The Influence of Pregnancy on the Location of the Center of Gravity in Standing Position

Abstract

The Influence of Pregnancy on the Location of the Center of Gravity in Standing Position
The purpose of the study was to compare the average location of the center of gravity vertical projection in sagittal plane in women at the beginning of and in advanced pregnancy as well as after delivery. The experiment was performed with the use of a force platform during four test sessions. A group of 44 women (8-16 weeks of pregnancy) participated in the initial test session. In the following sessions the number of the subjects reduced mainly due to medical and childcare problems: 33 women were tested in late pregnancy (2-3 weeks before delivery), and 39 women were tested two and six months after delivery.
The results showed the statisticaly significant (p<0,05) posterior displacement of the projection of the center of gravity of the lenth of approximately 4 mm in late pregnancy comparing to the beginning of pregnancy. The displacement may be the result of the body's adaptation to the increased mass in the anterior trunk area in late pregnancy. No discrepancy was found when comparing the average center of gravity location in the early pregnancy and after delivery.
We concluded that the change of the center of gravity location in late pregnancy is temporary and two months after delivery the vertical projection of the center of gravity is located as it was at the beginning of pregnancy.

Full text


Authorssubmittedtheircontributionofthearticletotheeditorialboard.
AcceptedforpintinginJournalofHumanKineticsvol.26/2010onSeptember2010.
JournalofHumanKineticsvolume262010,5‐11
SectionI‐Kinesiology
5
Kinesiology

TheInfluenceofPregnancyontheLocation
oftheCenterofGravityinStandingPosition
by
AgnieszkaOpala‐Berdzik1,BogdanBacik2,JoannaCieślińska‐Świder¹,
MichałPlewa¹,MonikaGajewska¹
The purpose of the study was to compare the average location of the center of gravity vertical projection in
sagittal plane in women at the beginning of and in advanced pregnancy as well as after delivery. The ex-
periment was performed with the use of a force platform during four test sessions. A group of 44 women (8-
16 weeks of pregnancy) participated in the initial test session. In the following sessions the number of the
subjects reduced mainly due to medical and childcare problems: 33 women were tested in late pregnancy
(2-3 weeks before delivery), and 39 women were tested two and six months after delivery.
The results showed the statisticaly significant (p<0,05) posterior displacement of the projection of the cen-
ter of gravity of the lenth of approximately 4 mm in late pregnancy comparing to the beginning of pregnancy.
The displacement may be the result of the body’s adaptation to the increased mass in the anterior trunk area
in late pregnancy. No discrepancy was found when comparing the average center of gravity location in the
early pregnancy and after delivery.
We concluded that the change of the center of gravity location in late pregnancy is temporary and two
months after delivery the vertical projection of the center of gravity is located as it was at the beginning of
pregnancy.
Key words: pregnancy, center of gravity, standing



1 - Academy of Physical Education, Faculty of Physiotherapy, Katowice, Poland
2 - Academy of Physical Education, Faculty of Physical Education, Katowice, Poland
Introduction
Theweightgainduringpregnancyisbetween9
and14kg.Consideringsegmentbodymassthis
weightgainisuniquebecauseitismainlylocatedin
thetrunk.Themeanrateofincreaseforthelower
trunkmassis0.29kgperweek(Jensenetal.1996).
Asaresultoftheweightgainintheanteriortrunk
areatheabdominalmusclesbecomeoverstretched.
Duetothestructuraladaptationsthefunctionofthe
abdominalmusclesisaffectedandtheybecomein‐
sufficient(Fastetal.1990,GilleardandBrown1996).
Theincreaseofthebodyweightandtheinsuffi‐
ciencyofabdominalmusclestogetherwiththein‐
creasedligamentslaxityandjointsmobilityobserved
asearlyasinthesecondtrimesterofpregnancy
(DumasandReid1997,Marnachetal.2003)mayall
leadtoadaptationalposturechanges.Theincreased
lumbarlordosis(FranklinandConner‐Kerr1998,
Otmanetal.1989),increasedcervicallordosis,pro‐
tractionoftheshouldergirdle,hyperextensionofthe
knees(GleesonandPauls1988,Konkler1990)and
increasedextensionoftheanklejoints(Friesand
Hellebrandt1943)arethemostfrequentlymentioned
posturechangesinpregnancy.
Becausethegreatestincreaseofthemassisinthe
anteriortrunkareaitappearsthatthepostureadap‐
tationsmustalsooccurinpregnantwomento
maintainposturalstabilitywhilestanding.Consid‐

6 TheInfluenceofPregnancyontheLocationoftheCenterofGravityinStandingPosition

JournalofHumanKineticsvolume262010, http://www.johk.awf.katowice.pl

Kinesiology

eringthesebodyadaptationsthelocationofthe
centerofgravity(CoG)maychangeaspregnancy
progresses.
AccordingtoFriesandHellebrandt(1943)and
Konkler(1990)thereisanincreaseintheheightof
theCoGattheendoftermcomparingtothefirst
trimesterofpregnancy.ItismentionedbyKonkler
(1990)andNobel(1995)thatthebody’sCoGmoves
forwardinsagittalplaneduetotheincreasein
weightofthepregnantuterusandtheweightshifts
towardtheheelstobringtheCoGtoamoreposte‐
riorposition.Thisopinionappearstobeunclearand
insufficientandmayleadtomisunderstandingofthe
problem.Theresultsofthestudyofthefootpressure
inthestaticmeasurementinpregnantwomen
showedsignificantlylowermaximalforefootpres‐
suresandhigherhindfootpressurescomparingto
theresultsofthenonpregnantwomen(Nyskaetal.
1997).FriesandHellebrandt(1943)intheirstudy
performedonasingleindividualinstandingposi‐
tionrecordedtheposteriordisplacementofthe
gravitationalcenterinthesagittalplaneduringthe
thirdtrimesterofpregnancy.Thisstudywasthe
onlyonefoundintheliteratureregardingtheloca‐
tionoftheCoGinpregnancyduringstance.Fotiet
al.(2000)intheirworkanalyzedgaitparameters
thrughoutpregnancyandobservedthechangesof
centerofmasslocationduringambulation.
Concerningtheinsufficientdatafrompublished
worksthepurposeofourstudyistocomparethe
CoGlocationinthesagittalplaneinthegroupof
womentestedatthebeginningofaswellasinad‐
vancedpregnancyandthantwoandsixmonthsaf‐
terdelivery.Ourpresumptionisthatthelocationof
theCoGmaychangeinlatepregnancy.Thechanged
postureinpregnancywhichoftenmaintainsas
learnedposturepostpartum(Konkler1990)maystill
havesomeinfluenceontheCoGlocationtwo
monthsafterdelivery.Weassumetheresultsre‐
cordedsixmonthsafterbirthshouldbesimilarto
thoseobservedatthebeginningofpregnancy.We
alsoassumethatthenumberofpregnancies(primi‐
gravidaandmultigravida)oratypeofdelivery
(naturalandC‐section)donotinfluencetheaverage
locationofverticalprojectionoftheCoGinwomen.
MaterialandMethods
Fiftyfivehealthypregnantwomen,allsingleton
gestationwereassignedtothestudy.Informedcon‐
sentwasgivenbyallsubjectsandthestudywas
acceptedbytheSenateEthicsCommitteeoftheKa‐
towiceAcademyofPhysicalEducation.Thewomen
weretested4times:atthebeginningofpregnancy
(8‐16weeks),attheendofpregnancy(2‐3weeks
beforedelivery),twomonthsandsixmonthsafter
delivery.Eightsubjectswithdrewafterfirsttestses‐
sionduetodisintrestandtheirdataandresultswere
notanalyzed.Threeotherparticipantswereex‐
cludedfromthestudyduetooverweightandobe‐
sity(BMI28.6,30.0and31.6kg/m²)recordedduring
Table1
Descriptionofthegroupofwomenoverthefourtestsessions:atthebeginningofpregnancy(1),attheendofpregnancy
(2),twomonthsafterdelivery(3),sixmonthsafterdelivery(4)
 TESTSESSION
PARAMETER^1(N=44)2(N=33)3(N=39)4(N=39)
Age[years]27.87±3.5628.19±3.6128.20±3.4527.85±3.58
Bodymass[kg]60.42±9.1972.64*±10.0462.65*±9.6060.82±9.13
Height[cm]165.69±5.80165.55±5.79165.60±5.84165.67±5.46
BMI[kg/m²]21.94±2.5726.42*±2.6922.75*±2.6422.11±2.72
^ - data given as mean and ± SD.*p<0.001 signifficance of differences between sessions 2,3,4 and session 1
P P
K K
S S
L L
B B
-100
-50
0
50
100
150
200
250
300
350
CoP
Figure1
The average location of the CoG vertical projection
within the base of support (force platform)
B–anteriorborderoftheplatform,L–linemarkedon
theplatforminfrontalplane–anteriorbordersofthefeet
(tipsofthetoes),P–posteriorbordersofthefeet(heels),
K–verticalprojectionoflateralmalleolus,S–CoP=the
averagelocationoftheCoGverticalprojectioninsagittal
p
lane

byAOpala‐Berdziketal.7

©EditorialCommitteeofJournalofHumanKinetics

Kinesiology

firstsession,becauseaccordingtoBłaszczyketal.
(2009)posturalcharacteristicsofoverweightand
obesewomenisdifferentfromthewomenwith
normalweight.Thereforethenumberofthesubjects
wasreducedto44.Finalyinthe1sttestsessionthere
were35primigravidaaged19‐35(mean±SD:
27.16±3.14)yearsand9multigravidaaged26‐38
(mean±SD:30.67±3.94)yearsparticipatinginthe
study.Someofthe44womenwerenotabletopar‐
ticipateinthe2nd,3rdor4thtestsessiondueto
medicalproblemsduringlatepregnancy,transpor‐
tationorcaregivingproblemswiththenewborn
child.Atthetimeofthe2ndsession33subjectswere
testedand39at3rdand4thsessions.Inthesessions
afterdeliverytherewere27womenafternatural
birthand12womenaftertheC‐section.Thedescrip‐
tionofthestadypatricipantsisshownintable1.
Accordingtotheliteraturetheprojectionofthe
CoGinnormalhealthyindividualsislocatedap‐
proximately4‐5cminfrontoftheimaginaryline
connectinglateralmalleolesofbothanklejoints
(Hellebrandtetal.1940).Basedontherecordedforce
ofthefeetpressureandtheirmomentumwhile
standingstillontheforceplatformthecomputer
calculatesthelocationofthecenteroffeetpressure
(CoP).TheCoPinthestaticmeasurementislocated
inthepointoftheaveragelocationoftheCoGverti‐
calprojectionwithinthebaseofsupport.
TorecordthelocationoftheCoPineachtestses‐
sionthewomenwereinstructedtostandwithboth
feetontheforceplatform(Kistler9281C),placingthe
tipsoftheirtoesdirectlybehindalinemarkedin
frontalplaneontheplatform,andtheycouldselect
theirpreferredstancewidth.Thewomenwerein‐
structedtostandquietlywitharmsattheirsides
lookingatthewallateyelevel.Eachtestsessionwas
conductedfor30s.Theforceplatformdatawere
filteredandtransmittedthroughAC/DCconverterto
thecomputer.Thecalculationofthelocationofthe
CoPinthestaticstandingpositionwasperformed
withtheuseofthemodifiedversionofthecomputer
program“Platforma2”.Inordertocalculateavalue
ofthedistancebetweentheCoPandtheaxisofrota‐
tionintheanklejoints(lateralmalleolus)thesub‐
jects’feetlengths[mm]andthedistancesbetween
thecentersoftheheelsandtheprojectionsofthe
lateralmalleoles[mm]weremeasuredinsagittal
plane(figure1).
AtthebeginningofstatisticalanalysistheWilk‐
Shapirotestwasusedtoanalysethedatadistribu‐
tionanditwasinaccordancewiththenormaldistri‐
bution.Thesignificanceofdifferencesbetween
primigravidaandmultigravidagrupswerecalcu‐
latedusinganalysisofvariance(ANOVA).Thedif‐
ferencesbetweentheinitialsessionmeasurements
(beginningofpregnancy)andfurthersessionsmeas‐
urements(latepregnancy,twoandsixmonthspost‐
birth)wereanalysedwiththeuseoftheStudentt‐
testforthecorrelateddata.Becausethistestcanbe
onlyusedtocomparetheequalnumberofthe
populations,thestatisticalanalysisconcernedonly
thosewomenwhoparticipatedinbothtestsessions:
1and2,1and3,1and4.Thestatisticalanalysisalso
includedthecalculationofthesignificanceofdiffer‐
encesbetweensubjectsafternaturalbirthandtheC‐
sectionusinganalysisofvariance(ANOVA).The
acceptedplevelwas<0.05.Allstatisticalanalyses
wereperformedusingtheStatistica6.0software
(StatSoftInc.,USA)andMicrosoftExcel2002.
Table2
TheaveragelocationofCoGprojection[mm]inprimigravidaandmultigravidawomenatthebeginningofpregnancy
(1)andsixmonthsafterdelivery(4).ANOVA.Statistica6.0.
PRIMIGRAVIDAMULTIGRAVIDA
SESSION
NMEANSDNMEANSDFP
135 47.413.08955.612.852.88NS
431 46.515.88855.516.711.91NS
NS–nonstatisticallysignificantatp>0.05
Table3
TheaveragelocationoftheCoGprojection[mm]atthebeginningofpregnancy(1)andattheendofpregnancy
(2),Studentt‐test.
SESSIONMEANSDNDIFFERENCEP
158.012.6733 
253.913.07334.1S
S–statisticallysignificantatp<0.05

8 TheInfluenceofPregnancyontheLocationoftheCenterofGravityinStandingPosition

JournalofHumanKineticsvolume262010, http://www.johk.awf.katowice.pl

Kinesiology

Results
Thecomparisonoftheprimigravidaandmulti‐
gravidasubjectsresultsatthebeginningofpreg‐
nancyandsixmonthsafterdeliveryusingANOVA
didnotshowanydiscrepancies(p>0.05)inaverage
locationoftheCoGprojectionwithinthebaseof
supportinsagittalplanesothewholegroup’sresults
wereconsideredinfurtheranalyses(table2).
Thedataanalysisshowedthesignificantposte‐
riordisplacementoftheprojectionoftheCoGwithin
thebaseofsupportinlatepregnancy(p<0.05)com‐
paringtothebeginningofpregnancy(table3).The
differenceoftheCoGlocationbetweenearlyandlate
pregnancyalthoughstatisticallysignificant,amounts
onlytothelengthofapproximately4mm.Compar‐
ingthelocationoftheCoGverticalprojectionin
sagittalplanebeforeandafterbirth,nodiscrepancies
werefoundintheresultstwoandsixmonthsafter
deliveryincomparisontothebeginningofpreg‐
nancy.(table4and5).Onlythevaluesofthedis‐
tancebetweentheCoPandtheankleaxisofrotation
intheadvancedpregnancywereshortercomparing
tothreeothertestsessions.Theresultsindicatethat
twomonthsafterdeliverytheCoGislocatedagain
asitwasatthebeginningofpregnancy.Therewas
nodifferencesfoundinthelocationoftheCoGpro‐
jectionwithinthebaseofsupportbetweenthere‐
sultsofthewomenafternaturaldeliveryandtheC‐
sectiontwoandsixmonthspostbirth(table6).
Discussion
Thisstudyappearstobethefirsttoanalysethe
changesoftheaveragelocationoftheCoGprojec‐
tionwithinthebaseofsupportinthestaticstanding
positioninagroupofwomentestedatthebeginning
ofpregnancy,inlatepregnancyandafterdelivery
withtheuseoftheforceplatform.Thesignificant
posteriordisplacementoftheCoGinadvanced
pregnancy(p<0.05)comparingtoearlypregnancy
foundinourstudy,indicatesitisacommonphe‐
nomenon.FriesandHellebrandt(1943)presenteda
studyofasingleindividualoverninesessions(3
monthsofpregnancyto6weekspostpartum).In
ordertorecordthelocationoftheCoGtheyuseda
kymogram(aninstrumentthatrecordedmovement
bystylusandrotatingdrum).Aplanimetricaverage
oftheCoGshiftingforthesinglestanceperiodwas
calculatedandprojectedintothefootprints.Experi‐
mentallydeterminedgravitylineswereerectedinto
Table4
TheaveragelocationofCoGprojection[mm]atthebeginningofpregnancy(1)andtwomonthsafterdelivery(3),Student
t‐test.
SESSIONMEANSDNDIFFERENCEP
159.613.9339 
359.814.1339‐0.2NS
NS–nonstatisticallysignificantatp>0.05

Table5
TheaveragelocationofCoGprojection[mm]atthebeginningofpregnancy(1)andsixmonthsafterdelivery(4),
Studentt‐test.
SESSIONMEANSDNDIFFERENCEP
159.113.5339 
458.516.42390.6NS
NS–nonstatisticallysignificantatp>0.05

Table6
TheaveragelocationofCoGprojection[mm]afternaturalbirthandtheC‐sectiontwo(session3)andsix(session4)
monthsafterdelivery.ANOVA.Statistica6.0.
NATURALDELIVERYC‐SECTION
SESSION
NMEANSDNMEANSDFP
32760.213.471258.916.120.06NS
42757.615.991260.718.100.27NS
NS–nonstatisticallysignificantatp>0.05

byAOpala‐Berdziketal.9

©EditorialCommitteeofJournalofHumanKinetics

Kinesiology

eachphotographtakenevery15s.Theirworkas
wellastheresultsofourstudyindicatetheposterior
displacementoftheCoGprojectiononthebaseof
supportinwomeninthethirdtrimesterofpreg‐
nancy.AlthoughNyskaetal.(1997)didnotanalyse
thelocationoftheCoG,theyfocusedonthemeas‐
urementofthefootpressureinthestaticstancein
full‐termpregnantwomenandfoundsignificantly
lowermaximalforefootpressuresandhigherhind‐
footpressurescomparingtothenonpregnantcontrol
group’sresults.Itmaybeconcludedthattheirfind‐
ingsarealsoinaccordancewithourresultsregard‐
ingtheposteriordisplacementoftheCoGinthe
thirdtrimesterofpregnancy.
TheobservedadaptationalchangeoftheCoGlo‐
cationaspregnancyprogressesmayberelatedto
posturalstabilityandenergyexpenditureinstand‐
ingposition.Atypicalcompensatorymechanismto
improveposturalstabilityandreducetheriskof
fallingisananteriordisplacementoftheCoGob‐
servedinelderlyindividuals(Woodhull‐McNeal
1992)andpatientswithbalanceimpairements
(Błaszczyketal.2007).ThischangeoftheCoGloca‐
tionmaybenecessaryforanindividualtousethe
“step‐initiatingstrategy”inordertoregainstability.
Thereforeweassumetheoppositephenomenonob‐
servedinourexperimentisnotrelatedtocompen‐
satoryimprovementofposturalstability.The
mechanismoftheposteriordisplacementoftheCoG
attheendofpregnancymayberathertheresultof
themassincreaseespeciallyintheanteriortrunk
area.Thismechanismmaybenecessarilytodecrease
thebody’sgravityforcemomentumintherelationto
theanklejointsaxisofrotation.Theenergyexpen‐
diturerequiredforthemaintenanceofthestanding
positionmaybedecreasedduetotheequalisationof
thegravityforcemomentumbythemomentumof
thefeetplantarflexors(calfmuscles)force.Thesig‐
nificantincreaseoftheBMIfrom21.9kg/m²inearly
pregnancyto26.4kg/m²inadvancedpregnancymay
leadtotheadaptationalposteriordisplacementof
theCoG.Ontheotherhandalthoughthedifference
betweenBMI=21.9kg/m²atthebeginningofpreg‐
nancyandBMI=22.7kg/m²twomonthsafterdeliv‐
erywasalsostatisticallysignificant,theCoGlocation
wasnotchanged.Thereforeitseamstheremaybea
bordervalueoftheproportionsofthebodymass
distributionthatleedstoadaptationalchangesofthe
CoGlocation(table1).
Apsychologicalfactorconcerningtheprotection
ofthefetusagainstariskofinjuryincaseoffore‐
wardfallingmayalsoberelatedtothesignificant
posteriordisplacementoftheCoGinlatepregnancy.
Theresultsofthepresentstudyindicatethe
changeofthewholebody’slocationintherelationto
thebaseofsupportonlyinlatepregnancy.Two
monthsafterdeliverytheCoGisbackattheinitial
location(beginningofpregnancy).Itisinaccordance
withtheresultsofthestudyofFriesandHellebrandt
(1943)whoreportedinasinglewoman6weeks
postpartumthereturnoftheCoGlocationtothe
earlypregnancystate.Itmeansthatthechanged
posturewhichusuallydoesnotcorrectspontane‐
ouslyandmaintainsaslearnedposturepostpartum
(Konkler1990)hasnoinfluenceonthewholebody’s
locationintherelationtothebaseofsupportafter
delivery.Thereforeitisnotsurprisingthatthere
werenosignificantchangesintheCoGlocationsix
monthsafterchildbirth.
Theweaknessofthepresentedstudymaybea
singlemeasurementoftheCoPduring30sstanding
positiontrialsontheforceplatformuponeachtest
session.Someauthorsrecomendtheeveragingofthe
repeatedmeasurementsinordertoachivethemost
reliableresults(CollinsandDeLuca1993).Inthe
studyofbalanceduringpregnancyJangetal.(2008)
calculatedtheaverageoften30strialsperformedby
thewomemontheforceplatformforeachsession.
Butleratal.(2006)computedtheaverageofthree30s
trialsintheirworkwiththeuseoftheforceplatform
concerningposturalequilibriumduringpregnancy.
OntheotherhandCollinsandDeLuca(1993)sugest
itmaysometimesbedifficulttoperformadditional
repetitionsofthetrialsinclinicalorscientificinves‐
tigationinposturographyduetotheundesiredrisk
offatigueofthepatient.Theystateitmaybenesse‐
serlytoacceptsometrade‐offbetweenreliabilityand
experimentalpracticality.InthestudyofNagaietal.
(2009)concerningthecharacteristicsofthestanding
posturecontrolduringpregnancyitisnotmen‐
tionedwhetherthemeasurementsofthetrialsper‐
formedontheforceplatformwererepeated.The
authorsoftheaboveworksinvestigatedtheCoP
displacementsintheaspectofposturalswayandnot
theaverageCoPlocationduringstandingposition.
OnlyFriesandHellebrandt(1943)measuredthe
averagelocationoftheCoGprojectionwithinthe
baseofsupportduringpregnancyandpostpartum
inasinglewoman.Theauthorsdidnotdescribethe
measuringdeviceintheirstudy.Webelivethereli‐
abilityofourstudywithasinglemeasurementdur‐
ing30strialsishighercomparingtothestudyper‐

10 TheInfluenceofPregnancyontheLocationoftheCenterofGravityinStandingPosition

JournalofHumanKineticsvolume262010, http://www.johk.awf.katowice.pl

Kinesiology

formedover65yearsagoduetothebiggernumber
ofparticipants.
Conclusions
Thereisaposteriordisplacementoftheaverage
locationoftheCoGverticalprojectionwithinabase
ofsupportinasagittalplaneinlatepregnancy.The
adaptationalchangeoftheCoGlocationinthead‐
vancedpregnancyistemporary.Twomonthsafter
deliverythelocationofCoGisasitwasintheearly
pregnancyanditisthesameinwomenafternatural
childbirthandtheC‐section.
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©EditorialCommitteeofJournalofHumanKinetics
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Kinesiology
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OtmanAS,BeksacMS,BagozeOTheimportanceof’lumbarlordosismeasurementdevice’applicationduring
pregnancy,andpost‐partumisometricexercise.EurJObstetGynecolReprodBiol.1989,31(2):155‐62.
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Acknowledgements
ThisprojectwassupportedbyaPolishMinistryofScienceandHigherEducationgrant2P05D05227
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
Correspondingauthor
AgnieszkaOpala‐Berdzik,
AcademyofPhysicalEducation,
FacultyofPhysiotherapy,Katowice,Poland
ul.Mikołowska72B,40‐065Katowice
Phone:+48322075100
Fax:+48322075200
E‐mail:aga.berdzik@wp.pl
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