The Role of Core Stability inAthletic FunctionW. Ben Kibler,1 Joel Press2 and Aaron Sciascia1 1 Lexington Clinic Sports Medicine Center, Lexington, Kentucky, USA2 Rehabilitation Institute of Chicago, Chicago, Illinois, USAThe importance of function of the central core of the body for stabilisation andAbstractforce generation in all sports activities is being increasingly recognised. `Corestability' is seen as being pivotal for efficient biomechanical function to maximiseforce generation and minimise joint loads in all types of activities ranging fromrunning to throwing. However, there is less clarity about what exactly constitutes`the core', either anatomically or physiologically, and physical evaluation of corefunction is also variable.`Core stability' is defined as the ability to control the position and motion ofthe trunk over the pelvis to allow optimum production, transfer and control offorce and motion to the terminal segment in integrated athletic activities. Coremuscle activity is best understood as the pre-programmed integration of local,single-joint muscles and multi-joint muscles to provide stability and producemotion. This results in proximal stability for distal mobility, a proximal to distalpatterning of generation of force, and the creation of interactive moments thatmove and protect distal joints. Evaluation of the core should be dynamic, andinclude evaluation of the specific functions (trunk control over the planted leg)and directions of motions (three-planar activity). Rehabilitation should include therestoring of the core itself, but also include the core as the base for extremityfunction.1. What is the Core?to its local functions of stability and force genera-tion, core activity is involved with almost all ex-The musculoskeletal core of the body includestremity activities such as running, kicking andthe spine, hips and pelvis, proximal lower limb andthrowing. Therefore, the position, motion and con-abdominal structures. The core musculature in-tributions of the core must be evaluated and treated

cludes the muscles of the trunk and pelvis that areas part of the evaluation and treatment of extremityresponsible for the maintenance of stability of thespine and pelvis and help in the generation andinjuries.transfer of energy from large to small body partsThis article provides a general functional defini-during many sports activities.[1,2]The muscles andtion of core stability, describes the anatomy andjoints of the hip, pelvis and spine are centrallyphysiology of core muscles, discusses core stabilitylocated to be able to perform many of the stabilisingin function and dysfunction, provides principles offunctions that the body will require in order for theclinical evaluation of core stability, and describesdistal segments (e.g. the limbs) to do their specificrehabilitation and conditioning programmes to max-function, providing the proximal stability for theimise the effect of core stability on athletic function.distal mobility and function of the limbs. In addition

2. Definition of Core Stabilitysingle-joint segmental stabilisation that allow thelonger, multi-joint muscles to work more efficientlyCore stability is an important component max-to control spine motions.[5]This combination ofimising efficient athletic function. Function is mostmuscle activations helps create the `neutral zone'often produced by the kinetic chain, the coordinated,control of the spinal segments. In this `neutral zone'sequenced activation of body segments that placesthe ligaments see minimal tension.[5-8]the distal segment in the optimum position at theThe abdominal muscles consist of the transverseoptimum velocity with the optimum timing to pro-abdominus, the internal and external obliques, andduce the desired athletic task.[2]The core is impor-rectus abdominus. Contracting the transverse ab-tant to provide local strength and balance and todominus increases intra-abdominal pressure anddecrease back injury. In addition, since the core istensions the thoracolumbar fascia. The transversecentral to almost all kinetic chains of sports activi-abdominals have been shown to be critical in

ties, control of core strength, balance and motionstabilisation of the lumbar spine.[9,10]Abdominalwill maximise all kinetic chains of upper and lowermuscle contractions help create a rigid cylinder,extremity function.enhancing stiffness of the lumbar spine.[11]It isThere is no single universally accepted definitionimportant to note that the rectus abdominus andof core stability. A general definition of core stabili-oblique abdominals are activated in direction-spe-ty that will be used in this article is the ability tocific patterns with respect to limb movements, thuscontrol the position and motion of the trunk over theproviding postural support before limb move-pelvis and leg to allow optimum production, transferments.[3,12-14]Contractions that increase intra-ab-and control of force and motion to the terminaldominal pressure occur before initiation of largesegment in integrated kinetic chain activities.segment movement of the upper limbs.[15,16]In thismanner, the spine (and core of the body) is stabilised3. Anatomy, Physiologybefore limb movements occur to allow the limbs toand Biomechanicshave a stable base for motion and muscle activa-tion.[17]Clinically, it has been shown that only a very3.1 Anatomysmall increase in activation of the multifidi andabdominal muscles is required to stiffen the spinalThe core acts as an anatomical base for motion ofsegments (5% of maximal voluntary contraction forthe distal segments. This can be considered `proxi-activities of daily living and 10% of maximal volun-mal stability for distal mobility' for throwing, kick-tary contraction for rigorous activity).[18]ing or running activities.[2,3]Most of the prime mov-Core stability requires control of trunk motion iner muscles for the distal segments (latissimus dorsi,all three planes. In order to provide stability in all

pectoralis major, hamstrings, quadriceps and iliop-planes of motions, muscles may be activated insoas) attach to the core of the pelvis and spine. Mostpatterns that are different from their primary func-of the major stabilising muscles for the extremitiestions. For example, the quadratus lumborum (QL)(upper and lower trapezius, hip rotators and glutei)muscle functions mainly as a stabiliser of frontalalso attach to the core.plane flexion and extension activities. However, theNumerous muscles make up the complex knownQL is attached from the transverse processes of theas core muscles. Some are small, short muscles withspine and the 12th rib to the iliac crests. This orien-small lever arms to span single joints. These aretation allows QL muscle activation that occurs inactivated in `length dependent' muscle activationassociation with flexion, extension and lateral bend-patterns.[4]Others span numerous spinal segmentsing activities to buttress shearing of the spine in theand function as prime mover muscles to integrateplane of movement, making it more than just aseveral joints and produce force. They are activatedfrontal plane stabilising muscle.[19]in `force dependent' activation patterns.[4]Coordina-tion of both activation patterns is required in theThe roof of the core muscle structures is themulti-segmented structure like the spine. The mul-diaphragm. Simultaneous contraction of the dia-tifidi are an example of short muscles that providephragm, the pelvic floor muscles, and the abdominal2006 Adis Data Information BV. All rights reserved.Sports Med 2006; 36 (3)

Previous muscles, is required to increase intra-abdominal 3.2 Physiology pressure, providing a more rigid cylinder for trunk

support, decreasing the load on the spine muscles Muscle activation in kinetic chain function is based on pre-programmed patterns of muscle activa- and allowing increased trunk stability. [16,18,20] The tion that are task oriented, specific for the athletic diaphragm contributes to intra-abdominal pressure activity, and are improved by repetition. These pat- before the initiation of limb movements, thereby

terns are grouped into the following two classes: assisting spine/trunk stability. This activation occurs � length-dependent patterns, which confer stability independently of the respiratory actions. [21] around one joint, are mediated by gamma affer- At the opposite end of the trunk component of the ent input and involve reciprocal inhibition of core muscles are the pelvic floor muscles. Because muscle to provide stiffness around a joint; of the difficulty in directly assessing these muscles, � force-dependent patterns integrate activation of they are also often neglected or ignored with respect multiple muscles to move several joints and de- to musculoskeletal rehabilitation. Synergistic acti- velop force, and are mediated by Golgi tendon

receptors. [4] vation patterns exist involving the transverse Force-dependent patterns of activation are abdominus, abdominals, multifidi and pelvic floor demonstrated in many aspects of core-related activi- muscles that provide a base of support for all the ties. Evaluation of muscle activation patterns in trunk and spinal muscles. [16] association with rapid arm movement shows that the The hips and pelvis and their associated struc- first muscles to be activated are the contralateral tures are the base of support for the core structures. gastrocnemius/soleus, [3] and that patterns of activa- Critical to functioning of the hip and pelvis are the tion proceed up to the arm through the trunk. [14,26] many major muscle groups in this area. These mus- Maximum foot velocity in kicking is more highly cles have large cross-sectional areas and, in addition related to hip flexor muscle activation than knee

extension. [3] A study of baseball throwing demon- to their stabilising role, can generate a great deal of strated that in all levels of pitching there is a pattern force and power for athletic activities. The glutei are of muscle activation that starts from the contralateral stabilisers of the trunk over the planted leg and external oblique and proceeds to the arm. [26]

provide power for forward leg movements. [2,22] The These muscle activation patterns also result in hip/trunk area also contributes around 50% of the increased levels of muscle activation in the extremi- kinetic energy and force to the entire throwing mo- ties, improving their capability to support or move tion. [23] the extremity. Maximum gastrocnemius plantarflex- The thoracolumbar fascia is an important struc- or power is generated by use of the hip muscles. ture that connects the lower limbs (via the gluteus Twenty-six percent more activation can occur in the maximus) to the upper limbs (via the latissimus ankle as a result of proximal muscle activation. [22] Similarly, a 23�24% increase in maximal rotator dorsi). This allows the core to be included in inte- cuff activation occurs when the scapula is stabilised grated kinetic chain activities such as throwing. [24] It by the trapezius and rhomboid muscles, either in covers the deep muscles of the back and trunk asymptomatic or symptomatic individuals. [27,28] In including the multifidi. The thoracolumbar fascia addition, the distal muscle activity can be more also has attachments to the internal obliques and directed towards precision and control, rather than transverse abdominus muscles, thus providing power generation, when proximal muscle activation three-dimensional support to the lumbar spine and is maximum. This can be seen in the function of the aiding core stability. [24] It helps to form a `hoop' elbow muscles in throwing. [26] around the abdomen, consisting of the fascia poste- Core muscle activation is used to generate rota- riorly, the abdominal fascia anteriorly, and the ob- tional torques around the spine. Most studies of lique muscles laterally, which creates a stabilising muscle activation demonstrate a differential pattern corset effect. [25] of intensity and timing of muscle activation, starting 2006 Adis Data Information BV. All rights reserved.

Sports Med 2006; 36 (3)

that minimise internal loads at the joint. There aremany examples of proximal core activation provid-ing interactive moments that allow efficient distalsegment function. They either provide maximalforce at the distal end, similar to the cracking of awhip, or they provide precision and stability to thedistal end. Maximum force at the foot segment inkicking is developed by the interactive moment re-sulting from hip flexion.[2]Maximum shoulder inter-nal rotation force to rotate the arm is developed bythe interactive moment developed by trunk rota-tion.[2]Maximum elbow varus torque to protectagainst elbow valgus strain is produced by the inter-active moment resulting from shoulder internal rota-tion.[2]Maximal fast ball speed is correlated with theinteractive moment from the shoulder that stabiliseselbow and shoulder distraction[29]and produces el-bow angular velocity.[30]Accuracy of ball throwingis related to the interactive moment at the wristproduced by shoulder movement.[30]As a result of the activations and interactivemoments, there is a proximal to distal developmentFig. 1. One-leg stance. No verbal cues are given. Attention shouldbe paid to alterations in posture or arm position.of force and motion, according to the `summation ofspeed' principle[2]that includes core activation. Thison the contralateral side, that creates rotation as wellis not always a purely linear development strictlyas force generation.[16,24,26]from one segment to the next. In the tennis serve,Finally, core muscle activation provides stiffnessto the entire central mass, making a rigid cylinderthat confers a long lever arm around which rotationcan occur and against which muscles can bestabilised as they contract.[16,24,25]3.3 BiomechanicsPhysiological muscle activation results in several

biomechanical effects that allow efficient local anddistal function. The pre-programmed muscle activa-tions result in anticipatory postural adjustments(APAs), which position the body to withstand theperturbations to balance created by the forces ofkicking, throwing, or running.[3,14]The APAs createthe proximal stability for distal mobility.The muscle activations also create the interactivemoments that develop and control forces and loadsat joints. Interactive moments are moments at jointsthat are created by motion and position of adjacentsegments.[2]They are developed in the central bodysegments and are key to developing proper force atdistal joints and for creating relative bony positionsFig. 2. Trendelenburg position, with the contralateral hip dropped.2006 Adis Data Information BV. All rights reserved.Sports Med 2006; 36 (3)

the moment of inertia in these distal areas is less,allowing the summation of higher velocities. Final-ly, by allowing joint force control to be largelyinfluenced and controlled by pre-programmed mus-cle activation patterns and interactive moments de-veloped through core activation, instead of beingbased on local ligament size or feedback-based localmuscle activation, the ligaments can be smaller insize, and the smaller local muscles can be activatedfor precision and control of performance variables.5. Examples of Failure of Core StabilityAssociated with DysfunctionWeak hip muscles and resulting alteration of hip/trunk position are a common finding associated withknee injury. Weak hip abductors and tight hip flex-ors are seen in association with anterior knee painand chondromalacia.[33,34]Alterations in hip muscleFig. 3. `Corkscrewing' � using hip rotators to stabilise the trunk overthe planted leg in the presence of hip abductor weakness.activity are associated with increased hip varus andhip flexion positioning and increased knee valguselbow maximal velocity is developed before maxi-positioning in squatting or landing manoeuvres, allmal shoulder velocity. However, this general patternof which increase load on the anterior cruciate liga-of force development from the ground through thement. A recent longitudinal study looked at core

core to the distal segment has been demonstrated instability parameters and found that weakness in hipthe tennis serve,[23,31]baseball throw[26]and kick.[2]external rotation was correlated with incidence ofForce control is also maximised through the core.knee injury.[35]Based on these associations, mostThe trunk is essential in re-acquiring the forwardrehabilitation and conditioning programmes for themomentum in throwing,[24]and approximately 85%knee now emphasise core stabilisation and hipof the muscle activation to slow the forward-movingstrengthening.[33,34,36]arm is generated in the periscapular and trunk mus-cles, rather than the rotator cuff.[32]4. Advantages of Core Stabilityin FunctionCore stability creates several advantages for inte-gration of proximal and distal segments in generat-ing and controlling forces to maximise athletic func-tion. The larger, bulkier muscles in the central corecreate a rigid cylinder and a large moment of inertiaagainst body perturbation while still allowing a sta-ble base for distal mobility. In addition, it placesmost of the `engine' of force development in thecentral core, allowing small changes in rotationaround the central core to effect large changes inrotation in the distal segments, similar to the crack-ing of the end of a whip. Because of the need forrelatively smaller mass in the peripheral segments,Fig. 4. Sagittal plane evaluation. The shoulders should barely touchthe wall.2006 Adis Data Information BV. All rights reserved.Sports Med 2006; 36 (3)

patient stand 8cm away from the wall, and progresslike the sagittal plane test from bilateral weightbear-ing to single-leg stance and alternately touch oneshoulder then the other just barely against the wall(figure 6). Quality of motion and speed can beassessed. With lesser degrees of core strength, thereis a greater breakdown in the ability to maintainsingle-leg stance and the ability to just barely touchFig. 7. Horizontal side support exercises.the wall. This test will assess transverse plane mo-tions that incorporate abdominal muscles, hip rota-partial quarter to half squats with no other verbaltors and spine extensors. Therapy can then be insti-cues. Similar deviations in the quality of the move-tuted based on the muscles and planes of motion thatment are assessed as in the standing balance test. Aare found to be deficient.Trendelenburg posture, which may not be noted onstanding balance, may be brought out with a single-7. Principles of Core Rehabilitationleg squat. The patient may use their arms for balanceRehabilitation of the core should concentrate onor may go into an exaggerated flexed or rotatedboth the intrinsic needs of the core for flexibility,posture (`corkscrewing') in order to put the glutealstrength, balance and endurance, and on the functionor short rotator muscles on greater tension to com-of the core in relation to its role in extremity func-pensate for other muscular weakness (figure 3).tion and dysfunction. A thorough pre-rehabilitationThree-plane core testing is an attempt to quantifyexamination including the core and the extremity iscore control in the different planes of spine and corerequired prior to rehabilitation.motion. Reliability and validity studies have notThe early focus is on the deficiencies discoveredbeen done on specific tests. Clinical experience hasduring the pre-rehabilitation examination. Thesedemonstrated that this battery of tests does giveusually include both altered patterns of motion ofuseful information that allows specific rehabilitationthe hip, trunk and shoulder, and actual weakness orprotocols to be instituted for increased core func-inflexibilities of muscles, and may be seen locally ortion. Testing is done with the patient standing adistantly. Specific inflexibilities and weaknessesgiven distance (usually 8cm) away from a wall. Inmay be addressed locally by specific exercises, butsagittal plane testing, they will be facing away frommotion patterns should be addressed on a globalthe wall. They are asked to slowly move their bodylevel involving the entire kinetic chain motion. Re-

backwards, keeping their feet flat on the floor, to justbarely touch their head against the wall (figure 4).Initially, this can be done with both legs on theground, then progressed to partial weightbearing oneach side and ultimately to single-leg standing. Sag-ittal plane core strength testing creates eccentricactivation in the abdominals, the quadriceps and hipflexor muscles, and concentric activation in the hipand spine extensors. Frontal plane testing is done byhaving the patient standing with one side then theother 8cm away from the wall (figure 5). Whilestanding on the inside leg, they are asked to barelytouch their inside shoulder to the wall. This testevaluates eccentric strength of the quadratuslumborum, hip abductors, and some long spinalmuscles that are working in a frontal plane. Finally,transverse plane motion is tested by having t

of rehabilitation is not only to restore core functionby itself, but also is the first stage of extremityrehabilitation.Progressions for lower extremity rehabilitationinclude forward and side lunges, integrated trunkrotation/hip rotations, and knee flexion/extensionswith trunk rotations.[34,36]The frequency and intensi-ty of each of the exercises will depend on the indi-vidual response to the exercises.Shoulder and upper extremity rehabilitation maybegin with a posture of bilateral leg stance, hipextension and trunk extension.[36]Activation pat-terns may start with ipsilateral muscles and proceedto contralateral activations. Diagonal patterns in-Fig. 9. Diagonal trunk rotation around a stable base: starting posi-tion.volving trunk rotation around a stable base imitatethe throwing motion (figure 9, figure 10, figure 11).habilitation for extremity injury should start withThe lower extremity activation drives the scapularcore emphasis.and shoulder activation. All upper extremity exer-In order to create a stable base, the rehabilitationcises should end in the same position of trunk/hipprotocols start with the primary stabilising muscula-extension. One method to assure this posture is toture such as the transverse abdominus, multifidus,ask the patients to end with `the elbows in the backand the quadratus lumborum. Due to their directpockets'. Progression for upper extremity rehabilita-tion include integrated scapular retraction/arm ab-attachment to the spine and pelvis, they are responsi-duction/external rotation and `low rows', integratedble for the most central portion of the core stability.trunk extension/scapular retraction/arm extensionExercises include the horizontal side support (figure(figure 12) and hip/trunk rotation with scapular re-7) and isometric trunk rotation (figure 8). This stagetraction (figure 13).Core stabilisation should avoid emphasising theuse of single planar exercises that isolate specificmuscles or specific joints. They may be used attimes during the rehabilitation protocol, but empha-sis should be on early progression to functional

positions, motions and muscle activation sequences.In this manner, normal physiological activations arerestored, which lead to restoration of normal biome-chanical motions. Exercises may be started with theextremity close to the body, decreasing the momentarm, then progressing to more abducted positions,increasing the forces and loads. The goal is toachieve coordination of activation of the segmentsthroughout the entire kinetic chain.Rehabilitation should be viewed as a `flow' ofexercises that build a base of stability and forcegeneration, and then proceed distally to establishcontrol of the forces while allowing maximal mobil-ity of the distal segment.[42]The core is the centralpart of this flow. It acts as the base of stability andthe `engine' of force generation, and also acts as thecontroller to regulate the forces. Since it is involvedin many aspects of all athletic activities, it should beevaluated as part of the workup of any extremityinjury, and should be rehabilitated prior to rehabili-tation of the injured extremity.Fig. 13. Hip/trunk rotation with scapular retraction. These may bedone in various planes of arm elevation.8. Conclusionbe approximated by evaluations that reproduce thethree-planar motions that are used by the core toCore stability is a pivotal component in normalaccomplish its functions. Better understanding ofathletic activities. It is best understood as a highlythe complex biomechanics and muscle activationsintegrated activation of multiple segments that pro-will allow more detailed evaluations and more spe-vides force generation, proximal stability for distalcific rehabilitation protocols.mobility, and generates interactive moments. It isdifficult to accurately quantify by isolating individu-Acknowledgementsal components, but its function or dysfunction canNo sources of funding were used to assist in the prepara-

tion of this review. The authors have no conflicts of interestthat are directly relevant to the content of this review.

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