Ci a ion: A onso, J.; Peña, J.; Sá, M.;
Vi gile, A.; Ga cía-de-Alca az, A.;
Bishop, C. Why Spo s Should
Emb ace Bila e al Asymme y: A
Na a i e Re iew. Symme y 2022,14,
1993. h ps://doi.o g/10.3390/
sym14101993
Academic Edi o : Chia ella S o za
Recei ed: 23 Augus 2022
Accep ed: 20 Sep embe 2022
Published: 23 Sep embe 2022
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symme y
S
S
Re iew
Why Spo s Should Emb ace Bila e al Asymme y:
A Na a i e Re iew
JoséA onso 1,* , Ja ie Peña 2, Má io Sá3, Adam Vi gile 4, An onio Ga cía-de-Alca az 5,6 and Ch is Bishop 7
1Cen e o Resea ch, Educa ion, Inno a ion, and In e en ion in Spo (CIFI2D), Facul y o Spo ,
Uni e si y o Po o, 4200-450 Po o, Po ugal
2Spo and Physical Ac i i y S udies Cen e (CEEAF), Uni e si y o Vic–Cen al Uni e si y o Ca alonia,
08500 Vic, Spain
3Facul y o Human Kine ics, 1649-004 Lisboa, Po ugal
4Depa men o Nu sing and Heal h Sciences, Uni e si y o Ve mon , Bu ling on, VT 05405, USA
5Facul y o Educa ional Sciences, Uni e sidad de Alme ía, 04120 Alme ía, Spain
6SPORT Resea ch G oup (CTS-1024), CERNEP Resea ch Cen e , Uni e si y o Alme ía, 04120 Alme ía, Spain
7Facul y o Science and Technology, London Spo Ins i u e, Middlesex Uni e si y, London NW4 1RL, UK
*Co espondence: [email p o ec ed]
Abs ac :
(1) Backg ound: Asymme y is ubiqui ous in na u e and humans ha e well-es ablished
bila e al asymme ies in hei s uc u es and unc ions. Howe e , he e a e (mos ly unsubs an ia ed)
claims ha bila e al asymme ies may impai spo s pe o mance o inc ease inju y isk. (2) Objec i e:
To c i ically e iew he e idence o he occu ence and e ec s o asymme y and spo s pe o mance.
(3) De elopmen : Asymme y is p e alen ac oss se e al spo s ega dless o age, gende , o com-
pe i i e le el, and can be e i ied e en in appa en ly symme ic ac ions (e.g., unning and owing).
Assessmen s o bila e al asymme ies a e highly ask-, me ic-, indi idual-, and spo -speci ic; luc u-
a e signi ican ly in ime (in magni ude and, mo e impo an ly, in di ec ion); and end o be poo ly
co ela ed among hemsel es, as well as wi h gene al pe o mance measu es. Assessmen s o spo s-
speci ic pe o mance is mos ly lacking. Mos s udies assessing bila e al asymme ies do no ac ually
assess he occu ence o inju ies. While inju ies end o accen ua e bila e al asymme ies, he e is no
e idence ha p e-exis ing asymme ies inc ease inju y isk. While aining p og ams educe ce ain
bila e al asymme ies, he e is no e idence ha such educ ions esul in inc eased spo -speci ic
pe o mance o educed inju y isk. (4) Conclusions: Bila e al asymme ies a e p e alen in spo s, do
no seem o impai pe o mance, and he e is no e idence ha sugges s ha hey inc ease inju y isk.
Keywo ds:
symme y; bila e al asymme y; in e limb asymme y; la e ali y; inju y isk; pe o mance
1. In oduc ion— he Ine i abili y and Omnip esence o Asymme y
In spo s aining and i ness con ex s, symme y in ui i ely seems o be a concep
ha p ac i ione s chase in he hope ha i may enhance pe o mance and/o educe inju y
isk. Such is he s eng h o his belie ha se e al s udies associa e lowe limb bila e al
asymme ies (e.g., g ound eac ion o ces and kinema ic analyses) wi h inc eased inju y
isk despi e no ac ually ha ing assessed any ac ual isk [
1
–
10
], i.e., he occu ence o inju ies
was no egis e ed in such s udies. Such unsubs an ia ed discou se misleads he eade s
and may e en de ac om analyzing o he ( ele an ) a iables o hose wo ks. T aining
p og ams a e o en designed o dec ease in e limb asymme y and may achie e small
o mode a e (bu commonly non-signi ican ) educ ions [
11
,
12
] and, mo e a ely, small
o la ge educ ions in asymme y [
13
]. When such p og ams a e success ul in achie ing
wha hey had p oposed o achie e (i.e., educing in e limb asymme ies), he au ho s may
(e oneously) assume ha inc eased symme y is bene icial, wi hou ac ually assessing he
e ec o hese changes on inju y isk [
2
]. E en a sys ema ic e iew s a ed ha in e limb
asymme ies in se e al so-called “ unc ional asks” (e.g., isokine ic knee ex ension and
Symme y 2022,14, 1993. h ps://doi.o g/10.3390/sym14101993 h ps://www.mdpi.com/jou nal/symme y
Symme y 2022,14, 1993 2 o 18
lexion, single-leg e ical jump, and single-leg hop) we e undesi able and should be
p ope ly emedied [5], despi e inju y isk no being o mally assessed in any capaci y.
This sea ch o bila e al symme y (which includes bila e al unk asymme ies and
also in e limb asymme ies) seems o de i e mo e om deeply held belie s han om
empi ical indings [
14
], and isks placing pe o mance and inju y p e en ion p og ams on
he w ong ack. Inciden ally, c aniocaudal and do so en al asymme ies a e p omp ly
accep ed (i.e., people unde s and ha we need one head on op and wo ee on he bo om,
and ha we ha e wo eyes in on o he head bu none in he back). Somehow, i is wi h
bila e al asymme y in spo ha some in ou p o ession seem o s uggle o app ecia e.
Howe e , he e is hope: he 10 h edi ion o he ACSM guidelines o exe cise es ing and
p esc ip ion makes no men ion o symme y in gene al exe cise p esc ip ion [
15
], unlike
he 9 h edi ion [
16
]. Ac oss he en i e 10 h edi ion o he guidelines, he only e e ence
o symme y is he sugges ion o symme ical and hy hmical mo emen o people wi h
a he oid ce eb al palsy [15].
Biologically, he heal hy de elopmen o human emb yos equi es b eaking se e al
symme ies, including do so en al and c aniocaudal [
17
]; unsu p isingly, p ope emb y-
ologic de elopmen al also equi es b eaking le – igh symme y in e eb a es, including
humans [
17
–
21
]. Consequen ly, human ana omy is abundan wi h bila e al asymme ies.
Despi e an appa en ex e nal symme y, he e is conside able asymme y in ho acic and
abdominal o gans [
19
,
21
]. Fo example, he diaph agm is asymme ic, usually eaching
one ib highe on he igh side (due o he in luence o he li e ) and one lumba e eb a
lowe on he igh side (due o a longe pilla ) [
22
]. The ao a (which is also asymme ic)
p esses agains he e eb al column and usually p oduces a sligh ho acic cu a u e wi h
con exi y owa ds he igh side [
22
]. Many o he asymme ies can be pinpoin ed wi h
espec o in e nal o gans (e.g., he li e , hea , lungs, colon, panc eas, e c.), which a e all
commonly known. Howe e , he e a e many addi ional bila e al asymme ies in ascula
ne wo ks, he lympha ic d ainage sys em, and neu al pa hways [
22
]. In some cases, ne es
no only a e se dis inc pa hs on he igh and he le side o he body, bu hey also
p esen his ological di e ences, such as in he case o he ecu en la yngeal ne e in
humans [23,24].
Impo an ly, bila e al ana omical asymme ies in humans ex end o he uppe and
lowe limbs as well (e.g., wid hs, leng hs, neu omuscula pa hs, he di e en numbe
o muscle bellies, and among many o he ea u es) and ha e been widely desc ibed in
he
li e a u e [22,25–29].
A ecen e iew has syn hesized in e limb asymme ies in he
hams ings’ s uc u e and unc ionali y, highligh ing ha impo an in aindi idual asym-
me ies may exis (some o which a e no changeable wi h aining) [
30
]. Na u ally, la e ali y
(i.e., a la e al p e e ence o using one limb o e he o he in ce ain asks) coupled wi h
in e limb ana omical asymme ies sugges s ha common ac i i ies such as walking and
unning should esul in signi ican in e limb asymme ies in gai (kine ic and kinema ic) in
heal hy popula ions and a hle es [
31
–
33
], and ha spo s may ac ually equi e magni ying
ce ain asymme ies [14,34] (e.g., spo s wi h highly unila e ally-biased ac ions).
Gi en all he abo e, asymme y seems o be pa o na u e and in eg al o human
biology and beha io . In spo s, in e limb asymme ies a e highly ask- and me ic-
speci ic [
7
,
11
,
12
,
14
,
34
–
43
] and so should be analyzed wi hin p ope con ex . The goal
o his na a i e e iew was o p o ide an accoun on he ole o asymme y in spo s,
ocusing on i s ela ionships wi h pe o mance and inju y occu ence. We chose o pe o m
a na a i e e iew o deli e a b oad and c i ical o e iew o hese opics, ocusing on key
concep s and ying o ell a compelling s o y.
2. Bila e al Asymme y Is Ubiqui ous in Spo s Pe o mance
Bila e al asymme y (a b oad e m e e ing o bila e al unk asymme ies o in e -
limb asymme ies) should be expec ed o be and is (p obably) no e en clinically ele an
below he a bi a y h esholds o 5–15% [
5
,
7
,
11
,
44
–
47
], wi h an emphasis on he a bi a i-
ness o such h esholds since hey a e ask- and me ic-speci ic [
7
,
11
,
12
,
34
–
39
,
42
,
48
–
50
]
Symme y 2022,14, 1993 3 o 18
and likely also indi idual-speci ic [
42
,
49
–
53
]. La e ali y (including, bu no limi ed o,
handedness, mas ica ion, and eye dominance) is a well-es ablished ea u e in he animal
kingdom, including humans, and has been shown o be associa ed wi h inc eased unc-
ional pe o mance [
54
–
59
]. Impo an ly, bo h limb dominance and p e e ence may also be
ask-speci ic [
34
]. The e o e, in e limb asymme ies should no be au oma ically associa ed
wi h pe o mance impai men s [
14
,
36
,
60
]. Team spo s such as baske ball, handball, o ol-
leyball, o name some no o ious examples, may boos uppe limb asymme ies due o hei
highly specialized and di e en ia ed demands, and he same could be said o acque spo s
such as ennis, padel, o badmin on. In he case o he lowe limbs, bila e al asymme ies
a e he no m in bo h gene al popula ions and in a hle es [
12
]. Fo example, in 38 male
adul socce playe s, es ing ensiomyog aphy assessmen s showed signi ican in e limb
asymme ies [
61
]. The as us medialis con ac ion ime, ec us emo is sus ained ime and
hal - elaxa ion ime, and biceps emo is sus ained ime we e g ea e in he dominan limb,
while as us la e alis con ac ion ime and delay ime we e g ea e in he non-dominan
limb [61].
Mo eo e , we should conside he “noisy” cha ac e o asymme y and he inconsis-
ency o ou comes, whe eby in es – e es s udies he g oup mean asymme y o en appea s
o be ela i ely s able, bu he s anda d de ia ion (SD) o he mean is almos always e y
la ge (i.e., 50–100% o he mean asymme y alue) [
38
,
49
,
52
,
62
]. In con as o he mean,
when indi idual da apoin s a e conside ed, changes in asymme y’s magni ude and di ec-
ion a e usually qui e la ge [
38
,
39
,
49
,
50
,
52
,
53
,
62
–
66
], sugges ing cau ion when in e p e ing
he mean alue o any in e limb asymme y da a. In ac , in e limb asymme ies should
no e en be conside ed “ ue” asymme ies i hei alue is equal o o smalle han he
inhe en a iabili y o any gi en es [
42
,
53
] (e.g., coe icien o a ia ion and ypical e o
o measu emen ). Addi ionally, limb p e e ence and limb dominance a e no necessa ily
coinciden ; we e e he eade s o he wo k o Vi gile and Bishop [
34
] o an explo a ion
o his opic. To uly elucida e he ele ance o asymme y in spo s pe o mance, we
mus unde s and: (1) he occu ence and ela ionships o bila e al asymme y wi h a hle ic
pe o mance, and (2) he e ec s o aining in e en ions on in e limb asymme ies.
2.1. Occu ence and Rela ionship o Bila e al Asymme y wi h A hle ic Pe o mance
A ecen sys ema ic e iew showed ha , among se e al popula ions (including
17 s udies
wi h a hle es), in e -limb s eng h asymme ies anged om nea symme y
o asymme ies la ge han 15%, wi hou clea ela ionships wi h independen pe o mance
asks (such as isokine ic dynamome y, jump es s, and sea ed sho pu , among o he s) [
46
].
Fu he mo e, addi ional esea ch has shown ha in spo s esea ch, no clea -cu associa-
ions exis be ween bila e al asymme ies and pe o mance [
14
]. In addi ion, any indings
om c oss-sec ional s udies should be aken wi h a pinch o sal , since he ela ionships
be ween in e limb asymme y (magni ude and di ec ion) and pe o mance a e no neces-
sa ily consis en ac oss a spo s season [
34
,
39
,
42
,
63
,
64
,
67
]. One s udy assessed he g ound
eac ion o ces (GRF) o 13 plyome ics- ained subjec s (23
±
3 yea s, 8 male and 5 emale)
a e a 45-cm d op jump whe eby he subjec s had o change hei lead leg (i.e., he leg
ini ia ing he mo emen o s ep o he box) [
1
]. Upon landing, he lead leg gene a ed
g ea e o ces han he ail leg. In e es ingly, pai wise compa isons showed ha signi ican
in e limb di e ences occu ed only when he igh leg led he mo emen , wi h he igh
leg making ea lie g ound con ac and gene a ing g ea e o ce and impulse han he le
leg. Limb symme y indices (LSI) gene ally showed asymme ies
≥
10% [
1
]. This shows
ha e en in a simple d op jump ask (wi h low con ex ual in e e ence), a hle es exhibi
ma ked in e limb asymme y in hei pe o mance. Howe e , he d op jump heigh was no
assessed, and so i is unclea how he obse ed asymme ies impac ed jump pe o mance.
In high-le el male (n= 38) and emale judokas (n= 23), he maximum isome ic
s eng h o he shoulde ex e nal o a o s was signi ican ly supe io (albei wi h a small
e ec size o
η2
= 0.03) in he dominan side in compa ison wi h he non-dominan side, bu
he same was no obse ed o in e nal o a ion [
4
]. Asymme ies a o ing he dominan
Symme y 2022,14, 1993 4 o 18
side we e also de ec ed o he unila e al sea ed sho pu es (small e ec ,
η2
= 0.07), bu no
o he Y balance es [
4
]. In summa y, uppe limb dominance (i.e., a bila e al asymme y)
seemed o a ec he pe o mance o shoulde ex e nal o a ions and unila e al sea ed
sho pu , bu he e ec s we e e y small. In 26 male handball playe s (U18), in e limb
asymme y ac oss a a ie y o jump and change-o -di ec ion (COD) es s anged om ~3.7
o ~12.7% [
41
]. Mode a ely signi ican co ela ions (
ρ
= 0.41–0.51) we e ound be ween
isoine ial c osso e s ep asymme y (bu no la e al shu le s ep) and a COD o 90
◦
and
COD o 180
◦
in bo h limbs, as well as 20 m sp in ing. Howe e , in e limb asymme ies we e
also quan i ied in he se e al single leg jumps and COD es s, which we e independen o
pe o mance [41].
In a s udy in ol ing young eli e ennis playe s (n= 41; ~50% emales) and sex- and
age-ma ched con ols (n= 41), signi ican asymme ies we e egis e ed in bo h g oups
ac oss all se en uppe and lowe limb es s [
68
], which b oadly assessed s eng h, eac ion
ime, and COD. While ennis playe s we e signi ican ly mo e asymme ic han he con ols
wi h espec o handg ip s eng h, hey we e less asymme ic han he con ols in he
single leg coun e mo emen jump (CMJ) and 6 m single leg hop es s. These asymme ies
we e es -speci ic and, since he di e en es s we e no examined o possible co ela ions,
i is unclea how asymme ic pe o mance in one es could ha e a ec ed a di e en es .
Signi ican in e limb mo phological asymme ies (e.g., ci cum e ences and wid hs) we e
also ound o bo h ennis playe s and he con ols, al hough hese we e signi ican ly highe
in he ennis playe s o he uppe limbs (bu no he lowe limbs), which is o be expec ed
gi en he na u e o he spo [68]. Mo eo e , he da a om ha s udy sugges ed ha bo h
mo phological and unc ional asymme ies we e no signi ican ly ela ed o each o he (i.e.,
he mo phological asymme ies did no a ec pe o mance in he s eng h and COD- ela ed
es s) and should bes be in e p e ed independen ly [
68
]. Simila esul s we e ound by he
same esea ch g oup o a di e en sample o 22 high-le el emale ennis playe s [69].
When pe o ming he olleyball spike (which is a highly asymme ic ac ion, bo h o
he lowe and uppe limbs), a sys ema ic e iew showed ha a acke s landed asymme i-
cally >65% o imes, mos ly on he le leg, and signi ican in e limb kinema ic asymme ies
we e no ed o he ankle, knee, and hip join s upon landing (e.g., join lexion angle, ange
o mo ion, maximal lexion eloci y, and angle upon ini ial con ac wi h loo ) [
70
]. E en in
highly con olled expe imen al se ings whe e he a hle es we e explici ly asked o land
on bo h ee simul aneously [
71
–
73
], a unila e al landing was s ill he no m. The e o e,
asymme ic landing s a egies seem o be p e e ed in olleyball [
70
]. E en in supposedly
mo e symme ic ac ions such as blocking, playe s ha e been shown o be as e mo ing
owa ds he igh side han o he le side o he cou e en in highly-con olled asks
wi h educed con ex ual in e e ence [
74
]. Howe e , i is cu en ly unclea how hese
asymme ies a e ela ed o pe o mance ou comes, such as a ack o block jump heigh .
Fu he , i is unknown whe he olleyball playe s’ landing asymme ies a e associa ed wi h
a posi i e, neu al, o nega i e e ec on he pe o mance o hese key game ac ions.
In socce , in e limb asymme ies anging om
−
11.01
±
12.38% (U18) o
−18.43 ±12.11%
(U16) ha e been obse ed in 68 socce playe s when pe o ming 90
◦
COD asks [
75
]. No e
he e y la ge SDs, indica ing a la ge wi hin-g oup a iabili y. The COD ime was signi i-
can ly lowe o he dominan han o he non-dominan side (no e ec size a ailable) [
75
].
In addi ion, in socce , a s udy o 46 male p o essional playe s (26
±
6 yea s o age) e -
i ied 8.4
±
6.6% and 9.0
±
7.1% in e limb asymme ies in isome ic maximal olun a y
con ac ion (MVC) o que and isokine ic knee peak ex ension o que. These in e limb
asymme ies we e smalle han hose egis e ed o he a e o ea ly, in e media e, la e, and
peak o que de elopmen (RTD), which anged om 12.3
±
9.9% o 20.6
±
14.3% (no e
he e y la ge SDs) [
8
]. Howe e , he only asymme ies ha we e signi ican ly co ela ed
(albei weakly, i.e.,
ρ≤
0.36) wi h he In e na ional Knee Documen a ion Commi ee and
he Lysholm knee-sco ing scales we e ea ly and in e media e RTD [
8
], deno ing ha he
di e en asymme ies we e la gely independen om pe o mance.
Symme y 2022,14, 1993 5 o 18
In 16 male socce playe s (14.7
±
0.2 yea s), no signi ican ela ionships we e obse ed
be ween in e limb asymme y (single-leg Abalako es , 10 m (5 + 5) and 20 m (10 + 10)
COD, and isoine ial powe es ) and pe o mance es s (10, 20, and 30 m sp in s plus
CMJ) [
43
]. The a e age in e limb asymme ies a ied om 3.02
±
1.74 % (20 m COD) and
21.68
±
18.85% (peak powe in he isoine ial es ), bu he e we e e y la ge in e indi idual
a iabili ies (e.g., 0.14 o 57.37% in peak powe in he isoine ial es ) [
43
]. Ano he socce -
ela ed s udy ollowed 18 male U23 playe s o one season [
67
]. Du ing bo h he p eseason
and midseason, no signi ican ela ionships we e ound be ween in e limb asymme ies
and pe o mance in sp in and COD es s, no we e he e signi ican co ela ions be ween
changes in asymme y and changes in sp in and COD pe o mance ac oss he en i e
season [
67
]. In he au ho s’ own wo ds, “sugges ions o he educ ion o asymme y ha
may indi ec ly enhance a hle ic pe o mance canno be made” (p. 787) [67].
In young emale baske ball playe s (n= 29, 15.7
±
1.34 yea s o age), signi ican in e -
limb di e ences we e iden i ied o all neu omuscula asks (single leg CMJ in di e en
di ec ions, s a excu sion balance es (SEBT), and sp in wi h a COD) [
35
]. The e was poo
ag eemen (35 o 52%) be ween he mo e skilled lowe limb and he limb subjec i ely iden-
i ied as being he dominan limb, which suppo s p e ious sugges ions abou di e ences
be ween limb dominance and limb p e e ence [
34
]. While he bila e al asymme y indices
be ween he mo e and less skill ul limbs a ied om 3.33
±
2.49% (la e al CMJ dis ance)
and 14.11
±
8.62% ( e ical CMJ heigh ) [
35
], he au ho s did no assess he co ela ions
be ween he magni udes o asymme y and he es pe o mances. The e o e, while he e
we e bila e al asymme ies in pe o mance in each es , i is unknown i asymme ies in one
es co ela ed wi h pe o mance in ano he es , i.e., i he asymme ies we e ask-speci ic
o ansla ed in o mo e gene al pe o mance.
Ano he s udy in ol ing 11 emale baske ball playe s (U19) pe o med se e al es s
(10 m sp in , COD 90
◦
, COD 135
◦
, and single leg CMJ in h ee di ec ions) plus ou
exe cises on a lywheel machine (accele a ion s ep, decele a ion s ep, c osso e s ep, and
sides ep) [
50
]. Signi ican in e limb asymme ies we e shown o all he es s, anging om
1.26
±
0.94 % (COD 135
◦
) o 11.75
±
7.79% ( e ical single-leg CMJ). The au ho s co ela ed
he in e limb asymme y sco es in he lywheel exe cises ( ou exe cises, bo h in concen ic
and eccen ic o ms, in a o al o eigh condi ions) and he pe o mance wi h espec o
COD and unila e al jumps o bo h he highes and lowes pe o ming limbs (in a o al
o 14 condi ions). Signi ican co ela ions (
ρ
=
−
0.61 o 0.81) we e ound o only 5 o he
112 cells (i.e., combina ions) [
50
], all o which e e ed o co ela ions wi h he concen ic
sides ep. When co ela ing in e limb asymme y sco es in he COD and unila e al jump
es s wi h he lywheel exe cises, no signi ican co ela ions we e ound. By and la ge,
in e limb asymme ies we e, he e o e, no co ela ed wi h pe o mance es s.
One s udy compa ed 18 p o essional male socce playe s o 23 p o essional c icke
playe s wi h espec o pe o ming single-leg CMJ and d op jumps, a 10 m sp in , and
a 505 COD [
76
]. The c icke a hle es had signi ican ly g ea e in e limb asymme ies
in jump heigh (11.49% s. 6.51%) and in he eac i e s eng h index (RSI; 10.37% s.
5.95%) han he socce playe s [
76
], sugges ing ha in e limb asymme ies may also be
spo -speci ic (i.e., ce ain spo s may p omo e enhanced asymme ies due o hei speci ic
pe o mance demands). While hese asymme ies in jump heigh and RSI we e mode a ely
co ela ed wi h slowe 505 COD imes in he c icke playe s ( = 0.56–0.74), hey we e no
co ela ed wi h o he pe o mance es s [
76
]. No signi ican co ela ions be ween in e limb
asymme ies and pe o mance es s we e ound o he socce playe s [76].
Kine ic and kinema ic in e limb asymme ies a e equen e en in cyclic, bila e al
ac ions such as unning and cycling [
31
–
33
,
77
]. In U15 swimme s (n= 38; hal emale), he
di ec ion o asymme y (i.e., a o ing one limb o he o he ) was a ely consis en be ween
single-leg CMJ and single-leg s anding long jump pe o mances [
78
]. This implies ha
he assessmen o in e limb asymme ies may be highly ask-speci ic, in line wi h o he
wo ks [7,8,11,12,35–38,43,50,63,67]. The asymme y sco es we e no signi ican ly di e en
be ween males and emales and we e no associa ed wi h 25 m and 50 m on c awl
Symme y 2022,14, 1993 6 o 18
swimming pe o mance [
78
]. Conside ing he means plus he SDs, he asymme y le els
we e oughly wi hin he p e iously men ioned 10–15% ange [
78
]. Howe e , i should be
no ed ha a ecen e iew unde lined he la ge SDs p esen in many symme y- ela ed
s udies, sugges ing ha impo an in e indi idual a iabili y almos always exis s [
11
]. This
may pa ly explain why ela ionships be ween bila e al asymme y (noisy and luc ua ing)
and pe o mance (mo e s able) a e o en weak o negligible.
The kinema ic and kine ic join pa ame e s we e assessed in 10 eli e-le el male owe s
on a owing e gome e [
7
]. An a e age o 5–10% in e limb asymme y was obse ed when
assessing he kinema ic pa ame e s o he ankle and he kine ic pa ame e s o he hip
and knee join s (i.e., accele a ions) [
7
]. Asymme ies > 10% we e obse ed o he kine ic
ankle pa ame e s, including esul an o ce and ankle join accele a ion [
7
]. In his s udy,
he kine ic asymme ies we e unco ela ed wi h kinema ic asymme ies and wi h lowe
limb leng h asymme y [
7
], u he con ibu ing o he no ion ha asymme y assessmen s
may be ask- and me ic-speci ic [
7
,
8
,
11
,
12
,
35
–
39
,
41
,
43
,
53
,
65
]. The au ho s p oposed ha
a low in e -s oke a iabili y in asymme y could p o ide a mo e s able and e icien
pe o mance [
7
]. The in e -s oke a iabili y was said o be associa ed wi h 5–10% in e limb
asymme ies wi h espec o he accele a ion o he hip and knee join s and in he ankle
join angle, as well as wi h >10% asymme ies in he esul an o ce and in he accele a ion
o he ankle join [7], bu no alues we e p o ided o co obo a e hese claims.
In a is ic gymnas ics, all beam ou ines o he quali ica ion ound (19 gymnas s) o he
B Wo ld Cup ’2014 we e analyzed [
79
]: he igh lowe limb ini ia ed 42.3% o all ac ions,
he le limb 29.1%, and bo h limbs 28.6%, deno ing he asymme ic/la e al p e e ences
o op-le el gymnas s. Mo eo e , 60% o he ac ions on he beam implied a unila e al
ake-o and/o landing [
79
]. An analysis o six high-le el emale a is ic gymnas s showed
signi ican kine ic and kinema ic in e limb asymme ies in he uppe limbs upon con ac
wi h he loo when pe o ming he o wa d handsp ing on loo [
80
]. Howe e , in bo h
s udies [
79
,
80
], he ways in which hese asymme ies ela ed o pe o mance we e no
epo ed. In summa y, asymme ies appea o be he no m in spo s, bu we need a
deepe unde s anding o how hey ela e o pe o mance in di e en es s. As he e idence
p esen ed he ein shows, he ela ionship be ween pe o mance in s anda dized es s and
spo s-speci ic pe o mance is o en missing in he a ailable li e a u e o da e.
2.2. E ec s o T aining In e en ions o Reduce Bila e al Asymme ies in Spo s
The p e ious sec ion illus a ed ha bila e al asymme y seems o be he no m in
spo s. I a aining p og am aims o educe bila e al asymme ies (mos commonly, in e -
limb asymme ies), how success ul is i ? A s udy aimed o educe in e limb asymme ies in
24 male adul socce playe s (ama eu le el) [
13
]. The pa icipan s we e andomized in o a
6-week ( wice weekly) unila e al s eng h- and powe - aining p og am o in o a con ol
g oup. Despi e he expe imen al g oup ha ing imp o ed pe o mance, he in e limb asym-
me y was unchanged o he 505 COD es and wo o he h ee jump es s (single-leg CMJ
and single-leg b oad jump) [
13
]. The e was a mode a e educ ion in in e limb asymme y
in he single-leg d op jump s i ness (g= 1.11) and RSI (g= 1.00) [
13
]. Ano he andom-
ized s udy compa ed an 8-week ( wice weekly) s eng h- and powe - aining p og am
( ocused on he lowe limbs and unk) wi h con ols in 37 emale U17 socce playe s [
81
].
The in e en ion esul ed in no meaning ul di e ences in in e limb asymme ies om
p e- o pos - es ing [
81
]. Simila esul s we e ound wi h a compa able sample o emale
socce playe s (n= 36; U14), albei wi h a sligh ly longe in e en ion ( wice weekly o
10 weeks) [82].
A non- andomized s udy wi h 20 adul male socce playe s compa ed a 6-week co e
s abili y aining egimen wi h con ols pe o ming a “s anda d” wa m-up [
83
], consis ing
o jogging, dynamic s e ching, and mobili y [
83
]. The in e limb asymme ies in single-leg
CMJ we e educed a e he aining p og am o he “co e” aining g oup, bu inc eased in
he con ols, while no di e ences in in e limb asymme y we e obse ed o isokine ic knee
es ing [
83
]. Gi en he de ails o he ac ual “co e” aining egimen ha was implemen ed,
Symme y 2022,14, 1993 7 o 18
i is possible o specula e ha he educ ions in asymme y in he jump es s may ha e
come om he weake limb gaining s eng h and ha e no hing o do wi h co e s abili y.
Mo eo e , he SDs shown in able V o he o iginal s udy [
83
] a e su p isingly small in
compa ison wi h mos o he s udies on he opic.
In 34 male handball playe s (U17), an 8-week andomized in e en ion compa ed he
e ec s o isoine ial and cable- esis ance aining on in e limb asymme ies [
84
]. In e limb
asymme y was educed in only one o he eigh pe o mance es s (unila e al CMJ) [
84
].
In 22 U16 o U19 baske ball playe s, a andomized s udy compa ed unila e al and bila e al
esis ance aining o 6 weeks [
85
]. Bo h g oups imp o ed in he pe o mance es s such
as he CMJ and 25 m sp in , among o he s. Only he unila e al g oup showed educed
in e limb asymme ies (no e ec size p o ided) as assessed by he bila e al di e ence in he
maximum powe in an inc emen al unila e al squa es [
85
]. Howe e , he s anda dized
be ween-g oup di e ences c ossed ze o (i.e., he e was no signi ican di ec ion o he
e ec s) [
85
], e en while applying 90% con idence limi s, which a e na owe han he mo e
common 95% limi s. A andomized 8-week ( wice a week) s eng h- aining p og am was
con as ed wi h a olume-equa ed spo -speci ic aining egimen in 31 male olleyball
playe s (aged 14.5 ±0.5 yea s) [86]. Despi e signi ican changes in pe o mance (dynamic
balance es , single leg hop, CMJ, and back squa one epe i ion maximum), no di e ences
we e obse ed wi h espec o in e limb asymme y as assessed by he single-leg hop
es s [86].
A andomized s udy wi h U12 male weigh li e s applied a 6-week ( wice weekly)
hams ing eccen ic aining p og am e sus con ols; no changes in in e limb asymme y
(de i ed om bila e al di e ences in he pe o mance o he single leg hop es ) we e no ed
om p e- o pos -in e en ion [
87
]. In a andomized s udy wi h wen y- h ee U14 ennis
playe s (male and emale), one g oup pe o med 12 sessions ( wo sessions pe week o e
6 weeks) ocused on balance aining, while he o he g oup pe o med only ennis-speci ic
d ills o he same pe iod [
88
]. The lowe limb bila e al asymme ies we e signi ican ly
educed in he balance- aining g oup only o he h ee es s ha we e pe o med (single-
leg hop, side-hop and side s eps, and o wa d 4.115 m es [4m-SSF]) [
88
], bu he e was no
assessmen o he e ec size o quan i y he magni ude o he e ec s. Howe e , no pos - es
imp o emen s we e no ed wi h espec o he COD and speed es s in his g oup (i.e., he
asymme ies we e educed du ing jumping, bu he playe s did no become as e ).
O e all, he e icacy o aining p og ams designed o dec ease in e limb asymme y
a e he e ogeneous [
11
–
13
]. E en when aining p og ams educe in e limb asymme y
wi h a concomi an inc ease in he pe o mance o he selec ed es s (e.g., CMJ and 10 m
sp in ), he indings a e o en mixed [
12
]. I is also possible ha he indings a e neglec ing
an impo an con ounde : he ceiling e ec [
14
,
53
,
89
]. The “weake ” limb may be u he
om i s maximum capaci y, being mo e sensi i e o aining s imuli and showing g ea e
imp o emen han he “s onge ” limb [
14
,
89
]. This could esul in educed in e limb
asymme y, and is e en mo e likely o occu in he case o al eady inju ed a hle es [
53
].
Beyond ha , he p e iously men ioned p oblems wi h he eliabili y o es – e es assess-
men s o lowe limb asymme ies and wi h only analyzing he a e age alues ( he eby
neglec ing o accoun o he in e indi idual a iabili y) limi ou unde s anding o how
well such p og ams wo k on an indi idual le el. Addi ionally, he a o emen ioned aining
p og ams mos ly used s anda dized physical es s. The ew spo -speci ic assessmen s we e
pe o med unde e y analy ical condi ions. The e o e, i is unclea whe he any educ ions in
in e limb asymme y we e posi i e o desi able o enhanced a hle ic o spo ing pe o mance.
2.3. Wha Does This All Mean?
The sum o hese indings shows ha in e limb asymme ies a e an in insic pa o
high-le el pe o mance, e en in supposedly mo e symme ic ac ions, and he e ec s o
aining in e en ions on asymme y and hei consequences o pe o mance a e unclea .
S ill, i is possible o specula e ha e en i i was es ablished ha asymme ies con ibu e
Symme y 2022,14, 1993 8 o 18
posi i ely o spo s pe o mance, hey could p oduce a ade-o , inc easing inju y isk [
90
].
Tha is he subjec o he nex sec ion.
3. Asymme y Is No Necessa ily Rela ed o Inju y Occu ence
Exp essions such as “inju y isk” and “inju y p e en ion” b ing abou complex con-
cep s such as “p edic o s” o inju y. P edic ion equi es ha a p ospec i e s udy asce ains
a ela ionship be ween he pa ame e and inju y occu ence, and ha a causal ela ionship
is es ablished. Fu he mo e, o hose causal ela ionships o be in e p e ed as p edic i e,
hey mus be eplica ed and exhibi a good i in di e en samples. Commonly, all we ha e
a e ela ionships es ablished o a speci ic sample a e he inju ies ha e occu ed. Mo e
de ailed accoun s o his deba e can be ound elsewhe e [
91
–
93
]. Fo he emainde o he
manusc ip , we will e e o inju y occu ence ins ead o inju y isk.
I is in ui i ely accep ed ha inju ies may cause inc eased asymme ies [
45
,
53
,
94
],
while i is no so clea ha p e-exis ing asymme ies inc ease inju y occu ence. The e o e,
obse a ional s udies compa ing heal hy e sus inju ed o p e iously inju ed subjec s a e
po en ially misleading, as hey easily in i e he eade s o in e causali y, e en when
he au ho s s a e ha only an associa ion was measu ed. Se e e inju ies (such as muscle-
endinous up u es) a e likely o inc ease he na u ally occu ing asymme ies in a hle es,
esul ing in excessi ely la ge in e limb asymme ies, owing o he ac ha one limb is
incapaci a ed [
53
]. Hence, we unde s and he a emp o minimize such asymme ies
du ing ehabili a ion and e u n o spo o e u n o play p ocesses [
53
]. Rega dless, we
ques ion i ixed, a bi a y in e limb symme y h esholds should always be achie ed,
as was p e iously discussed. Un o una ely, se e al s udies ela e asymme ies o inju y
occu ence wi hou e en p o iding basic associa i e da a [
1
–
10
], po en ially due o a
eliance on p e-concei ed assump ions and belie s on he subjec . Fo example, in a
p e iously men ioned s udy whe eby a hle es pe o med a d op jump ask [
1
], he po en ial
implica ions o he landing asymme ies owa ds inju y occu ence we e discussed a leng h.
Howe e , his s udy did no di ec ly assess inju y occu ence, and so such asymme ies
a e me ely he ealiza ion o a ac and should no be used o in e subsequen inju y
occu ence. In his sec ion, we will del e in o obse a ional s udies i s , and p ospec i e
s udies second.
3.1. Wha Obse a ional S udies Really Say abou Bila e al Asymme y and Inju y Occu ence
In p o essional socce and baske ball a hle es wi h unila e al ch onic ankle ins abili y
(CAI), he e was in e limb asymme y be ween he inju ed and non-inju ed ee in he
baske ball a hle es wi h espec o he e ical ime o s abiliza ion du ing he pos u al
sway upon landing wi h bo h legs (~1.3 s equi ed o he inju ed oo ), bu no in he
socce a hle es [
3
]. The au ho s concluded ha balance exe cises should be p o ided o
baske ball and socce a hle es o educe ecu en ankle sp ains. Howe e , no assessmen
o inju y ecu ence was p o ided and so hese sugges ions emain pu ely specula i e.
In he p e iously ci ed s udy wi h high-le el male and emale judokas [
4
], in e limb
asymme ies in ex e nal o a ion s eng h we e no mal and he e was no an assessmen o
inju y occu ence. Ne e heless, he au ho s s a ed ha p e en ion p og ams should apply
simila wo kloads on bo h uppe limbs and a emp o achie e g ea e symme y.
An associa ion s udy wi h 33 badmin on playe s (19 male and 14 emale;
14.4 ±1.2 yea s
o age) assessed in e limb asymme ies in uppe and lowe limbs h ough ine ial mea-
su emen uni s (IMUs) and compa ed hose wi h inju y his o y [44]. No signi ican uppe
o lowe limb asymme ies we e de ec ed o he p e iously bila e ally inju ed g oup.
Al hough signi ican asymme ies we e shown o he non-inju ed (<10%) and o he
unila e ally inju ed g oup (>10%), he be ween-g oups di e ence in he asymme y mag-
ni udes was no s a is ically di e en . These esul s sugges ha some deg ee o in e limb
asymme y should always be expec ed and is no necessa ily associa ed wi h inju y occu -
ence. In he same ein, knee lexion and ex ension s eng h, hop dis ance, and e ical jump
heigh we e compa ed be ween 47 a hle es wi h ACL econs uc ion and 46 non-inju ed
Symme y 2022,14, 1993 9 o 18
a hle es [
45
]. The a hle es wi h ACL econs uc ion we e mo e asymme ic wi h espec
o knee lexion maximal s eng h han he non-inju ed a hle es (~17% s. ~9% in e limb
asymme y, espec i ely). No be ween-g oup di e ences we e ound wi h espec o he
in e limb asymme y o he emaining h ee es s. Fo he jump es s, his is no su p ising,
gi en ha measu es o dis ance and heigh o en mask biomechanical and s a egy-based
in o ma ion [
95
,
96
]. Indeed, only a mino i y o a hle es (24% in he non-inju ed g oup and
17% in he inju ed g oup) showed symme ic (i.e., <10% asymme y) pe o mances in all
ou es s [45].
In socce , a s udy showed ha 15 young male playe s had signi ican plan a p essu e
asymme ies (e.g., hallux, i h me a a sal, and medial ea oo ), wi h g ea e p essu es
applied using he non-p e e ed oo [
6
]. These asymme ies we e no obse ed in he
15 ma ched con ols. Despi e he au ho s ela ing such asymme ies o he isk o s ess
inju ies (i is e en in he i le o he pape ), inju y occu ence was no di ec ly assessed.
In a s udy wi h 159 p o essional socce playe s, in e limb asymme ies in isokine ic knee
lexion and ex ension we e un ela ed o compe i i e le el o a his o y o inju ies [
97
]. S ill,
he au ho s p oposed p esc ibing exe cise o co ec o he obse ed asymme ies. Wi h
ega d o isokine ic assessmen s o he knee, in e limb asymme ies o 19.5–31.7% in he
knee ex ension s eng h and 36.6–51.2% in knee lexion ha e been shown o eli e U16
male socce playe s [
47
], bu his s udy once again did no assess any ela ionships wi h
inju y occu ence.
In 25 eenage c icke as bowle s (16 wi h and 9 wi hou lowe back pain [LBP]), he
symme y o he abdominal muscle mo phology (i.e., he combined hickness o he ex e nal
and in e nal obliques and ans e sus abdominis) was ac ually associa ed wi h LBP [
98
],
while he playe s wi hou LBP had a supe io hickness o he abdominal wall con ala e al
o he dominan uppe limb (i.e., he heal hy a hle es we e mo e symme ic). Possibly,
hese unc ional asymme ies a e he by-p oduc o he epea ed la e al lexion o he unk
du ing as bowling ac ions and his ein o ces he concep ha some spo s may equi e
abo e-no mal bila e al asymme ies o unc ion in a gi en ask [
14
,
34
], which has been
p e iously sugges ed in o he spo s as well [99].
One s udy asked 6 owe s wi h a his o y o back inju ies and 19 non-inju ed owe s o
pe o m 30 s okes wi h maximal exe ion in a owing machine [
100
]; he lowe limb GRF
asymme ies we e no signi ican ly di e en in he owe s wi h o wi hou a his o y o back
inju y [
100
]. In 12 p o essional unne s, an associa ion be ween in e limb asymme ies in
he g ound con ac imes and he inju y his o y o he lowe limbs in he p e ious 24 mon hs
was es ablished o he 400 m dis ance, bu no o he o he dis ances (600 m, 800 m, and
1000 m) [
33
]. I was unclea whe he such asymme ies induced inc eased inju y occu ence
o we e caused by p e ious inju ies (i.e., ankle sp ains, Achilles endinopa hy, and shin
endinopa hy) [33].
This is no o say ha any deg ee o asymme y is desi able, as i is possible ha
when su passing ce ain h esholds (subjec o in e - and in aindi idual a ia ion) inju y
occu ence may inc ease [
60
]. Pa ien s wi h p ima y ACL econs uc ion ha unde wen
6-mon h unc ional and isokine ic es ing and a minimum 2-yea ollow-up we e assessed
o a seconda y ACL inju y [
101
]. O he 344 pa ien s, 59 (17%) had a seconda y ACL inju y
~5 yea s a e he p ima y inju y. The LSIs in isokine ic es ing we e signi ican ly g ea e in
he e-inju ed g oup wi h espec o 60
◦
/s knee ex ension and lexion. Howe e , ega ding
knee ex ension, bo h g oups p esen ed an in e limb asymme y
≥
18%, wi h only a 7%
be ween-g oup di e ence. Fo knee lexion, bo h g oups p esen ed in e limb asymme ies
ha migh no be conside ed clinically ele an (
≤
8%), wi h only a 5% di e ence be ween
he wo g oups. No be ween-g oup di e ences we e ound o in e limb asymme ies in he
single-leg hop and iple hop dis ances [
101
]. Howe e , ano he s udy ound an inc eased
deg ee o in e limb asymme y in male a hle es wi h ACL econs uc ion (n= 26) e sus
heal hy con ols (n= 22) [
94
], while in e limb asymme ies o 23% (single leg d op jumps)
and 17% (single leg e ical jumps) whe e egis e ed in inju ed subjec s, and only 0–2%
asymme ies we e obse ed in heal hy a hle es.
Symme y 2022,14, 1993 16 o 18
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