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Balance control dur ing lateral load t ransfers over a

slippery surfaceRobert D. Caten a

a, Angela DiDomenico

b, Jacob J. Banks

b& Jack T. Dennerl ein

cd

aDepart ment of Physical Therapy, University of Evansvil l e, Evansvil l e, IN, USA

bCent er f or Physical Ergonomi cs, Liber t y Mut ual Research Inst it ut e for Safet y, Hopkint on,

MA, USAc

Department of Environmental Health, Harvard School of Public Health, Boston, MA, USAd

Departm ent of Ort hopaedic Surgery, Brigham and Women's Hospit al and Harvard MedicaSchool , Bost on, MA, USA

Available online: 25 Oct 2011

To cite t his art icle: Robert D. Caten a, Angela DiDomeni co, Jacob J. Banks & Jack T. Dennerl ein (2011): Balance controlduring lateral load transfers over a slippery surface, Ergonomics, 54:11, 1060-1071

To link t o thi s arti cle: ht t p : / / dx .doi .org/ 10.1080/ 00140139.2011.618229

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Balance control during lateral load transfers over a slippery surface

Robert D. Catenaa

, Angela DiDomenicob

*, Jacob J. Banksb

and Jack T. Dennerleinc,d

aDepartment of Physical Therapy, University of Evansville, Evansville, IN, USA; bCenter for Physical Ergonomics, Liberty MutualResearch Institute for Safety, Hopkinton, MA, USA;

cDepartment of Environmental Health, Harvard School of Public Health, Boston,

MA, USA;d

Department of Orthopaedic Surgery, Brigham and Womens Hospital and Harvard Medical School, Boston, MA, USA

(Received 5 November 2010; final version received 23 August 2011 )

Few studies have measured balance control during manual material handling, and even fewer with environmentalcofactors. This study examined the effect of different surface frictions during a stationary manual material handlingtask. Thirty-six healthy participants completed 1808 lateral transfer tasks of a load over high- and low-frictionsurfaces (m 0.86 and m 0.16, respectively). Balance measures, stance kinematics and lower extremity muscleactivities were measured. Success during the novel slippery surface dichotomised our population, allowing us toinvestigate beneficial techniques to lateral load transfers over the slippery surface. Stance width reduction by 8 cmand 158 of additional external foot rotation towards the load were used to counter the imbalance created by theslippery surface. There was no clear alteration to lower extremity muscular control to adapt to a slippery surface.

Changes in stance seemed to be used successfully to counter a slippery surface during lateral load transfers.

Statement of Relevance: Industries requiring manual material handling where slippery conditions are potentiallypresent have a noticeable increase in injuries. This study suggests stance configuration, more so than any othermeasure of balance control, differentiates vulnerability to imbalance during material handling over a slipperysurface.

Keywords: falls; balance control; manual materials handling; slips; friction

Introduction

According to the US Bureau of Labor Statistics (BLS

2009), there is a particularly high rate of falls in

industries requiring manual material handling, such as

delivery personnel and transportation cargo handlers.Fall incidence rates are about 100% higher in ground

freight transportation compared with ground passen-

ger transportation occupations. Activities within man-

ual material handling industries require a variety of

movements and manoeuvres with a load. Balance and

fall related research has primarily addressed biome-

chanical aspects of walking during load-carrying

situations (Holbein-Jenny et al . 2007, Birrell and

Haslam 2009, Gillette et al. 2010). Load transfer

manoeuvres have not been examined extensively as a

challenge to balance control and we are not aware of

any detailed epidemiological data to indicate the risk

of different load transfer manoeuvres. The previous

balance control studies that have examined load

transfers have emphasised sagittal plane reaching

(Kozak et al. 2003, Row and Cavanagh 2007, Liao

and Lin 2008) and lifting (Commissaris and Toussaint

1997, Toussaint et al. 1998, Kollmitzer et al. 2002),

where the centre of gravity (COG) is not expected to

excessively deviate towards the lateral directions

(Kollmitzer et al. 2002).

Our research group has recently endeavoured to

examine the lateral load transfer manoeuvre. This task

requires the acquisition, movement and placement ofa load from one side of the body to the other. This is

similar to tasks performed by air and truck cargo

handlers and deliverers. The body COG is required to

travel closer to the sides of the base of support (BOS)

as load distance is increased during such a manoeuvre.

Relocation of the COG near the edges of the BOS

may be an important cause of imbalance during

manual material handling (Streepey and Angulo-

Kinzler 2002). Our first examination (Catena et al.

2010) focussed on the effects of weight during the

lateral load transfer. We found that increased weight

led to greater vulnerability to a loss of balance.

Individuals in turn constricted movement about the

ankle joints by stiffening the joints through

co-contraction of antagonist shank musculature.

Stiffening the bodys posture is a commonly used

conservative adaptation to environmental

perturbations (Santello and McDonagh 1998, Lark

et al. 2003, Nielsen et al. 2004).

*Corresponding author. Email: [email protected]

Ergonomics

Vol. 54, No. 11, November 2011, 10601071

ISSN 0014-0139 print/ISSN 1366-5847 online

2011 Taylor & Francis

http://dx.doi.org/10.1080/00140139.2011.618229http://www.tandfonline.com


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Besides highlighting the importance of co-contrac-

tion to control balance, these previous studies highlight

the need to examine the effects of a variety of

environmental factors (e.g. icy surface, unstable loads

or distracting attention) in balance control, as not all

factors are countered with the same control mechan-

isms. This remains true for occupational tasks such asthe lateral load transfer, where there has been limited

research involving environmental factors such as

reduced friction. Load transferring in slippery condi-

tions has focussed more on basic tasks such as walking

(Myung and Smith 1997) and simple grasping tasks

(Cooper et al. 2005), as opposed to lateral movements

with the load. One study (Holbein and Redfern 1997)

suggests that holding loads to the side in more slippery

conditions probably leads to choosing a more con-

servative posture, by reducing the travel area of the

COG within the BOS. The more dynamic task of

walking with a load in slippery conditions has been

shown to result in imbalance (Myung and Smith 1997,Cham and Redfern 2002).

While imbalance is certainly more prevalent in

walking, since the COG has greater motion and is

more prevalently located outside of the BOS, a fall

during walking is commonly in the anterior or poster-

ior (A/P) direction (Bakken et al. 2006). A fall in the A/

P direction has easily employed countermeasures such

as stepping manoeuvres, joint flexions and bracing by a

leg and arms (Bakken et al. 2006). Imbalance in a

medial or lateral direction does not permit the same

countermeasures (Lo and Ashton-Miller 2008). Step-

ping manoeuvres to counter a fall to same side as the

loaded leg are blocked by the loaded leg requiringrotation of the entire body around it. Lower extremity

lateral joint movements are limited to hip and ankle

abductions and bracing for impact after a fall is limited

to the arm on the side of the lateral fall. A fall in the A/

P direction can, however, be more easily countered

during lateral movements of the body. Stepping

manoeuvres could be accomplished with either foot

(depending on the distribution of the body weight over

each foot). During this stepping manoeuvre, all lower

extremity joints are available to help decelerate the

body. If the step does not succeed in preventing a fall,

then both arms are available to help brace for impact

with the ground. Occupational tasks that require

lateral motions of the whole body must be examined

more closely if we hope to get a full understanding offalls in the workplace.

In this study, we evaluated how balance control

during lateral load transfers was affected by the

friction of the standing surface. The first attempt on a

novel slippery surface was of particular interest. Based

upon our previous analyses of weight during the lateral

load transfer, we hypothesised that rather than

demographic characteristics or kinematic factors,

inappropriate muscular control would be the

predominant factor in loss of balance. Secondly, we

sought to describe how individuals adapt to a slippery

surface given enough practice. We hypothesised that

muscular activity in the lower extremities might bealtered to control imbalance sufficiently. Findings from

this research will demonstrate how a slippery surface

can be negotiated safely when a lateral load transfer is

required and how individuals might alter normal

lateral load transfer performance when a slippery

surface is not necessarily expected, but occasionally

present.

Methods

Participants

Thirty-six healthy working age (2066 years of age)

adults without material handling experienceparticipated in this study (Table 1). Informed consent

was approved by the Institutional Review Boards of

both Harvard School of Public Health and Liberty

Mutual Research Institute for Safety. The informed

consent was read and signed by each participant prior

to data collection.

Participants wore a tank top, shorts, below-ankle

socks and below-ankle hiking shoes (Nike Bandolier

Table 1. Demographic variables across the two groups: those who successfully completed LCOF-1 (SL-1) and those who wereunsuccessful (UL-1).

Variable SL-1 UL-1 p-value

Gender: female, male 8, 15 6, 7 0.501Age (years): mean (SD) 44.6 (14.8) 43.0 (15.9) 0.871Height (m): mean (SD) 1.70 (0.10) 1.68 (0.08) 0.598Weight (kg): mean (SD) 77.6 (17.7) 70.6 (11.0) 0.150BMI (kg/m2): mean (SD) 26.6 (4.0) 25.0 (3.4) 0.202Arm length (% body height): mean (SD) 34.9 (1.1) 35.3 (1.2) 0.353Pelvis width (% body height): mean (SD) 15.4 (1.6) 15.2 (1.4) 0.654Hand dominance: right, left 18, 5 11, 2 0.644Foot dominance: right, left 20, 3 10, 3 0.438

Ergonomics 1061


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II) for instrumentation purposes. All shoes were

purchased at the beginning of this study and were

only worn for this study; therefore, the tread was that

of brand new hiking shoes. A harness connected to an

overhead fall arrest system was worn during box

transfer tasks for safety purposes. The harness was

adjusted to allow for unobstructed side-to-side motion.Slack in the harness cable was adjusted to provide

resistance only when needed to catch the person from

falling (Figure 1).

Procedure

Each participant completed 1808 lateral transfers of a

41.5 (width) cm 6 41.5 (depth) cm 6 32.5 (height) cm

box loaded to 5% (+ 0.01 kg) of their body weight

with semi-oval, reinforced cut-out handles. Two

different flooring surfaces were presented to the

participants. A high friction surface (m 0.86) was

created between the rubber shoe soles and plywoodflooring painted with a granular white paint high

coefficient of friction surface (HCOF). A low friction

surface (m 0.16) was created between Teflon tapeadhered to the bottom of the shoes and Teflon flooring

low coefficient of friction surface (LCOF). The two

surfaces were the same colour and had a similar gloss

so that they were nearly indistinguishable. The surface

and shoe modification was performed in such a way as

to not alert the participants about the change.

The task required the participants to transfer the

box from one pedestal to the other, whose heights wereadjusted such that the boxs handles were at the

participants standing elbow height. The pedestals top

surfaces were 11.5 cm wider and longer than the box

and covered with perforated rubber matting, which

encouraged the participants to lift rather than slide the

box off the surface. Prior to data collection, each

participant was given the opportunity to practice the

lateral load transfer over the high friction surface.

Participants were instructed to perform the transfer at

a self-selected pace. The purpose of this practice

session was to familiarise them with the process and

instructions. At the same time, we asked that they

determine their self-imposed maximum transferdistance that could be completed safely while still

adhering to our instructions.

For consistency, specific instructions on how to

complete the task were provided to all participants.

Figure 1. The start position for a HCOF trial. The objective is for the participant to transfer the box from its pedestal tothe opposing pedestal without walking. A researcher monitors the feet from behind during the trial. Lasers on the floor mark thestarting stance configuration. This figure has been adopted from Catena et al. (2010) with written consent from the authors andpublisher.

1062 R.D. Catena et al.


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These were: (1) you are not allowed to lift a foot

completely off of the ground, drag your feet or slide

your lead foot, but you are allowed to pivot on your

feet. (2) When lifting the box, make sure you have your

little fingers in the handles before lifting, dont drag the

box, dont rest your weight on it, and dont push

yourself up by pushing into it. (3) When placing thebox, make sure that the box is completely on the

pedestal and again dont push against the box to push

yourself up. The final purpose of the practice sessions

was for the participants to determine their preferred

starting stance configuration. The starting stance

configuration was marked. After each collected trans-

fer trial, the participant would be instructed to return

to the starting stance configuration.

Data collection started with six successful trials

performed at the participants practiced maximum

distance over a HCOF. While the participants were

sequestered away from the testing area, researchers

modified the flooring surface to the Teflon surface.When the participants returned to the testing area they

put on the modified shoes, which were not slippery

unless in contact with the Teflon surface. The second

test condition required a single transfer of the box at

the same distance and starting stance configuration

over the new and unidentified LCOF-1. There were no

practice trials for this condition. The third condition

started after adequate familiarisation and practice with

the modified surface. This practice session included

modifications to the transfer distance and starting

stance configuration as deemed necessary by the

participant. Once the modifications and practice were

completed, the new start stance configuration wasmarked and the participants completed six successful

transfer trials LCOF-2. The conditions were not

randomised because the novelty of the first low friction

condition and adaptation to the slippery surface were

of particular interest.

Measurements and parameters

All COFs were measured with a Brungraber Mark II

tribometer (Chang et al. 2003). Testing for the high

friction condition involved a cut section of a shoe sole

against the plywood floor surface. Testing for the low

friction condition involved a cut section of a shoe sole

with Teflon taped to the bottom against the Teflon

floor surface.

A load cell (Scaime C9418) was located underneath

the top of each pedestal and sampled at 1000 Hz to

capture the timing of the box transfer. The gradual

acquisition and release of the box (per cent of box

weight on pedestal) was measured from these data,

allowing for calculations of the total time to complete

the box transfer and inclusion of the box weight into

the body centre of mass (COM). The distance the box

was transferred was calculated from the displacement

of the box COM as measured by the motion analysis

system described below, and normalised to the stature

of the participant.

Three-dimensional marker data were collected by a

12-camera motion capture system (MotionAnalysis,Santa Rosa, CA). Thirty-four markers were placed on

the body to identify 13 individual body segments,

similar to previous marker sets used for measuring

balance control (Hahn and Chou 2004, Catena et al.

2009, Catena et al. 2010). Eight markers (four for each

foot) attached to the shoes at the most anterior,

posterior, medial and lateral projections of the sole

allowed us to measure elevation of each part of the

foot off of the ground. These same markers were used

for BOS measurements. Marker data were collected at

100 Hz for a time sufficient for the participant to

complete the task and filtered with a zero-lag forth

order low pass Butterworth filter at 8 Hz.Thirteen body segment COMs were calculated

based on anthropometric data of segment COM

locations and weights from previous research using the

appropriate age, gender and body mass index

information for each participant (Plagenhoef et al.

1983, Winter 1990, de Leva 1996, Pavol et al. 2002,

Durkin and Dowling 2003). The weighted sum of

segment COMs, harness and electromyography

(EMG) transmitter determined the whole body COM

at each time frame for the entire trial. The weight of

the box was proportionally accounted for in the COM

calculation as load cell measures in each pedestal

changed.The BOS was calculated from the four markers on

each foot. As a portion of a foot was raised, BOS sides

connected to the corresponding marker were

disregarded. If a foot was raised entirely off the ground

then that trial was excluded from our analyses. Stance

kinematics was measured through calculations of

displacement of the ankle joint centres in the lateral

direction (stance width) and the external rotation of

the support and contralateral feet with respect to pelvis

rotation (combination of foot and hip external

rotation). The minimum distance of the COG to each

of the six edges (front, back and lateral forefoot and

rear foot of the right and left feet) of the BOS (E1:

where x2, y2 and x1, y1 are coordinates of adjacent BOS

markers and x0, y0 are coordinates for the COG at a

single point in time) was used to indicate balance

control. Variability in balance control was determined

by measuring the total area of COG path (estimated as

an oval using the most A/P and M/L points)

normalised to the area of the BOS and the COM path

distance during the transfer normalised by the

displacement of the box during the transfer.

Ergonomics 1063


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dx2 x1y1 y0 x1 x0y2 y1ffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffiffiffi

x2 x12 y2 y1

2q : E1

Bilateral activity of the biceps femoris, vastus

medialis, gastrocnemius and tibialis anterior were

measured by a telemetry electromyography system(Noraxon, Scottsdale, AZ). All electrode placements

were in accordance with those described in the literature

(Hermens et al. 1999) and confirmed via palpation and

observing the EMG signal during isometric contrac-

tions. The skin was prepared beforehand by treatment

with alcohol and gentle abrasion. One author was

responsible for all skin preparation and electrode

placement for all participants during this study.

Data from maximum voluntary contractions

(MVCs) were collected to normalise the amplitude of

the EMG signal. A MVC for 5 s was collected while

the lower limb was manually restrained by a researcher

at a specific joint angle (08 ankle plantar flexion forgastrocnemius and tibialis anterior; 308 knee flexion

for biceps femoris; 158 knee flexion for vastus

medialis). This was performed three times for each

muscle with 1 min breaks in between.

All EMG signals were pre-amplified, band passed

filtered (10500 Hz), sampled at 1000 Hz and synchro-

nised with motion capture data. EMG signals were

rectified and smoothed using a 100-ms moving window

about each frame. The signals collected during the

lateral transfers were normalised by the maximum

signal amplitude measured during the MVC protocols.

The eight muscle activities were grouped into four

agonistantagonist pairs: the support and contralateralshank (gastrocnemius and tibialis anterior) and the

support and contralateral thigh (biceps femoris and

vastus medialis). The support leg was defined as the leg

on the ipsilateral side of the load at a particular time

and the contralateral leg was opposite of the support

leg. For each pair, we calculated the average intensity

(average muscle activity of the segment muscles) and

co-contraction percentage (lesser muscle activity as a

percent of greater muscle activity) of agonistantago-

nist muscle pairs for 800 ms time windows around

acquisition and release of the box to determine support

leg muscle activity and muscle activity of the contral-

ateral leg. We previously used this method to measure

co-contraction activity and percentage separately (Ca-

tena et al. 2010). Our separate equations are derived

from the single encompassing equation proposed by

Winter (1990).

Statistical analyses

This study initially had one independent variable

(task), which had three levels [high friction (HCOF),

novel low friction (LCOF-1), and practiced low

friction (LCOF-2)]. However, our balance control

variables were unobtainable for LCOF-1 in a group of

13 individuals who were unable to successfully

complete the first slippery trial. Therefore, we

dichotomised groups by successful (SL-1) vs.

unsuccessful (UL-1) completion of LCOF-1. Fouranalyses were performed: (1) group comparison, (2)

between-group comparison during HCOF, (3) within-

group comparison of LCOF-1 and HCOF in the SL-1

group and (4) a group-by-task comparison with the

two groups and HCOF and LCOF-2 tasks.

All assumptions for analysis of variance (ANOVA)

were considered appropriate except for muscle activity

variables and age, which displayed non-parametric

distribution. Muscle activity was examined using the

KruskalWallis test for group comparisons, Savage

one-way analyses for task comparison main effects and

Wilcoxon two-sample tests for task pair-wise

comparisons in SAS (Cary, NC). Age was analysedwith a MannWhitney test in SPSS 12.0 (Chicago, IL).

Parametrically distributed stance and balance

variables were examined using the Proc MIXED

function of SAS with age as a covariate to fit

ANCOVA models with subject as a random effect.

Pair-wise comparisons with Bonferroni adjustments

were performed when statistical significance was

calculated from main effects. Analyses performed on

group demographics and the first slippery trials were

analysed with chi-squared analyses and t-tests, when

respectively appropriate, in SPSS 12.0. Alpha levels for

all statistical analyses were set a priori at 0.05.

Results

First slippery trial analyses

Group comparisons

By analysing performance during the first slippery

trial, we hoped to determine how balance is affected by

lateral load transfer over a novel slippery surface. Of

the 36 participants, 13 of them were unable to

successfully complete the first slippery trial. Two

individuals fell (fall arrested by harness) during their

first trial with the slippery surface (Figure 2). Six

individuals took a step to maintain balance during

their first trial with the slippery surface. The five

remaining individuals did not lose balance in any way,

but chose to stop the trial after first attempting it,

stating that they believed they would be unable to

complete the task given the instructions described

previously without falling. This group of 13 individuals

made up the unsuccessful during first low friction trial

(UL-1) group. We found no significant differences

between genders, hand dominance, foot dominance,



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age, height, weight, body mass index (BMI), pelvis

width normalised by height or arm length normalised

by height for the two groups (Table 1).

Between-group comparison during HCOF

Our next attempt to determine what differentiatedgroup success examined initial performance of

individuals in the HCOF trials. By doing this, we

hoped to learn what tactics each group was initially

employing and would supposedly continue to employ

if unaware of work environment changes. Independent

sample t-test results indicated that there were no

differences between groups in how far the box was

transferred or the transfer time for HCOF trials.The UL-1 group started with an 8.3-cm wider

stance than SL-1 (p 5 0.001). Stance width remained

statistically wider for the UL-1 group throughout the

load transfer (Figure 3). Table 2 shows the results only

for start and load acquisition times, but results were

similar at load release and the end of the trial. While

the two groups did not differ significantly in the

amount of foot external rotation that they initially

started with, the UL-1 group used 138 less external

rotation when they manoeuvred to acquire the box

compared with the SL-1 group (p 5 0.001). During

this manoeuvre, the load side foot rotates out about

the hip and then the pelvis rotates about thelongitudinal body axis, and often past that of the

externally rotated foot, but does even more so in the

UL-1 group as this group keeps the foot in a relatively

static position compared with SL-1 individuals. The

UL-1 group did not contract agonistantagonist

muscles as uniformly across lower extremity joints

(co-contraction) as the SL-1 group (Table 3); however,

Figure 2. Example raw data of a LCOF-1 trial for an UL-1individual transferring a box from right to left. Top view ofthe COG movement throughout the trial within the BOS.The BOS shown is specific to the instance of pickup. The symbol shows the COG location at pickup. Notice how theCOG quickly trails straight backwards at the end of the trial.This individual failed due to a loss of balance backward atthe end of the trial that resulted in a fall into the harness.

Figure 3. The average BOS transition from start (A) to pickup (B) of the box between SL-1 (black) and UL-1 (grey) forthe HCOF trials. Stance width for the two groups is indicated in the middle of the feet and the difference between lead footrotation angles is indicated between the dashed lines.

Ergonomics 1065


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the UL-1 group did use more agonistantagonist

muscular intensity at the support thigh (p 0.006),contralateral thigh (p 5 0.001) and contralateral

shank (p 0.046).Ultimately, only a few outcome balance measures

proved to be different between the two resultant

groups in the HCOF condition (Table 4). We did

find that the COG of the UL-1 group travelled 1.4 cm

closer to the edge of their lateral-rear foot side of the

BOS during box acquisition compared with the SL-1

group (p 0.037). On the other hand, the UL-1 groupstayed 1.2 cm further away from the front boundary of

their BOS compared with the SL-1 group (p 0.024).

Within-group comparison of LCOF-1 and HCOF

in the SL-1 group

Our final attempt to determine what differentiated

group success examined initial performance of

successful individuals in the HCOF trials compared

with their performance during LCOF-1. By doing this,

we hoped to learn whether initial tactics used by this

group either continued to be used with success or were

abandoned in favour of more appropriate tactics

specifically for the slippery surface. Box transfer

distance and start stance properties did not change

from HCOF to LCOF-1 because they were held

constant between tasks to emulate unawareness to a

change in the work environment.

While there were no differences in start stance,

there was a change in stance almost immediately as the

individuals manoeuvred to pick the load up (Table 2).

There was over a 2-cm increase in stance width at load

acquisition in LCOF-1 compared to HCOF

(p 5 0.001), which became a 5-cm increase by the

end of the trial in LCOF-1 (p 0.037). During LCOF-1, the SL-1 group also rotated their feet more with the

pelvis as it rotated towards the direction of the pedestal

Table 2. Stance kinematic variables across the three conditions.

Variable Group HCOF LCOF-1 LCOF-2

Stance width at start (cm) SL-1 54.4 (9.9)B 55.5 (10.4) 51.6 (13.7)UL-1 62.7 (17.1)A X 54.9 (17.7)Y

Stance width at load acquisition (cm) SL-1 60.4 (8.1)B Y 63.0 (8.5)X 63.3 (10.8)X

UL-1 65.7 (16.0)A 62.8 (15.8)

External foot rotation* of lead foot at start (degree) SL-1 15.94 (9.03) 14.86 (10.8) 16.78 (9.83)UL-1 18.49 (15.0) 19.34 (14.4)External foot rotation* of lead foot at load acquisition (degree) SL-1 716.84 (22.3)A Y 75.90 (14.2)X 72.58 (13.6)X

UL-1 729.89 (23.6)B 712.15 (16.6)X

Notes: *Relative to perpendicular position of pelvis ASIS; A group 4 B group during the specified task (p 5 0.05); X task 4 Y task inthe specified group (p 5 0.05). Significant pairwise comparison results are presented with superscript letters and shaded. Values are given as mean(SD.)

Table 3. Agonistantagonist muscle pair co-contraction parameters over the 800 ms window for the support leg andcontralateral leg.


Support shank co-contraction intensity{ SL-1 53.7 (24.7)A 55.1 (24.4) 52.3 (24.7)A

UL-1 49.5 (23.5)B

49.3 (26.8)B

Support shank average intensity{ SL-1 26.5 (13.0) 28.7 (13.2) 25.8 (12.2)UL-1 29.5 (24.9) 35.6 (33.5)

Support thigh co-contraction intensity{ SL-1 50.2 (24.8)A 54.7 (25.0) 50.2 (22.8)A

UL-1 44.4 (22.8)B 47.2 (26.3)B

Support thigh average intensity{ SL-1 21.0 (13.2)B Y 24.1 (14.2) 23.6 (13.2)B X

UL-1 24.2 (13.1)A Y 27.1 (17.2)A X

Contralateral shank co-contraction intensity{ SL-1 32.1 (26.5)A X 21.4 (17.7) 23.7 (22.3)A Y

UL-1 17.3 (18.7)B 18.9 (20.7)B

Contralateral shank average intensity{ SL-1 9.8 (6.5)B 10.6 (7.9) 8.8 (7.7)B

UL-1 17.1 (10.3)A 16.5 (17.1)A

Contralateral thigh co-contraction intensity{ SL-1 37.4 (26.9) 40.3 (26.8) 38.4 (24.8)UL-1 35.3 (23.7) 38.1 (26.4)

Contralateral thigh average intensity{ SL-1 9.4 (7.2)B 10.3 (8.3) 9.7 (7.9)B

UL-1 11.0 (9.1)A 11.2 (11.4)A

Notes: A group 4 B group during the specified task (p 5 0.05); X task 4 Y task in the specified group (p 5 0.05);{

% of the larger muscleactivity; {% MVC. Significant pairwise comparison results are presented with superscript letters and shaded. Values are given as mean (SD).



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to either pick up or drop off the box (p 5 0.001).

There were no statistical differences between lower

extremity muscle involvements in each task for theSL-1 group (Table 3).

The outcome balance measures of individuals after

introduction of a slippery surface changed significantly

from a high friction surface for the successful group

(Table 4). The SL-1 group took 22% longer (more

than half a second longer) to complete the box transfer

in LCOF-1 compared with the HCOF condition

(p 0.010). During this time, there was an increasein COM travel distance (p 0.003). The COG of theSL-1 group stayed 1.6 cm further from the edge of

their lateral-fore foot side of the BOS during box

acquisition with the slippery surface (p 5 0.001). On

the other hand, the COG travelled closer to theanterior and posterior sides of the BOS during the

task with a slippery surface (p 5 0.001 and p 0.017,respectively).

Group-by-task comparison with HCOF and

LCOF-2 tasks

By analysing performance during the HCOF and

LCOF-2, we hoped to determine how the two groups

adapted to a slippery surface to accomplish the lateral

load transfer. This in turn provided some additional

information of what differentiated success in the first

slippery trial. Eventually, all individuals from the UL-1

group made sufficient modifications to accomplish the

lateral load transfer over the slippery surface. This

occurred after several practice trials during which the

pedestal distance was initially shortened, and then

increased as a new transfer technique was employed

successfully. By the end of the practice trials, there

were no statistical differences in transfer distance

between the two groups or between surface conditions.

For both groups, the maximum transfer distance for

LCOF-2 was determined with half as many practice

trials as for HCOF (p 5 0.001), but there was no

group difference in the number of practice trials used(p 0.815).

While the UL-1 group had a significantly wider

start stance than SL-1 in the high friction condition (as

documented in a previous section), the group

difference was not present in the adapted low friction

condition (Table 2). The UL-1 group had decreased

their start stance width by 7.8 cm during LCOF-2

compared to HCOF (p 0.002), while there was nosignificant change in the SL-1 group. As previously

mentioned, the SL-1 group had more external rotation

of the foot towards the box just before picking it up

during HCOF compared to the UL-1 group. This

remained true during LCOF-2 (p 0.027), but bothgroups increased external rotation of the pickup foot

even more so during this task (p 5 0.001). Likewise,

when individuals turned to drop the box off on the

opposing pedestal, both groups increased the external

rotation of the foot on that side by more than 158

(p 5 0.001). The two groups modified muscular

control going from HCOF to LCOF-2 similarly and

group difference remained similar between the two

tasks. The one difference between the two groups was

that the SL-1 group decreased contralateral shank

co-contraction during LCOF-2 (p 0.008), while theUL-1 group did not (Table 3).

Occasional imbalances still occurred after

adaptation to the slippery surface. There was no

difference in the number of unsuccessful trials between

the two groups during HCOF (Table 4), but the UL-1

group had significantly more unsuccessful trials

during LCOF-2 than the SL-1 group (p 0.026). Wefound an increase in the area of the BOS covered by

the COG for both groups during LCOF-2 compared to

HCOF (p 5 0.001). The COG of all individuals

travelled closer to the back (p 5 0.001) and front

Table 4. Balance control variables across the three conditions.


Total area of COG (% of start BOS area) SL-1 25.0 (9.8)Y 27.7 (9.8) 28.8 (9.5)X

UL-1 24.6 (9.0)Y 29.8 (9.8)X

COM path distance during transfer (% of box displacement) SL-1 28.5 (4.4)Y 30.0 (4.3)X 30.0 (4.6)X

UL-1 29.7 (4.9) 29.3 (4.4)

Minimum distance between COG and front of BOS (cm) SL-1 7.18 (3.8)

A X

5.19 (3.2)

Y

4.52 (3.0)

Y

UL-1 8.38 (3.4)B X 6.22 (3.0)Y

Minimum distance between COG and back of BOS (cm) SL-1 7.88 (2.4)X 7.13 (2.2)Y 5.67 (2.4)Y

UL-1 7.83 (2.2)X 6.66 (2.0)Y

Practice trials prior to the given task SL-1 13.83 (5.8)X 7.09 (1.5)Y

UL-1 13.69 (5.9)X 7.38 (2.3)Y

Average number of foot lifts(trials excluded during analysis of other variables)

SL-1 0.74 (1.1) 0.39 (0.8)B

UL-1 0.69 (1.0) 1.15 (1.1)A

Notes: A group 4 B group during the specified task (p 5 0.05); X task 4 Y task in the specified group (p 5 0.05). Significant pairwisecomparison results are presented with superscript letters and shaded. Values are given as mean (SD).

Ergonomics 1067


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(p 5 0.001) of the BOS during LCOF-2 compared to

HCOF.

Discussion

The purpose of this study was to evaluate balance

control during lateral load transfers to determine (1)characteristics of individuals able to maintain balance

on a novel slippery surface and (2) to see how

individuals adapted to a slippery surface, particularly

when at first they were unable to maintain balance. We

had originally hypothesised that the key to maintain-

ing balance during the lateral load transfer over a

slippery surface was increased joint stiffness as

scientific literature and our own past research had

suggested. We found that this was not the case for a

slippery surface. The most beneficial technique to

maintaining balance was to increase intended move-

ment towards and with the box, as evident in the

success individuals had after allowing greater footrotations with stance width adjustments. These find-

ings suggest that individuals already incorporating

increased body motion towards the direction of

interest will be less likely to succumb to a loss of

balance on the occasional low friction surface experi-

enced during a lateral material handling task.

Balance control

Balance control was measured several ways in this

study, the most direct being a loss of balance. A

complete loss of balance was rare. Foot lifts to quickly

readjust the BOS to encompass the body COG weremuch more prevalent. All individuals at some point in

testing or practice trials used such a strategy. However,

the UL-1 group had a significant increase in these

needed BOS adjustments during the required comple-

tion of six successful low COF trials. This indicated

that adaptation to the slippery surface did not

completely eliminate imbalance and that the UL-1

individuals may face an increased chance of a loss of

balance. In fact, we never observed UL-1 performance

completely equivalent to SL-1 performance. Future

studies might examine performance over more trials to

determine if there is ever complete convergence of the

two groups.

By examining balance through measures of the

COM, we were able to identify how balance was

possibly lost in many of these situations. While the

lateral load transfer task has large movements from

side-to-side, the BOS is appropriately widened for this

movement. However, by widening the BOS to such a

degree, anterior and posterior control is possibly

reduced through a significant alteration in normal

muscle functioning. We were unable to find any

scientific literature describing balance with a very

wide stance (450 cm), but when stance width is

increased to such a degree muscle pathways and

lengths across the hip, knee and ankle are drastically

altered. Some muscles such as the gluteals and the

tibialis anterior are shortened when stance is wide,

while hamstrings and peroneal muscles are lengthened.The lengthtension relationship then dictates that these

muscles will have reduced force production (Nordin

and Frankel 2001). We suspect muscle tensions played

a role in anterior/posterior imbalance seen.

When individuals stood with a 45-cm stance width,

there were no deleterious effects on balance (Kirby

et al. 1987). However, when individuals rotate their

pelvis towards the box to pick it up, they are now

actually standing with less mediolateral stance width

and more A/P foot displacement. When individuals

stood with 30 cm of A/P foot separation, there were

significant increases in mediolateral deviation of the

centre of pressure (Kirby et al. 1987), similar to A/Pmeasures in our study because our measures are based

on a global reference. Since anterior and posterior

COG movement increased in the slippery surface

conditions, it seems apparent that movements in these

directions are important to examine as a direct cause of

loss of balance during lateral load transfers. Something

about the SL-1 group allowed them to explore these

areas of the BOS, similar to the UL-1 group, without

the same detrimental balance effects that the UL-1

groups faced.

Stance kinematicsOne possible area to analyse group difference in

imbalance in the A/P direction is kinematics of the

lateral load transfer manoeuvre. There was clear

evidence of group differences in transfer manoeuvres.

The UL-1 group chose a wider BOS to transfer this

box over a high friction surface. A wider BOS may

indicate even further reductions in force production of

lower extremity muscles. When used during the

slippery surface condition, this wider BOS may not be

as easily controlled as a narrower BOS when friction is

not supplying a lateral stopping force against the sole

of the foot and hip adductor muscles are solely

responsible for maintaining the stance width. This is

similar to hip adductor use to decelerate the foot in

push off during skating (Chang et al. 2009). One of the

adaptive techniques that the UL-1 group chose to

employ was a narrowed stance width in LCOF-2. The

hypothesis that stance width determines success then

leads to questions about strength as a contributing

factor. A narrower stance width would require more

low-back extensor activity to reach a load because of a

need for increased torso flexion. Greater hip adductor



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strength would allow individuals to maintain a slightly

wider BOS over a slippery surface to avoid torso

flexion.

Another important kinematic change was the

external foot rotation towards the load. In the high

friction conditions, the SL-1 group rotated the

ipsilateral foot more towards the load than the UL-1group. External rotation was even greater during the

first slippery trial. UL-1 individuals similarly increased

the amount of ipsilateral foot rotation to the load in

the adaptive low friction trials. One possible reason

this may be beneficial is that this manoeuvre positions

the lower extremities in such a way that flexion at the

ankle, knee and hip is towards the load rather than

only having a hip abduction moment available to

control movement in the direction of interest as would

be the case if the body remained directed forward with

the load still by the side. This hypothesis emphasises

the importance of muscles that were unfortunately not

measured in our study (e.g. gluteus medius and tensorfascia latae) so we can only speculate that the rotation

manoeuvre decreased the reliance on less effective hip

abductors. A second reason external rotation might be

significant is not that it just signifies what is specifically

needed for better performance in this particular task,

but it might also be an indicator that the groups

benefited from a strategy that utilised body movement

to their advantage. Foot rotations may be just one

indicator of a surfer movement over slippery surfaces;

a motion altered to glide with body momentum over

the slippery surface, rather than fighting against the

motion unreduced by a lack of surface friction

(Marigold and Patla 2002). This alteration reducesthe need for shear forces applied to the surface for

movement. To completely analyse this hypothesis,

future studies will look at whole body kinematics.

The unfavourable effect of rotating feet towards to

the load is that the body is put into a more tandem-like

stance configuration. This form of stance has been

demonstrated to be more unstable than side-by-side

stance (Winter 1995). We believe that this contributed

to the increased motion towards the forward and

backward directions of the BOS. Doing this would

require increased activation of lateral balance control

musculature that was unfortunately not measured in

our testing.

Muscular control

Initial and continued group differences in lower

extremity muscle activity would seem to indicate that

the UL-1 group stiffened lower extremity joints in an

apparent conservative strategy to avoid falling. This is

similar to previous literature that shows that many

different populations regard joint stiffening as a way to

avoid falls (Santello and McDonagh 1998, Lark et al.

2003, Nielsen et al. 2004). However, muscular control

that we measured had no apparent effect on success in

the lateral load transfer over the slippery surface. If

anything, it was beneficial to loosen the joints with less

lower-extremity muscle activity. Their continued slight

increase in joint stiffness may be an explanation as towhy the UL-1 group required more step manoeuvres to

avoid falling even after adaptation to the slippery

surface. This was contradictory to our hypothesis and

to our previous findings that indicated stiffening of the

ankle joint to counter increased load weight during the

lateral load transfer (Catena et al. 2010). This was also

contradictory to others interpretation of the benefit of

ankle stiffness. However, some researchers have found

that while joint stiffening is employed, it is not always

successful in maintaining postural control (Reeves

et al. 2006, Cenciarini et al. 2010, Reynolds 2010),

which indicates task dependency, or that success may

depend on the joint and task being examined (Chamand Redfern 2001). Future studies of the lateral load

transfers should include measurements of muscles

involved in coronal and transverse plane movements to

complete the analysis of muscular control.

Demographic characteristics

We found no indication that simple demographic

factors determined successful performance in lateral

load transfer over a low friction surface. Among the

examined factors, age (Winter 1995) and BMI (Hue

et al. 2007, Teasdale et al. 2007) have been touted as

correlating factors to balance control. Our sample ofindividuals contained a range of BMIs (17.734.2 kg/

m2) that we think was adequate to assess it as a non-

factor in balance during the lateral load transfer for

most of the working population performing such tasks.

We had a normal distribution of BMIs that made up

our sample. Unfortunately, we do not believe our

sample of age (2066 years) was completely adequate

to state for sure that it is a non-factor in balance

during the lateral load transfer. Rather than a normal

distribution, ours was a uniform distribution of age. It

also failed to include enough individuals of a balance-

effected age, older than 50 (Winter 1995). We do

believe that age will certainly be a factor past 60, but

our findings do show that the gradual age increase was

not a factor in the working age population.

Limitations

(1) A list of instructions were given to the

participants and specified in the Procedure

section. These instructions were meant to limit

Ergonomics 1069


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the variability between individuals. While this

process allows us to specifically analyse the

variables of interest (low friction and balance),

we risked missing the natural or desired

manoeuvres individuals might use if not con-

strained. Specifically, not allowing stepping

would be highly unlikely to occur in a naturalsetting, especially for such a transfer distance.

Our requirement that the maximum box

transfer distance be attempted was also meant

to reduce the variability between individuals.

This requirement meant that individuals at-

tempted lateral load transfer much further than

they would normally. Observed compensations

for this unnatural distance were a wider stance

in the UL-1 group and more trunk flexion in

the SL-1 group. An interesting future study

might use information from this study to

compare with natural performance of the

same task without said restrictions.(2) There are a couple of limitations to using the

study cohort. We have no reason to believe

that our cohort is anthropometrically charac-

teristic of the manual material handling popu-

lation. Application of our results should

consider this. Also, we did not attempt to

control was athletic ability. We had one

individual in our study that was a collegiate

athlete, and several others that would be

considered athletic. These individuals were

able to complete the first slippery trial success-

fully, but since there were very few of them we

could not analyse them in a statistical analysis.Athletes have been previously shown to recover

from imbalance situations better than non-

athletes (Brauer et al. 2008). Increased strength

and coordination of an athlete would imagin-

ably have some beneficial effect on perfor-

mance of the lateral load transfer.

(3) Muscle activity was a key factor in our previous

study of lateral load transfers (Catena et al.

2010). Specifically, flexor/extensor co-contrac-

tions were found to be a factor in performance.

Our current study continued with these mea-

sures but was not able to draw any significant

conclusions from them. Since we did not

anticipate transverse and coronal plane kine-

matics as being important, we made no

measurements of major contributors to move-

ment in these planes. The result was a lack of

information that would possibly be beneficial

to our analysis. Likewise, no measure of

strength was recorded, which would be helpful

in making more concrete conclusions about the

consequences of stance width adjustments.

Conclusions

This study has shown that a slippery surface has

adverse effects on individuals using a wide stance and

constricted movement to perform a lateral load

transfer. Even though the task is mostly side-to-side

movement, the balance control results suggest that loss

of balance will likely occur in the A/P directions. Theoften employed strategy of maintaining balance

through increased joint stiffness was not used

successfully to maintain balance in the slippery

condition, was not increased any further in adaptation

to the slippery condition, and in some cases was even

decreased during the slippery condition. Only stance

kinematic performance was able to distinguish success

in the slippery conditions. Whole body kinematics will

be analysed in future studies to determine the extent

that body kinematics plays in successful completion of

the lateral load transfer over a slippery surface.

Having shown that simple modifications to stance

kinematics can determine balance control over aslippery surface during lateral material handling,

further research strategies could be taught to

individuals having to occasionally perform such an

occupational task. It might also be the case that a

decreased reliance on muscle control might be

beneficial for such a task. However, we caution any

immediate use of the techniques described in this

article without further analysis on whole body joint

kinematics to detect any additional modifications to

the lateral manoeuvre that might be needed

accompaniments.

AcknowledgementsThis research was supported by the Liberty Mutual-Harvard School of Public Health Postdoctoral Program,as a postdoctoral fellowship was awarded to Dr. Catena.The authors also thank Simon Matz for his help withstatistical analyses.

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