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Neu omechanical adap a ions in he gas ocnemius muscle a e Achilles endon
up u e du ing walking
© 2024 The Au ho (s). Published by Else ie L d.
Published e sion
Khai , Raad M.; Wa , Jadyn; Sukanen, Ma ia; C onin, Neil J.; Finni, Taija
Khai , R. M., Wa , J., Sukanen, M., C onin, N. J., & Finni, T. (2025). Neu omechanical
adap a ions in he gas ocnemius muscle a e Achilles endon up u e du ing walking. Jou nal
o Elec omyog aphy and Kinesiology, 80, A icle 102962.
h ps://doi.o g/10.1016/j.jelekin.2024.102962
2025
Neu omechanical adap a ions in he gas ocnemius muscle a e Achilles
endon up u e du ing walking
Raad M. Khai
a,*
, Jadyn Wa
c
, Ma ia Sukanen
a
, Neil J. C onin
a,b
, Taija Finni
a
a
Facul y o Spo and Heal h Sciences, Neu omuscula Resea ch Cen e , Uni e si y o Jy ¨
askyl¨
a, Jy ¨
askyl¨
a, Finland
b
School o Educa ion & Science, Uni e si y o Glouces e shi e, Glouces e shi e, UK
c
Uni e si y o Wisconsin-Madison, Madison, WI, USA
ARTICLE INFO
Keywo ds:
Ope a ing leng h
Fascicle
Rup u e
Gai
Flexo hallucis longus
ABSTRACT
Al hough some Achilles endon up u e (ATR) pa ien s egain unc ion in low- o ce le els ac i i ies, i is no ye
well known how neu omuscula and s uc u al al e a ions a e ATR mani es du ing e e yday-locomo ion. This
s udy assessed medial gas ocnemius (MG) ascicle sho ening du ing walking 1-yea a e ATR. Addi ionally, we
explo ed neu omuscula al e a ions in la e al gas ocnemius (LG), soleus and lexo hallucis longus (FHL)
muscles.
We obse ed 3.1 pp (95 %CI 0.8–5.4 pp) highe a e age and 14.5 pp (95 %CI 0.5–28.5 pp) highe peak LG
su ace elec omyog aphy ampli ude in he inju ed compa ed o he un-inju ed du ing walking, bu no di e -
ences we e obse ed in soleus o FHL. The inju ed limb ascicles we e 12.9 mm sho e while s anding compa ed
o he un-inju ed limb. In absolu e e ms, MG sho ening in he inju ed limb was 2.8 mm (95 %CI 0.96–4.6 mm)
smalle compa ed o he un-inju ed limb. Howe e , when no malized o s anding ascicle leng h, he amoun o
sho ening was no di e en be ween he limbs.
Ou esul s showed ha 1-yea a e ATR, MG muscle had emodelled, which mani es ed as sho e ascicle
leng h du ing s anding. Du ing walking, inju ed and un-inju ed MG ascicles showed simila sho ening ela i e
o he s anding ascicle leng h, sugges ing ha MG could unc ion e ec i ely a he new mechanical se ings
du ing e e yday locomo ion.
1. In oduc ion
The Achilles endon (AT) emains one o he mos equen ly inju ed
endons in he human body wi h inc easing incidence in he ecen de-
cades (Holm e al., 2015; Lan o e al., 2015). AT up u e (ATR) equi es
a long eco e y pe iod and leads o pe manen mo phological changes,
such as endon elonga ion and sho e ascicles in he medial gas oc-
nemius (MG) muscle (Bax e e al., 2018; Peng e al., 2017; Bax e e al.,
2018; Peng e al., 2017; Silbe nagel e al., 2012; S ensson e al., 2019;
an Dijk e al., 2023; S ensson e al., 2019; an Dijk e al., 2023). A e
ATR, pa ien s o en expe ience long- e m unc ional de ici s cha ac e -
ized by limi ed heel aise capabili y and weakness a he end ange o
plan a lexion (Mullaney e al., 2006; Mullaney e al., 2006; Silbe nagel
e al., 2012). Al hough some pa ien s egain capaci y o pe o m in slow
mo ion ac i i ies ha equi e low o ce le els, unc ional impai men s
a e obse ed in high demand asks such as jogging o hopping (Jandacka
e al., 2017; Willy e al., 2017). I is no ye well known how
neu omuscula and s uc u al adap a ions in he iceps su ae (TS)
muscle– endon uni a e ATR mani es du ing e e yday locomo ion.
Muscle a ophy and sho e ascicles a e obse ed in he MG a e
ATR due o he inc eased leng h o he endon and p olonged ankle
immobiliza ion in a sho ened posi ion (Bax e e al., 2018; Khai e al.,
2022). Tendon elonga ion shi s he ope a ing leng h le wa ds on he
ascending limb o he o ce–leng h ela ionship (Hoe ne e al., 2023;
S ¨
audle e al., 2020), limi ing o ce p oduc ion capaci y and educing
he excu sion o he MG muscle. The muscle adjus s o he ex a slack o
he endon by educing sa come es in se ies o main ain op imum ac in-
myosin o e lap compa able o p e-inju y se ings (Hoe ne e al., 2023).
This mechanism migh amelio a e he loss o o ce p oduc ion capa-
bili y, acili a ing o ce gene a ion wi hin a smalle ope a ing ange
(S ¨
audle e al., 2020). S ¨
audle e al., (2022) epo ed ha MG con ac ile
beha iou was simila be ween he up u ed and con ala e al leg du -
ing walking a a speed o 1.5 m⋅s
−1
2 yea s a e ATR. Due o ela i ely
as adap a ion o muscles, i is easonable o assume ha al eady 1-yea
* Co esponding au ho a : Facul y o Spo and Heal h Sciences, Uni e si y o Jy ¨
askyl¨
a, PO BOX 35, Finland.
E-mail add ess: [email p o ec ed] (R.M. Khai ).
Con en s lis s a ailable a ScienceDi ec
Jou nal o Elec omyog aphy and Kinesiology
jou nal homepage: www.else ie .com/loca e/jelekin
h ps://doi.o g/10.1016/j.jelekin.2024.102962
Recei ed 22 Augus 2024; Recei ed in e ised o m 11 No embe 2024; Accep ed 22 No embe 2024
Jou nal o Elec omyog aphy and Kinesiology 80 (2025) 102962
A ailable online 23 No embe 2024
1050-6411/© 2024 The Au ho (s). Published by Else ie L d. This is an open access a icle unde he CC BY license (
h p://c ea i ecommons.o g/licenses/by/4.0/ ).
a e ATR, MG muscle has egained su icien plan a lexion o ce
equi ed o locomo ion ac i i ies such as walking.
Rega dless o he ea men s a egy, s udies ha e shown inc eased
su ace elec omyog aphy (EMG) no malized ampli ude o he TS mus-
cles du ing plan a lexion (Khai e al., 2023; Wenning e al., 2021) and
highe mean equency (McHugh e al., 2019) a e up u e. Wenning
e al., (2021) ound highe no malised EMG ac i i y o he gas ocnemii
muscles du ing he push-o phase o gai bu no di e ences we e
obse ed in he soleus muscle (Wenning e al., 2021). Simila ly, Suydam
e al., (2015) obse ed highe no malized EMG du ing gai a 6 mon hs
in he la e al gas ocnemius (LG), and MG a 12 mon hs a e up u e
(Suydam e al., 2015). Neu omuscula con ol is u he al e ed so ha
deep lexo s such as lexo hallucis longus (FHL) a e ec ui ed o
po en ially alle ia e ensile o ces ansmi ed h ough he AT (Masood
e al., 2016). This mechanism is hough o s a ea ly in he eco e y
s age and pe sis due o he sho ened posi ion o he TS (Finni e al.,
2006; Heikkinen e al., 2017; Khai e al., 2023). Few s udies ha e
in es iga ed MG dynamic beha iou a e Achilles endon up u e
(S ¨
audle e al., 2022; an Dijk e al., 2023), while no s udies ha e
explo ed he neu omuscula al e a ion and unc ional con ibu ion o
FHL in pa ien s wi h ATR du ing walking.
Due o he high p e alence o ATR and long- e m unc ional
impai men associa ed wi h he inju y; i is impe a i e o de e mine how
TS muscle– endon uni emodelling mani es s in e e yday locomo ion.
The e o e, he objec i e o his s udy was o in es iga e he ascicle
sho ening o he MG du ing walking 1-yea a e non-su gically ea ed
ATR. Addi ionally, we examined he EMG ac i i y o LG, soleus and FHL.
I was hypo hesized ha he inju ed limb MG would ha e a compa able
no malised ascicle sho ening o he un-inju ed limb du ing walking.
Mo eo e , we expec ed he ela i e EMG ac i i y o LG, soleus and FHL
o be highe in he inju ed limb compa ed o he con ala e al limb.
2. Ma e ials and me hods
Twen y- ou pa icipan s (21 males, 3 emales) wi h unila e al ATR
we e ec ui ed o his c oss-sec ional s udy wi hin a clinical coho
s udy “Non-ope a i e ea men o Achilles endon Rup u e in Cen al
Finland: a p ospec i e coho s udy – NoARC, ial egis a ion:
NCT03704532”. Rup u e was diagnosed in acco dance wi h he Ame -
ican Academy o O hopaedic Su geons (AAOS) based on a minimum o
2 o he ollowing 4 c i e ia: a posi i e Thompson es , dec eased plan-
a lexion s eng h, p esence o a palpable gap, and inc eased passi e
ankle do si lexion wi h gen le manipula ion (Su geons, 2009). All pa -
icipan s (means ±SD age: 40.7 ±10.6 yea s, heigh : 177.2 ±8.3 cm,
mass: 85.2 ±15.2 kg) we e ea ed non-su gically wi h ea ly mobiliza-
ion (Rei o e al., 2017). Da a om one yea a e p esen ed, wi h pa -
icipan s es ed (mean ±SD) 13 ±1 mon hs a e up u e.
2.1. Lab p o ocol
Upon a i al a he labo a o y, pa icipan s we e p epa ed o su -
ace elec omyog aphy (EMG) measu emen s. The skin was sha ed and
ab aded wi h alcohol pads o educe impedance. Disposable dual su ace
sil e -sil e elec ode Ambu BlueSenso N elec odes (Ambu A/S, Bal-
le up, Denma k) we e placed on he TS muscles wi h an in e elec ode
dis ance o 22 mm acco ding o SENIAM ecommenda ions (S egeman
and He mens, 2007). Fo FHL, elec odes we e placed be ween he so-
leus inse ion and he FHL muscle– endon junc ion whe e only he FHL
muscle belly lies (P´
e e e al., 2015), wi h an in e elec ode dis ance o
16 mm. Ul asound imaging o isola ed big oe lexion was pe o med o
con i m he FHL elec ode placemen . EMG signals we e collec ed a
1500 Hz using a No axon wi eless EMG sys em (No axon Inc., Sco s-
dale, AZ, USA) ia a 16-bi A/D boa d (Powe 1401, Camb idge Elec-
onic Design, Camb idge, UK) connec ed o he compu e . Signals we e
eco ded using Spike2 so wa e (Camb idge Elec onic Design, Cam-
b idge, UK).
Pa icipan s we e hen sea ed in a cus om-made ankle dynamome e
(Uni e si y o Jy ¨
askyl¨
a, Finland) wi h hip a 120◦, knee ully ex ended
0◦, ankle a 90◦, and i s me a a sophalangeal join s a 0◦. The oo and
high we e secu ely ixed wi h a s ap o p e en any heel li o pos u al
changes. Once pa icipan posi ion was secu ed, a se ies o submaximal
con ac ions we e pe o med o p econdi ion he pa icipan s and
amilia ize hem wi h he equipmen . S a ing wi h he un-inju ed limb,
pa icipan s we e asked o pe o m a leas h ee isome ic maximal
olun a y con ac ions (MVC) wi h an o e all con ac ion and elaxa ion
ime o 6 s, and he highes MVC o que was used o subsequen EMG
no maliza ion. To que was sampled a 1 KHz ia a ansduce in he oo
pedal o he ankle dynamome e .
Following he MVC’s, pa icipan s we e asked o walk wi h a speed
anging om 1.46 – 1.76 m⋅s
−1
, which co esponds o he a e age
walking speed o young adul s (S en o h e al., 2017) ac oss a 6-me e
walkway wi h pho ocells a bo h ends. A 6-cm linea ul asound p obe
(96-elemen −7 MHz – EchoBlas e 128; Telemed, Vilnius, Li huania o
LV8-5 N60-A2; 5–8 MHz – ARTUS EXT-1H, Telemed, UAB) was placed
i s on he mid-belly o he MG o he inju ed limb o measu e he MG
ascicle beha iou du ing walking (Lich wa k e al., 2007). A e he
pa icipan s had pe o med a amilia isa ion ial o ge acquain ed wi h
he walking pa h, 2–3 walking ials was pe o med and sa ed o o line
analysis. Then, he ul asound p obe was a ached o he MG o he un-
inju ed limb and he walking ials we e epea ed.
2.2. Ul asound
MG ascicle leng h changes du ing gai cycles we e analysed using a
alida ed semi-au oma ed acking algo i hm (C onin e al., 2011) in
MATLAB R2022b, combined wi h manual key ame co ec ions o
educe d i . B ie ly, he muscle egion o in e es was selec ed o
encompass he en i e muscula issue, hen a ascicle ep esen ing he
a e age ascicle o ien a ion in he mid- egion o he muscle belly was
selec ed. In cases whe e he ascicle ex ended ou side he ield o iew,
ex apola ion was used o es ima e he loca ion o he ascicle endpoin s.
Fascicle leng h changes we e acked using a Lucas-Kanade op ical low
algo i hm wi h a ine op ic low ex ension (C onin e al., 2011). A e
checking he quali y, we excluded ul asound da a om six pa icipan s
whe e ascicles we e no clea ly isible o mo ed ou o he ul asound
imaging plane.
2.3. EMG
EMG da a we e il e ed using a ou h o de Bu e wo h il e be-
ween 30 – 350 Hz wi h MATLAB R2022b. Highe co ne equency was
used o augmen a i ac s bu accep ing ha il e ing educes EMG signal
(De Luca e al., 2010). Roo mean squa e (RMS) en elopes we e
compu ed using a mo ing 50 ms window and no malized o he RMS o
maximal EMG ampli ude. Maximal EMG ampli ude was es ima ed om
a 1-sec window a ound peak o que du ing MVC. All EMG signals we e
isually inspec ed o de ec mo emen a i ac s o noise. Upon quali y
assessmen , soleus da a om one pa icipan and LG da a om wo
pa icipan s we e excluded om analysis. Fu he mo e, da a om ou
FHL gai cycles we e disca ded due o excessi e a i ac s. Addi ionally,
in wo pa icipan s he inse ion o soleus muscle was excep ionally low,
hence, hose cases we e excluded o a oid po en ial c oss- alk be ween
FHL and soleus muscles (P´
e e e al., 2015). Only a i ac - ee cycles
we e p ocessed (Fig. 1). P ocessed EMG da a we e in e pola ed om
each gai cycle o 100 da a poin s hen a e aged o cycles wi hin a ial.
The walking EMG da a was no malised o MVC and exp essed as a
pe cen age (%) o he maximal EMG ampli ude. Peak and a e age EMG
ampli udes we e calcula ed om he chosen cycles o u he analysis.
Compa isons be ween limbs a e exp essed as pe cen age poin s (pp).
R.M. Khai e al.
Jou nal o Elec omyog aphy and Kinesiology 80 (2025) 102962
2
2.4. Gai cycle de ec ion
Walking ials included he accele a ion and decele a ion phases,
hence he i s and las cycles we e excluded. F om ul asound e-
co dings, da a om 2 o 3 cycles in he middle o he ial we e manually
iden i ied and in e pola ed o 100 da a poin s. Cycles we e de e mined
based on EMG. Cessa ion o he no malized LG muscle EMG ac i i y
below 2 % was used o ma k he app oxima e end o he s ance phase
(Fig. 1) (Islam e al., 2020). A i ac ee cycles we e a e aged and
ascicle sho ening was de e mined as maximal ascicle sho ening by
sub ac ing minimal ascicle leng h om maximal ascicle leng h du ing
he gai cycle (Fig. 2). Re e ence ascicle leng h was es ima ed as he
leng h o he ascicles in s anding posi ion be o e ini ia ion o walking
( an Dijk e al., 2023). No malized ascicle sho ening was exp essed as
pe cen age (%) o ascicle leng h change du ing walking ela i e o he
e e ence s anding ascicle leng h. Compa isons be ween limbs a e
exp essed as pe cen age poin s (pp).
2.5. S a is ical analysis
Pai wise T- es was used o compa e maximum MVC o que, s anding
ascicle leng h, MG ascicle sho ening and neu omuscula p ope ies
Fig. 1. Illus a ion o he me hod used o iden i y accep ed s ance phases (g een) and exclude s eps whe e a i ac s ( ed) we e de ec ed based on ascicle leng h and/
o EMG da a. Accep able cycles we e de e mined o begin once he EMG ac i i y o he LG muscle ceased, ma king he app oxima e end o he s ance phase. All EMG
signals we e isually inspec ed o de ec mo emen a i ac s o noise, and cycles ha we e a i ac - ee we e included. In his example, a e excluding he i s s ep
ha included accele a ion, he cycles in he middle we e inspec ed o noises and a i ac s and he clean one (g een) was accep ed o u he analysis.
Fig. 2. Rep esen a ion o he calcula ion o ascicle sho ening. Sho ening
leng h was calcula ed as he di e ence be ween maximal and minimal ascicle
leng h (do ed ho izon al lines) du ing he gai cycle no malized o s anding
ascicle leng h (L
ascicle
). The cycle in he igu e s a s wi h he swing phase
ollowed by he s ance phase.
R.M. Khai e al.
Jou nal o Elec omyog aphy and Kinesiology 80 (2025) 102962
3
be ween he un-inju ed and inju ed limbs a e checking ku osis and
skewness o he da a. I skewness o he da a was de ec ed, Wilcoxon
signed ank es was used. S a is ical analysis was pe o med using JASP
(JASP, Ams e dam, Ne he lands). The le el o signi icance was se a p
<0.05 and desc ip i e da a a e p esen ed as mean ±s anda d de ia ion.
3. Resul s
O he 24 pa icipan s 6 we e excluded due o da a quali y and he
analysis we e done on he ollowing sample: 18 pa icipan s o MG
ascicle sho ening du ing walking, 16 o LG EMG, 17 o soleus EMG,
and 12 o FHL EMG.
Maximal o que was lowe in he inju ed limb (mean ±SD: 171.7 ±
51.5 Nm) compa ed o he un-inju ed limb (226.4 ±65.2 Nm), wi h a
mean di e ence (95 %CI) o 54.7 Nm (33.6 o 75.8 Nm). Pa icipan s
walked a he same speed when he un-inju ed (mean ±SD 1.73 ±0.55
m⋅s
−1
) and inju ed limb (1.74 ±0.57 m⋅s
−1
) was assessed o MG
ascicle sho ening. The un-inju ed limb showed longe ascicles du ing
s anding wi h a mean di e ence (95 % CI) o 12.9 mm ([7.7 o 18.3
mm], d =17, =5.2; p <0.001) compa ed o he inju ed limb (Fig. 3).
In absolu e e ms, he a e age sho ening o he MG in he inju ed leg
was smalle wi h a mean di e ence o 2.8 mm ([95 % CI 0.96 o 4.6], d
=17, =3.2; p =0.004) compa ed o he un-inju ed limb. Howe e ,
when no malized o s anding ascicle leng h, he amoun o sho ening
did no di e (d =17, =0.7; p =0.436) be ween he inju ed (mean ±
SD 12.0 ±5.1 %) and un-inju ed limbs (13.3 ±4.8 %).
3.1. EMG ac i i y
Du ing walking, LG showed highe EMG peak ampli ude in he
inju ed limb wi h a mean di e ence o 14.5 pp ([95 %CI 0.5 o 28.5 pp],
d =15, = − 2.87; p =0.011) and highe a e age ampli ude wi h a
mean di e ence o 3.1 pp ([95 %CI 0.8 o 5.4 pp], d =15, = − 2.19; p
=0.043) compa ed o he un-inju ed limb. No s a is ical di e ences
we e obse ed in soleus o FHL (Fig. 4).
4. Discussion
This s udy aimed o unde s and neu omuscula unc ion o he cal
muscles a e ATR. The e was a highe EMG ampli ude in he inju ed LG,
bu he EMG ampli ude was no di e en in soleus o FHL du ing he gai
cycle. The MG ascicle sho ening du ing walking di e ed be ween
limbs in absolu e e ms bu no when ela ed o he s anding ascicle
leng h. This indica es ha e-modelling o he MG muscle a e ATR –
wi h po en ial emo al o sa come es in se ies – migh be su icien o
enable adequa e o ce p oduc ion in low- o ce asks such as walking.
In his s udy, MG ascicle leng h du ing s anding was on a e age 13
mm sho e in he inju ed limb. This is likely due o an inc ease in
endon leng h a e up u e. Since he TS muscle– endon complex e-
mains he same leng h, he ex a endon slack is accompanied by e-
modelling o he MG muscle o sho e ascicles (Bax e e al., 2018;
Hoe ne e al., 2023; Hull ish e al., 2019; Peng e al., 2017). This
ascicle adap a ion is linked wi h changes in sa come e leng hs in lu-
encing o ce p oduc ion capaci y (Hoe ne e al., 2023). In long e m,
inc ease in endon s i ness along wi h MG muscle adap a ions would
allow muscle o p oduce mo e o ce in he new ope a ing leng h (S ¨
audle
e al., 2020).
Absolu e MG ascicle sho ening was lowe in he inju ed limb du ing
walking. Ne e heless, when no malised o s anding ascicle leng h, MG
sho ening was compa able be ween limbs. This could indica e ha he
inju ed MG was ope a ing wi hin a simila egion o he o ce–leng h
cu e a a sa come e le el o he un-inju ed limb du ing walking.
S ¨
audle e al., (2022) in es iga ed walking a 1.5 m⋅s
−1
on a eadmill
and ound ha con ac ile beha iou o he MG was simila be ween
limbs app oxima ely 4-yea s pos ATR in su gically ea ed pa ien s. I
should be no ed ha he ope a ing leng h in S ¨
audle e al., (2022) was
no malised o he op imal sa come e leng h a he pla eau egion o he
human sa come e o ce–leng h ela ion (He zog e al., 1991). While we
no malized o ascicle leng h du ing s anding, ou indings a e consis-
en wi h hose o S ¨
audle e al., (2022), as we obse ed simila
no malized MG sho ening du ing walking in non-su gically ea ed
pa ien s 1-yea a e ATR. Jandacka and colleagues epo ed unc ional
de ici s in ac i i ies ha ha e highe o ce equi emen s such as unning,
bu did no ind any asymme y du ing o e g ound walking (Jandacka
e al., 2018). I is plausible ha MG e-modelling ia he educ ion o
sa come es in se ies main ains op imum ac in-myosin o e lap compa-
able o p e-inju y se ings (Hoe ne e al., 2023), p ese ing low o ce
p oduc ion in e e yday ac i i ies such as walking. Howe e , limi a ions
may pe sis in high demand asks such as heel aises ( an Dijk e al.,
2023), jogging o hopping (Jandacka e al., 2017; T o a e al., 2017;
Willy e al., 2017). These limi a ions possibly esul om a educ ion in
he ange o mo ion a which o ce can be p oduced (S ¨
audle e al., 2020)
(Fig. 5), and dec eased wo k capaci y o he plan a - lexo muscles o he
inju ed limb (Bax e e al., 2019).
Fig. 3. MG ascicle leng h du ing s anding (le ), absolu e MG sho ening du ing walking (middle), and MG ascicle sho ening du ing walking no malized o
s anding ascicle leng h ( igh ). G ey do s ep esen each indi idual (n =18) and colou ed do s mean wi h line leng hs indica ing SD. *Signi ican di e ence be ween
limbs (P <0.05).
R.M. Khai e al.
Jou nal o Elec omyog aphy and Kinesiology 80 (2025) 102962
4
Du ing walking, mean and peak LG EMG ampli udes we e highe in
he inju ed limb compa ed o he con ala e al limb, bu he EMG
ampli ude o soleus and FHL did no di e be ween limbs. Conco dance
wi h hese esul s, p e ious s udies epo ed highe ac i i y o LG and
MG bu no in soleus du ing gai (Suydam e al., 2015; Wenning e al.,
2021). Wenning e al., (2021) epo ed highe ac i i y o hese muscles
du ing he push-o phase o gai . The inc eased leng h o he endon
a e up u e migh necessi a e a s onge muscle con ac ion du ing he
push-o phase o gai o c ea e adequa e o ce a he ankle join , since
he TS muscle is p obably ac ing ou side he op imum egion on he
o ce–leng h cu e (Fig. 5), whe e g ea e ac i i y is needed o achie e
he same absolu e o que as he con ala e al limb (Nou bakhsh and
Kukulka, 2004). Rega ding soleus muscle, i s physiological c oss
sec ional a ea is almos 5 imes g ea e han ha o LG (Wa d e al.,
2009), so i is plausible ha soleus is s ill capable o su icien con i-
bu ion du ing walking despi e po en ial a ophy ollowing ATR
(Au we be e al., 2020; Nicholson e al., 2020). I mus also be no ed
ha soleus is a la ge and complex muscle wi h mul iple compa men s,
and due o he inhe en na u e o su ace EMG, he elec odes only
measu e ac i i y om a speci ic a ea. I is unknown, o example,
whe he deep compa men s compensa e o he al e ed o ce p oduc-
ion caused by he inc eased leng h o he endon. Fu he mo e, soleus
con ains mo e slow- ype muscle ibe s which may adap mo e slowly
han in he as e LG muscle (Liebe and F id´
en, 2000; Wa d e al.,
2009).
Con a y o he ini ial hypo hesis, FHL EMG ampli ude in he inju ed
limb was no di e en compa ed o he o he limb du ing waking o heel
aise. Al hough highe EMG ampli ude in he FHL muscle could be
obse ed in Fig. 4 ela i e o he con ala e al limb, he e was la ge
in e indi idual a iabili y in FHL EMG ampli udes, wi h no clea
dis inc ion be ween g oups o FHL use s and non-use s. Based on p e-
ious s udies looking a isome ic con ac ions (Finni e al., 2006; Khai
e al., 2023; Masood e al., 2016), FHL was expec ed o show compen-
sa o y ac i i y in he up u ed limb. Howe e , i may be ha he
inc eased CSA o he FHL muscle in pa ien s wi h ATR (Heikkinen e al.,
2017), may enhance FHL o ce gene a ing capaci y, hence educing he
need o inc eased EMG ac i a ion in o de o achie e a gi en o ce
ampli ude. Fu he mo e, he FHL compensa o y mechanism may no be
ope a i e in all pa ien s wi h ATR and da a would need o be looked a
indi idually. Acco ding o p e ious indings (Khai e al., 2023), i could
be hypo hesized ha pa ien s wi h excessi e AT leng hening a e
up u e ha e highe FHL EMG ac i i y han pa ien s wi h lowe in e -
limb di e ence in AT leng h.
Fig. 4. Mean EMG ampli ude da a du ing walking (solid lines) and s anda d de ia ion (shaded a eas) o all pa icipan s (LG n =16, soleus n =17, and FHL n =12).
*Signi ican di e ence be ween limbs (P <0.05).
Fig. 5. Schema ic ep esen a ion o he TS muscle o ce–leng h cu e in he un-inju ed and inju ed limb a e e-modelling o sho e muscle leng h. Fo ce cu es
we e es ima ed based on Hill’s equa ion (Go don e al., 1966; Liebe and F id´
en, 2000) wi h smalle wid h and 24% less maximal o ce gene a ion capaci y
de e mined om his coho . Do ed lines show equi ed EMG ac i a ion a a gi en o ce magni ude, in inju ed ( ed) and un-inju ed limbs (blue) (Suydam e al.,
2015). The black do illus a es ha he inju ed limb ope a es a sho e leng h whe e g ea e ac i i y is needed o achie e a gi en absolu e o que as he con a-
la e al limb.
R.M. Khai e al.
Jou nal o Elec omyog aphy and Kinesiology 80 (2025) 102962
5
4.1. Limi a ions
This s udy is no wi hou limi a ions. Fi s , acking o he ascicle
beha iou om a wo-dimensional ield o iew migh no ully cap u e
he complex h ee-dimensional non-uni o m beha iou o he MG
muscle (Rana and Wakeling, 2011). Manual ame co ec ion was used
o minimize e o s and limi d i . Secondly, due o male-dominance o
Achilles endon up u e incidence (ˇ
C e nik and F ank, 2004; Houshian
e al., 1998; Leppilah i and O a a, 1998), we we e unable o ha e
adequa e ep esen a ion o bo h sexes bu in e -limb analysis wi hin
indi iduals should no a ec he in eg i y o he analysis, since no di -
e ence be ween sexes is expec ed a e ATR in non-su gically ea ed
pa ien s (G ¨
a a e Silbe nagel e al., 2015). Addi ionally, EMG and ul-
asound measu emen s we e pe o med simul aneously du ing he
walking ials, and i is challenging o main ain he ul asound p obe in
an app op ia e loca ion and educe undesi ed EMG a i ac s du ing
walking (Vigo sky e al., 2018). In a ew cases, excessi e mo ion o EMG
wi es du ing walking gene a ed a i ac s. Hence, all da a we e isually
inspec ed and ials ha did no mee he quali y check we e disca ded.
This led o a smalle sample size, which educed he s a is ical powe o
he compa ison be ween limbs, especially in he case o FHL. Las ly,
using isome ic plan a lexion o no malize EMG may no ully ac i a e
all examined muscles, which is an inhe en p oblem wi h EMG s udies.
In spi e o hese limi a ions, ou indings a e consis en wi h p e ious
s udies ha in es iga ed MG con ac ile beha iou (S ¨
audle e al., 2022)
and EMG ac i i y in he TS muscles a e ATR (Suydam e al., 2015;
Wenning e al., 2021).
5. Conclusion
MG unc ion du ing gai is ela i ely p ese ed 1-yea a e ATR in
non-su gically ea ed pa ien s. This sugges s ha e-modelling o he
MG muscle o sho e ascicles compensa es o he well-documen ed
MG a ophy and endon elonga ion a e ATR and migh be su icien
o enable adequa e o ce p oduc ion in daily ac i i ies such as walking,
whe e TS muscles ope a e a sub-maximal o ce le els.
Funding sou ces
This wo k was suppo ed by Academy o Finland (UNRESAT, g an
#323168/Taija Finni), and (ACHILLES, g an #355678/Taija Finni).
JW ecei ed unding om he E ick Laine In e na ional In e nship
Schola ship h ough he Uni e si y o Wisconsin-Madison. The unding
o ganiza ion had no ole in collec ion, analysis and in e p e a ion o he
da a, o publica ion.
CRediT au ho ship con ibu ion s a emen
Raad M. Khai : W i ing – e iew & edi ing, W i ing – o iginal d a ,
Visualiza ion, In es iga ion, Da a cu a ion. Jadyn Wa : W i ing – e-
iew & edi ing, Da a cu a ion. Ma ia Sukanen: W i ing – e iew &
edi ing, In es iga ion. Neil J C onin: W i ing – e iew & edi ing, Su-
pe ision, So wa e, Concep ualiza ion. Taija Finni: W i ing – e iew &
edi ing, Supe ision, Resou ces, Funding acquisi ion.
Decla a ion o compe ing in e es
The au ho s decla e ha hey ha e no known compe ing inancial
in e es s o pe sonal ela ionships ha could ha e appea ed o in luence
he wo k epo ed in his pape .
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