onto

pallets for transport and require a significant amount of manual handling by workers. This specific task of

manual bag handling has been associated with the development of musculoskeletal disorders (MSDs),

especially low back disorders. This study evaluates the biomechanical demands of different work layouts

when performing manual palletizing of small bags, and evaluates the biomechanical stresses associated

with different stacking techniques. Results indicate that peak forward bending moments as well as spinal

compression and shear forces are higher when the pallet is situated at the side of the conveyor as

opposed to the end of the conveyor. At low levels of the pallet, controlled bag placement results in higher

peak forward bending moments than stacking at higher levels and when dropping the bag to lower

levels. The results of this study will be used to inform the development of an audit tool for bagging

operations in the mining industry.

Relevance to industry: In many cases for workers loading small bags, compression forces exceed the

NIOSH criterion of 3400 N. Orientation of the pallet has a significant impact on spinal compression, and

positioning the pallet at the end of the conveyor reduces the estimated compressive loading on the

lumbar spine by approximately 800 N.

Published by Elsevier B.V.

International Journal of Industrial Ergonomics 47 (2015) 84e92

Contents lists available at ScienceDirect

International Journal of Industrial Ergonomics

journal homepage: www.elsevier.com/locate/ergon

The effects of operator position, pallet orientation, and palletizing

condition on low back loads in manual bag palletizing operations

Sean Gallagher a

a Department of Industrial and Systems Engineering, Auburn University, 3304 Shelby Center, Auburn, AL 36849-5346, USA

, John R. Heberger b,

b National Institute for Occupational Safety and Health (NIOSH), Office of Mine Safety and Health Research, 626 Cochrans Mill Road, Pittsburgh,

PA 15236-0070, USA

*

*	 Corresponding author. Tel.: þ1 412 386 6620.

E-mail addresses: seangallagher@auburn.edu (S. Gallagher), JHeberger@cdc.gov

(J.R. Heberger).

1. Introduction

Many mining commodities are packaged and shipped using

bags. These may be small bags that are manually handled or bulk

bags that may weigh several hundred kilograms (kg). Small bags

(typically weighing 23 kg but with weights up to 46 kg) are usually

loaded onto pallets for transport and require a significant amount

of manual handling by workers. While the loading of small bags

onto pallets has been automated in some loading facilities, at many

operations the repetitive job of loading small bags onto pallets is

still performed manually. Manual handling is associated with the

development of musculoskeletal disorders (MSDs), especially low

back sprains and strains (Dempsey and Hashemi, 1999). This is

particularly true if the workplace layout is poorly designed and/or

http://dx.doi.org/10.1016/j.ergon.2015.03.005

0169-8141/Published by Elsevier B.V.

appropriate lifting aids (such as lift tables) are not provided

(Keyserling et al., 1988).

In the United States, the Mine Safety and Health Administration

(MSHA) requires all mines to report all injuries, illnesses, and fatalities. These data are in the public domain, and are provided in

statistical analysis software format (IBM SPSS, Somers, NY) by the

National Institute for Occupational Safety and Health (http://www.

cdc.gov/niosh/mining/data/default.html). For this study, accident,

injury, and illness reports from MSHA were obtained for the calendar years 2007e2011. After filtering for cases that occurred only

in mills and preparation plants and that were considered non-fatal

injuries with days lost, the MSHA database contains 217 injuries

that can be classified as occurring during bag palletizing. The

number of days lost and restricted activity days due to palletizingrelated injuries over this time period was 10,047, with a median of

17 days per injury. Overwhelmingly, the specific mineworker activity at the time of injury was handling material or rock, accounting for over 88% of all accidents. The predominant nature of

injury was sprains and strains (68%), with a few scattered contusions and fracture cases. Overexertion was the predominant
S. Gallagher, J.R. Heberger / International Journal of Industrial Ergonomics 47 (2015) 84e92 85

accident type (70%). The back was the part of body most frequently

injured when handling bags (34% of cases), followed by shoulders

(15%), and hands/fingers (11%).

To begin to address the worker safety concerns revealed by

these numbers, this study evaluated the biomechanical demands of

different work layouts when performing manual palletizing of

small bags. Specifically, investigators observed during field visits

that manual palletizing operations in which bags were delivered via

conveyor were typically performed by workers stacking bags onto

two different pallet orientations relative to the conveyor: pallet at

the end of the conveyor, or pallet at the side of the conveyor. Thus,

one purpose of this study was to evaluate the biomechanical

stresses on workers performing bag palletizing tasks with the

pallets in these two orientations. Furthermore, field visits revealed

that some workers maintained their grasp on the bag through the

final placement on the pallet, while others would drop the bag into

place, particularly at the lower layers of the pallet. A secondary

purpose of the study was to evaluate the biomechanical stresses

associated with these techniques. Finally, the effects of the lift

destination height and worker position (left or right side) with

respect to the pallet were investigated.

One of the issues of interest in this study was the in uence fl of

pallet positioning on spinal loading during palletizing tasks. Spinal

compression is traditionally assumed to be the principal biomechanical mechanism associated with occupationally related low back

disorder (LBD) (Granata and Marras, 1999; Waters et al., 1993).

However, Granata and Marras (1999) found that the biomechanical

sources of low back pain (LBP) are dynamic, multifaceted, and

multidimensional, with spinal shear and torsion loading also playing

roles. Occupational low back injury prevention research has focused

on the effects of reducing extreme torso flexion and the external

moment, with little emphasis on torso twisting and lateral bending

(Jorgensen et al., 2005). Torso twisting has also been identified as a

risk factor for occupational LBP (Hoogendoorn et al., 2000; Kelsey

et al., 1984; National Institute for Occupational Safety and Health,

1997; National Research Council, 2001; Punnett et al., 1991).

Though previous studies have examined the torso kinematics

and biomechanical loading associated with changes in pallet position with loading boxes, no studies have looked at these factors

with respect to positioning of workers at the side versus the end of

the conveyor when palletizing bags. Thus, this study examined the

effect of operator position relative to the conveyor on lumbar

loading, and also evaluated the effects of control of the load during

lifting (dropping versus controlled placement) and lift destinations

(high vs low levels of pallet) on loading of the lumbar spine.

2. Methods

2.1. Experimental design

A split-split-split plot experimental design was employed to

evaluate the physical demands of lifting bags off a conveyor and

placing them onto a pallet. Ground reaction force and kinematic

data were used to drive a biomechanical model that estimated joint

forces and moments and low back compression experienced during

the lifting task.

This study evaluated torso twisting in two different conveyor

configurations. From the motion analysis data collected in this

experiment, the spinal compression and shear can be estimated

and compared with the dynamic lifting components.

2.2. Study population and participant inclusion criteria

Eight male participants from the National Institute for Occupational Safety and Health (NIOSH) in Pittsburgh, PA, participated in

this study. The average ± standard deviation of the age and weight

were 33 ± 5.3 years and 88.6 kg ± 10.5 kg, respectively. Two participants were left-handed and six were right-handed. Participants

were healthy with no symptoms for cardiovascular disease and no

history of hand, wrist, arm, back, and neck or shoulder injuries.

Before participating, each participant read and signed an informed

consent form approved by the NIOSH Human Subjects Review

Board.

2.3. Independent variables

Several independent variables were examined in this study

(Table 1). First, the orientation of the pallet relative to the conveyor

(variable name of pallet orientation with values End versus Side)

was of interest. Pallet orientation is directly related to the location

of the operator. When the pallet is on the Side, the operator

removes the bags from the side of the conveyor. When the pallet is

on the End, the operator removes the bags from the end of the

conveyor. There were two operator positions: Position1, in which

the operator is on right of pallet and needs to move to his left to

place bag on pallet, and Position2, in which the operator is on left of

pallet and needs to move to his right to place bag on pallet. Examples of these different scenarios can be seen in Fig. 1. Next, three

levels of bags were stacked on the pallet in each trial (see Fig. 2):

Level1 (the bottom three bags), Level2 (two bags, laid on top of and

in a perpendicular orientation to Level1), and Level3 (three bags, laid

on top of Level2, placed as in Level1). Bags were stacked in one

column at the part of the pallet closest to the operator. Additionally,

three palletizing conditions were examined: a lower pallet level

600 (Level1 above floor level) with controlled bag placement

(LPLcontrol), a lower pallet level while dropping the bag into place

on the pallet (LPLdrop), Fig. 2A, and an upper pallet level (Level1 3000

above the floor) with controlled bag placement (UPLcontrol),

Fig. 2B. Finally, bag destination (which is horizontal lifting distance)

for the closest and farthest bag from the conveyor for each level

(with values Near or Far), was an independent variable. For this

variable, the middle bags of Level1 and Level3 were omitted from

the analysis.

2.4. Dependent variables

Moments calculated about L5-S1 were the primary dependent

variables in the study. These included the Peak Forward Bending

(PFB) moment, Peak Left Lateral Bending (PLLB) moment, Peak

Right Lateral Bending (PRLB) moment, Peak Left Twisting (PLT)

moment, and Peak Right Twisting (PRT) moment. Estimates of the

Compression and AeP Shear Forces acting about L5-S1 were obtained through the use of a regression equation developed by Van

Dieen and Kingma (2005) which are based on the value of the

net L5-S1 moment. Data from each operator position (Position1 and

Position2) were analyzed separately, as bending and twisting moments would be occurring in opposite directions in these two

positions.

2.5. Data collection procedure

Participants were positioned on two force plates and then performed twelve lifting tasks (two pallet orientations [Side or End],

three palletizing conditions [UPLcontrol, LPLcontrol, LDLdrop], and

two operator positions [Position1 or Position2]) in a completely

randomized order.

Each task consisted of 8 lifts of 11.3-kg (25-lb.) bags off of a

conveyor and onto a pallet. The bag weight of 11.3 kg was used due

to NIOSH Human Subjects Review Board restrictions. The bags were

obtained from a mining company and dimensions were 2200
86 S. Gallagher, J.R. Heberger / International Journal of Industrial Ergonomics 47 (2015) 84e92

Table 1

Variable names, values, and descriptions used in this study.

Variable name Values Definition

Pallet orientation End Pallet located at end of conveyor

Side Pallet located at side of conveyor

Operator position Position1 Operator is on the right side of the pallet

Position2 Operator is on the left side of the pallet

Bag level Level1 Bottom three bags on pallet

Level2 Middle two bags

Level3 Top three bags

Palletizing conditions UPLcontrol Top of pallet is 3000 above floor; bag is placed on pallet

LPLcontrol Top of pallet is 600 above floor; bag is placed on pallet

LPLdrop Top of pallet is 600 above floor; bag is dropped onto pallet

Bag destination Near Bags on the pallet that are closest to conveyor

Far Bags on the pallet that are farthest from conveyor

long x 1500wide x 5.500high. The bags were packed with filler material to achieve the desired weight and fullness. Each participant

completed a total of 96 lifts. For each lifting task, the participants

stacked the eight bags on the pallet with three bags on the bottom

layer stacked lengthwise (Level1). The next two bags were laid

perpendicular to the first layer (Level2), and finally three were laid

lengthwise again as the third layer (Level3), as shown in Fig. 2. For

each pallet orientation and operator position, there were three

palletizing conditions: control placement, drop placement, and

high placement. The controlled placement (LPLcontrol) and drop

placement (LPLdrop) occurred with the bottom layer (Level1)

starting on top of the pallet on the floor (approximately 600 above

the floor). The high placement (UPLcontrol) had the bottom layer

starting on a tabletop at 3000 above the floor.

Bags were sent down the conveyor at intervals of 10 s. Participants were instructed on how the lifting tasks should be performed

(dropping bags and controlled placement) and were allowed to

practice until they felt comfortable with the tasks. After completion

of each task, the participant was given a rest period of at least two

minutes per the recommendations of Caldwell et al. (1974) for

studies involving physical exertion.

Two force plates (0R6-5-2000, Advanced Mechanical Technology, Inc. [AMTI], Watertown, MA) were used to capture ground

reaction forces and center of pressure data of both feet. Researchers

watched the participants' feet to make sure they did not move off of

the force plates and touch the floor; however, participants were

allowed to lift a leg and stand on one force plate if needed. The force

plate data was collected at 600 Hz using the EvaRT 5.0.4 software

(Motion Analysis Corporation, Santa Rosa, CA) through an analogto-digital board (PCI-6071E, National Instruments, Austin, TX).

Fig. 1. A) Pallet orientation at End of conveyor for Position2 (operator is on left of pallet

and needs to move to his right to place bag on pallet). B) Pallet orientation End for

Position1 (operator is on right of pallet and needs to move to his left to place bag on

pallet). C) Pallet orientation Side, for Position1. D) Pallet orientation Side for Position2.

A motion capture system (Eagle Digital Real Time System;

manufactured by Motion Analysis Corporation, Santa Rosa, CA)

utilizing retro-reflective markers placed on the surface of the skin

(or on shoes, socks, or clothing) was used to determine the orientation of the body at a sampling rate of 60 Hz. Kinematic motion

analysis data was collected using EvaRT software from Motion

Analysis Corporation. Retro-reflective markers were placed on

various joints of the body using the modified Helen Hayes marker

set (Fig. 3) (Davis et al., 1991). A total of 31 markers were affixed

with double-sided adhesive electrode collars. The motion analysis

system was calibrated at the beginning of each day of testing. After

each participant completed the twelve lifting tasks, the captured

motion analysis data was post-processed to make sure the markers

were labeled correctly and to make sure no markers were missing.

Random noise is usually characterized by high-frequency content while the movement signal is generally limited to a band of

low frequencies. In MATLAB, a fourth-order 10 Hz low pass Butterworth filter was used to remove the high-frequency (noise)

components and retain those of the low frequency (movement

signal) in both the motion analysis and force data.

2.6. Biomechanical model

Biomechanical models employing both bottom-up and topdown approaches were used to analyze the data; however, only

the bottom-up model results are presented here. The 14 body

segments and local coordinate system for L5-S1 are shown in

Fig. 4A. Mass distributions for each body segment are based upon

data provided by Dempster (1955), as corrected for fluid loss by

Clauser et al. (1969). Three-dimensional forces, moments, and

center of pressure data obtained from AMTI force plates were used

to calculate moment estimates for L5-S1. Axes established for the

Fig. 2. A) The three bags on the bottom row (Level1) are 6 inches above the floor. The

two bags in the middle row make up Level2, and the top three bags make up Level3

Note the stacking pattern in which Level2 is perpendicular to Level1 and Level3. This

resembles palletizing conditions for LPLcontrol and LPLdrop. B) Palletizing condition

UPLcontrol, in which Level1 is 30 inches above the floor. In this condition, bags are

placed onto the pallet.

.
S. Gallagher, J.R. Heberger / International Journal of Industrial Ergonomics 47 (2015) 84e92 87

Fig. 3. Modified Helen Hayes marker set used to obtain motion data in the study.

Fig. 4. A) The fourteen body segments and local coordinate system for L5-S1. B) Axe

established for the force plates using the right-hand rule.

s

force plates use the “right-hand” rule and are illustrated in Fig. 4B.

The position estimate for L5-S1 is operationally defined as a point

lying 40% of the distance (posterior to anterior) from the sacral

motion analysis marker (V.SACRAL) to a point bisecting the line

connecting markers on the right and left anterior superior iliac

spines (R.ASIS and L.ASIS) as described in Gallagher et al. (2009). To

estimate the forward lumbar bending moment, for example, the

following calculation was made:

MyL5-S1 1⁄4 [(xfp1 - xL5-S1) x Fzfp1] þ [(xfp2 - xL5-S1) x Fzfp2]

- (zL5-S1 x Fxfp1) - (zL5-S1 x Fxfp2) - Myrt - Myrs

- Myrf - Mylt - Myls - Mylf

where MyL5-S1 is the estimated forward bending moment about the

lumbosacral joint; xfp1,2 is the center of pressure in the x direction

calculated from force plate 1 or 2, Fzfp1,2 is the ground reaction force

in the z direction measured by the force plate 1 or 2, zL5-S1 is the

position of L5-S1 in the z direction, Fxfp1,2 is the force measured by

force plate 1 or 2 in the x direction, and Myrt ... Mylf represent the

moments about y associated with various links of the legs (thigh,

shank, foot). The total force (including calculated inertial forces for

each body segment) of these segments was used in the low leg link

corrections.

Inertial forces were calculated using a dynamic model described

by Huston et al. (1976) and Huston (2013). Since participants

pivoted during the performance of the palletizing tasks, moments

about L5-S1 were rotated based on the position of the markers

L.ASIS and R.ASIS so that consistent moment estimates about the

local coordinate system at L5-S1 were maintained.

2.7. Data analysis

Data was analyzed using a 2 x 3 x 3 x 2 (pallet orientation [Side

vs. End] x palletizing condition [High controlled vs. Low drop vs. Low

controlled] x bag level [Level1 vs. Level2 vs. Level3]) x bag destination (Near vs. Far) split-split-split plot analysis of variance

(ANOVA) with no between subjects variable. Separate ANOVAs

were run for conditions where participants were in different

operator positions (Position1 vs. Position2) due to the fact that when

participants were on the right side of the pallet they would bend or

twist in one direction, but would bend or twist in the opposite

direction when positioned on the left side. Measures such as lateral

bending and twisting would thus be occurring in opposite directions (positive in one versus negative in the other), which made

estimates of effects problematic due to cancellation of directional

influences. Variance-covariance matrix equality and form were

assumed. Tukey's Honestly Significant Difference (HSD) post-hoc

tests were performed for significant omnibus F tests. To control

for alpha inflation associated with the multiple dependent variables, a Bonferroni-corrected alpha level was used. The Bonferroni

correction required that F-tests for each variable achieve p < 0.01 to

be considered significant, thus maintaining a family-wise Type I

error rate of 0.05.

3. Results

Results of ANOVA were examined and ANOVA assumptions

tested (including examination of equality of variances, normality of

residuals, and outliers) and no violations were observed for any

variable. Tables 2 and 3 provide summaries of significant effects on

forward bending, lateral bending, and twisting moments when

participants were located in Position1 and Position2, respectively.

Effect sizes (h2

) are also provided in these tables.

3.1. Operator Position1 (operator is on the right side of pallet and

moves towards the left to place bag on pallet)

Peak Forward Bending (PFB) moments were affected by pallet

orientation, palletizing condition, bag level, and bag destination

(Table 2). PFB moments were significantly higher (p < 0.01) during

bag transfers when pallets were oriented at the Side of the conveyor

as opposed to being oriented at the End of the conveyor. The PFB

moment averaged 38% higher when lifting to a pallet on the

conveyor Side (Fig. 5A).
88 S. Gallagher, J.R. Heberger / International Journal of Industrial Ergonomics 47 (2015) 84e92

Table 2

Significant main effects and interactions when operator was in Position1. Interactions not included in the table were not significant for any dependent variable. Statistical

significance is indicated by asterisks (*) with * meaning p < 0.01, and ** indicating p < 0.001. Effect size (h2

) is provided for significant effects, except for main effects that are

involved in an interaction due to the unreliability of the effect size estimate in such cases.

Pallet orientation

(End vs Side) (A)

Palletizing condition

(upper control vs low

control vs low drop) (B)

Bag level (Level1 vs

Level2 vs Level3) (C)

Bag destination

(Near vs Far) (D)

A*D B*D C*D

Peak forward bending moment *

2 h 1⁄4 0.242

**

2 h 1⁄4 0.125

**

2 h 1⁄4 0.013

**

2 h 1⁄4 0.017

Peak left lateral bending moment ** *

2 h 1⁄4 0.026

Peak right lateral bending moment * **

2 h 1⁄4 0.034

Peak left twisting moment * * **

2 h 1⁄4 0.026

Peak right twisting moment ** *

2 h 1⁄4 0.015

** **

2 h 1⁄4 0.053

Table 3

Significant main effects and interactions when the operator was in Position2. Interactions not included in table were not significant for any dependent variable. Statistical

significance is indicated by asterisks (*) with * meaning p < 0.01, and ** indicating p < 0.001. Effect size (h2

) is provided for significant effects, except for main effects that are

involved in an interaction due to the unreliability of effect size estimates in such cases.

Pallet orientation

(End vs Side) (A)

Palletizing condition

(upper control vs low

control vs low drop) (B)

Bag level

(Level1 vs Level2

vs Level3) (C)

Bag destination A*B

(Near vs Far) (D)

A*C A*D B*D C*D A*C*D

Peak forward bending

moment

** **

2 h 1⁄4 0.018

** *

2 h 1⁄4 0.053

Peak left lateral bending **

moment

**

2 h 1⁄4 0.030

**

2 h 1⁄4 0.053

Peak right lateral

bending moment

* ** **

2 h 1⁄4 0.031

*

2 h 1⁄4 0.020

*

Peak left twisting

moment

**

2 h 1⁄4 0.043

 

2 h 1⁄4 0.107

** 

2 h 1⁄4 0.026

Peak right twisting

moment

** **

2 h 1⁄4 0.017

PFB moments were also affected by palletizing condition (upper

vs. low) (p < 0.001). Post-hoc tests indicate that a controlled

stacking technique (maintaining control of the bag to the pallet)

when stacking below knee level resulted in higher forward bending

moments than stacking at waist level or dropping the bag into

position on the pallet (Fig. 5B). The latter conditions were not

statistically different from one another in terms of PFB moment

(p > 0.05). The bag level also affected PFB moments, with the lowest

level of stacking resulting in higher PFB moments compared to the

middle or top levels of stacking (regardless of whether palletizing

condition was at the lower level or upper level) (p < 0.001). Finally,

PFB moments were greater when the bag destination was farther

from the conveyor as opposed to nearer (p < 0.001).

Peak Left Lateral Bending (PLLB) moments was affected by interactions between pallet orientation and bag destination (p < 0.01)

as shown in Table 2. Peak Right Lateral Bending (PRLB) moments

was affected by an interaction between bag level and bag destination (Near vs. Far) (p < 0.001) as shown in Table 2.

Peak Left Twisting (PLT) moments in Position1 were affected by

an interaction between palletizing condition and bag destination

(p < 0.001). In this interaction, it appears that the Near distances

resulted in lower PLT moments in waist level and dropping the bag

below knee level; however, with controlled stacking below knee

level, twisting moments were similar with both Near and Far bag

destinations.

Peak Right Twisting (PRT) moments were affected by bag level,

with Level2 resulting in lower PRT moments than Level1 or Level3

(p < 0.01). PRT moments were also affected by an interaction between pallet orientation and bag destination (p < 0.001). This

interaction is shown in Fig. 6 and the pallet on the Side of conveyor

condition resulted in higher PRT moments, but these moments

varied according to the respective interacting variable.

Fig. 5. A) Peak forward bending moments for Position1. L5-S1 moments were significantly higher when the pallet was placed on the Side of the conveyor (b) compared to the End of

the conveyor (a). B) A controlled stacking technique when stacking below knee level resulted in significantly higher forward bending moments (b) than stacking at waist level or

dropping the bag into position on the pallet (both a).
S. Gallagher, J.R. Heberger / International Journal of Industrial Ergonomics 47 (2015) 84e92 89

Fig. 6. Peak right twisting moment was affected by an interaction between pallet

orientation and bag destination. L5-S1 moments were higher when pallet orientation

was at the Side of the conveyor compared to the End of the conveyor, but the benefit o

pallet orientation at the End of the conveyor was minimized with bag destinations that

were further away.

f

Estimated peak spinal compressive and peak spinal shear forces

for Position1 (based on the net L5-S1 moment) were both affected

by the main effects of pallet orientation, palletizing condition, bag

level, and bag destination, with no significant interactions to report.

This difference between lifting at the side versus the end of the

conveyor resulted in a 840 N increase on the estimated peak

compressive forces on the spine and a 90 N increase in estimated

shear forces on the spine (Fig. 7).

3.2. Operator Position2 (operator is on the left side of pallet and

moves towards the right to place bag on pallet)

PFB moments for operator Position2 were affected by the main

effect of bag level (p < 0.001) and an interaction between pallet

orientation and palletizing condition (p < 0.01). The bag level main

effect found that PFB moments were greater at the lowest pallet

level (240.9 Nm) as opposed to medium and high levels (216.3 Nm

and 222.8 Nm, respectively). The interaction demonstrated that the

PFB moments were very similar between pallet orientations with

Fig. 7. Estimated peak compressive and shear forces on the spine for Position1.

Compression and shear forces on the spine were higher when the pallet orientation

was on the Side of the conveyor.

palletizing conditions of high placement and low level palletizing

when dropping the bag on the pallet (UPLcontrol and LPLdrop);

however, with low controlled palletizing (LPLcontrol), higher PFB

moments were seen when the operator was positioned at the Side

versus End of the conveyor.

PLLB moments in Position2 were affected by an interaction of

pallet orientation and bag destination (p < 0.001) and an interaction between palletizing condition and bag destination (p < 0.001)

as seen in Fig. 8. For the former interaction, PLLB moments were

generally higher at the End than at the Side orientation; however,

PLB moments increased with bag destination when at the End of

the conveyor, but decreased with bag destination when at the

conveyor's Side. The crossover interaction between palletizing

condition and bag destination indicated that PLLB moments were

higher with Far lifts in the UPLcontrol condition, and higher with

Near lifts in the LPLcontrol condition, while both Near and Far lifts

resulted in equivalent PLB moments in the LPLdrop condition.

For PRLB moments, interactions were observed between bag

level and bag destination (p < 0.001) and pallet orientation by bag

destination (p < 0.001). The bag level x bag destination interaction

was characterized by a generally high PRLB moments when bag

destination was Far; however, as bag level increased (low to high)

the magnitude of this difference diminished. The interaction of

pallet orientation by bag destination indicated a large effect of bag

destination when pallet orientation was at the End of the conveyor;

however, PRLB moments were decreased when pallet orientation

was at the Side of the conveyor, and the magnitude of the bag

destination (Near or Far) was much smaller when positioned at the

Side of the conveyor.

Table 3 shows that PRT moments in Position2 were affected by a

triple (p < 0.01) interaction. This interaction did not demonstrate

any clear or consistent pattern of response to the combinations of

independent variables.

Estimated peak spinal compressive and peak spinal shear forces

for Position2 were both affected by an interaction of pallet orientation and palletizing condition (p < 0.001). Fig. 9 shows the

interaction between pallet orientation and palletizing condition for

spinal compression and spinal shear forces. The End and Side pallet

orientations have similar compressive and shear forces for the low

(drop) (LPLdrop) palletizing condition. However, the Side of

conveyor pallet orientation is significantly higher than the End

pallet orientation for the low (control) palletizing condition

(LPLcontrol); while the opposite result is observed for the high

palletizing condition (higher spinal compression and shear forces

when positioned at the end of the conveyor).

4. Discussion

Design of operator workstations (such as a bagging workstation)

can be an effective method of reducing the risk of worker injury. Of

particular interest in the current study is the impact of the orientation of the pallet at the End of conveyor as opposed to the Side of

the conveyor when performing palletizing operations with bags

commonly found in industry. Results of this analysis clearly

demonstrate that positioning the pallet at the End of the conveyor

results in a significant reduction in loading on the lumbar spine

compared to positioning the pallet on the Side of the conveyor. This

may be due to the ability to use the momentum of the bag as it

comes off of the conveyor when the pallet is at the conveyor end, as

opposed to having to forcefully redirect the bag from its course

along the conveyor when the pallet is located on the Side. For

example, having the pallet at the End of the conveyor resulted in

compression estimates that were over 840 N lower than when the

pallet was located at the Side in Position1. This represents a 19%

change in lumbar spine loading e a significant decrease.
Fig. 8. Peak left bending moments were generally higher at the End than at the Side orientation; however, PLB moments increased with bag destination when at the end of the

conveyor, but decreased with bag destination when at the conveyor's Side (left). The crossover interaction (right) between palletizing condition and bag destination showed that PLB

moments were greater with Far lifts in the high condition, and higher with Near lifts in the low controlled condition, while both the Near and Far lifts resulted in equivalent PLB

moments in the low drop condition.

Not surprisingly, the technique of dropping the bag to lower

levels (often observed in industrial bag palletizing operations)

appeared to convey significant benefits when loading the lower

levels of the pallet, when compared to controlled lifts to the lower

pallet levels. In some cases, this technique was statistically “tied” in

terms of the lowest lumbar stresses with lifting to the higher pallet

level. In other situations it was rated as second-best to lifting to the

higher pallet levels. Estimates of lumbar compression for the drop

technique overall were approximately 600e800 N lower than those

for controlled placement at the lower levels. Though some wastage

of material could result from this practice, the benefits in terms of

reduced spinal loading might still be a favorable trade-off for

manufacturers.

Recent evidence has suggested that low back disorders (and

most likely all MSDs) are likely the result of a process of fatigue

failure in affected tissues (Gallagher and Heberger, 2013; Barbe

et al., 2013). All known materials experience fatigue failure (i.e.,

material failure at submaximal levels of loading), including biomaterials tested in in vitro studies (Brinckmann et al., 1988;

Schechtman and Bader, 1997; Gallagher et al., 2005, 2007). It

would be surprising if biomaterials in vivo did not also share this

inherent material property. Although no currently available ergonomics assessment tools have used fatigue failure theory to assess

MSD risk, several tenets of fatigue failure theory may help provide

some context to the results presented in this paper.

One fundamental precept of fatigue failure theory is that if a

tissue is loaded at a high level of stress relative to its Ultimate Stress

(US) e the stress at which it will fail in one cycle e it will fail in a

rather limited number of loading cycles. If the material is loaded

repetitively at 80% of its US, it can still be made to fail, but it may

take 100 cycles to do so. Loading at 50% of ultimate stress may cause

failure in 1000 cycles. Interestingly, for many materials there exists

a so-called “endurance limit” (usually at around 30% US) where

materials can be loaded for a very large number of cycles e in some

cases indefinitely e without failure (Ashby et al., 2010). It is also

important to recognize that since the fatigue failure curve is

indexed to the US of a material, individuals with different US values

will incur damage at different rates when exposed to the same

absolute load. For example, exposure to a 3000 N load may be

acceptable for an individual with a spine whose US is 12 kN, but the

same load may lead to more rapid damage accumulation for an

individual with a spine whose US is 5 kN.

When considering the effects of bagging workstation design

demonstrated in this paper, it is clear that certain decisions in the

setup of a workstation may lead to significantly increased (or

decreased) risk of LBDs in manual bag palletizing. The 840 N

decrease in lumbar spine loading associated with placing the pallet

at the end of the conveyor will decrease the rate of cumulative

damage incurred by spinal tissues, as would the 600e800 N

decreased spinal loading associated with dropping the bags when

Fig. 9. A) and B) The interaction of pallet orientation and palletizing condition for spinal compression forces and spinal shear forces indicates compression and shear forces at the

side-of-conveyor orientation are significantly higher than the End-of-conveyor orientation at the low controlled palletizing condition; however, the opposite occurs with the high

palletizing condition.

A Peak Spinal Com pression 
 

P osition 2 
 

5500

5000

End o f Conveyor 
 

S ide o f Conveyor 
 

N ) (

4500

c e For

4000

3500

3000

H igh Low (D rop) Low (C on trol)

P alle tizing C ondition

B Peak S pinal Shear
 

P osition 2 
 

700 End o f Conveyor

S ide o f Conveyo r

650

)N(

600

ecF o r

550

500

450

H igh Low (D rop) Low (C on trol)

P alle tizing C ondition

90 S. Gallagher, J.R. Heberger / International Journal of Industrial Ergonomics 47 (2015) 84e92
S. Gallagher, J.R. Heberger / International Journal of Industrial Ergonomics 47 (2015) 84e92 91

palletizing the lower levels (compared to controlling the bag at the

end of the lift). Furthermore, maintaining a lifting level at or around

waist level (similar to the high lifting conditions in this study) will

reduce lumbar loading to a similar degree, as demonstrated previously (Jorgensen et al., 2005; Marras et al., 1997; Davis et al.,

2010). The reduced loading associated with such design changes

will reduce the rate of cumulative damage in spinal tissues in

accordance with fatigue failure theory. Nevertheless, it is important

to recognize that reducing the rate of damage does not necessarily

translate to lack of development of cumulative trauma in the

lumbar spine. However, if the compressive loading is reduced to a

low percentage of an individual's US (e.g., 30% US), damage can be

minimized and healing of damaged tissue may be possible.

It is apparent that significant loads on the lumbar spine can be

experienced even when palletizing 11.3-kg bags. However, this load

is much lower than is usually handled in actual bagging operations,

where bags can range from 23 to 46 kg. As manual bag palletizing is

a task that involves both high loads on the lumbar spine and high

rates of repetition, the risk of LBDs would be quite significant based

on previous epidemiology data (Gallagher and Heberger, 2013), and

fatigue failure theory. Clearly the preferred method of bag palletizing is through the use of robots or use of mechanical aids such as

vacuum hoists (Gallagher et al., 2011). However, if no such method

is available, several aspects of the bag stacking workplace design

can help to reduce the loads on workers. This study suggests that

positioning of the pallet at the end of the conveyor (as opposed to

the side) can reduce lumbar loading significantly (>800 N). If

possible, pallets should be positioned at the end of the conveyor.

Lumbar loads are also lower when dropping the bag to lower levels

of the pallet rather than placing the bag with control. Such a

technique is recommended if lift tables are not available.

The main limitations of this study include the use of 11.3-kg

bags, rather than bags found in industry that usually weigh

23e46 kg. Using a heavier bag would have resulted in significantly

increased loads on the lumbar spine. In addition, the study participants were not experienced with palletizing tasks and their

methods may differ from more experienced materials handlers.

Loads on the spine may also be influenced by such differences in

lifting technique.

Results of this study will be used to inform the development of

an audit tool for bagging operations in the mining industry as

described by Dempsey et al. (2012), as well as to make recommendations for design of bag palletizing workstations. In particular,

based on our conclusions it is recommended that where possible,

companies with manual palletizing operations design their systems

so that pallets and workers who are palletizing are positioned at the

end of the conveyor belt.

5. Conclusion

On the basis of the results of this study, the following conclusions are drawn:

1.The peak forward bending moment experienced when the

pallet is oriented on the side of the conveyor is significantly

higher than when the pallet is located at the end of the

conveyor. Placing the pallet on the side of the conveyor

increased the estimated compressive loading on the spine by

over 800 N.

2. Controlled stacking at the lower levels of the pallet resulted in

higher peak forward bending moments than stacking at the

higher pallet levels or dropping the bags to the lower pallet

level. The difference in estimated lumbar compressive loading

ranged from 600 to 800 N.

3. Estimates of average peak lumbar compressive forces exceeded

the 3400 N Action Limit recommended by NIOSH even when

lifting 11.3-kg bags. Bags in industry may weigh up to 46 kg,

which would lead to extremely dangerous compressive forces.

4. Proper design of the palletizing workstation can help to reduce

lumbar loading to a degree; however, manual bag palletizing

(even when designed properly) is a task that tends to involve

high repetition and high lumbar loads, the combination of

which leads to a dramatic escalation of LBD risk (Gallagher and

Heberger, 2013).

Disclaimer

The findings and conclusions in this report are those of the

authors and do not necessarily represent the views of the National

Institute for Occupational Safety and Health.

Acknowledgment

The authors would like to acknowledge Niccole LaBranche for

her many hours of post-processing and assistance with data

collection and the NIOSH employees who volunteered to participate in this study.

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