Three-dimensional morphology and strain of the Achilles free tendon immediately following ...

24 Pages·2015·1.01 MB·English

by  Steven Obst

©2015.PublishedbyTheCompanyofBiologistsLtd|JournalofExperimentalBiology(2015)218,3894-3900doi:10.1242/jeb.127175
RESEARCHARTICLE
Three-dimensional morphology and strain of the human Achilles
free tendon immediately following eccentric heel drop exercise
StevenJ.Obst*,RichardNewsham-WestandRodS.Barrett
ABSTRACT short-termchangesintendontransversedimensionscouldhaveon
thelocalmechanicalandbiologicalenvironment (Heinemeierand
OurunderstandingoftheimmediateeffectsofexerciseonAchillesfree
Kjaer,2011;Shimetal.,2014;Smithetal.,2013),thereisaneedto
tendontransversemorphologyislimitedtosinglesitemeasurements
better understand the acute effects of exercise on the tendon 3D
acquiredatrestusing2Dultrasound.Thepurposeofthisstudywasto
morphologyandstrain.
provideadetailed3DdescriptionofchangesinAchillesfreetendon
There is evidence from 2D ultrasound (2DUS)-based
morphologyimmediately following a singleclinicalbout ofexercise.
measurements that resting Achilles free tendon antero-posterior
Freehand3DultrasoundwasusedtomeasureAchillesfreetendon
(AP)diameterisreducedforupto2.5hafterdynamicexerciseand
length, and regional cross-sectional area (CSA), medio-lateral (ML)
thiseffectismorepronouncedfollowingeccentriccomparedwith
diameterandantero-posterior(AP)diameterinhealthyyoungadults
concentric exercise (Grigg et al., 2009), and in healthy compared
(N=14)atrestandduringisometricmusclecontraction,immediately
with injured tendons (Grigg et al., 2012). These short-term
beforeandafter3×15eccentricheeldrops.Post-exercisereductionsin
reductions in AP diameter after exercise have been suggested to
transverse strain were limited to CSA and AP diameter in the mid-
reflect fluid exudation from the tendon core to the peri-tendinous
proximalregionoftheAchillesfreetendonduringmusclecontraction.
space as a result of the creation of positive hydrostatic pressure
ThechangeinCSAstrainduringmusclecontractionwassignificantly
correlated to the change in longitudinal strain (r=−0.72) and the withinthetendonwhentendonfibresstretchandpackundertensile
load (Grigg et al., 2009; Hannafin and Arnoczky, 1994). While
changein AP diameter strain(r=0.64). Overallfindingssuggest the
invitrostudiesdemonstrateload-dependentchangesinfluidcontent
Achillesfreetendonexperiencesacomplexchangein3Dmorphology
and tendon dimensions in response to repeated loading (Hannafin
followingeccentricheeldropexercisethatmanifestsundercontractile
and Arnoczky, 1994; Helmer et al., 2006; Wellen et al., 2005),
butnotrestconditions,ismostpronouncedinthemid-proximaltendon
invivostudiesofthehumanAchillestendonreportmixedfindings
andisprimarilydrivenbychangesinAPdiameterstrainandnotML
withrespecttoexercise-inducedchangesintendonvolume(Pingel
diameterstrain.
etal.,2013a,b;Shalabietal.,2004;Syhaetal.,2013)andcross-
KEYWORDS:3Dultrasound,Cross-sectionalarea,Diameter,Creep sectionalarea(CSA)(Burgessetal.,2009;Farrisetal.,2012;Neves
etal.,2014;Ooietal.,2015);discrepanciesthatmayreflectdifferent
INTRODUCTION exercise interventions, populations or imaging methods used to
Repeatedexposuretohightensilestrainsduringexercisehasbeen assess tendon morphology. It is also currently unclear whether
showntoinduceimmediatechangesinAchillestendonmechanical changesintransversemorphologyandstrainfollowingexerciseare
andmorphologicalpropertiesthatcouldimpactontendonfunction uniformly distributed acrossthe length of the tendon, occuralong
and injury risk, and be important for long-term tendon adaptation themedio-lateral(ML)diameter,relatedtochangesinlongitudinal
(Obstetal.,2013).Themechanicalandmorphologicalpropertiesof deformation,oraremorepronouncedundertensileload.
the distal Achilles tendon (i.e. Achilles free tendon) appear Thepurposeofthisstudywastoexaminetheimmediateeffectof
particularlysusceptible to change following acute exercise (Grigg asingleboutofeccentricheeldropexerciseonthe3Dmorphology
et al., 2009, 2012; Lichtwark et al., 2013; Obst et al., 2015). of the Achilles free tendon at rest, and during a submaximal
Increased strain of the Achilles free tendon measured at the same isometric plantarflexion contraction, using a 3D ultrasound
external load after exercise has been demonstrated following (3DUS)-based method (Obst et al., 2014b). We selected the
y
running(Lichtwarketal.,2013)andrepeatedeccentricheeldrops eccentric heel drop as it remains an integral part of Achilles g
o
(Obstetal.,2015),andissuggestiveofearlystagetendonfatigue tendon rehabilitation (Beyer et al., 2015; Habets and van Cingel,
l
o
due to mechanical creep (Fung et al., 2010). Because increased 2015; Kjaer and Heinemeier, 2014), induces high tendon strains i
tendon longitudinal strain during tensile loading is closely (∼8.5%;Jeongetal.,2014)andhasbeenshowntoaffectimmediate B
l
associated with changes in tendon transverse morphology and changes in mechanical (Obst et al., 2015) and 2D morphological a
t
strain(Obstetal.,2014b;Pokhaietal.,2009;ReevesandCooper, (Griggetal.,2009)propertiesoftheAchillesfreetendoninhealthy n
e
2014;Vergarietal.,2011),asimilarrelationshipmightbeexpected adults. We expected that transverse morphology and strain (CSA, m
for the changes in tendon morphology and strain that occur in APdiameterandMLdiameter)oftheAchillesfreetendonwouldbe i
r
response to an exercise bout. In view of the high rate of exercise- altered immediately following exercise and correlated to the e
p
related injuries of the Achilles free tendon and the potential role correspondingchangeinlongitudinalstrain. x
E
f
o
SchoolofAlliedHealthSciencesandCentreforMusculoskeletalResearch, MATERIALSANDMETHODS
MenziesHealthInstituteQueensland,GriffithUniversity,Australia. Participants al
n
*Authorforcorrespondence([email protected]) Fourteenrecreationallyactiveparticipants(8males,6females;age28.1± r
u
4.1years, height 172.8±9.0cm and body mass 70.0±15.4kg) with no
o
Received23June2015;Accepted10October2015 historyoflowerlimborAchillestendoninjury/surgeryparticipatedinthe J
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RESEARCHARTICLE JournalofExperimentalBiology(2015)218,3894-3900doi:10.1242/jeb.127175
study. All participants provided written informed consent prior to transducer determined from four reflective markers rigidly affixed to the
participation. The study was conducted according to the Declaration of probe (see Obst et al., 2014a). Prior to data acquisition, the relationship
Helsinki and was approved by the institutional human research ethics betweentheimagecoordinatesystemandthemarkercoordinatesystemwas
committee. determinedusingasinglewallphantomcalibrationprocedure(Prageretal.,
1998).Followingcalibration,the3Dcoordinatesofapixelwithina2DUS
Experimentaldesignandeccentricexerciseprotocol image were known with an approximate error of ±0.5mm. All 3DUS
Participants were asked to avoid strenuousphysical activity 24h prior to calibration,acquisitionandimagesegmentationprocedureswereperformed
testing. Testing commenced with maximal and submaximal isometric usingtheStradwinsoftwarepackage(Stradwin4.71,CambridgeUniversity,
plantarflexion contractions (pre-exercise) during which freehand 3DUS UK; http://mi.eng.cam.ac.uk/∼rwp/stradwin/). Previous work has
scanswereperformedformeasurementofAchillesfreetendonparameters. established the accuracy and repeatability of freehand 3DUS for in vivo
Following this, participants completed 3×15 (2min rest between sets) measurement of human muscle morphology (length and volume) at rest
eccentric heel drop exercises at a constant speed of 3s guided by a (Barberetal.,2009)andAchillesfreetendonmorphology(length,volume,
metronome (60beatsmin−1) that equates to an average ankle angular CSA, AP diameter and ML diameter) under passive and active loading
velocityof∼20–25degs−1(Henriksenetal.,2009).Theoppositelegwas conditions(Lichtwarketal.,2013;Obstetal.,2014a,b,2015).
usedtoreturntothestartposition,ensuringthateccentric-onlycontractions All ultrasound acquisition and analysis were performed by one
oftheplantarflexorswereperformed.Verbalinstructionwasprovidedto investigator(S.J.O.)usinga58mmlineartransducerwith10MHzcentral
encouragefullanklerangeofmotionduringeachexerciseandtoensurethat frequency (L14-5W/60 linear, Ultrasonix) and standard image settings
thekneeremainedinfullextension.Allpost-exercisetendonassessments (framerate60Hz,depth40mm,gain50%,dynamicrange65dB,map4
wereperformedwithin2minoftheexercisebout(post-exercise). and power 0). For each ankle condition (i.e. rest and 70% MVIC), two
transversescanswereperformedovertheposteriorlegextendingproximally
Datacollectionandanalysisprocedures fromthedistalcalcaneustobeyondtheSOLmuscle–tendonjunction.To
Torqueandmuscleactivation enhancevisualisationoftheAchillestendoncross-section,a1.5cmthick
Anklejointtorque,muscleactivationandtendonmeasurementsweremade acoustic standoff pad (Ultra/Phonic Focus, Pharmaceutical Innovations
withparticipantspositionedpronewiththeirleftkneefullyextended(0deg Inc.,Newark,NJ,USA)washousedwithinathermoplasticcaseaffixedto
flexion)andleftankleplantarflexedto15deg.Thefootandanklewere theendofthetransducer.Totalscantimeforeachtrialwas8–12s(average
firmly secured to a foot plate that housed a torque transducer (TFF600, scanspeedof10mms−1)withanapproximatebetween-frameintervalof
Futek,Irvine,CA,USA).Thepositionofthefootandanklewascheckedto less than 0.2mm. Achilles free tendon length determined using a
ensurethatthetalocruraljointaxisofrotation(i.e.linebisectingthemedial combination of sagittal, transverse and frontal image re-slices and was
andlateralmalleoli)alignedwiththetorquetransduceraxisofrotation.In definedastheshortestdistancebetweentwo3Dpointscorrespondingtothe
thispositiontherestingnetanklejointtorquewasnegligible(meanankle mostdistaledgeofthecalcanealnotchandtheSOLmuscle–tendonjunction
torque<1Nm)andthusmeasurementsofrestingtendonmorphologywere (MTJ) (Obst et al., 2015, 2014b). Tendon cross-sections were manually
made at approximately zero stress (De Monte et al., 2006). Participants segmented between these two landmarks at ∼5–10mm intervals and
initiallycompletedfivemaximalvoluntaryisometriccontractions(MVICs) interpolatedusingashape-basedinterpolationmethodtocreatea3Dsurface
of ankle plantarflexion to pre-condition the Achilles tendon (Maganaris, representationoftheAchillesfreetendon(Treeceetal.,2000)(Fig.1A–C).
2003) and establish target plantarflexion torques for subsequent testing Thecorresponding3DpointcloudwasthenexportedintoMatlab(version
trials.Two3DUSscanswerethenperformedatrestandduringan8–12s R2012a,TheMathWorks,Natick,MA,USA)fordeterminationoftendon
isometric plantarflexion contraction at 70% MVIC. Real-time visual transversemorphologyusingmethodsdescribedbyObstetal.(2014b).Pre-
feedbackoftorquewasprovidedtoassistmaintenanceofthetargetankle and post-exerciselongitudinal and transverse strainsat 70%MVICwere
torque for the duration of the trial. A double differential surface normalisedtothecorrespondingpre-exerciserestinglengths.Alltransverse
electromyography (EMG) system (band pass 30–500Hz, gain 300dB, morphologicalparametersarepresentedastheaverageofeach10%interval
commonmoderejectionratio160dB)andcommercialamplifier(Bagnoli- and expressed relative to the normalised tendon length. Because of
8™,DelsysInc.,Boston,MA,USA)wereusedtomeasuremuscleactivity difficultiesvisualisingtheAchillesfreetendoncross-sectionasittraverses
ofmedialgastrocnemius(MG),lateralgastrocnemius(LG),soleus(SOL) the calcaneus, subsequent analysis of mean and regional transverse
andtibialisanterior(TA)foralltendonmeasurementtrials.Followingskin morphology and strain only includes data between 30% and 100% of
preparation, three 24mm surface electrodes (inter-electrode distance the normalised tendon length; this approximated the tendon region
20mm; H124SG, Covidien, Kendall, USA) were placed over each between the proximal edge of the calcaneus and distal edge of the SOL
muscleaccordingtoSENIAMguidelines(Hermensetal.,2000).Torque MTJ(Fig.1B).
and EMG signals were analog–digital converted at a sampling rate of
1000Hzandrecordedusingacustom-writtenLabViewprogram(LabView Statisticalanalysis
9.0, National Instruments, Austin, TX, USA) and USB data acquisition Differencesinregionaltransversemorphology(CSA,APdiameterandML
device(NI:USB-6259BNC,NationalInstruments).Pre-andpost-exercise diameter) measured before and after exercise were assessed separately at y
g
target plantarflexion torques were normalised against the pre-exercise eachtorquelevelusingatwo-way(measurementtime×tendonregion)full o
plantarflexion MVIC. EMG signals were amplified (×3000), band-pass factorialrepeatedmeasuresgenerallinearmodel.Whereasignificanttime- ol
filtered (30–500Hz), full-wave rectified and root mean squared using a by-region interaction or main effect of time was found, pairwise Bi
20msnon-overlappingwindow.AllEMGamplitudeswerenormalisedto comparisons were performed between measurement time points at each l
pre-exerciseMVICandaveragedovera3swindowduringthemiddleof tendonregionusingBonferroniposthoccomparisons.Thesamestatistical ta
n
eachtrial.EMGamplitudesforLG,MGandSOLwerealsoexpressedasa approach was used to assessthe effect ofexercise on regional transverse
e
ratio of total triceps surae activation to account for the potential effect a strainsat70%MVIC.Differencesinpre-andpost-exercisetendonlength m
change in synergistic muscle activation may have on tendon transverse and volume, ankle torque and EMG, were assessed at each torque level i
r
strain(Arndtetal.,1998). using a two-way (measurement time×torque level) full factorial repeated e
p
measures general linear model. Where a significant time-by-torque x
3DUSmeasurementofAchillesfreetendonmechanicaland interaction or main effect of timewas found, pairwise comparisons were E
morphologicalproperties performed between measurement time points at each torque level using of
Ourfreehand3DUSsystemconsistedofaconventionalultrasoundmachine Bonferroni post hoc corrections. Differences in pre- and post-exercise
l
(SonixTouch, Ultrasonix, Richmond, BC, Canada) and a five-camera tendon longitudinal strain at 70% MVIC were assessed using atwo-way a
n
optical tracking system (V100:R2, Tracking Tools v 2.5.2, NaturalPoint, pairedStudent’st-test.Relationshipsbetweenpre-andpost-exercisechange r
u
Corvallis,OR,USA)thatenabledsynchronousrecordingof2Dbrightness- inmeanCSAstrainandtendonlongitudinalstrain,meanCSAstrainand o
mode(B-mode)ultrasoundimageswith3Dpositionandorientationofthe mean AP diameter strain, and mean CSA strain and mean ML diameter J
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RESEARCHARTICLE JournalofExperimentalBiology(2015)218,3894-3900doi:10.1242/jeb.127175
A (P<0.001). The data show tendon CSA was decreased across the
measurement region (30–100% normalised length) following
exercise at rest (pre-exercise 59.5±7.4cm2, post-exercise 52.6
±6.8cm2,P=0.001)andat70%MVIC(pre-exercise62.1±8.5cm2,
post-exercise 50.9±7.2cm2, P<0.001). Post hoc comparisons
between time points at each tendon region revealed significant
differencesintendonCSAat70%MVICbetween60%and80%of
normalised tendon length (Fig. 2B, top), but no change in resting
CSA (Fig. 2A, top). There was a significant time-by-region
interactionandmaineffectoftimefortendonMLdiameteratrest
B (P<0.001) and 70% MVIC (P<0.001). Tendon ML diameter was

decreasedacrossthemeasurementregionfollowingexerciseatrest
AT cross-section(s) (pre-exercise 13.7±0.33mm, post-exercise 12.4±0.27mm,
Soleus MTJ P<0.001) and at 70% MVIC (pre-exercise 13.0±0.39mm, post-
exercise11.3±0.31mm,P<0.001).Posthoccomparisonsbetween
time points at each tendon region failed to reveal any significant
differences in ML diameter at either torque level (Fig. 2A,B,
middle). There was no significant time-by-region interaction for
Calcaneal notch
tendonAPdiameteratrestorat70%MVIC;however,therewasa
Proximal edge of calcaneus significant main effect of time at 70% MVIC (pre-exercise 5.4
±0.43mm, post-exercise 5.2±0.47mm, P<0.001). Post hoc
C
C comparisons between time points for each region revealed a
Reconstructed AT
significantdecreaseinAPdiameterat70%MVICbetween50%and
100% of the normalised tendon length (Fig. 2B, bottom). There
were no significant time-by-region interactions for any transverse
strainmeasure;however, therewas amain effect of time for CSA
strain(pre-exercise0.9±2.3%,post-exercise−2.5±2.3%,P=0.035)
and AP diameter strain (pre-exercise 10.4±1.75%, post-exercise
5.7±1.9%, P<0.001), but not ML diameter strain. Post hoc
comparisons between time points at each region revealed
significant reductions in CSA strain and AP diameter strain
Fig.1.3Dultrasoundanalysis.Exampleofareconstructed3Dstackof between 60% and 90%, and 40% and 100% of the normalised
ultrasoundimages(A)withcorrespondingmanualsegmentationoftheAchilles
tendon length, respectively (Fig. 3). Changes in mean CSA strain
freetendon(AT)cross-sections(B)andinterpolated3Dsurfaceimbedded
following exercise were strongly correlated to changes in
backintotheoriginalimagestack(C).TheATwasmanuallysegmentedfrom
thetransverseimagesat∼0.5–1cmintervalsfromthemostdistaledgeofthe longitudinal strain (r=−0.72, P=0.002) and mean AP diameter
calcanealnotchtothesoleusmuscle–tendonjunction(MTJ). strain(r=0.65,P=0.006)(Fig.4).
strainwereeachassessedusingthePearson’sproduct-momentcorrelation Effectofexerciseplantarflexiontorqueandmuscle
coefficient(r).AllstatisticalanalyseswereperformedusingSPSSStatistics activation
software(version20.0,SPSSInc.,Chicago,IL,USA)andthesignificance Therewasnotime-by-torqueinteractionormaineffectoftimeon
levelforalltestswassetatP≤0.05. ankle plantarflexion torque at rest (grand mean 0.97±0.53Nm)
or at 70% MVIC (grand mean 57.1±13.7Nm). There was a
RESULTS significant time-by-torque interaction for normalised EMG
EffectofexerciseonAchillesfreetendonlength, amplitudes for all muscles; however, main effects of time were
longitudinalstrainandvolume only found for MG (P<0.001), LG (P<0.001) and SOL
Therewasasignificanttime-by-torqueinteractionandmaineffect (P<0.001). Post hoc comparisons revealed no change in resting
y
of time for tendon length. Post hoc comparisons revealed a normalised EMG amplitude; however, significant increases were g
o
significant increase in post-exercise tendon length at rest (pre- found at 70% MVIC for MG (pre-exercise 0.54±0.13, post- l
o
exercise 61.4±17.7mm, post-exercise 61.9±17.4mm, P=0.03) exercise 0.72±0.24, P=0.002), LG (pre-exercise 0.59±0.17, post- i
B
and at 70% MVIC (pre-exercise 65.3±18.0mm, post-exercise exercise 0.89±0.31, P=0.004) and SOL (pre-exercise 0.5±0.17,
l
66.8±17.9mm,P<0.001).Whennormalisedtopre-exerciseresting post-exercise0.73±0.18,P<0.001).Nosignificanttime-bytorque a
t
length, these changes equated to a significant increase in tendon interaction or main effect of time was found for any of the EMG n
e
longitudinal strain at 70% MVIC (pre-exercise 6.5±2.6%, post- ratios. m
exercise 9.2±3.1%, P<0.001). There was no significant time-by- ri
e
torque interaction or main effect of time for tendon volume DISCUSSION p
measurements at rest (grand mean 4.1±1.3ml) or at 70% MVIC Thisstudyusedfreehand3DUStodeterminetheimmediateeffect x
E
(grandmean4.2±1.2ml). of isolated eccentric exercise on the 3D morphologyand strain of
f
the Achilles free tendon. Post-exercise reductions in regional o
EffectofexerciseonAchillesfreetendonregional transverse morphology were detected for CSA and AP diameter al
n
transversemorphologyandstrain during the 70% MVIC and were most pronounced in the mid- r
Therewasasignificanttime-by-regioninteractionandmaineffect proximal region of the tendon. In contrast, no differences in ML u
o
of time for tendon CSA at rest (P<0.001) and 70% MVIC diameter in response to the eccentric exercise were detected at J
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RESEARCHARTICLE JournalofExperimentalBiology(2015)218,3894-3900doi:10.1242/jeb.127175
100 A 100 B Fig.2.GroupdataforAchillesfree
Pre−exercise tendoncross-sectionalarea(CSA),
Post−exercise medio-lateral(ML)diameterandantero-
2) 80 80 posterior(AP)diameter.Datawere
m obtainedatrest(A)andduringa70%
m
A ( maximalvoluntaryisometriccontraction
CS 60 60 * * * (b7o0u%toMfeVcIcCe;nBtr)icbheefoerledraonpdeaxfetercriases.inAgllledata
wereaveragedover10%intervalsand
expressedrelativetonormalisedtendon
40 40
0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 length(0%=calcanealnotchand100%
=soleusMTJ).Databetween0%and30%
ofnormalisedtendonlength(seevertical
dottedline)wereexcludedfromthe
20 20
statisticalanalysis.Errorbarsrepresent±1
m) s.e.m.(N=14).*Significantposthoc
m pairwisedifferencebetweenpre-andpost-
er ( exercisevalue(P<0.05).
et 15 15
m
a
di
L
M
10 10
0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100
6.5 6.5
m) * * *
m *
er ( 5.5 5.5 *
et
m
a
di 4.5 4.5
P
A
3.5 3.5
0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100
Normalised tendon length (%)
70% MVIC or for any morphological measures evaluated at rest. to the superior calcaneal edge (equating to ∼60–80% normalised
Although post-exercise reductions in regional AP diameter under tendon length in Fig. 2A) immediately after a similar bout of
load were small (∼0.2mm), when expressed relative to pre- eccentricexerciseinhealthyindividuals(Griggetal.,2009,2012).
exercise resting values, they equated to a 50% reduction in mean It is possible that the inclusion of tendon pre-conditioning
AP diameter strain under load. Consistent with our hypothesis, contractions prior to baseline measurements in the present study
we also observed a strong inverse relationship between may have reduced the overall effect of our exercise intervention.
post-exercise change in longitudinal strain and mean CSA strain Consistent with this observation, previous studies that have not
(r=−0.70),thelatterofwhichwasprimarilyderivedfromreduced included tendon pre-conditioning contractions tend to report
AP diameter strain. Together, these results highlight the significant changes in transverse morphology immediately after
interaction between exercise-related changes in tendon length exercise(e.g.APdiameter;FahlstromandAlfredson,2010;Grigg
y
and transverse morphology, and the importance of characterising etal.,2009,2012;Wearingetal.,2013),whereasstudiesthathave g
o
tendon morphology in 3D. included pre-conditioning report little or no effect (e.g. CSA; l
o
Burgessetal.,2009;Farrisetal.,2012;Lichtwarketal.,2013;Obst i
B
EffectofexerciseonAchillesfreetendontransverse etal.,2015).Inaddition,thelackofchangeinrestingAPdiameter
l
a
morphologyatrest could reflect differences in the measurement method between t
Consistent with previous in vivo studies, we found no change in studies.Wecomputedtendondiameteralongthetendonlengthby n
e
regionalCSAmeasuredatrestimmediatelyafterexercise(Burgess analysingtheshapeanddimensionsofeachconsecutive2Dtendon m
etal.,2009;Farrisetal.,2012;Lichtwarketal.,2013;Obstetal., cross-sectiondeterminedfrommanualsegmentationoftheoriginal ri
e
2015; Ooi et al., 2015). While our results did show a small but transverse B-mode images. As a consequence, while our method
p
consistentreductioninCSAacrossallmeasurementregions(∼7%), minimises measurement errors due to tendon obliquity (Fornage, x
E
post hoc comparisons failed to reveal any significant differences 1986; Sunding et al., 2014), the measures are inclusive of the
f
between time points at any of the tendon regions. We also found peri-tendinousspace(asopposedto Grigg etal.,2009, 2012)and o
nosignificantdifferencesinrestingMLdiameterorAPdiameterat therefore are unlikely to detect small changes in transverse al
n
any of the tendon regions. The latter result was unexpected, as dimensions due to possible movement of fluid from the tendon
r
previousstudieshavedemonstratedlargereductionsinAPdiameter coretotheperi-tendinousspace(Griggetal.,2009,2012;Wellen u
o
(∼0.9mmor∼20%)measuredatrestbetween2and4cmproximal etal.,2005). J
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RESEARCHARTICLE JournalofExperimentalBiology(2015)218,3894-3900doi:10.1242/jeb.127175
20 A 15 A
15 Pre-exercise r=−0.72
Post-exercise 10
%) 10
SA strain ( −−10550 * * * −055
C
−15 −10
−200 10 20 30 40 50 60 70 80 90 100 n (%) −15−2 −1 0 1 2 3 4 5
%) 5 B SA strai 15 B ∆ Longitudinal strain (%)
ain ( 0 ∆C 10 r=0.64
er str −5 5
et −10 0
m
a
di −15 −5
L
M −20 −10
0 10 20 30 40 50 60 70 80 90 100
−15
−15 −10 −5 0 5
20 C ∆ AP diameter strain (%)
%)
n ( 15 * * * * Fig.4.Correlations(±95%confidenceintervals)betweenchangesin
eter strai 150 * * A(sNtcr=ahi1nil4;le)P.s=(Af0r).e0Ce0S2teA)nasdntordani(nBp)(aΔCrCaSmSAAestsetrtrarsaininpa)onasdnt-deAxlPoendrgciaiitsmuedeiftnoearrlsestatrracaihinnp((ΔaΔArlotPincgdipiitaaumnditentaelr
m strain;P=0.006).r,Pearson’scorrelationcoefficient.
a
di 0
P
A −5 MVIC, we cannot discount small changes in volume (minimal
0 10 20 30 40 50 60 70 80 90 100 detectable change for 3DUS volume ∼0.2ml; Obst et al., 2014b)
Normalised tendon length (%) or redistribution of fluid along the tendon length (Sun et al.,
2015). Regardless,ourfindings demonstrate thatacute changes in
Fig.3.GroupdataforAchillesfreetendonstrainat70%MVICmeasured
tendon transverse morphology after exercise may be more
beforeandafterasingleboutofeccentricheeldropexercise.(A)CSA,(B)
MLdiameterand(C)APdiameterstrain.Alldatawereaveragedover10% pronounced when measured under tensile load, and that such
intervalsandexpressedrelativetonormalisedtendonlength(0%=calcaneal changes may largely be dependent on changes in tendon length
notchand100%=soleusMTJ).Databetween0%and30%ofnormalised due to mechanical creep.
tendonlength(seeverticaldottedline)wereexcludedfromthestatistical Consistent with our hypothesis, increased negative CSA strain
analysis.Errorbarsrepresent±1s.e.m.(N=14).*Significantposthocpairwise
measuredduringthe70%MVICappearedtobeprimarilydrivenby
differencebetweenpre-andpost-exercisevalue(P<0.05).
reduced positive AP diameter strain, with little change in ML
diameter strain. The latter result was somewhat surprising
EffectofexerciseonAchillesfreetendonlongitudinaland considering the inverse and proportional relationship that exists
transversestrainunderload betweenfreetendonAPdiameterandMLdiameterdeformationand
In contrast to the resting measurements, both CSA and AP strainundertensileloadduringisometricplantarflexioncontraction
diameter during the 70% MVIC were reduced immediately after (Obst et al., 2014b). These results could therefore represent
exercise and represent the first in vivo evidence of altered evidence of altered biaxial strain of the free tendon, during
transverse morphology and strain of the human Achilles free muscle contraction, in response to acute exercise. These changes
y
tendonmeasuredduringamusclecontraction.Consistentwithour werenotexplainedbypost-exercisedifferencesinmuscleactivation g
o
hypothesis, reduced mean CSA strain post-exercise was inversely patternsortorqueproduction,butcouldreflectnon-uniformfatigue l
o
correlated to increased longitudinal strain during the isometric or creep of Achilles tendon fascicles, due to differences in i
B
muscle contraction. Our results provide the first evidence that mechanical properties and/ortensileloading during eccentric heel
l
a
links acute changes in transverse morphology to corresponding drop(Arndtetal.,2011,1998;SlaneandThelen,2014).Irrespective
t
changes in tendon length after exercise. Short-term changes in ofthecause,acutealterationsinthenormalbiaxialstraincouldhave n
e
Achilles free tendon transverse morphology after exercise implicationsforintra-tendinousforcedistribution(Haraldssonetal., m
(Fahlstrom and Alfredson, 2010; Grigg et al., 2010, 2009, 2008), fluid flow (Reese et al., 2010; Yin and Elliott, 2004) and ri
e
2012; Ooi et al., 2015; Wearing et al., 2011, 2013, 2008) may tissuehomeostasis(Smithetal.,2013)andarethereforerelevantin
p
thereforebeindicativeoftransientmechanicalcreepofthetendon the context of exercise-dependent regional adaptation of Achilles x
E
(Lichtwark et al., 2013; Obst et al., 2015), whereby increased freetendonstructureandfunction.OurfindingsalsosuggestthatAP
f
collagen fibril packing linked to longitudinal creep and reduction diameter may be more responsive to change following exercise, o
of collagen crimp could promote fluid redistribution within, and compared with CSA or ML diameter, and support the use of AP al
n
out of, the tendon (Grigg et al., 2009; Hannafin and Arnoczky, diameter to evaluate acute, and possibly chronic, exercise-
r
1994; Wellen et al., 2005). It should be noted that although we dependentchangesinAchillesfreetendontransversemorphology u
o
didnotdetectanychangeintendonvolumeatrestorduring70% (Griggetal.,2012). J
3898
RESEARCHARTICLE JournalofExperimentalBiology(2015)218,3894-3900doi:10.1242/jeb.127175
Limitations response to tendon fatigue damage accumulation in a novel in vivo model.
There are a number of limitations that must be considered when J.Biomech.43,274-279.
Grigg,N.L.,Wearing,S.C.andSmeathers,J.E.(2009).Eccentriccalfmuscle
interpretingthefindingsofthepresentstudy.Firstly,becausewedid
exerciseproducesagreateracutereductioninAchillestendonthicknessthan
notmeasureregionaldifferencesinlongitudinalstrain,wecanonly concentricexercise.Br.J.SportsMed.43,280-283.
speculate upon the role non-uniform strains along and between Grigg,N. L.,Stevenson,N. J.,Wearing,S. C. andSmeathers,J.E. (2010).
Achillestendonfascicles,duringtheeccentricheeldrop,mayhave Incidentalwalkingactivityissufficienttoinducetime-dependentconditioningof
theAchillestendon.GaitPosture31,64-67.
hadonourregionaltransversestrainresults.Secondly,inthepresent
Grigg,N.L.,Wearing,S.C.andSmeathers,J.E.(2012).Achillestendinopathy
study,wewereunabletodelineatebetweentheepi-tendonandpara- hasanaberrantstrainresponsetoeccentricexercise.Med.Sci.SportsExerc.44,
tendon using3DUS and sowereunable toquantify possible fluid 12-17.
exudation from the tendon core into the peri-tendinous space in Habets,B.andvanCingel,R.E.H.(2015).Eccentricexercisetraininginchronic
mid-portion Achilles tendinopathy: a systematic review on different protocols.
response to exercise. Finally, caution is warranted in generalising
Scand.J.Med.Sci.Sports25,3-15.
thefindingsof thepresentstudytodifferentexercisemodes,time Hannafin, J. A. and Arnoczky,S. P. (1994). Effect of cyclic and static tensile
points,agegroupsortendinopathictendons. loadingonwatercontentandsolutediffusionincanineflexortendons:aninvitro
study.J.Orthop.Res.12,350-356.
Haraldsson,B.T.,Aagaard,P.,Qvortrup,K.,Bojsen-Moller,J.,Krogsgaard,M.,
Conclusions Koskinen,S.,Kjaer,M.andMagnusson,S.P.(2008).Lateralforcetransmission
Themid-proximalAchillesfreetendonexperiences areductionin betweenhumantendonfascicles.MatrixBiol.27,86-95.
CSA and AP diameter during active contractions performed Heinemeier,K.M.andKjaer,M.(2011).Invivoinvestigationoftendonresponses
tomechanicalloading.J.Musculoskelet.NeuronalInteract.11,115-123.
immediately following a clinical dose of eccentric heel drop
Helmer,K.G.,Nair,G.,Cannella,M.andGrigg,P.(2006).Watermovementin
exercise. These findings suggest that the free tendon has different tendon in response to a repeated static tensile load using one-dimensional
mechanicalpropertiesalongitslengthandalongtheAPcompared magneticresonanceimaging.J.Biomech.Eng.128,733-741.
with the ML axis (anisotropy) and/or experiences non-uniform Henriksen,M.,Aaboe,J.,Bliddal,H.andLangberg,H.(2009).Biomechanical
characteristics of the eccentric Achilles tendon exercise. J. Biomech. 42,
strainsalongitslengthandalongtheAPcomparedwiththeMLaxis
2702-2707.
duringheeldropexercise.TheobservedreductionsinCSAandAP Hermens, H. J., Freriks, B., Disselhorst-Klug, C. and Rau, G. (2000).
diameter strain after eccentric heel drop exercise were correlated Development of recommendations for SEMG sensors and sensor placement
withlongitudinalstrainandarelikelytoreflectfluidredistribution procedures.J.Electromyogr.Kinesiol.10,361-374.
Jeong,S.,Lee,D.-Y.,Choi,D.-S.andLee,H.-D.(2014).Acuteeffectofheel-drop
alongthetendonandacorrespondingincreaseinpackingdensityof
exercise with varying ranges of motion on the gastrocnemius aponeurosis-
collagenfibrilsunderload. tendon’smechanicalproperties.J.Electromyogr.Kinesiol.24,375-379.
Kjaer,M.andHeinemeier,K.M.(2014).Eccentricexercise:acuteandchronic
Competinginterests effectsonhealthyanddiseasedtendons.J.Appl.Physiol.116,1435-1438.
Theauthorsdeclarenocompetingorfinancialinterests. Lichtwark,G.A.,Cresswell,A.G.andNewsham-West,R.J.(2013).Effectsof
running on human Achilles tendon length-tension properties in the free and
gastrocnemiuscomponents.J.Exp.Biol.216,4388-4394.
Authorcontributions
Maganaris,C.N.(2003).Tendonconditioning:artefactorproperty?Proc.R.Soc.B
S.J.O.,R.N.-W.andR.S.B.allcontributedtotheconceptionanddesignofthestudy,
Biol.Sci.270Suppl.1,S39-S42.
interpretationoftheresultsanddraftingofthemanuscript.S.J.O.undertookalldata
Neves,K.A.,Johnson,A.W.,Hunter,I.andMyrer,J.W.(2014).DoesAchilles
collectionandanalysis,andcreationoffigures.
tendoncrosssectionalareadifferafterdownhill,levelanduphillrunningintrained
runners?J.SportsSci.Med.13,823.
Funding Obst,S.J.,Barrett,R.S.andNewsham-West,R.(2013).Immediateeffectof
S.J.O.wassupportedbytheGriffithUniversitypost-graduateresearchscholarship exerciseonAchillestendonproperties:asystematicreview.Med.Sci.Sports
(GUPRS)scheme.InfrastructuresupportwasprovidedbyGriffithUniversity. Exerc.45,1534-1544.
Obst,S.J.,Newsham-West,R.andBarrett,R.S.(2014a).Invivomeasurementof
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