la importancia del control sensorio motor en la estabilidad de core articulo de revisión

7/23/2019 La Importancia Del Control Sensorio Motor en La Estabilidad de Core Articulo de Revisin

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. . . . . . . Ap>.ri ii Sports Me a 2008; 38 11); 893.916EVIE W RTICLE 0 1 1 2 1 M 2 / 0 8 / 0 0 1 1 . 0 8 9 3 / S 4 8 . 0 0

2008 Adls Data Information B V Ail rights reserve d.

The Importance o Sensory Motor Controlin Providing Core StabilityImplications for Measurement and Trainingan Borghuis ^ At L . Hof an d Koen A.P.M. Lemmink^

1 Center for Hu ma n Movement Sciences U niversity Medical Center Groningen University of GroningeGroningen the Netherlands

2 School of Sports Studies Ha nze University of Applied Sciences G roningen the Ne therla nds

Contents

Abstract 8941 A bo ut Core S tability 895

1.1 The Co re 8951.2 Co re Sta biiity 8961.3 im p or ta n t Structures in iVlaintaining Co re S tabiiity 8971.4 Loca i a n d Gio ba i iViuscie Systems 8971.5 iHip M uscuia ture 8991.6 S tabiiity versus M obiiity 8991.7 Stren gtin, End uran ce an d Sensory-iViotor Co ntro i 8991.8 Senso ry-Motor Con troi 900

2. Care Stabiiity, Ath letic Perform an ce a n d injury 9012.1 Care Stability a n d Athietic Perfarma nce 9012.2 Co re Stabiiity a n d injuries 9012.3 The Raie af th e iHip Mus cuiature in injury Occ urre n ce 9022.4 Streng th versus Endurance 9022.5 Sp inal instabiiity Caus ed by Neuramuscular im ba ian ce

in th e Locai M uscie System 9032.6 Spinai Instabiiity Caus ed by Neuromuscuiar Im baia nc e

in th e Gio bai M uscle System 9032.7 Sen sary-M atar Controi an d injuries 904

3. Care Stabiiity, Neuram uscuiar Core Con troi a n d Baia nce 9053.1 D eia yed M uscie Refiex Response in Patients with Low Back Pain 9053.2 Neuromuscuiar im ba ian ce in Patients with Law Baci< Pain 9063.3 Baiance Pe rforma nce in Reiation ta Ca re Stabiiity 9073.4 Reiationship betwe en Baiance Perform ance a n d

Neuramuscuiar Care Cantrai 9074. Training Co re S tability 908

4.1 Functian ai Training of Both th e Laca l a n d Gia bai M uscie System 9084.2 Care Streng thening : Caa rdina tive an d Prapriaceptive Training 9094.3 Swiss-Baii Train ing 909

5. M eas uring Care Stabiiity 9105.1 M easuring Care Strength an d Endurance 9105.2 M easuring Neuromuscular Cantrai a n d Caard inatian 911

6 C i i d R d ti f F th R h 913


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894 Borghuis

bstr ct Although the hip musculature is found to be very impo rtant in connectithe core to the lower extremities and in transferring forces from and to tcore, it is proposed to leave the hip musculature out of consideratiowhen talking about the concept of core stability. A low level of co-contration of the trunk muscles is important for core stability. It provides a levof stiffness, which gives sufficient stability against minor perturbationNext to this stiffness, direction-specific muscle reflex responses are alimpo rtant in providing core stability, particularly when encountering suddperturbations.

It appears that most trunk muscles, both the local and global stabilizati

system, must work coherently to achieve core stability. The contributions the various trunk muscles depend on the task being performed. In the searfor a precise balance between the am oun t of stability and m obility, the rolesensory-motor control is much more important than the role of strength endurance of the trunk muscles. The CN S creates a stable foundation movement of the extremities through co-contraction of particular muscleAppropriate muscle recruitment and timing is extremely important in prviding core stability.

No clear evidence has been found for a positive relationship between costability and physical performance and more research in this area is needeOn the other hand, with respect to the relationship between core stability ainjury, several studies have found an association between a decreased stabiland a higher risk of sustaining a low back or knee injury. Subjects with suinjuries have been shown to demonstrate impaired postural control, delaymuscle reflex responses following sudden trunk unloading and abnormtrunk muscle recruitment patterns. In addition, various relationships habeen demonstrated between core stability, balance performance and activtion characteristics of the trunk muscles. Most importantly, a significacorrelation was found between poor balance performance in a sitting balantask and delayed firing of the trunk muscles during sudden perturbation. was suggested that both phenomena are caused by proprioceptive deficits.

The importance of sensory-motor control has implications for the deveopment of measurement and training protocols. It has been shown thchallenging propriocepsis during training activities, for example, by makiuse of unstable surfaces, leads to increased demands on trunk musclethereby improving core stability and balance. Various tests to directly indirectly measure neuromuscular control and coordination have been dveloped and are discussed in the present article. Sitting balance performanand trunk muscle response times may be good indicators of core stability. light of this, it would be interesting to quantify core stability using a sittinbalance task, for example by making use of accelerometry. Further researchrequired to develop training programmes and evaluation methods that asuitable for various target groups.


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Sensory-Motor Control and Core Stability 895

prevention of the back and the lower extremities

and what effect core stability has on power andendurance during athletic performance. How-ever, when talking about the concept of corestability, it is relevant to first have a commongeneral definition, thereby assuring that the dis-cussion surrounds one and the same concept.

The purpose of the present article is to givean overview of the existing literature with respectto several issues related to core stability. Thereare various notions about the composition andfunctioning of the core. Several attempts to de-fine core stability have been found in the litera-ture. This article looks at the importance of corestrength within the concept of core stability. Anoverview is given of the relevant body structuresand tissues that are important in providing corestability with special attention to the muscular

;system, Furthermorey the role of strength, en-durance and sensory-motor control is reviewed.

In the third part of this article, literaturefindings will be discussed regarding the relation-

ship between core stability on the one hand andathletic performance and injury on the otherhand. The role of deficiencies in sensory-motorcontrol with respect to clinical instability andlower extremity and low back injuries will bediscussed in further detail.

In the existing literature, core stability is oftenassociated with the maintenance of balance,especially in measurement and training proce-dures. To clarify this relationship, an overview is

given about the association between core stabi-lity, balance performance and the results of var-ious studies in which electromyographic (EMG)measurements of the trunk muscles have beenmade during perturbation tasks. First, some stu-dies are discussed in which muscle reaction timesand muscle recruitment patterns have been in-vestigated in patients with low back pain. Subse-quently, the relationship between core stabilityand balance is taken into consideration andeventually an overview is presented about the

correlations found between balance performanceand EMG-measurement results .

is given to the effects of exercises in which a Swis

ball is used as a training aid.Finally, we look at the current ways in whiccore stability is being measured. First, somissues are discussed with respect to the measurement of trunk muscle strength and enduranceEventually, several tests will be considered thahave been used to directly or indirectly measurneuromuscular control and coordination,

PubMed was used to search for articlesSearch terms included 'core stability', 'trunk stability', 'lumbar spine stability', 'core strengthand 'neuromuscular core control ' and these wercombined with terms such as 'measurement' o'training'. Various review articles were founcontaining useful references. The present articlis based on a selection of the most valuabland relevant of these articles,

1 About Core Stability

The term 'core stabihty' has received a lot oattention, especially in the past few years. It stated that core stability is a key component ithe training programmes of individuals who araiming to improve their health and physical finess, but core stability is also an important concept in clinical rehabilitation and in the traininof competitive ath letes, ' ' '

1,1 The C ore

Particular attention has been paid to the corbecause it serves as the centre of the functionakinetic chain. The core is seen as a musculacorset that works as a unit to stabilize the bodand in particular the spine, both with and withoulimb movement,[^1 In the alternative medicinworld, the core has been referred to as the 'powehouse', the foundation or engine of all limmovement,'^^ Kibler et al,'^' also stressed the importance of the core in providing local strengtand balance, in decreasing back injury and i

maximizing force control.The core of the body includes both passiv


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Borghuis et

Akuthota and Nadler^^ described the core as a

box with the abdominals in the front, the para-spinals and gluteals in the back, the diaphragmas the roof and the pelvic floor and hip girdlemusculature as the bottom, Kibler et al,^^^ statethat the musculoskeletal core of the bodyincludes the spine, hips and pelvis, abdominalstructures and also the proximal lower limb. Ac-cording to them, the core musculature includesthe muscles of the trunk and pelvis,f- Thesemuscles are responsible for the maintenance ofstability of the spine and pelvis and help in thegeneration and transfer of energy from large tosmall body parts during many sports activities.So, in addition to its stabilizing function, the coremusculature also has a mobilizing function,( l

1,2 Core Stability

There is no single universally accepted defmi-tion of core stability, Panjabi'^^ presented a con-ceptualization of core stability that is based onthree subsystems: the passive spinal column,active spinal muscles and a neural control unit.Based on this conceptualization, Liemohn et al,''^defmed core stability as "the functional integra-tion of the passive spinal column, active spinalmuscles and the neural control unit in a mannerthat allows the individual to maintain the inter-vertebral neutral zones within physiological lim-its, while performing activities of daily living,"Kibler et \P' defmed core stability as "the abilityto control the position and motion of the trunkover the pelvis, thereby allowing optimum pro-duction, transfer and control of force and motionto the terminal segment in integrated athletic,kinetic chain activities," Leetun et aL^^' stressedthe importance of the passive structures to a lesserdegree and stated that core stability can be seen asthe product of motor control and muscular ca-pacity of the lumbo-pelvic-hip complex. Thisdefmition stresses the importance of coordinationin addition to core strength and endurance.

Although the terms core stability and corestrength are sometimes used interchangeably,core strength is just one part of the core stability

McGill and Cholewicki'^' formulated a b

mechanical foundation for stability that giuseful insights in the complex interactions tare involved when stabilizing the core. Ththeory was based on and elaborated on the wof Bergmark,[*l who mathematically formalithe concepts of energy wells, stiffness and stality. A ccording to McGill and Cholewickithe foundation of core stability begins with concept of potential energy. For musculoskeleapplication, the focus is mainly on elastic pottial energy. Elastic bodies possess potentenergy by virtue of their elastic deformatunder load and this elastic energy is recoverwhen the load is removed,'^' The greater stiffness, the more stable the structure. Thstiffness creates stability,'^^ Joint stiffness creases rapidly and nonlinearly with musactivation, so that very modest levels of musactivity create sufficiently stiff and st ble jointsFurthermore, joints possess inherent joint stiff-ness through their ligaments and other capsu

structures. These structures contribute to stiff-ness, which increases towards the end rangejoint motion,'^' All stabilizing musculature mwork coherently to achieve stability,'^'

For the purpose of their study, Zazulet al, '*'"'^ developed a more operational deftion. They defined core stability as "the bodability to maintain or resume an equilibriposition of the trunk after perturbation,"

It should be considered here that, while

focus of McGill and Cholewickil^l is mainly the anticipating reaction of the core structuduring small, expected perturbations, Zazuet al,'^'"^^ also assume greater, unexpected dturbances of the core, in which the creationstiffness does not suffice to maintain stabilIf the initial spine stability is insufficient relation to the external load applied by a pertbation, a fast and strong reflex response ccompensate, in order to constrain trunk motwithin a safe boundary. Such a direction-specimuscle reflex response may be crucial in pventing large intervertebral displacements


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1.3 Importan t Structures in Ma intaining

Core StabiiityLumbar spine stability is provided by bone,

disc, ligaments and muscle restraints.1'^1 As notedabove (section 1.2), stability of the lumbar spinerequires both passive stiffness, through theosseous and ligamentous structures and activestiffness, through muscles.'^l If any of the activeor passive components are impaired in function,instability of the lumbar spine may occur. It hasbeen shown that the musculature is most im-portant in maintaining spinal stability undervarious conditions.''^^ Panjabi'^] suggested thatmuscle activity is used to compensate for a loss ofpassive stability. It has been shown that musclescan contribute to stabihty of the trunk throughco-contraction.['' ' Healthy subjects increaseco-contraction in response to conditions thatthreaten spinal stability. This adaptation is trig-gered by information from both mechan-oreceptors and nociceptors.t'^^ Co-contraction

further connects the stability of the upper andlower extremities via the abdominal fascial sys-tem. This effect becomes particularly importantin overhead athletes, because the created stableconnection acts as a torque-counter torque ofdiagonally related muscles during throwing.t^' Toacquire this co-contraction, precise neural inputand output are needed. Arokoski et al. ' ' '*' identi-fied that the stability of the spine was increasedwith either increased flexor-extensor muscle co-activation or increased intra-abdo mina l p ressure.In the temporal sequence of many athletic tasks,core muscle activity precedes lower extremitymuscle activity. Hodges and Richardson,' '^^ forexample, demonstrated that trun k m uscle activityoften occurs bef'ore the activity of the lowerextremity m usculature. This implies that the CN Screates a stable foundation for movement of thelower extremities through co-contraction of par-ticular muscles.''^^

Kavcic et al. ' '* conducted a systematic bio-mechanical analysis in order to assess the poten-tial stabilizing role of individual lumbar muscles.

and these control patterns change as the spin

loading patterns change.' '^^ The same result wafound by means of biomechanical analysiconducted by Cholewicki and McGill' '^1 anCholewicki and VanVliet,' '*' who suggested thano single muscle possesses a dominant responsibility in providing lumbar spine stability. Generally, those muscles that were antagonist tothe dominant moment of the task were moseffective at increasing stability.''^'

It appears that most trunk muscles ar

important in providing core stabihty, their importance depending on the activity being performed.'^l These include muscles that attach directly to the vertebrae: the uni-segmental multifidus muscles and multi-segmented quadratulumborum and longissimus, and muscles that dnot: iliocostahs and the abdominal wall. Acrosthe various trunk muscles, the mechanical advantage to provide stability to the lumbar spinvaries. It should be born in mind that this varietis functional. This can be illustrated by the example of a long-guyed mast. Guy wires, wirerunning from the top and a few levels below tthe ground, are needed to keep the mast uprightbu t the ma st should a lso have sufficient stiffnesby itself to prevent buckling. In the back, thesfunctions are provided by the global stabilizationsystem (GSS) and local stabilization system(LSS), respectively.

1.4 Locai a nd Global Muscle Systems

Comerford and Mottram''^^ stated that almuscles have the ability to concen trically shorteand accelerate motion for mobility function, tisometrically hold or eccentrically lengthen anddecelerate motion for stabihty function and tprovide afferent proprioceptive feedback to thCNS for regulation and coordination of musclfunc tion. B ergmark'^1 pro po sed a classificationscheme that groups core muscles into either thGSS or the LSS. The larger muscles of the trunkare the chief contributors to the GSS and thsmaller muscles are the main contributors to th


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898 Borghuis et

larger masses and longer moment arms of force,

provide more forceful movements.t'l The LSSmuscles are closer to the spinal column and thuscan provide varying degrees of segmental contro l.For example, the intertransversarii mediales, in-terspinales and rotators are very close to thecentre of rotation of the spinal segments. Theirhigh density of muscle spindles and their verysmall physiological cross-sectional area suggeststhat they may act primarily as position transdu-cers of the spinal column.' ''^ Following the workof Bergmark,i*l Comerford and Mottramt'^l havealso proposed a classification system for musclefunction. They have characterized muscles aslocal stabilizers, global stabiUzers and globalmobilizers. The function of the local muscle sys-tem is to provide sufficient segmental stabilityto the spine, whereas the global muscle systemprovides general trunk stabilization and enablesthe static and dynamic work necessary for dailyliving and sport activities.'^'^ A symmetrical ac-tivation of the local muscles has been shownduring the performance of low load, asymmetriclifting tasks, which suggests that these musclesplay a stabilizing role during these activities. Theglobal muscles, however, show asymmetric pat-terns of activation during the same tasks, sup-porting their role of global stabilizers and primemovers.t^'^ It has been identified that the multi-fidus, transversus abdominis and the internalobliques are part of the local stabilizing system,whereas the longissimus thoracis, rectus abdo-

minis and external obliques constitute a part ofthe global stabilizing system.t^'l

Recent work has focused on the functionalcontribution of different trunk muscles to pos-tural stabilization of the lumbar spine as well astheir respective changes in the presence of acuteand chronic pain. Although the sensory-motorcontrol of spinal stability is provided in a mutualinteraction among all muscles of the trunk,Ebenbichler et al.t'^' described four major func-

tional groups of muscles that contribute via dif-ferent mechanisms to the postural stabilization ofthe spine: (i) local paravertebral muscles that

resulting forces on the spine; (iii) muscles t

contribute to pressure facilitation within abdominal cavity, thereby providing glostabilization of the spine; and (iv) muscles tfacilitate the pressure within the fascia tusystem of the back.''^'

Cholewicki and VanVliet^'^] reported that trunk muscles, including abdominal as wellback musculature, contribute to core stabilThe relative contributions of each muscle grocontinually change throughout an athle

task.'^ The abdominals serve as a vital comnent of the core. In particular, the transverabdominis has received a lot of attention.Contracting the transversus abdominis increaintra-abdominal pressure and tensions the thacolumbar fascia. The thoracolumbar fascia isimportant structure that connects the lower lim(via the gluteus maximus) to the upper lim(via the latissimus dorsi). It helps to form a 'hoaround the abdomen, consisting of the fasposteriorly, the abdominal fascia anteriorly athe oblique muscles laterally.'^^ This way, a sbilizing corset effect is created. Together, internal oblique, external oblique and transverabdominis increase the intra-abdominal pressinside the hoop formed via the thoracolumfascia, thus creating functional stability of lumbar spine.^^' Contractions that increase inabdominal pressure occur before initiation large segment m ovement of the upper limbs.'this manner, the spine is stabilized before li

movements occur, thereby allowing the limbshave a stable base for motion and muscle actition. Thus, abdominal muscle contractihelp in creating a rigid cylinder, thereby enhaing stiffness of the lumbar spine. It is aimportant to note that the rectus abdominis oblique abdominals are activated in directispecific patterns with respect to limb movemethus providing postural support before limovements.''^

According to Ebenbichler et al.,^'^ the bmuscles are clearly divided into two magroups: (i) the deep muscles of the lumbar sp


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intertransversarii mediales and laterales; and (ii)

the long erector spinae muscles that span manysegments. ^] These two distinct functional musclegroups have large differences in innervation,which indicates significant functional differences.The stabilizing role of the paravertebral musclesaims mainly at protecting the articular structures,discs and ligaments from excessive bending,strains a nd injury.t ^ Ac cording to Bergm ark, the role of the long, multisegmental back musclesis to provide general trunk stabilization and tobalance external loads, thereby helping to unloadthe spinal segments.

1.5 Hip M usculature

At the opposite end of the trunk co mpo nent ofthe core muscles are the pelvic floor muscles.Most of the prime mover muscles for the distalsegments (latissimus dorsi, pectoralis major,hamstrings, quadriceps and iliopsoas) attach tothe core via the pelvis and spine. Most of themajor stabilizing muscles for the extremities(upper and lower trapezius, hip rotators andglutei) also attach to the core. - Because of thedifficulty in directly assessing these muscles, theyare often neglected or ignored with respect tomusculoskeletal rehabilitation. The glutei arestabilizers of the trunk over the planted leg andprovide pow er for forward leg movements. l Th ehip mus culatu re plays a significant role within thekinetic chain, particularly for all ambulatoryactivities, in stabilization of the trunk and pelvis

and in transferring force from the lower ex-tremities to the pelvis and spine. Altho ughsome authors - ] include the glutei a s pa rt of thecore, being an integral part of core functioning,in the present article these muscles are seen asconnections between the core and the lowerextrem ities. Kible r et al. l stated tha t the g lutei,as major stabilizing muscles for the extremities,attach to the core, implying that they do notconstitute part of the core.

1.6 S tability versus M ability

A di K ibl l ^^ l

multi-joint muscles to provide stability and pro

duce motion. This integrated core muscle activitresults in pro xim al stability for d istal mobility. - There is a proximal to distal patterning of forcgeneration and a proximal to distal patterning ithe creation of interactive moments that movand protect distal joints . Interactive moments aremoments at joints that are created by motioand position of adjacent b ody segments. - Theare developed in the central body segmentsInteractive moments are important for developing proper force at distal joints and for creatinrelative bony positions that minimize internaloads at the joint. And erson and Behm ^ statethat much is known about how muscles maintaistatic equilibrium, but little is known about howthey maintain dynamic balance when exertinan external force. Exerting external forces whilattempting to maintain dynamic balance formthe base of success in the majority of sports and is a necessity in the activities of daily living. ^The cost of coping with instability is an increas

in co-contractions, which results in a decrease iexternal force. However, in many instances, thtask could not be performed without this coactivation. ^ l Th us, this stabilization procesconsists of establishing active muscular constraints to minimize the degrees of freedomwithin one or several joints and results in stabilization of the excessive mobility of the extremities. ^ l Sufficient stab ility is bo th a co mplex concept and a desirable objective for whicoptimal balance between stability and mobility irequired. However, the objective is constraineby the need for a mo dest am ou nt of extra stabilitto form a margin of safety, but not so much as tcom prom ise the spine with the additiona l load. The art, especially for athletes, is to enhancmobility, while at the same time preservingsufficient stability.

1.7 Strength, Endurance and Sensory-MotorControl

Cholewicki and Mc Gill ^ dem onstrated thatff b l f h l b


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900 Borghuis et

muscles. Thus, maintaining sufficient stabihtywhen performing tasks, particularly the tasks ofdaily living, is not compromised by insufficientmuscle streng th,'' It has been shown that only avery small increase in ac tivation of the abdominalmuscles is required to stiffen the spinal segments(5% of maximal voluntary contraction for activ-ities of daily living and 10% of maximal voluntarycontraction for rigorous activity),''I Further-more, it has been suggested that back musclecontractions as low as 25% of maximal voluntarycontraction are able to provide maximal jointstiffness,'^^^ A low percentage of maximal volun-tary isometric contraction from the trunk mus-culature thus stabilizes the spine during normalmovements. This implies that, alongside musclestrength, muscular endurance and, in particular,sensory-motor control are important aspects inproviding sufficient core stability,'^'^ For ex-ample, the trunk flexor-to-extensor ratio may beas or more important than absolute strength andendurance, because this ratio has been shown to

be abnormal in people with back pain,'^^'

1,8 Sensory-Motor C ontrol

In the last few decades, there has been anincreasing awareness of the importance of the spe-cialized and integrated action of the muscle sys-tem in maintaining stability and optimal functionof the movement system. Efficient movementfunction and the maintenance of balance duringdynamic tasks are more complex than merelyadequate force produc tion from the muscles. Themuscle actions must be precisely coordinated tooccur at the right time, for the correct durationand with the right combination of forces,'^"*' Thiscoordinated action occurs within groups of sy-nergistically acting muscles and is also imp ortantin the interactions between agonist and antago-nist muscles. It requires sensory, biomechanicaland motor-processing strategies along withlearned responses from previous experience and

anticipation of change,f *' A primary sensorymechanism for motor control is proprioceptionfrom the muscles Gandevia et al '^^' stated that

the joints, sensation of the perceived timingmuscle contraction and sensation of force, efand heaviness of workload.

It is important to note that the dynamic bility of th body, or any specific joint such asknee, depends on neuromuscular control ofdisplacement of all contributing body segmduring movement,''"' Core stability is relatedthe body 's ability to control the trunk in respoto internal and external disturbances. Theseclude forces generated from distal body segmas well as forces generated from expectedunexpected perturbations,''"' When a limbmoved, reactive forces are imposed on the spacting in parallel and opposing those producthe movement,''^' Due to its multi-segmenature and the requirement for muscle conttion to provide stability of the spine, the spinparticularly prone to the effect of these reacforces. This indicates the importance of musccontrol of the spine during limb movement,''

Radebold et al,'^*' stated tha t, in general, this a combination of three levels of motor con(spinal reflex, brain stem balance, and cogniprogramming) that produces appropriate muresponses. The first one, the spinal refiex pway, uses proprioceptive input from musspindles and Golgi tendon organs. For the amatic control of the motion segment, the sence of a ligamento-muscular reflex has bproposed,''^' The y-spindle system facilitatesoc-motor neurons that control the slow tw

muscle fibres. The second level of moto r co ntthe brain stem pathway, coordinates vestiband visual input, thereby using propriocepfrom joint receptors,'^^1 Cognitive programmis based on stored central commands, which lto voluntary adjustments,'^^' Pre-programmmuscle activations result in so-called anticipapostural adjustments,'-'' These adjustments ption the body to withstand the perturbationbalance created by the forces of actions suchkicking, throwing or running, Ebenbichler et afound that, when reactive forces due to lmovement challenged the stability of the tru

l d b f h l


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create the proximal stability for distal mobility,

as mentioned earlier (section 1,6), The muscleactivations also create the interactive momentsthat develop and control forces and loads at

Ebenbichler et al, ' '^' wrote about the presenceof two parallel systems in the con trol of v olun tarymovements, one to control the intended volun-tary element of the movement and one to initiatecorrective forces necessary for maintaining equi-librium. It has been shown that there exists aninverse relationship between the length of thevoluntary reaction time and the degree of pos-tural stability in a certain situation,I'^l A shorterreaction time for a specific task implies an in-crease in the postural stability. Findings fromstudies on trunk motor control demonstratedthat the CNS immediately interrupts an ongoingvoluntary motor programme to prioritize thepostural control programme,''^1 So, from theseresults, it can be concluded that appropriatemuscle recruitment and timing is extremely im-

portant in the control of spine equilibrium andmechanical stability,

2. Co re Stability Athletic Perform anceand Injury

In an article by Leetun et al,,'^ it was statedthat core stability has an important role in injuryprevention. Decreased lumbo-pelvic stability hasbeen suggested to be associated with a higher

occurrence of lower extremity injuries, particu-larly in females. This highlights the impo rtanc e ofproximal stabilization for lower extremity injuryprevention. In addition, Anderson and Behm^'^suggested that a lack of trunk stabilization mayalso be a major contributor to the occurrence oflow back pain,

2,1 Core Stability and Athietic Perfornnance

Besides its local functions of stability and forcegeneration, core activity is involved in almostall extremity activities such as running, kicking

d h i Si h i l l

all kinetic chains of upper and lower extremit

function,'^]A few studies have been found that investigated whether improved core stability is associated with better physical performance, Stantoet al,P' ' did n ot find a positive rela tions hibetween core stability and running performancas measured by maximal oxygen uptake or running economy, nor did they find an improveposture during treadmill running to volitionaexhaustion with increased core stability. In addition, Tse et al,^^ found no increased functionaperformance in college-aged rowers after exposing them to an 8-week core endurance traininprogramme.

Considering the wide variety of movementassociated with various sport activities, athletemust possess sufficient strength in hip and trunmuscles to provide stability in all three planes omotion,[*i More and more, scientists are including assessment of join t mecha nics proximal andistal to the sites where injuries tend to occu

This is because of the closed chain nature oathletic activities. Motion at one segment wilinfiuence that of all other segments in the chainHowever, the infiuence of proximal stability olower extremity structure and pathology remainlargely unknown,'^^ Given the wide range oindividuals and physical demands, questionremain as to what is the optimal balance betweestability, motion facilitation and moment generation. And there are questions about how muc

muscular co-contraction is necessary to achievstability and how it is best achieved,^^1

2,2 Core Stabiiity a nd Injuries

Zazulak et al, ' ' '^ showed that proprioceptivdeficits in the body's core may contribute tdecreased active neuromuscular control of thlower extremity, which may lead to valgus angulation and increased strain on the ligaments of thknee. Such findings, in addition to years of em

pirical evidence, have led to the suggestion thathe knee may be a victim of core instability wit

t t l t it t bilit d li


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902 or huis et al.

described a so-called 'position of no

retu rn' that is characterized by hip adduction andinternal rotation, which in turn leads to kneevalgus and tibial external rotation. In addition,the same alignment tendency seems to be relatedto repetitive injuries such as patellofemo ral p ainsyndrome and iliotibial band friction syn-

With respect to direct injuries of the body'score, Nadler et al.'^"' noted tha t a thletes with ac-quired ligamentous injuries or lower extremityoveruse were significantly more likely to requiretreatment for low back p ain during the followingyear. In addition, various other factors have beenshown to be associated w ith low back pain, u nderwhich are poor m uscle enduran ce, altered m usclefiring rates and muscular imbalance. '^ ' ' The oc-currence of low back pain in an athletic popula-tion has been well documented in various sports,including football, golf gymnastics, running,soccer, tennis and volleyball. Between 5 and15 % of all athletic injuries consist of low back

pain.'^^i Most sport injuries related to the lumbarspine are soft tissue injuries such as musclestrains, ligament sprains and intervertebral discinjuries.['^J These injuries often prevent the ath-lete from regular training and competition.Moreover, low back injuries have become anincreasing problem, especially in relation torecreational activities with high demands on theback such as racquet sports, golf handball ,baseball, volleyball or rowing. In amateur ath-letes, these injuries often mean an end to thosesporting activities and a prolonged disability towork. "]

2 3 The Role of the Hip Musculaturein injury Oc currence

Some authors, who considered the hip mus-culature to be par t of the body's core, haveinvestigated several characteristics of the hipmuscles in relation to the occurrence of lowerextremity or low back injuries. Some of theirresults are discussed below.

abductor (gluteus mdius) muscles have b

identified.i^'l With regard to

muscular influencon low back pain, the hip musculature plays significant role in transferring forces from thlower extremity towards the spine and thmay influence the development of low bacinjuries.'-'' ' ' With respect to knee injuries, wehip muscles are a com m on finding associatwith knee injury. For example, weak hip abdutors and tight hip fiexors are seen in associatiowith anterior knee pain.t^' Leetun et al.'^^ haconducted a prospective study in which costability measures were compared between athletes who reported an injury during their seasversus those who did not. They looked fostrength measures that could be used to identiathletes at risk for lower extremity injury. It wasfound that athletes who sustained an injury ovthe course of a season displayed significantly lhip abduction and external rotation strength thuninjured athletes.'^' Hip external rotatiweakness most closely predicted injury sta

These results are in accordance with the abovdescribed findings by Ireland,'^^) who showthat hip abductors and external rotators play aimportant role in the alignment of the lowextremities. They assist in the prevention omovement into hip adduction and internrotation during single limb support.

However, as Leetun et al.'^^ also arguthemselves, hip external rota tion strength is onone element of core stability and other aspec

not included in the study may also have p redictthe occurrence of lower extremity injury, espcially because of the low coefficient of detemination that was found for the hip externrotation strength.

2 4 Strength versus Endurance

In section 1 of this article, it has been showthat besides muscular strength, endurance andespecially sensory-motor control are very impotant aspects in providing sufficient core stabiliLet us first have a look at the endurance aspec


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ensory-Motor Con trol and Core Stability 903

muscle endurance appears to be reduced in pa-

ients with acute and chronic low back pain.McGill et alJ^^l suggested that trunk muscleendurance is of greater importance in the pre-vention of low back pain than the ability ofhese muscles to generate force. In agreement

with this suggestion, the endurance of the trunkextensors has been found to predict the occur-rence of low back pain in 30- to 60-year-oldadults.[*1 However, it has to be said that theamount of muscle activation needed to ensuresufficient stability depends on the task.^^^^ Gen-erally, for most tasks of daily living, very modestlevels of abdominal wall co-contraction are suf-ficient. B ut if a jo in t has lost passive stiffnessdue to damage, more co-contraction is needed tocompensate for the deficiency. Besides, whenencountering unpredictable activities such as asudden load to the spine, a fall or quick move-ments, a strength reserve is needed. In sportactivities and during heavy physical work, thereare increased demands on both strength and

endurance.t^^l However, according to McGill,^^'a review of the evidence suggests that greaterranges of spine motion are associated with in-creased risk of future problems and that en-durance, more than strength, is related to reducedsymptoms.

control of the neutral zone, resulting in anabnormal increase in the range of motion. Hedefined the neutral zone as the range of inter-vertebral motion within which there is minimalinternal resistance.^^^l It is hypothesized thatchanges in a spinal segment that allow for ex-cessive motion cause poor spinal stability andback pain. Structural changes that contribute tothis instability are, among other things, disc dis-ease, muscular changes such as weakness andpoor endurance and ineffective neural control.'^^^With respect to muscular changes, a significant

reduction of cross-sectional area has beendemonstrated in various local muscles, which issupposed to be associated with either failure ofnormal recruitment or with atrophy of the mus-cle.^"1 Excessive motion of the lumbar segmentresults in the loss of sensory-motor control in aspine's segment neutral zone. So an increasedneu tral zone has been suggested as an indicato r ofclinical instability, although no objective quanti-tative measurements for clinical use are currently

available to assess this indicator.I'^l To maintainmechanical stability of the lumbar spine, com-pensation is required by the trunk musculatureIt has been shown that effective muscle controlcan return the neutral zone within physiologica

2.5 Spinal Instability Caused byNeuromuscular Imbalance inthe Local Muscle System

In section 1 of the present article, a distinc-tion between the local and the global musclesystem has been observed. Dysfunction ofmovem ent a roun d a joint can be a local or aglobal problem,'^ although both frequently occurtogether.

Local problems can be caused by a dysfunc-tion of the recruitment and motor control of thedeep segmental stability system resulting in poorcontrol of the neutral joint position.^^''"^''] The

motor recruitment deficits present in two ways,namely altered patterns of recruitment andaltered timing (a delay in muscle response time)

2.6 Spinal Instability Caused byNeuromuscular Imbalance inthe G iobai Muscle System

So far, we have only considered neuromus-cular dysfunctions in the local muscle system, bufunctional stability is dep end ent on integratedfunction of both the local and global musclesMechanical spinal stabiUty dysfunction can occuin the form of segmental (articular) or multi-segmental (myofascial) dysfunction. These dysfunctions present as combinations of restriction ofnormal motion and compensations to maintainfunction.^"

The role of the global spinal muscles is tocontrol range of movement and alignment. Dysfunction of these muscles is caused by an


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904 orghuis e

bi-articular mobility muscles.'^'*' Comerford andMottram'^"*] noted that there are many clinicallyconsistent neuromuscular imbalances betweensynergistic and antagonistic muscles. These arecharacterized by the early and dominant recruit-ment of the multi-articular mobilizing trunkmuscles, while the mono-articular stabilizing sy-nergist recruitment is delayed or these muscleslack efficiency in their shortening capacity. Thisimbalance can result in abnormal over-pull andunder-pull by the muscles around a motion seg-men t, so tha t there is give (excessive join t mo tion)

in the direction of over-activity and restriction (aloss of joint motion) in the direction of the lessactive global muscles.I'^-^''] The result of thisfaulty movement is abnormal accessory glides,which increase micro-trauma in the tissuesaro un d the join t, leading to dysfunction andpain.' '^l In the normal functioning musculature,there exist complex motor control processes thatregulate relative stiffness or flexibility in linkedmulti-chain movements.'^' ' The movement sys-tem has a great ability to adapt to changes. Whensignificant restriction of motion occurs at a joint,the body will attempt to maintain function at allcosts. To achieve this, some other joint or musclemust compensate by increasing relative mobility,which often results in tissue damage.'^"' In sum-mary, dysfunction in the global system presentsin three interrelated forms, namely length-asso-ciated change related to muscle function, im-balance in recruitment between synergistic andantagonistic muscles and direction-dependent

relative stiffness and compensation.

2.7 Sensory-Motor Control and Injuries

Now that we have seen the importance ofsensory-motor control in providing stability, letus consider what is known about the relationshipbetween deficient core neuromuscular controland the occurrence of injuries.

In all activities of daily living, a human body ismoved through three dimensions at differingvelocities while experiencing varying torques

d f E i ll i t ti iti t

neuromuscular system may not adapt welthese demands, resulting in impaired peman ce or even injury.'^' We have seen tha t spmuscles provide stability and that murecruitment patterns significantly affect loaon the intervertebral joints. Imbalanced muactivation can lead to inappropriate magnitof muscle force and stiffness, thereby loathe spine incorrectly and inducing low back and musculoskeletal injury.'^] Brown McGill''**'] stated that, under conditions of sequilibrium, the stiffness produced by a muwill function in a stabilizing manner, whileforce can function in either a stabilizingdestabilizing manner, depending on the orietion of the muscle abo ut the join t. Con siing that the relationship between force stiffness is non-linear and a situation in wthe orientation of a muscle is such thatinstantaneous tension acts in a destabilima nne r ab ou t a join t, there ma y exist a criforce level at which any additional increas

force becomes dominant over the corresping stiffness increase, thereby reducing the slizing potential of the muscle.''*''^ ComerfordMottram'^''] stated that there is a clear between reduced proprioceptive input, disturslow motor unit recruitment and the developmof chronic pain states. Deficient core neuromcular control may predispose athletes to low binjuries as well as injuries of the lower extrem

With respect to low back problems, a delareflex response of trunk muscles is found to bpre-existing risk factor for sustaining a low binjury in athletes.' '"] Furthermore, because >9of sports-related low back injuries occur fself-initiated actions such as jum pin g, run ningcutting, it is likely that a deficit in motor conis a causative factor in these injuries.'^^' Tstatem ent is supp orted by the findings of Gill Callaghan,'^"] who re ported a significant decrin repositioning ability in patients with low bpain. They concluded that precise muscle spin

input is a vital aspect for accurate positioninthe pelvis and lumbo-sacral spine.'^" In additsubjects with low back pain have been shown


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unloading and abnormal trunk muscle recruit-ment patterns,f'^ Furthermore, athletes with ahistory of low back pain continued to demon-strate motor control deficits of the trunk, evenafter clinical recovery and return to their priorevel of competition,''"^

With regard to knee injuries, Zazulak et al,['lshowed that decreased neuromuscular control ofthe body's core, measured during sudden trunkunloading and trunk repositioning tasks, is as-sociated with an increased risk of knee injury inathletes. Dynamic stability of an athlete's kneeis defined as the ability of the knee joint tomaintain intended trajectory after internal orexternal disturbance,t'i It depends on accuratesensory input and appropriate motor responsesto deal with rapid changes in trunk positionduring manoeuvres such as cutting, stoppingand landing. Deficits in neuromuscular control ofthe body's core may compromise dynamic stabi-lity of the lower extremity, resulting in increasedabduction torque at the knee,t'l As a result of

this, strain on the knee Ugaments is increased,leading to injury.In the study by Zazulak et al,,['l the strongest

predictor of injury in the female athletes wasfound to be the magnitude of displacement, inparticular laterally, during sudden trunk un-loading. In addition, active proprioceptive re-positioning error and history of low back painwere also related to a higher risk of sustaining aknee injury,['"1 Zazulak et al,^'*' reported deficitsin active proprioceptive repositioning in womenwith knee injuries and ligamental or meniscal in-juries, compared with uninjured women. Thesedeficits are measured prospectively, indicatingthat they may predispose female athletes to kneeinjury. In contrast, no differences in propriocep-tive repositioning error were found between in-jured and uninjured men,I"

Thus, in female athletes, impaired core pro-prioception may lead to impaired control of thecore, which in turn negatively affects control of

the knee and consequently may lead to kneeinjury,''*^ With respect to male athletes, Zazulak

t l ' '"^ f d th t th i ti d fi

history of low back pain was shown to be thestrongest predictor of knee injury,f'l

3. Co re Stability Neurom uscuiarCore Controi and Balance

In section 2, we have seen that dysfunction ofspinal structures, dysfunction of trunk musclesor neuromuscular deficits can result in spinalinstability. Instability of the spine is an importantaspect of low back pain, since it can lead toexcessive tissue strain and consequent pain,'''*'Comerford and MottramP'* stated that the mus-cles in the local system do not demonstrateconsistent strength deficits or changes in length.The importance of the neural control over thetrunk muscles was underlined by Barr et al,,'^'^who noted that back pain has been found to beassociated with deficits in spinal proprioception,balance and with deficits in the ability to react tounexpected trunk perturbation. We have also seenbefore that deficits in neuromuscular control of the

body's core may lead to uncontrolled trunk dis-placement during athletic mo vement. T his, in turnmay increase knee abduction motion and torqueplace the lower extremity in a valgus position andresult in increased strain on the knee ligaments andin anterior cruciate ligament injury,''"' Sections3,1-3,4 discuss the relationship between impairedmuscular core control, poor balance performanceand spinal stability, by considering various resultsof EMG and balance studies, mainly conducted inpatients with low back pain,

3,1 Delayed Muscle Reflex Responsein Patients with Low Baci< Pain

Deficiencies in motor control of the lumbarspine have been proposed as one of the factorspredisposing a person to experience a low backinjury. This is supported by findings that patientswith low back pain, who were being exposed tosudden trunk loading, exhibited longer trunk

muscle response latencies than healthy con-trols,' '"^ In addition, Ebenbichler et al' '^^ notedthat the timing of feed-forward contractions o


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906 Borghuis

patients. Whereas healthy subjects tend to con-tract the transversus abdominis before othermuscles to stabilize the spine in anticipation oflimb movement, patients with low back painshow a delayed contraction of this muscle.t^^^Radebold et alt'^^J and Cholewicki et al,'^^ mea-sured refiex respo nses from 12 majo r tru nkmuscles during sudden force release experimentsin subjects with chronic low back pain and inathletes with a history of an acute low back in-jury. These responses were short-latency refiexesmost likely associated with muscle spindle activ-

ity. It was shown t ha t subjects with low back painhad significantly longer latencies, both in theoffset of agonistic and in the onset of anta gon isticmuscles,'-'^'''^] These longer latencies were seen inresponse to sudden force release in fiexion, ex-tension and lateral bending directions. In com-parison with healthy controls, the individualmuscle reaction times of the patients showedgreater variabinty,^'*^ In addition, Cholewickiet al.I^^l also found that athletes with a recenthistory of an acute low back injury shut offsignificantly fewer muscles. This was supportedby the findings of Cholewicki et al, '"' that ath-letes with low ba ck injuries s hu t off significantlysmaller number of muscles in trunk fiexion.Furthermore, athletes with a history of low backinjury switched on a smaller number of trunkextension muscles than athletes without such ahisto ry. T he re sults of Cho lewick i et al, ""' alsoshowed that delayed switch-off latencies of theabdominal muscles in fiexion and lateral bending

are a significant predictor of a future low backinjury in athletes. All these results enhance ourunde rstanding of the mechanisms underlying lowback injuries. They are in favour of the hypoth-esis that a delayed muscle refiex response in-creases the vulnerability of the spine to injuryunder sudden loading conditions.

An alternative to the hypothesis of delayedmuscle refiex response as a risk factor is the hy-pothesis that the delayed response is caused by

the injury or pain itself Damage to the receptorswithin the soft tissues of the lumbar spine couldimpair the feedback control and in turn delay

compen sation mechanism, adopted by the pawith low back pain to compensate for an injand unstable spine or to avoid pain. HowCholewicki et al, '"' ' found no significant chin muscle refiex latencies following a low injury among athletes who reported no histoinjury, indicating that delayed muscle responsudden trunk loading is a significant predictoa future low back injury,

3,2 Neurcmuscuiar Imbalance in Patientswith Law Back Pain

An adequate response to sudden loadepends also on correct muscle recruitmpatterns to assure the mechanical stability oflumbar spine,t"*^ Besides altered timing, mcontrol deficits also present as altered patternrecruitment, Comerford and MottramP'' stthat there is evidence of alteration of normacruitment, both in peripheral and in local trstability muscles, which is associated with pai

pathology. These alterations are present unormal functional movement conditions. only consistent evidence of failure of the muin the local system is in the regulation of mutension to control segmental motion and tocruit prior to load ing of the join t system, theenha ncing stabihty durin g function,P" The result of this altered recruitment pattern is a of functional or dynamic stability, Renkaet al,[^^l found distinct neuromuscular imbalabetween right and left erector spinae at the bar level during maximum voluntary trextension among athletes with low back pwhereas there were no significant EMG-actiimbalances in subjects without low back pThese results support the hypothesis that nemuscular imbalance is associated with low bpain, especially in athletes participating in spwith high demands on the back. HoweRenkawitz et al,t^^l also stated that not evneuromuscular imbalance is a pathological f

ing. For achieving top athletic performaunilateral neuronal and muscular adjustmare sometimes important preconditions


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chronic low back pain were compared with those

n healthy control subjects. They found that theatios of antagonist over agonist and the ratiosof lumbar over thoracic erector spinae EMGamplitude were greater in the patients than inhe control subjects.^'^^ In addition, Radebold

et al.['*^l de m on str at ed a significantly differentmuscle recruitment pattern in response to suddenoad release between patients with chronic low

back pain and healthy control subjects. Thepatients maintained agonistic muscle contrac-ion while their antagonistic muscles became

concurrently activated, whereas the electro-myograms of healthy control subjects showed aswitch from agonistic to antagonistic musclecontraction, not exhibiting co-contraction inmuscle recruitment patterns. Furthermore,patients showed large variability in the recruit-ment pattern of individual muscles comparedwith the healthy control group.' ' '^'

Without a prospective study it is hard toanswer the question whether neuromuscularimbalances are a result or a cause of low backpain. In their study, Radebold et al.'^'^l consideredthe differences exhibited within the chronic lowback pain group to represent specific muscleresponse patterns, necessary as a compensationmechanism to stabilize their lumbar spine inresponse to sudden loading. In addition. VanDien et al.I' 1 suggested tha t the chan gesin muscle activity in patients with low backpain should be regarded as functional adapta-

tions in response to a reduced spinal stabih ty. B uteven if the muscle co-activation pattern aftersudden loading is an adaptation mechanism, italso is an indicator of abnormal function forwhich the individuals need to compensate.'^^l

3.3 Balance Performance in Relationto Core Stability

Now that the relationship between neuro-muscular control and core stability has been dis-cussed, let us now consider what is known aboutbalance performance in relation to core stability

to the prevention of injury. Stabilization of the

trunk is crucial for maintaining static or dynamicbalance, especially to provide a solid base whenattempting to exert forces upon external ob-jects. '^ ' ' However, there is little research doc-umenting the effects of balance on performancemeasures such as, for example, force and power.

Within the human body there are a number ofneuromuscular mechanisms that are responsiblefor the maintenance of balance. Balance isachieved through an interaction between centralanticipatory and reflexive actions and these actionsare assisted by the active and passive restraintscaused by the muscular system.'^'' There existcontinuous afferent and efferent control strategieswithin the sensory motor system, using feedbackfrom somatosensory, vestibular and visual inputs,with the vestibular system being considered as themain controller.'^' ' Standing on an unstable sup-port calls upon higher levels of the control systemand requires an essential change in the mode ofutilization of incoming proprioceptive informa-

tion. Kornecki et al.''*-'' reported that, when stand-ing on an unstable support, the myopotentials ofthe stabilizing muscles precede the insta nt of forceapplication. Slijper and Latash'' ' reported such ananticipatory increase in activity of, among othermuscles, the erector spinae and the rectus abdo-minis. These anticipatory postural adjustmentsminimize the subsequent postural destabilization.Two mechanisms are proposed to underlie thenegative effect of postural instability on balance,namely an alteration of proprioceptive messagesat the peripheral level and alterations in centralprocessing.'^'' Spinal proprioception and balancehave been found to be abnormal in patients withchronic low back pain.''*^''*^' In addition, posturalstability and one-foot balance have been found tobe significantly reduced in these patients, suggesting that they posses poor central and peripheralbalance control mechanisms.' '^'

3.4 Reiationsliip be twe en BaiancePertormance and NeuromuscularCore Controi


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908 Borghuis

and trunk muscle properties. In this study, it was

investigated whether trunk muscle response toquick force release was correlated with balanceperformance in unstable sitting. It was shownthat patients with chronic low back pain havepoorer postural control of the lumbar spine thanhealthy control subjects and this differenceincreased with increasing task level. Most im-portantly, in the absence of visual feedback, poorbalance performance correlated significantly withlonger trunk muscle onset times in response tosudden force release. The authors noted thatthis finding suggests the existence of a commonpathology underlying both phenomena.'^^1

In this study by Radebold et al.,P^] two dif-ferent motor control pathways were addressed,namely the spinal reflex and the brain stempathway. These pathways are both dependenton proprioception and other sensory inputs, oncentral information processing and on appro-priate motor output. It is quite likely that poorpostural control and delayed muscle responseare due to a deficit in one or more of these

The significant correlation between the aver-age muscle onset time and balance performancewas only found in the eyes closed condition. Themore pronounced deficiency in postural controlduring this condition was suggested to exist be-cause of the remaining sensory input systemsbeing more challenged in the absence of visual

It is also interesting to note that the healthycontrol subjects controlled their posture better inthe sagittal plane than in the lateral direction,although the patients with low back pain showedno difference between the two directions.l^^^ Itwas suggested that fine postural adjustmentsmight be easier in the sagittal plane, because alljoints have a much greater range of motion oreven move exclusively in that direction. H owever,in the case of a disturbed proprioception, as inpatients with low back pain, those subtle dif-ferences may disappear. Therefore, balanceperformance, measured in the anterior/posteriord h b d b

4 Training Core Stability

Strength and endurance of the trunk mulature and torso balance are said to be impofor core stability, appropriate posture and mimal performance during sports.' '^ To enhathletic performance and to prevent or rehatate various lumbar spine and musculoskedisorders, strengthening or facilitation of themuscles has been advocated.'^^ There is a nedevelop exercise programmes and therapstrategies based on academic and clinical dence. Although the use of core strengtheprogrammes is widespread, little researchbeen conducted on the efficacy of these grammes. The goal of core stability exercise grammes is enabling performance of high-lactivities in daily life and sports, while keethe spine stabiUzed.t^^'

In sections 4.1-4.3, some proposals are hlighted about what aspects of the core shoultrained and how to train them. Subsequentlyimportance of proprioceptive training wildiscussed with special attention towards the of wiss ball training in improving core stabi

4.1 Functional Training of Both the Loca lan d Giob ai Muscie System

Rehabilitation strategies include specific bilization of articular and connective tissue

strictions to regain myofascial extensibilityRetraining of the global stability muscles isquired to control myofascial compensations the local stability system should be trainedappropriate muscle recruitment to control mentai motion by increasing muscle stiffnessBesides the deep and the global musculatother components that can be improved byercise include muscles that increase intradominal pressure to increase lumbar stabilitythe precise neural control of the lumbar musso that they fire in a normal and efficient mner.^' The focus of core stability training sho


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ensory-Motor Control and Core Stability 909

One of the greatest challenges in training the

ore is the integration of specific training regi-mens into functional activities. Isolation of spe-ific muscles or joints should be avoided in coretabilization exercises and the emphasis should ben the training of muscle activation sequences inunctional positions and motions.'-' ' This way,ormal biomechanical motions are restoredhrough normal physiological activations. Theventual goal is to make the required muscle re-ruitment automatic and to achieve an adequateoordination of activation of the segments thatre part of the kinetic chain.'^^

4.2 Core Strengthening: Co ordina tivean d Proprioceptive Training

Anderson and Behm'^'^ noted that resistanceraining, besides its effect of increasing musculartrength, also increases the coordination ofynergistic and antagonist muscle activation,hereby improving stability. It is known that

trength gains can be due to both increases inhe cross-sectional area of the muscles involvednd due to improvements in neuromuscularoordination. The neural adaptations occurringn the early phase of a resistance training pro-ramm e lead to an improved c oordina tion of sta-

bilizing muscles. ' '^ Com erford and M ot tr am '' ' 'tated that motor control and recruitment arehe priority in stability retraining. In addition,

Akuthota and Nadler'^' stated that motor re-earning may be more important than strength-ning in patients with low back pain. Emphasiz-ng the improvement in neuromuscular function

of the trunk muscles may have positive effectswith respect to the prevention of repeated injurieso the lower back or in reducing recovery time.'^^'

Fu rth erm ore , H ewe tt et al. ' ^' suggested tha tneuromuscular core training would also improvedyna mic stability of the knee join t. Z azu lak

t al.'' ' ' stated that there is strong evidence forhe use of neuromuscular training to improve

neuromuscular control of the trunk and lowerxtremity. Research by Caraffa et al.''*'' showedhat neuromuscular control can be enhanced by

sport-specific skill training. Perturbation pro-

grammes challenge the propriocepsis, for ex-ample by using wobble boards, roller boards,discs and Swiss balls. The sensitivity of afferentfeedback pathways is increased through balanceand motor skill training. By improving the sen-sitivity of the position sense of muscle and jointreceptors, the onset times of stabilizing musclesis imp rove d.'^' ' Wilder et al. '^' '' showed tha t, aftera rehabilitation lasting only 2 weeks in whichthe back extensors were actively trained, musclereaction times in patients with chronic low backpain decreased significantly down to a level si-milar to that of healthy volunteers. In addition.Hides et al. '^' ' found that patients with acute lowback pain who received training in co-contract-ing the multifidi and transversus abdominismuscles had much less chance of recurrence oflow back pain than a control group who did notreceive this training. Renkawitz et al.'^^' foundthat both the number of tennis players with lowback pain and the occurrence of neuromuscular

imbalance in the lumbar region decreased sig-nificantly as a result of dynamic neuromuscularchanges after a sport-specific home exercise pro-gram me lasting 7 weeks. Based on these findings,it may be helpful to evaluate athletes for pro-prioceptive deficits before competition and totarget them for specific active neuromusculartraining when needed.

4.3 Swiss-Baii T raining

Training on labile surfaces will challenge themusculature and by training the body to handleunexpected perturbations, balance and proprio-ception may improve. '^ ' Unstable trainingenvironments stress the stabilizing role of themusculature at the expense of functional forceproduction. The goal of such training methods isto accommodate to an unstable environment,thereby diminishing the loss of force.'^' ' Numer-ous training aids have been developed to create

a training environment in which functionalperformance can be enhanced, one of them beingthe so called Swiss ball Several studies have


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910 Borghuis

core stability, Marshall and Murphy'^-^^ found

increased activity of the rectus abdominis, trans-versus abdominis and the internal obliques whileperforming different core stabihty exercises(single-leg hold and press-up) on a Swiss ball,compared with exercising on a stable surface. Inaddition, Behm et al,'^^ showed an increase inthe activation of the deep abdominal, stabilizersas well as the lumbo-sacral and upper lumbarerector spinae during trunk strengtheningexercises on a Swiss ball. Besides the increase inE M G activity, C osio -L im a et al,''*^^ also d em on -strated an improved performance on a staticbalance task after a 5-week functional trainingprogramme with a Swiss ball compared withconventional floor exercises in untrained women,Stanton et al,t^''l assessed certain strength andendurance aspects of the core as measured by theSa hrm ann test (see section 5 for further d escrip-tion) and it appeared that 6 weeks of Swiss-balltraining significantly improved performance onthe test. From these findings, it can be concluded

that the use of unstable surfaces such as a Swissball stresses the propriocepsis and increases theextent of activation of the trunk muscles that areimportant for balance and stability in sport.

5 Measuring Core Stability

This section looks at the variou s ways in which

core stability has been assessed in the past fewyears. Usually doctors and therapists manuallyperform clinical testing of segmental spine stabi-lity,''^^ but to date no objective quantitativemeasurements are available for clinical use.In section 1 of this article, we have seen th atsome authors stress the importance of muscularstrength in providing core stability, Kibleret al, '^' stated that no standard way has beendescribed to measure core strength. Section 5,1discusses some issues related to the measurementof core strength and end urance and subsequently,various methods will be presented in which

l l d di i f h

5,1 Measuring Co re Strength an d Enduran

Several investigators have used different niques in trying to determine the relstrengths of specific core muscles via isomdynamometer values and EMG data, '^^ Tdata can give an estimate of core strength. lua tion of any specific single mu scle as a refepoint is questionable because, to provide strength, numerous muscles fire in task-spepatterns. In their review, Kibler et al,''I propto assess core strength by qualitatively looki

one-leg standing balance ability and a onsquat and by conducting a standing, three-pcore strength test. Three-plane core testing attempt to quantify control of the core indifferent planes of spine motion,''I Patients seither on one leg or on both legs, a given dis

,away from a wall. Starting from different ipositions, they have to slowly move their toward the wall, without hitting it. Redability to maintain single-leg stance and redability to just barely touch the wall are assocwith decreased core strength,'^ Although cliexperience has demonstrated that this battetests gives useful information, allowing the dof specific rehabilitation protocols for increcore function, no specific studies have beenducted to determine reliability and validity othree-plane core strength test, '^'

Therapy should focus on the muscles woin the planes of motion that are found to b

ficient,'- '^ The observation of posture is of tional value with respec t to specific flexibilitystrength testing. Patients should be evaluatecommon muscle imbalances that affect the abto maintain a neutral spine position,t^^l Forpurpose, the use of ultrasound or fine-nE M G m ay become of great value, although to be noted that this measurement method isimpractical to use in a clinical setting. There consensus about the question whether stretesting of the abdominals and spine extenso

clinically valuable. One reason for this variain the literature ma y be the different strerequirements that patients have '^^'


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Study in which subjects were required to adopt a

push-up position with the elbows locked and thetoes placed on the vertical apex of a Swiss ball, sothat the subject was parallel to the ground. Whenthe hip fiexion angle reached a deviation of >10from the angle determined at the start of the test,the time of failure was recorded. In addition, aclinical measure of the strength and endurancecapacity of the core was obtained using theSa hrm an n co re stability test. ^^ Du rin g this test,while the subject is lying supine, an infiatable padof a Stabilizer Pressure Biofeedback Unit isplaced in the natu ral lordo tic curve and is infiatedto 40 m m Hg , Th e test consists of five levels inwhich the subject has to make certain leg move-ments, with each level increasing in difficulty.During each level, i the pressure on the Biofeed-back Unit is rioted and a deviation of >10mmHgfrom a particular baseline value indicates thatlumbo-pelvic stability is lost.^^^

Kavc ic et al, condu cted a study of which thepurpose was to quantify tissue loading char-acteristics and lumbar spine stability resultingfrom the muscle activation patterns that weremeasured when selected stabilization exerciseswere performed. During the investigation, tenmale subjects performed a series of eight differentexercises, while external forces, 3-dimensionallumbar motion and electromyography were mea-sured. In order to calculate a measure of L4-L5compression and spine stability, the measureddata were input into a series of biomechanical

models. The value for stability (stability index)was obtained by calculating a level of potentialenergy in the lumbar spinal structure for each ofthe 18 degrees of freedom (three rotatio nal axes atsix lumbar joints). This stability index resultedfrom the combined potential energy existing inboth the passive and active spinal structures,minus any work added by external loads. Thisway, 18 values of potential energy were obtainedthat were formed into an 18x18 Hessian matrixand diagonalized. T he index of spine stability w asrepresented by the determinant of this matrix.Based on this index, together with muscle activa-

5,2 Measuring Neuromuscular Cantrol

an d C oordinationAs me ntioned in section 1 of this article, sen

sory motor control of the tissues around the lumbar spine plays an important role in providincore stability. In their study, Marshall andMurphyt^^l considered optimal stabilizatioto be increased muscle activation of the ventrolateral abdominals compared with the rectuabdominis activation. They calculated thratio of the ventrolateral abdominal and erecto

spinae muscle activity expressed relative to threctus abdominis, based on the percentage omaximum voluntary contraction, to determinthe synergistie relationship between these muscles,[^^ Liem ohn et al,t cond ucted a study owhich the major purpose was to develop a measurement schedule, enabling quantification ocore stability. They noted that coordination anbalance are key elements in core stability tra ininactivities and so they chose to measure core sta

bility through balance tests in which actual corstability training postures were replicated, 1 Fothis purpose, a stability platform was used owhich balance had to be maintained in thredifferent postures, namely kneeling arm raisequadruped arm raise and the bridging postureThe duration of the balance tasks was 30 secondand the tilt limits of the balance board were set a5 to either side. The number of seconds that thsubject could not maintain balance within thrange of the tilt limits was recorded, ^ Radebolet al, ^^ also assessed core stability thro ug h balance task, namely an unstable sitting tesSubjects were placed on a seat equipped with foot support, thereby preventing movement othe lower extremities (figure 1), Polyester hemspheres of varying diameter were attached underneath the seat, providing four levels of seainstability. The seat was placed on a force plate athe edge of table. Displacements of the centre opressure undern eath the seat were measured wit

the force plate , while the subjects performed triawith eyes open and closed. The sitting task wachosen to verify that deficits in the postural con


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912 Borghuis et

postural control of lower body joints. This test

was designed to assess the brain stem posturalcontrol pathway.' ^'The same authors'^*' also used a test to assess

the spinal reflex motor control pathway. Duringthis test, subjects were placed in a semi-seatedposition in an apparatus that prevented motionof the lower extremities (figure 2). They exertedisometric trunk flexion, extension and lateralbending at a force level corresponding to 30% ofmaximal isometric trunk exertion. Subsequently,the resisted force was suddenly released with anelectromagnet and the agonistic and antagonisticresponse time of 12 major trunk muscles (rectusabdominis, external and internal obhque, latissi-mus dorsi, thoracic and lumbar erector spinae)

Force plate CoP

Variable diameterhemisphere

ig 1. Sitting balance task. The subject is seated while arms aridlegs are fixed so that postural adjustments are only possible throughtrunk m otion. The seat instability ievei is increased by decreasing the

ig 2. Trunk perturbation task. While the subject exerts isomettrunk flexion, extension or lateral bending, the resisted force is sudenly reieased by an electromagnet. Response times of 12 trunmuscles are measured using surface electromyography [EM(reproduced from Radebold et al., ^^ with permission).

was measured using surface EMG.'^^' Agonimuscles were defined as muscles that are acbefore the force release and are expected to off after the release. Antagonistic muscles

inactive before the force release and are expecto respond with increased electrical activity athe release.' *'' In addition to the EMG equment, Zazulak et al.''' used a Flock of Birelectromagnetic device to record trunk motafter the force release. The sensor was placedthe back at approximately the T5 level. Althothe semi-seated position is not a functional letic position, this posture was chosen to confor other potential neuromuscular response stegies by movement around the lower extremjoints.'^^'

In controlling the muscles around the lum


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and passive proprioceptive repositioning using

an apparatus designed to quantify trunk pro-prioception. The apparatus produced passivemotion of the lumbar spine in the transverseplane. Subjects were seated on this apparatus sothat rotation took place around a vertical axisextending through the L4/L5 vertebrae. The seatwas driven by a stepper motor at a steady, slowrate, thereby m inimizing tactile cueing. The focusof the test was mainly on feedback from m uscula rand a rticular m echa nore cep tors of the trunk. *

Since the upper body remained fixed to thebackrest with a seatbelt, the contribution of thevestibular system was eliminated. Th e lower bodymoved in the plane parallel to the ground. Sub-jects were initially rotated (27sec) 20 away fromthe neutral spine posture and stayed in that po-sition for 3 seconds. In the passive test, the sub-jects were slowly rotated (17sec) back towardsthe original position by the motor. In the activetest, the subjects rotated themselves after theclutch was disengaged from the motor drive.When the subjects perceived themselves to be inthe original, neutral position, they stopped theapparatus by pressing a switch and subsequentlythe repositioning error was recorded. ^ In theactive test, trunk muscles generate the movementand therefore muscle spindle feedback is in-volved. However, during the passive test, whenmuscles are not active, sensory feedback frommu scle spindles is decreased. * Th eref ore, inp utfrom joint and cutaneous receptors likely plays a

greater role in sensory feedback during passiverepositioning. Hence, the level of input from themuscle spindles differed between the active andpassive tests. *

6 Conclusions an d Rec omm endationsfor Further Research

The purpose of this article was to give anoverview of the existing literature with respectto several issues associated with core stabiHty. Indefining the core, it was found that some authorsinclude the hip musculature as being part of the

related to the core, it is proposed to leave the hip

musculature out of consideration with respecto the concept of core stability, although thehip musculature is found to be very importanin connecting the core to the lower extremitieand in transferring forces from and to the coreIn section 1 of this article, it was found th aco-contraction of the trunk muscles, therebycreating stiffness which in turn creates sufficienstability, is very important in providing core stability. Besides stiffness, direction-specific muscl

activations are also important in providing corstability, particularly when encountering suddenperturbations. The contributions of the varioutrunk muscles depend on the task being performed. Particularly for athletes, it is of greasignificance to find a precise balance between thamount of stability and mobility. It is also shownthat, in the search for this balance, the role osensory-motor control is much more importanthan the role of strength or endurance of thetrunk muscles. Future research should furthereveal the complex biomechanics and musclactivations, thereby allowing more detailed evaluation methods and more specific training orehabilitation protocols.

No positive relationship has been found in theliterature between core stability and physicaperformance. More research in this area is needewith adequate training programmes and sufficiently sensitive measurement protocols. Withrespect to the association between core stability

and injury, various studies have found that a decrease in core stability is related to a higher risk osustaining a knee injury or low back pain. Furthermore, it is found that deficient neuromuscular core control predisposes athletes to lowback and lower extremity injuries. In additionstudies conducted to correlate hip muscle characteristics with injury also found that decreasedstrength, poor endurance and delayed firing areassociated with lower extremity and low back

injuries. Based on these findings, it would beinteresting for future research to look at therelationship between the activation speed o


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914 Borghuis et

sport seasons, by making use of prospective

longitudinal studies, to further investigate therelationship between core stability and injuryrisk.

Several studies demonstrated that delayed fir-ing of the trunk muscles and neuromuscular im-balance are associated with low back pain. Inaddition, decreased balance performance wasalso found to be related to the occurrence of lowback pain. Only one study has been found inwhich the relationship between balance perfor-

mance and trunk muscle response times is in-vestigated. A significant correlation was foundbetween poor balance performance in a sittingbalance task and delayed firing of the trunkmuscles during sudden perturbation and it wassuggested that both phenomena were beingcaused by proprioceptive deficits. Further re-search is needed to lay a stronger foundation forthis relationship. In addition, investigation is re-quired to look for the cause of delayed muscleresponses and to see whether decreased muscleresponse times actually result in a reduced risk ofinjury.

With respect to the training of core stability, itis shown that stressing the propriocepsis duringtraining activities leads to increased demands ontrunk muscles, thereby improving core stabilityand balance, which are important aspects insport. In this respect, creating unstable surfaces,for example through the use of a Swiss ball, is aclever way to stress the trunk muscles. In addi-tion, in various articles the importance of func-tional training is emphasized. Further investiga-tion is warranted to validate the use of wiss ballsin physical training programmes and future re-search is required to develop specific, functionaltraining protocols in various sport domains andin the field of rehab ilitation.

In section 5, several tests were discussed inwhich balance, trunk muscle activation char-acteristics and proprioception have been mea-

sured. One study even tried to quantify core sta-bility by calculating an index of spine stability.On the basis of findings discussed in section 3, it

research in this domain is needed to fur

ground the various relationships. Simple quatative test procedures have to be designed evaluated that are of clinical use and that refthe sensory-motor control aspects of the nemuscular system surrounding the lumbar spTaking practical issues into consideration, it be noticed that the use of EM G measuremeninvestigate trunk muscle reaction times is qdemanding and expensive. It would be interesto quantify core stability using a balance tfor example by measuring centre of pressure-placements or by making use of acceleromFuture research is required to develop such a and to adjust it to the specific demands of vartarget groups, such as patients or athletes. development of sensitive measures can leadthe identification of neuromuscular risk facthat predispose athletes to low back and loextremity injuries. On the basis of such evations, interventions can be developed to mothe risk factors, thereby decreasing the riskinjury.

cknowledgements

No sources of funding were used to assist in the paration of this review. The authors have no conflicinterest that are directly relevant to the content ofreview.

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