Effects of transcranial direct current stimulation on joint flexibility and pain in sedentary male individuals

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SCISPO33471—8

Pleasecitethisarticleinpressas:LinsV,etal.Effectsoftranscranialdirectcurrentstimulationonjointﬂexibilityand

paininsedentarymaleindividuals.Scisports(2019),https://doi.org/10.1016/j.scispo.2019.01.005

Disponibleenlignesur

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ORIGINAL

ARTICLE

Effects

of

transcranial

direct

current

stimulation

on

joint

ﬂexibility

and

pain

in

sedentary

male

individuals

Effets

de

la

stimulation

directe

sur

la

des

articulations

et

la

douleur

chez

des

personnes

masculines

V. Lins

a,∗

,

E. Lattari

a

,

D. Monteiro

b,c

,

L. Cid

b,c

,

Q2

G. Albuquerque

Neto

d

,

S. Machado

a,e a

PhysicalActivityNeuroscienceLaboratory,PhysicalActivitySciencesPost-GraduateProgram(PGCAF), SalgadodeOliveiraUniversity(UNIVERSO),

RJ,Brazil

b

SportScienceSchoolofRioMaior-PolytechnicInstituteof

Portugal

c

ResearchCenterinSport,HealthandHumanDevelopment(CIDESD),Portugal

d

PhysicalActivitySciencesPost-GraduateProgram(PGCAF),SalgadodeOliveiraUniversity(UNIVERSO), RJ,Brazil

e

IntercontinentalNeuroscienceResearchGroup

Q3

Received3April2018;accepted25January2019

KEYWORDS Non-invasivebrain stimulation; Transcranialdirect currentstimulation; Flexibility; Pain;

Primarymotorcortex

Summary The aim of this study was to analyzethe effects of cathodal tDCS (c-tDCS) on

jointﬂexibilityandpainperceptioninasedentarymale.Eightmalehealthy,sedentary

right-leg-dominant and novice in muscle stretching aged between 19 and 30 years (24.0±4.0

years) wererecruited. Subjectsperformed three experimental conditions inarandomized,

double-blindedcrossoverdesign:anodalstimulation(a-tDCS),c-tDCSandsham-tDCS(2mAfor

20minutestargetingthebilaterallymotorcortex).Beforeandimmediatelyafterthe

experi-mentalconditions(baselineand post-condition,respectively),subjectscompleted therange

ofmotion(ROM) ofright hiptest andtheVisualAnalogicScale for Pain (VAS pain; levelof

signiﬁcanceP<0.05).In post-condition, c-tDCS was greater than toa-tDCS (P<0.001), and

sham-tDCS(P<0.001)intherighthipROM.HipROMincreasedinthepost-conditioncompared

tobaselineinthe c-tDCS condition(P<0.001). In thea-tDCS condition, hip ROMdecreased

in the post-condition compared to baseline (P<0.001). For VAS pain, in post-condition,

∗

Correspondingauthor.LaboratoryofPhysicalActivityNeuroscience,PhysicalActivitySciencesPost-graduateProgram,SalgadodeOliveira University,22061-021Niterói,RiodeJaneiro,Brazil.

E-mailaddress:[email protected](V.Lins).

https://doi.org/10.1016/j.scispo.2019.01.005

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SCISPO33471—8

c-tDCS wasless than a-tDCS (P<0.001), andsham-tDCS (P<0.001). Inthe c-tDCS condition,

theVASpaindecreasedinthepost-conditioncomparedtobaseline(P<0.001).Thisstudy

sug-geststhatc-tDCSappliedtoSM1maypromoteincreasedinROMofthehipanddecreasedand

perceptionofpain.

MOTS Stimulationcérébrale noninvasive; Stimulation courantcontinu; Flexibilité; Douleur; Cortexmoteur primaire

Lebutdecetteétude étaitd’analyserleseffetsdu tDCScathodique(c-tDCS) sur

la ﬂexibilité des articulations et la perception de la douleur chez un homme sédentaire.

Huithommesenbonnesanté,sédentaires,dominantsdelajambedroiteetnovicesen

étire-mentmusculaire

de19

30ans (24,0±4,0ans)ontétérecrutés.Lessujetsont réalisé

trois conditions expérimentales dans un plan croisé randomisé

double insu : stimulation

anodale(a-tDCS),c-tDCSetsham-tDCS(2mApendant20minutesenciblantlecortexmoteur

bilatéral). Avant et immédiatement

lesconditions expérimentales (de base et

post-condition,respectivement),lessujetsontcomplétél’amplitudedemouvement(ROM)dutest

dela hanche droiteet l’échelle visuelle-analogiquedela douleur(douleur EVA;niveau de

signiﬁcationp<0,05).Enpost-condition,c-tDCSétaitsupérieur

a-tDCS(p<0,001)et

sham-tDCS (p<0,001)dansla ROMdelahanchedroite.LaROMdelahanche aaugmentédans la

Q4

post-condition par rapportauxvaleurs debasedansla conditionc-tDCS (p<0,001). Dansla

conditiona-tDCS,laROMsoushancheadiminuédanslapost-conditionparrapportauniveau

debase(p<0,001).Pourladouleurliée

l’EVA,enpost-condition,lec-tDCSétaitinférieur

a-tDCS(p<0,001)etausham-tDCS(p<0,001).Danslesconditionsc-tDCS,ladouleurliée

l’EVA

diminuait

laconditionparrapport

lasituationinitiale(p<0,001).Cetteétude

quelec-tDCSappliqué

SM1pourraitfavoriseruneaugmentationdelaROMdelahancheet

unediminutiondelaperceptiondeladouleur.

1. Introduction

Stretching refers to a movement applied to increase the

Q5

range of motion(ROM) of joints, i.e.ﬂexibility, andhave

beenusedbysportscoachesforperformanceenhancement

andinjurypreventionbyregainingjointROM[1].

Tradition-ally,stretching exercises havebeen studied in relationto

injury[2],performance [3], andmuscle soreness [4].The

stretchingisaneffectivemethodtochronicallyincreasethe

joint range of motion [5]. This increase in ROM hasbeen

related to both neural [6,7] and mechanical [8] factors.

Morerecently,asensorytheoryhasbeenproposed

suggest-inginsteadthatincreasesinmuscleextensibilityaredueto

amodiﬁcationofsensationonly[9].

Different forms of stretching exercises are an

effec-tive way of improving ROM in healthy individuals [10,11]

andothertechniqueshavebeenproposedforimprovement

of the ROM as the yoga [12] and transcranial direct

cur-rentstimulation[13].Transcranialdirectcurrentstimulation

(tDCS) consists of a non-invasive electrical stimulus that

canpromoteexcitation,throughtonicdepolarizationofthe

membrane resting potential(anodal stimulus, a-tDCS),or

cortical inhibition, byhyperpolarization ofthe membrane

restingpotential (cathodalstimulus, c-tDCS) [14,15].This

non-invasive neuromodulatorytechnique has been usedas

anergogenicinhealthysubjectstoinducechangesin

phys-ical performance, such as increase in muscular strength

[16,17],andjoint ﬂexibility[18],andreduction inratings

perceivedexertion[16,18],andpain[19].

OnlyonestudyinvestigatedtheeffectsoftDCSonjoint

ﬂexibilityinhealthysubjects[13].Inthisstudy,c-tDCSover

thesensorimotorcortex(SM1),withacurrentof10minutes

andintensityof2.0mA,resultedina10.5%increaseinROM

oftheankleineightmalehealthy[13].Theauthor’s

sugges-tion that theSM1 was involvedin joint ﬂexibilitybecause

thepassivetorquedidnotchangeand thismayhavebeen

affected neuralfactors, suchas perceptionof joint angle

or pain. Corroborating thishypothesis, thec-tDCS applied

over corticalregionsofthepainneuromatrix, astheSM1,

increased pain thresholds in healthy adults [20]. To test

the hypothesisthattheSM1 is involvedin jointﬂexibility,

weinvestigatedwhetherc-tDCSoverSM1bilaterally

modi-ﬁeshipROManddecreasepainperceptioninthesedentary

male.Thus,theaimofthisstudywastoinvestigatewhether

theeffectsofc-tDCSonjointﬂexibilityandpaininsedentary

male,wouldenhancehipROM,anddecreasepainperception

incomparisontoa-tDCSandsham-tDCS.

2. Methods

2.1. Subjects

Eight male healthy, sedentary [21] and novice in

mus-cle stretching, right-leg-dominant, and aged between 19

30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105

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SCISPO33471—8

Q1

and 30 years (24.0±4.0 years) were recruited.

Regard-ing anthropometric measurements, participants averaged

84.1±17.6kg of bodymass and 173.6±4.1cm of height.

Were excluded subjects that had neuropsychiatric,

car-diovascular, or osteoarticular diseases, used any kind of

neuropsychiatricdrugs,andusedanycaffeinatedbeverage

onthedayoftheexperimentoralcoholicbeveragesinthe

daybefore.Thesamplesizewascalculatedusing G*Power

software (version 3.1). For analysis we use the following

commands:Testfamily=F-tests,Statisticaltest=analysisof

variance (Anova): repeated measures between factors, a

errorprobability=0.05,power(1-berrorprobability)=0.80,

andeffect sizewasset withd=0.54[13].This effectsize

was calculated according to the meanand standard

devi-ation dataof c-tDCS and sham-tDCS interventions [13]. A

total of5 participantsin each condition were needed for

thisstudy.Eachparticipantsignedawrittenconsentform,

andtheexperimentwasapprovedbytheinstitutionalethics

committeeoftheSalgadoOliveira University,accordingto

the Norms ofConduct in Human Research (CNSresolution

466/2012).

2.2. Anthropometricmeasurements

Participants’ body mass and height were measured with

a weighingscale and stadiometer (Filizolamodel 31;

Fili-zolaS.A.,

Paulo,Brazil),followingtherecommendations

proposed bythe InternationalSociety for Advancementof

Kinanthropometry[21].

2.3. Applicationoftranscranialdirectcurrent

stimulation(tDCS)

Thesubjectsremainedseatedcomfortablyinachairlocated

within the laboratory. The electric current of 2mA was

appliedusingapairofpadssoakedinsalinesolution(NaCl

140mmol dissolved in Milli-Q water) comprising the two

5×5cm electrodes, connected to a directcurrent

stimu-lationdevice(TCT,China)andpositionedusingelastics.The

stimulationprocedureshadthedurationof20minutes.The

proceduresforplacingtheelectrodesfollowedthe

propos-alsofMizunoetal.[13].Forc-tDSC,thecathodeelectrode

isplacedovertheSM1bilaterallyandanodeelectrodewas

placedovertheoccipitalcortex(OC),bothlocatedon

elec-trodeareaCzandOz,inaccordancewiththeinternational

10—20system EEG [22]. For a-tDCStheanodewas placed

over theSM1 and cathodewas placedover theOC.Inthe

sham-tDCS condition, the electrodes were placed in the

samepositionsofthea-tDCS.However,thestimulator was

turnedoff after30seconds, acting asa placebocondition

[23]. Patients usuallyreport tingling sensations or itching

fromtheinitialelectricalstimulationbutthereisevidence

thattherearenostimulationeffectshasthedeviceisturned

off during the remaining time. This procedure allows the

subjectstobecomeblindedtothetypeofstimulusthatthey

willreceiveduringtheexperiment[24].AlltDCSprocedures

wereconductedbythesameresearchassistant.

2.4. ROMofthehip(hipROM)

Toevaluatetherangeofmotionofthejointhip,anangular

testwasused,asproposedbytheAmericanCollegeofSports

Medicine(ACSM)[21],withtheuseofadigitalgoniometer

(IGAGING

®

,

Paulo,Brazil).The subjectwaspositioned

in dorsal decubitus,with the hip at zerodegrees of

ﬂex-ion,extension, adduction,abduction, and rotation.Aﬁrst

evaluatorkeptthesubject’s contralateral legﬁxedto the

stretcher while passively raised the other to the highest

levelofdiscomfortorpainreportedbythesubject,keeping

theknee extended,andthefoot inaneutralpositionbya

singleattempt.Then,thesecondevaluatorperformedthe

measurementoftheanglereachedbythemaximumROMin

thejoint righthip. The goniometer was positionedas

fol-lows:Fulcrus=majortrochanterofthefemur;Stabilization

branch=lateralmidlineofthepelvis;Mobilebranch=lateral

midline of thefemur, using the lateralepicondyle as the

referencepoint.

2.5. VisualAnalogicScaleforPain(VASpain)

TheVASpainisaunidimensionalmeasure ofpain intensity

valid, reliable and appropriate for use in clinical

prac-tice [25]. For pain intensity, the scale is most commonly

anchoredby‘‘nopain’’(scoreof0)and‘‘painasbad asit

couldbe’’or‘‘worstimaginablepain’’(scoreof10[100-mm

scale])[26].

2.6. Experimentalprocedures

Each participant had 4 visits to the laboratory. On the

ﬁrstvisit, subjectsassigned the consent form, completed

asocio-demographicquestionnaire,andweresubmittedto

anthropometric measurement. The subjects had to

per-form awarm-upon the cycleergometer,with duration of

5minutes, 70rpm and the initial load (kg) was adjusted

according to body weight before performing the ROM of

therighthip test.After warm-up,thesubjectsperformed

theROMoftheright hiptestandduringthemaximumHip

ROM, the pain scale was measured through the VAS Pain

[26].The proceduresof theﬁrstvisitwereused as

famil-iarizationtothelaterexperimental procedures. Following

theinitialvisit,with48to72hoursoftheintervalsbetween

thevisits, subjects attended thelab for the three

exper-imental conditions (c-tDCS, a-tDCS, or sham-tDCS), with

sessionorderrandomlycounterbalancedacrossparticipants.

TherandomizationschemewasgeneratedbyusingtheWeb

siteRandomization.com(http://www.randomization.com).

Beforeandaftertheexperimentalconditions(baseline,and

post-stimulation),subjectsperformedthewarm-up,ROMof

thehip(hipROM)andtheVASpainduringtestperformance

(Fig.1).Allsessionswereperformedintheafternoon(i.e.,

14:00—17:00-hour p.m.) to avoidcircadian effects on the

ﬂexibility.Theambient temperatureranged from21

◦

Cto

23

◦

Candrelativehumidityrangedfrom55to70%.Subjects

werealsoinformedtomaintaintheirregularfoodand

hydra-tiondietbeforeperformingthevisitsandwerediscouraged

toconsume ergogenicbeverageslikecoffee.TheHipROM

andVAS Painwereconductedbytwo researchersandtDCS

wasconductedforanotherresearchassistant.

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Figure1 Experimentaldesign.

2.7. Statisticalanalyses

Atwo-wayanalysisofvariance(Anova)withrepeated

mea-sures with the entrance for condition (c-tDCS, a-tDCS,

and sham) and time (baseline, and post-condition) were

performed for the hip ROM and VAS pain. The

spheric-ity assumption was tested using the Mauchly’s test and

the Greenhouse-Geisser correction was used whenever

data sphericity was violated. Post-hoc comparisons were

performed using the Bonferroni correction. Values were

reportedwithmeanandstandarddeviation.Thelevelof

sig-niﬁcancewassetatP≤0.05. Theanalysiswasundertaken

usingSPSS23,0.

A bivariate Pearson correlation (r) was performed, in

ordertodetermineapossibleassociationbetweenhipROM

andVASpainmeasuresforthec-tDCSanda-tDCSconditions.

This association was determined between

post-condition-minusbaselinescoresinbothmeasures.

Effect size analysis was conducted to report the

mag-nitude of differences between the c-tDCS and a-tDCS

conditions compared to sham for hip ROM and VAS pain.

Effect sizes were computed using the equation proposed

by Morris and De Shon [27], on the G*Power

soft-ware (version 3.1). Effect sizes were classiﬁed as trivial

(d<0.19),small(d=0.20—0.49),moderate(d=0.50—0.79),

large(d=0.80—1.29),andverylarge(d>1.30)[28].Ineach

condition,adescriptiveanalysiswasperformedfor

respon-dersvs.non-responders.Tobeconsideredresponder,each

participantneedtosimplyrespondtothetrainingprotocol

usedinthestudy,andnon-responderistheparticipantthat

simplydonotrespondtothetrainingbutthatdoesn’t

neces-sarilymeantheywouldnotrespondtoanytrainingprogram

[29].Weusechangesfromofthebaselinetopost-condition

ineachsubjectforthehipROMandVASpain.Theabsolute

valueswereexpressedforeachsubject.

3. Results

Two-way repeated measures Anova’s showed signiﬁcant

interactionsbetweenconditionandtime (F(2,14)=140.36;

Q6

P<0.001),and main effect for condition (F(2,14)=60.17;

P<0.001)inhipROM.Therewasnosigniﬁcantmaineffect

fortime (F(1,7)=3.01;P>0.05).Therewereno signiﬁcant

differencesbetweentheconditionsinthebaseline(c-tDCS:

109.0±4.3 ◦ ;a-tDCS:109.8±4.7 ◦ ;sham-tDCS:110.1±4.6 ◦ , P>0.05). In post-condition, c-tDCS (123.9±4.1 ◦ ) was

greater than a-tDCS (100.4±1.2

◦

, P<0.001), and

sham-tDCS (110.7±4.2

◦

, P<0.001). At this time, sham-tDCS

was greater than a-tDCS (P<0.001). There was

base-line to post-condition changes in both c-tDCS and a-tDCS

conditions. The hip ROM increased in the post-condition

compared to baseline in the c-tDCS condition (baseline:

109.0±4.3

◦

,post-condition:123.9±4.1

◦

,P<0.001).Inthe

a-tDCS condition, the hip ROM decreased in the

post-condition compared to baseline (baseline: 109.8±4.7

◦

,

post-condition:100.4±1.2

◦

,P<0.001)(Fig.2).

For VAS pain, two-way repeated measures Anova’s

showed signiﬁcant interactions between condition and

time (F(2,14)=57.49; P<0.001), main effect for

condi-tion (F(2,14)=94.25; P<0.001), and main effect for time

(F(1,7)=56.70; P<0.001). There were no signiﬁcant

dif-ferences between the conditions in the baseline (c-tDCS:

9.5±0.7;a-tDCS:9.6±0.5;sham-tDCS:9.6±0.5,P>0.05)

forpainperception.Inpost-condition,thepainperception

in the c-tDCS (5.1±0.8) was less than a-tDCS (9.5±0.5,

P<0.001), and sham-tDCS (9.6±0.5, P<0.001). In the

c-tDCScondition,theVASpaindecreasedinthepost-condition

compared to baseline (baseline: 9.5±0.7, post-condition:

5.1±0.8

◦

,P<0.001)(Fig.3).

No correlations was showed between

post-condition-minus baseline scores in thehip ROM andVAS pain

mea-suresforthec-tDCS(r=0.02,P>0.05)anda-tDCS(r=−0.12,

P>0.05)conditions.

MeansandstandarddeviationvaluesareshowninTable1.

Effectsizewasverylargeinthec-tDCSconditioncompared

to sham-tDCS(d=2.38) andtrivialinthea-tDCScondition

comparedtosham-tDCS(d=0.07)inhipROM.ForVASpain,

effectsizewasverylargeinthec-tDCSconditioncompared

tosham-tDCS(d=−5.05)andtrivialinthea-tDCScondition

comparedtosham-tDCS(d=0.16)(Table1).

Regarding responders and non-responders was showed

that thec-tDCS conditionprovided an increasein the hip

ROMinallsubjects(range:7.9to23.5

◦

).Thea-tDCS

condi-tionprovided adecreaseinhipROMinallsubjects(range:

−3.2 to −17.5

◦

). In the sham-tDCS condition occurred a

small increase(range: 1to 3

◦

)and decrease(range: −1.7

to−2

◦

)inhipROM(Fig.4).

Respondersandnon-responderswereshowedthatthe

c-tDCSconditionprovidedadecreaseinVASpaininallsubjects

215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300

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Figure2 EffectsoftDCSonHipROM.

*

c-tDCS>a-tDCS(P<0.001),andsham(P≤0.001);

**

sham>a-tDCS(P≤0.001);

#

c-tDCSin thepost-condition>c-tDCSinthebaseline(P<0.001);

##

a-tDCSinthebaseline>a-tDCSinthepost-condition(P≤0.001).

Figure3 EffectsoftDCSonpainperception.

*

c-tDCS>a-tDCS(P<0.001),andsham(p≤0.001);

**

c-tDCSinthepost-condition< c-tDCSinthebaseline(P<0.001).

Table1 DescriptivestatisticsandeffectsizesforROMoftherighthip,andpainperception.

Measures Baseline(M±SD) Post-condition(M±SD) ESvs.sham(classiﬁcation)

ROM(degrees) c-tDCS 109.0±4.3 123.9±4.1 2.38(verylarge) a-tDCS 109.8±4.7 100.4±1.2 0.07(trivial) sham 110.1±4.6 110.7±4.2 VASPain(mm) c-tDCS 9.5±0.7 5.1±0.8 −5.05(verylarge) a-tDCS 9.6±0.5 9.5±0.5 0.16(trivial) sham 9.6±0.5 9.6±0.5

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Figure4 Respondersvs.non-respondersforHipROM.

Figure5 Respondersvs.non-respondersforpainperception.A.U.:ArbitraryUnit.

(range: −3 to −6 A.U.). The a-tDCS condition showed an

increaseintwosubjects(3,and6;1A.U.foreachone), a

decrease inthree subjects(2,4,and 7;−1 A.U. foreach

one),and threeremained unaltered(subjects1,5,and 8)

for VAS pain. For sham-tDCS condition, occurred a small

increaseinthreesubjects(1,3,and7;1A.U.foreachone),

adecreaseinthreesubjects(2,5,and8;−1A.U.foreach

one),andtworemainedunaltered(subjects4and6)forVAS

pain(Fig.5).

4. Discussion

Theaimofthisstudywastoexaminewhethertheeffectsof

tDCSenhancehip ROM,and painperceptionincomparison

toa-tDCSandsham-tDCSinasedentarymale.Accordingto

ourinitialhypothesis,resultsshowedthatc-tDCScondition

improvedhip ROM,and decreasedVASpain incomparison

toa-tDCSandsham-tDCS.Our ﬁndingsshowed thatc-tDCS

overtheSM1bilaterallysigniﬁcantlyincreasedhipROMand

reducedVASpain,whilea-tDCSoverSM1bilaterallyledtoa

decreaseinhipROM,andsham-tDCShadnoeffect.

Justonestudyanalyzedtheeffectsofc-tDCSon

ﬂexibil-ity.Mizunoet al.[13]showed that2mAc-tDCS appliedto

SM110minutes,signiﬁcantlyincreasedankleROM,whereas

a-tDCS and sham-tDCS had no effect. Our results were

similar by Mizuno et al. [13], despite the different joint

assessed. Corroborating our ﬁndings, all subjects

demon-strated an increase in ROM following c-tDCS (our results,

range: 7.1—22.1%; Mizuno et al.’s results [13], range:

1.0—26.2%).

Afterc-tDCSapplication,hipROMwaslikelybecauseof

decreasedpainperception,consideringthattheindividuals

donot stopped hipﬂexion at thesamelevelof perceived

discomfort during thepre- and post-tDCS tests. However,

wedidnotﬁndapositiveassociationbetweentheincrease

in hip ROM and a reduction in pain perception. A

possi-ble explanation is that changes in pain perception could

causeadecreaseinSM1excitabilityafterc-tDCS.ManyfMRI

studieshavesubstantiatedtheinvolvement ofSM1 inpain

perception[30—34].Peyronetal.[32]revealedthat

signif-icant SM1activation afterpainfulstimulationand speciﬁc

nociceptive neurons are known to exist in SM1 [35].

Fur-thermore, Antal et al. [36] showed that c-tDCS over the

SM1decreasedlaser-stimulated subjectivepainperception

ofthehand,whereasanodalandsham-tDCShadnoeffect.

Theseﬁndingssuggestthatc-tDCSovertheSM1decreased

subjective pain perception. Previous results also indicate

thattheincreaseinhip ROMobservedinour studymaybe

basedondecreasedpainperceptionsecondarytodecreased

brainexcitability,whichwascausedbyc-tDCSovertheSM1

[19].

Inaddition,painperceptiondecreasedinallsubjects.In

fact,SM1seemstobeinvolvedwiththeperceptionofpain,

accordingto somestudies[37].Ourﬁndingsare similarto

severalstudiesthatappliedc-tDCStoSM1inhealthyadults,

observing adecrease inpain thresholds[20,33—36,38,39].

fMRIstudieshaveshownthatexperienceofpain coincides

withhyperactivityofSM1[33—36,38,39].Therationalefor

the reduction of pain by c-tDCS over SM1 is corroborated

bysomestudies[40,41].c-tDCSwhenappliedtoM1andS1

inhealthyindividuals[42—45],decreasesbrainexcitability

[46,47]and increasespainthreshold perceptionin healthy

individuals[44,45].

However,new evidence suggeststhat inhibitoryeffects

of c-tDCS may shift to excitatory effects by modulation

of c-tDCS parameters such as current intensity and

dura-tion [48], site of stimulation [49], and repetition [50].

This same argument could be used for a-tDCS and could

explain our result, i.e., no increase in pain perception.

Thus, tDCSintensity doesnot essentiallyincreaseefﬁcacy

of stimulation, however,may alsomodifythe directionof

excitabilityalterations[48—50].This should betaken into

account forapplicationsoftDCSusingdifferentintensities

anddurationsinordertoachievestrongerorlongerlasting

after-effects.

Somemethodologicallimitationswerepresentedinthe

study.First,thenumberofsubjectswassmall,althoughthe

effectsizecalculationwasperformed.Second,itispossible

thatthesiteofthestimulatedareahasbeenlarge,dueto

301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377

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SCISPO33471—8

thesizeoftheelectrode(35cm

2

).Third,theﬂexionofthe

hipwas performedwithkneeextended,limitingour

inter-pretationonbiarticularmusclesinvolvedinthismovement.

5. Conclusion

Thisstudysuggeststhatc-tDCSappliedtoSM1maypromote

increasedinROMofthehipanddecreasedandperceptionof

pain.tDCShasbeensteppingoutofthelaboratoryintothe

communityatlarge,includingthesportsandﬁtnessﬁelds.

Although its effects are variable between individuals and

withinindividuals,itisnotunreasonabletoclaimthattDCS

hasgreatpotentialas‘‘ergogenicresource’’forimproving

humanphysicalperformance, e.g.ﬂexibilityandpain

per-ception.Inanincreasinglysuccess-orientedsocietywithless

effortandimprovedperformance,tDCSseemstobeauseful

toolforuseinsportsandﬁtness,aswellassafewithregard

toitstoleranceandadverseeffects,relativelyinexpensive

andreadilyavailable.

Disclosure

of

interest

Theauthorsdeclarethattheyhavenocompetinginterest.

Uncited

reference

Q7

[51].

Acknowledgments

Serg ioMachadowassupportedbygrantfromCarlosChagas

FoundationfortheResearchSupportintheStateofRiode

Janeiro(FAPERJ),YoungScientistsfromtheStateofRiode

Janeiro2017.WethankDr.EricMurillo-Rodriguezforhishelp

inEnglishrevision.

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