etto core estabiliz

® National Strength and Conditioning AssociationVolume 29, Number 2, pages 10-25

Keywords: core; core musculature; core strength; core stability;lumbo-pelvic-hip complex; spinal stability; functional training;transversusabdominus; multifidus

Core Training: Stabilizing the ConfusionMark D.Faries and Mike Greenwood, PhD,CSCS,* D; FNSCABaylor University, Waco,Texas

s u m m a r y

Confusion exists regarding what the

core musculature is, how it is evalu-

ated, how it is trained, and how it is

applied to functional performance.

The core musculature is divided into

2 systems, local (stabilization) and

global (movement), with distinction

between core-strength, core-stabili-

ty,andfunctional exercises.

R ecently, an infomercial promisedits audience that the advertisedpiece of exercise equipment

would give anyone an "attractive core."There are unfortunate, and at times hu-morou.s, misconceptions associatedwith core muscle training and the ideaof someone having an attractive trans-versus abdominis or attractive multi-fidus muscle. To rhe traiined eye, itwas apparent that these advertiserswere referring to the potential chis-eled appearance of the rectus abdo-minis and perhaps external obliques.

but similar to matiy mispcrccptions,these individuals did not have a com-plete understanding of what the coretruly is. At times, the same confusionis noted in the exercise physiology, fit-ness, and strength and conditioningprofessions. The confusion runs fromthe specific anatomy of the core withregard to defining what it truly is, towhe[her particular exercises are de-signed to enhance core strength orcore stability, to the definition of coreexercise, to its separation from func-tional exercise, and finally to the ef-fects of core training on performanceoutcomes. Many times this confusionis as simple as pure semantics and/ordifferences in terminology, but in anycase, the confusion does more to di-vide the misunderstood topic than itdoes to combine research areas andtraining strategies. Using available re-search, this article attempts to educatethe readership on an extremely popu-lar, but controversial, topic. In hopesof eliminating much of che confusionassociated with the core musculature,the specific intent of this article is toprovide an idea of where current coreresearch resides, thereby enabling di-rection for future scientific researchand application in the strength andconditioning fields.

Core Strength VersusCore StabilityIt is wise to begin this section by describ-ing a general foundational overview ofthe core, and then di.scuss the differ-ences between core strength and corestability. The "core musculature" can bedefined generally as the 29 pairs of mus-cles that support the lumbo-pelvic-hipcomplex in order to stabilize the spine,pelvis, and kinetic chain during func-tional movements (26). The core is alsocommonly referred to as the "power-house" or the foundation of all limbmovement (I). These muscles are theo-rized to create this foundation for move-ment through muscle contraction thatprovides direct support and increasedintra-abdominal pressure to the inher-ently unstable spint- (10, 25, 33, 61). Toensure stability of the spine in order toproduce force and to prevent injury,trunk muscles must have sufficientstrength, endurance, and recruitmentpatterns (10).

"Strength," in reference to this article,can be defined as the ability of a muscleto exert or withstand force. Active con-trol of spine stability, in this case, isachieved through the regulation of thisforce in the surrounding muscles (16).When instability is present, there is a

April 2007 • Strength and Conditioning Journal

failure to maintain correct vertebralalignment, or, in other words, a failurein the musculature to apply enoughforce to stabilize the spine. So, "stabili-ty" describes the ability of the body tocontrol the whole range of motion of ajoint so there is no major deformity,neurological deficit, or incapacitatingpain (51, 53). In general, the goal of thecore musculature is to stabilize the spineduring functional demands, because thebody wants to maximize this stability (1,16). This level oi stability and kinematicresponse of the trunk is determined bythe mechanical stability level of thespine and the reflex response of thetrunk muscles prior to force being ap-plied to the body (16). Limb movementprovides cxertional force onto the spine,where the magnitude of reactive forces isproportional to the inertia of the limb(35, 37), whereas coactivation of the ag-onistic and antagonistic trunk muscleswork to stiffen the lumbar spine to in-crease its stability (16). There is a con-cern, which is discussed later, that toomuch strength or force from core mus-culature actually can cause greater insta-bility if it is not directed correctly. Also,there is evidence to support that en-durance is the more important trainingvariable when it comes to the core mus-culature (46,47).

With a general understanding of the goalof the core musculature to stabilize thespine against forces, one can begin to sep-arate the confusion between the terms"core stability" and "core strength," de-spite the limited research. When theterm "core stability" is used, reference isbeing made to the stability of the spine,not the stability of the muscles them-selves. Within the research, there hasbeen no reference to enhancing the sta-bility of a muscle, but rather its ability tocontract. When the term "core strength"is used, reference is being made to theability of the musculature to stabilizethe spine through contractile forces andintra-abdominal pressure. Cholewickiand colleagues confirm chat "accivc con-croi of spine stability is achieved

Table 1Muscle Characteristics

Local Global

Deeply placed

Aponeurotic

Slow-twitch nature

Active in endurance activities

Selectively weaken

Poor recruitment, may be inhibited

Activated at low resistance levels(30-40% maximal voluntarycontraction)

Lengthen

through the regulation of force in thesurrounding muscles. Therefore, coacti-vation of agonistic and antagonistictrunk muscles stiffens the lumbar spineand increases its stability" (16,p. 1380). Increases in mu.scle activationpotentially lead to greater spinal stabili-ty. In the same vein, confusion also mayarise as to whether a given exercise is acore-strength or a core-stability exercise.Core exercises do not aim to increase thestability of the musculature, but ratheraim to enhance the muscles' ability tostabilize the spine, particularly the lum-bar spine. The confusion between corestrength and core stability may be clari-fied further wich a proper understandingof the anatomy of the core musculature.

Anatomy of the CoreMusculature: Local andGlobal SystemsLeonardo da Vinci first described theconcept of muscle grouping around thespine. He suggested that the centralmuscles of chc neck stabilized che spinalsegtnents, whereas the more lateral mus-cles acted as guide ropes supporting thevertebrae (18). Bergmark first classifiedthe muscles acting on the lumbosacralspine as either "local" or "global" (9).Scientific modifications have been madeto these initial classifications (1, 51).The local and global muscles can be cat-egorized according to the varying char-acteristics between them (Table 1). The

Superficial

Fusiform

Fast-twitch nature

Active in power activities

Preferential recruitment

Shorten and tighten

Activated at higher resistance levels(above 40% maximal voluntarycontraction)

local musculature (Table 2) includes thetransversus abdominis (TrA), multi-fidus, internal oblique, medial fibers ofexternal oblique, the quadratus lumbo-rum, diaphragm, and pelvic floor mus-cles (61, 64). These muscles have shortermuscle lengths, attach directly to thevertebrae, and are primarily responsibleto generate sufficient force for segmen-cal stability of the spine (10, 26, 61). Re-cent research has promoted the TrA andmultifidi as the primary stabilizers of thespine (26, 50, 51). The TrA is the deep-est of the abdominal muscles, originat-ing at the iliac crest, inguinal ligament,and thoracic and lumbar spinousprocesses via the thoracolumbar fascia,then attaching anteriorly at the Uneaalba (49, 61). When contracted, it isable to increase tension of the thora-columbar fascia and increase intra-ab-dominal pressure, which increases spinalstiffness In order to resist forces actingon the lumbar spine (26, 52, 61). Themultifidi attach from the vertebral arch-es to the spinous processes spanningfrom sacral to cervical spine. Each mus-cle spans 1-3 vertebral levels, thus pro-viding the largest concribution to Inter-segmental stability (61). Because oftheir short moment arms, the multifidiare not involved with gross movement(1). The TrA and multifidi have beenfound to activate prior to Hmb move-ment in an attempt to stabilize the spinefor that movement (33-38). The TrA

April 2007 -Strength and Conditioning Journal

Table 2Core Musculature

Local muscles(stabilization system)

Primary

Transversus abdominis

Multifidi

Secondary

Internai oblique

Medial fibers of externaloblique

Quadratus lumborum

Diaphragm

Pelvic floot muscles

Iliocostalisandlognissimus(lumbar portions)

Global muscles(movement system)

Rectus abdominis

Lateral fibers of external

oblique

Psoas major

Erectorspinaelliocostalis (thoracicportion)

has been shown to activate up to 100milliseconds before the activation oflimb musculature during limb reactiontime tests (30). The TrA, specifically, isactivated regardless of direction of limbmovement (26, 33-36, 38). This activa-tion promotes spinal stability no matterthe direction and begins to confirm theprimary stabilizing function of the TrA.

Due to the lone stabilization functions ofthe TrA and multiPidi, the local systemcan be divided into primary and sec-ondary stabilizers (Table 2). The primarystabilizers are the TrA and multifidi, be-cause they do not create movement of thespine. The internal oblique, the medialfibers of the external oblique, and thequadratus lumborum function primarilyto stabilize the spine, but also functionsecondarily co move the spine (51).

The muscles primarily in charge of pro-ducing movement and torque of thespine are the global muscles (Table 2).Global muscles (sometimes categorizedas "slings") possess long levers and largemoment arms, making them capable ofproducing high outputs of torque, withemphasis on speed, power, and largerarcs of multiplanar movement, whilecountering external loads for transfer tothe local musculature (26, 61). Thesemuscles include the rectus abdominis,lateral fibers of the external oblique.

B

psoas major, and the erector spinae. Tra-ditional exercises such as the sit-up havefocused on enhancing the capacity ofthis global musculature. It is thoughtthat exercises that produce gross move-ment of the spine, such as the sit-up,emphasize the global system and not thelocal system. Tbese exercises emphasizethe global systems, not isolate the globalsystems, because both systems theoreti-cally work in synerglsm (17). With ref-erence CO fiber typing, the local systemcomprises mainly type I fibers, whereasthe global system mainly consists of typeII fibers (57, 61). Ic should be notedhere that there are other, less researchedmuscles not labeled in the classificationof local and global musculatures, andthese classifications may vary with newand much needed discoveries from re-search investigations. With the lack ofcurrent research in this area and most in-vestigations using populations withvariations of low back pain, it is difficultto make assumptions regarding the ap-plication of the core musculature to thestrength and conditioning populations.Nonetheless, these assumptions aremade.

Application of the CoreMusculatureCore and lumbo-pelvic-hip stabiliza-tion research began by investigating in-dividuals with low back pain, chronic

April 2007- Strength and Conditioning Journal

low back pain, spondylolysis, and spon-dylolischesis (22, 23, 34, 51, 52, 57,59, 64, 66). It has been shown that inindividuals with low back pain andlumbar instability, local stabilizingmuscles, including the TrA, are affectedpreferentially, resulting in inefficientmuscular stabilization of the spine(33-37, 52). Although In in vivoporcine studies, Hodges and colleagueshave shown ihe TrA to increase intra-abdominal pressure, thus reducinglumbar intervertebral displacementand increasing lumbar stiffness (33).Despite the lack of in vivo TrA researchin humans, other research has been ableto create a strong theory of its impor-tance, along with the other local mus-cles, in stabilizing the spine (1, 9, 16,22, 27, 33-38, 43). The core muscula-ture becomes especially important asthe application of forces onto the spineduring events of life and sport chal-lenges the musculacure's ability co sta-bilize and protect the spine.

As previously stated, the spine is inher-ently unstable. The ligamentous spine(stripped of muscle) will fail or buckleunder compression loads of as little as 2kg or 20 N (10, 46). Level walking canproduce up to 140 N of compressionforce to each side of the spine with eachstep (20). Holding an 80-lb object infront of the body while standing in neu-tral posture will produce large compres-sion forces of 2,000 N at the lower lum-bar levels (24). Compression was foundto be 3,230 N for straight-leg sit-upsand 3,410 N for bent-knee sit-ups,whereas shear forces were 260 and 300N, respectively (47). Rowing has beenshown to produce peak spinal compres-sion forces on the spine of 6,066 N formen and 5,031 N for women (3). Foot-ball blocking has been shown to produceaverage compression forces, anteropos-terior shear forces, and lateral shearforces of 8,679 ± 1,965 N; 3,304 ±116N;and 1,709 ±411 N, respectively (28).Half-squat exercises with barbell loadsin the range of 0.8-1.6 times bodyweight applied variant spinal compres-

sive loads between 6 and 10 times bodyweight (13). In other words, a 200-lbathlete lifting a barbell loaded to 320 lbsduring a half-squat would be applying2,000 lbs or almost 8,900 N of compres-sive force onto the lumbar spine.Cholewicki, McCill, and Normanshowed that the average compressiveloads on the L4-L5 joint of powerlifterswere estimated to be up to 17,192 N(15). Extreme lifting also has beenshown to produce loads on the lumbarspine of up to 36,400 N (29).

These types of compressive loads at thelumbar spine, from life and sport, ex-ceed those loads determined during fa-tigue studies to cause pathologicchanges in both the lumbar disk and thepars interarticularis, which contributeto conditions such as spondylolysis(28). Spine compression and lateralshear forces also have been shown to in-crease as the lift origin becomes moreasymmetric, with one-hand liftingchanging the compression and shearprofiles significantly (44). This knowl-edge is valuable, because much of lifeand sport requires not only extremeloading of the spinal musculature, butalso varying angles, positions, andspeeds. This understanding of the mul-tiplanar forces that life and sport placeon the spine and the Injury that couldensue have prompted individuals toseek methods to train the strength orstabilizing capacity, endurance, andneuromuscular reactive properties ofche core musculature. It has been sug-gested that focus should move paststrength alone to understand the speedwith which the muscles contract in re-action to a force (51). ft also has beensuggested that an individual whodemonstrates strong performance on astrength test of force may not necessari-ly display an equally strong perfor-mance on a test of endurance (43). Theindividual's history and the specificityof training should dictate the outcomesof assessment tools and subsequenttraining emphasis. As with other mus-cular assessment, measures of the core

should include various performancemeasures of force, endurance, andpower. This area, among many others, isone of needed future research.

Assessing the Core MusculatureThere is limited research on the assess-ment of core musculature, which addsto some of the confusion associatedwith this topic. Clinically, core activa-tion has been measured with ultra-sound, magnetic resonance, and elec-tromyography (3, 33-38, 50, 54, 64).One of the limitations in the clinical di-agnosis of lumbar instability revolvesaround the difficulty to accurately de-tect abnormal or excessive intersegmen-tal motion, with conventional radiolog-Ic testing often reported as beinginsensitive and unreliable (52). Therecould be possible advancements in theseareas, but current research with the coremusculature is lacking, to the authors'knowledge. Progress has been made co-ward simpler assessments of the coremusculature, with growing knowledgeof abdominal hollowing aiding thisprogress. Abdominal hollowing Is specif-ically the cocontraction of the local sys-tem, especially the TrA, multifidi, inter-nal oblique, diaphragm, and pelvicfloor musculature, while an individualisometrically contracts and draws in theabdominal wall or navel without move-ment of the spine or pelvis (5, 19, 22,52, 56, 57, 59, 60, 63). This drawing-inmaneuver Is designed to emphasize thedeep local muscle activity, because thereis minimal activation of the more super-ficial global muscles, such as the rectusabdominis (51). It has been shown thatabdominal hollowing, rather than thesit-up movement, activates a cocontrac-tion mechanism of the TrA. multifidus,and Internal obliques, rather than therectus abdominis and external obliques,with increased acclvacion of che TrAwhen lumbopelvic motion is limited(56, 59, 63). Abdominal hollowing alsohas been shown to increase the cross-sectional area of the TrA (19). The re-search of abdominal hollowing providesimportant feedback as to the design of

future core assessment programs by il-lustrating that many exercises that maybe performed as core exercises do notpreferentially activate the local stabi-lization system of the core, but ratheremphasize the global musculature. Agrowing number of researchers, howev-er, have concerns that abdominal hol-lowing during exercise can actuallycause injury and should not be advocat-ed. The newer suggestion for athletesappears to be the abdominal bracingtechnique. This growing controversy isdiscussed in more detail in the next sec-

This focus on activating the scabiliza-cion system of che core is thought tocarry into future prescription for ath-letes as well. As it is, the mosc commonlyutilized assessments and training aredone in the supine or prone position.They are designed to assess or to trainthe stabilizing system with minimal acti.-vation of the movement system, but aquestion arises when athletes do not typ-ically require spinal stabilization in asupine or prone position. Athletes andother individuals must be concernedwith spinal stability, including abdomi-nal hollowing, with the effects such asgravity, external forces, and momentum.To the authors' knowledge, there is nocurrent, valid test for the core muscula-ture in a plane or position other thansupine and prone, along with limited re-search in quantifying the activation ofboth stability and global systems in theathlete. Assuming the law of specificityapplies to the core musculature as well,ic may be beneficial for fucure researchCO assess and to quantify che activationof the stabilization system in positionsmore specific to a given sport, function,

or action.

Posterior pelvic tilting also has been ad-vocated to cause cocontraction of thelocal stabilization musculature. Never-theless, it is not suggested at times dueto the increased activation of the rectusabdominis and speculation of negativepreload effects on the lumbar spine that


Table 3Sahrmann Core Stability Test

Level 1 Begin in supine, crook-lying position while abdominal hollowing

Slowly raise 1 leg to 100° of hip flexion with comfortable knee flexion

Opposite leg brought up to same position*

Level 2 From hip-flexed position, slowly lower 1 leg until heel contacts ground

Slide out leg to fully extend the knee

Return to starting flexed position

Level 3 From hip-flexed position, slowly lower 1 leg until heel is 12 cm aboveground

Slide out leg to fully extend the knee


Level 4 From hip-flexed position, slowly lower both legs until heel contactsground

Slide out legs to fully extend the knees


Level 5 From hip-flexed position, slowly lower both legs until heeis 12 cm aboveground

Slide out legs to fully extend the knees


^Subsequent levels begin in this hip-flexed position.

often cause low back pain (22). For thepelvic tilt to be performed, the individ-ual contracts the lower abdominal mus-cles to rotate the pelvis posteriorly, sothat the lumbar spine flattens out. Acommon posture is hyperlordocic,which tilts the pelvis anteriorly or for-ward and is associated with the unbal-anced lengthening of the abdominalmuscles and gluteals combined withshortening of the hip flexors that maylead to lack of accurate segmental con-trol (51).

Researchers investigating simpler formsof core strength (its ability to stabilize)and endurance assessments have utilizedthese findings supporting the cocontrac-tion effects of abdominal hollowing onthe local stabilization musculature. Ab-dominal hollowing, especially in thesupine position, has been shown to in-crease the activity of the TrA (8, 63). Inresponse to this notion, researchers havebegun Co utilize an inflatable biofeed-

back transducer placed under the lum-bar spine in this supine position. TrA ac-tivation decreases as lumbopelvic move-ment increases, and thus stability of thespine can then be measured indirectlythrough changes in the pressure appliedto the transducer (63). A common testutilizing this biofeedback transducer, aswell as increased spine stabilization de-mands with lumbopetvic motion, is amodified Sahrmann lower abdominalassessment (1, 62).

The Sahrmann assessment protocol is Il-lustrated in Table 3 and begins in thesupine crook-lying position. Strength,endurance, and stability at the lumbarspine with the varying protocols, in-cluding the Sahrmann scale, are assessedusing an inflatable pressure tranducer orcuff, such as the Stabilizer (ChattanoogaPacific Pty. Ltd., Brisbane, Australia)(61). With the Sahrmann core .stabilitytest, the transducer is placed under theindividual's lumbar spine while he or she

is lying supine in a hook-lying position.The transducer then is inflated to 40mm Hg, while the individual activatesthe stabilizing musculature via the ab-dominal hollowing technique. Abdomi-nal hollowing, if performed correctly,will result in either no change in pres-sure or a slight decrease from the initial40 mm Hg (22). There are 5 levels in theSahrmann test. In order to advance to anew level, the lumbar spine positionmust be maintained, as indicated by achange of no more than 10 mm Hg inpressure on the analog dial of the pres-sure biofeedback unit (62). Pelvic tiltwith its flattening of the lumbar spineonto the cuff will increase the pressurereading. This pelvic tilting will increasethe pressure transducer to a point whereit does not move, thus indicating thatthe lumbar spine has maintained stabili-ty (61). The Sahrmann protocol couldpossibly be used as a scientifically basedprotocol that indirectly tests the abilityof the core musculature to stabilize thespine with and without motion of thelumbopelvic complex. This protocolmay provide an easier means for futureresearch to pre- and posttest the effect.sof training on the core musculature.Nevertheless, there is important re-search needed to validate the effective-ness of this assessment in varying popu-lations, as well as research investigatingmuscle activation and its application toperformance.

Quantifying the Core andOther ConcernsResearch has begun to further quantifythe muscles that contribute co stabilityunder spinal load, expanding on the fewstudies that have been done in this area(39, 40, 41). In other words, these re-searchers seek CO determine how muchmuscular stiffness is necessary for stabil-ity (11, 14, 47), typically by placing anumeric value co activity, compression,and resultant stability. Activation pat-terns are measured while certain exercis-es are performed at different spinalloads, and these patterns are quantifiedusing advanced biomechanical models


(43). Extensive discussion of these bio-mechanical models is out of the scope of[his article, but the growing area of re-search has brought valuable information[o the strength and conditioning profes-sion in regard to abdominal exercise pre-scription.

As stated previously, much research hasproposed the TrA as a major contributor[o spinal stability and has suggested ab-dominal hollowing or "drawing-in" as away to activate the TrA with minimal ac-civacion of the rectus abdominis andother global muscles. Abdominal hol-lowing has been shown to increase thethickness of the TrA (19) and promotesgreater sacroiliac joint stability (59). Be-cause most of this research, however, hasbeen performed with patients with lowback pain, questions have arisen regard-ing its application to a healthy individ-ual or advanced athlete. Many scientistsnow suggest that a more suitablemethod of stabilizing the spine may be.ibdominal bracing, due to its ability tococontract more abdominal muscles, in-stead of one muscle, such as the TrA,being activated for stability (40). Vera-(iarcia and colleagues showed that coac-livacion of all trunk abdominal muscles(abdominal bracing) increased the sta-bility of the spine and reduced lumbardisplacement after loading. All the torsomuscles appear to play an important rolein securing spinal stability and mustwork together to accomplish this stabili-ty. Many of these same scientists dis-agree with abdominal hollowing and theattempt to singularly activate the TrAjnd multifidus before dynamic, athleticmovements. Hollowing or drawing-inmay decrease activation of many mus-cles that are normally active during dy-namic movements, thus preventing thenatural abdominal coconcraction of allmusculature.

Not only has the interpretation of thescientific literature caused confusion ofproper abdominal activation technique(abdominal hollowing, drawing-in, orabdominal bracing), but these terms

have been misconstrued. There are manypopular fitness facilities, to remain un-named, thac teach che drawing-in ma-neuver to their trainers for subsequentprescription to clients. They use the term"draw-in" to describe che inward move-ment of the abdomen with abdominalcontraction, similar to the feeling whenall air is expelled forcefully. The activa-tion of the TrA will create a pull inwardagainst the abdominal viscera, thus beinga strong muscle of exhalation and expul-sion (32). By forcefully expiring all ofone's air, the activation of the TrA isthought to be optimal and the client canexperience the proper sensation of thetight abdominals during the drawing-inmaneuver. In this case, the sensation ofabdominal activation may simulate thatof abdominal bracing. Richardson andJull (58) originally described drawing-inby asking patients to "gently draw in tbeabdominal wall especially in the lowerabdominal area." Abdominal hollowingor drawing-in has been defined further asthe isometric contraction of the abdomi-nal wall without movement of the spineor pelvis (22) or as placing emphasis onanterolatera! abdominal muscle activityover the rectus abdominis by drawing thenavel up and in toward the spine (2). Thedraw-in maneuver is described different-ly than the abdominal bracing tech-nique, in which more of the externalobliques are activated (58). Abdominalbracing has been described more specifi-cally as coactivation of al! the abdomi-nals (2, 65) or as lateral fiaringof the ab-dominal wall (42, 63). The drawing-inor abdominal hollowing maneuver maybe better suited for static exercises thatfocus on training the local system, butmay be a poor suggestion for activatingabdominals during performance taskswhere the global system must be active.Conversely, abdominal bracing is not ap-propriate if the aim of the exercise is topreferentially activate the TrA or che In-ternal obliques (63). It seems that hol-lowing is being suggested currently forgreater TrA activity in a supine positionwith low back pain patients, whereas ab-dominal bracing may be more suitable

for more dynamic movements and exter-nal loading. It has been noted previouslythat future research will need to investi-gate whether or not these types of staticexercises translate to multiplanar, dy-namic situations.

Not only ha.s controversy arisen overwhat type of abdominal activation is op-timal for spinal stability, but researchhas begun to examine the potentiallyharmful effects of too much stability, inaddition to those of too little stability.Sufficient stability of the lumbar spinecan be achieved for a neutral spine inmost people with modest levels of coac-tivation of the abdominal wall (47).This "sufficient stabilicy" would be cheminimal level to assure spinal stabilitywithout imposing unnecessary loads onthe muscles and associated tissues (65).Because it appears that endurance maybe more important than strength andshould be trained before strength, train-ing may be better focused toward re-ed-ucating faulty motor control systems(46, 47) rather than toward stabilizationsystem strength, which may cause inap-propriate force to the spine.

Currendy, there seems to be no suchthing as an ideal set of exercises for all in-dividuals, but there are general sugges-tions for exercises that emphasize trunkstabilization in a neutral spine, while alsoemphasizing mobility at the hips andknees (4, 6, 47). Based on his quantify-ing research, McClil (46) has suggestedthe proper order of exercises to be the catstretch exercise, anterior abdominals andcurl-ups with hands under the spine tohelp maintain a neutral spine, lateralmusculature activation with side bridges,and finally, extensor exercises like thebird dog (4-point kneeling, oppositearm, opposite leg raise) exercises. McCillhas made further suggestions that theideal exercise would challenge the mus-cle while imposing minimum spine loadswith a neutral posture and elements ofwhole body stabilization (47, 48). Cau-tion should be used when implementingwhole body stabilization as a pure core


Figure 2. Marching.

Figure 3. Prone bridge.

exercise, because scientific observationshave not correlated balancing whilestanding on an unstable surface to train-ing spinal stabilizers (12). Future re-search should begin to examine thespinal stabilizers during these popularexercises and to quantify loads on thespine during real-time, real-life activi-ties. How do the loads presented inquantification studies compare with the

exercises and performance tasks of ath-letes? This research would fall in linewith suggestions of utilizing core exercis-es while standing, because specificitywould suggest such exercises to mimicthe demands of life and sport. Imple-mentation of core applications and/oradvanced abdominal exercises and beinginfused into fitness and sport perfor-mance protocols more rapidly than valid

research is being conducted. Quantifica-tion of the core appears to be a valid areaof future research for the strength andconditioning or fitness professional tostay up-to-date and to utilize in futureprescriptions.

Training the Core Musculature1 he main purposes of basic corestrength training (training the local sys-tem) is to increase stability and to gaincoordination and timing of the deep ab-dominal wall musculature, as well as toreduce and prevent injury (26, 66).Most of the research done on the appli-cation of the core musculature has fo-cused on limited bouts in order to exam-ine activation only. There has been anenormous tnedia frenzy that advocatesthe ability of core training to enhaticeperformance; unfortunately, there Islimited research to support these claims.Hagins and associates showed that a 4-week lumbar stabilization exercise pro-gram improved the ability to performprogressively difficult lumbar stabiliza-linn exercises (30). Six weeks of Swissball training specifically designed forcore activation improved the ability ofthe core musculature to stabilize thespine significantly, while also improvingcore endurance (62). Because the spineis mechanically unstable, stiffness maybe decreased at one joint accompaniedby muscles and a motor control systemchat is "unfii."This combination resultsin inappropriate muscle activation se-quences when performing even relative-ly simple tasks (46). Core training seeksto coordinate the kinetic chain (muscu-lar, skeletal, and nervous systems) to en-hance the synergism and function of thecore musculature. Panjabi (53) created aconvenient model For the core muscula-ture, categorizing the interaction of thespine into 3 systems: passive, active, andneural. The passive system consists ofthe vertebrae, intervertebral discs, zy-gapophyseal joints, and ligaments. Theactive system consists of the muscles andtendons surrounding and acting uponthe spinal column, including both localand global muscles. The neural system


describes the central nervous system(CNS) and accompanying nerves thatdirect efferent and afferent control overthe active system to provide dynamicstability during movement. These sys-tems work interdependencly, so that oneis able to make up or compensate fordeficits in another (53).

Active System: Local andGlobal MusculatureI he progression of training in the coremusculature typically and currentlyworks from the inside out. Training fo-cuses on optimizing the function of thelocal system before emphasizing move-ments that utilize the global system.Functional progression is the most im-portant aspect of the core-strengtheningprogram, which includes performancegoals, a thorough history of functionalactivities, varied assessments, and train-ing in all 3 planes of motion (1, 45). Aspreviously mentioned, the local or stabi-lizing system consists of mainly type Iionic musculature. The type I fibers ofthe local stabilization system tend toweaken by sagging (51). Specificity,then, would require local system exercis-es that involve little to no motionthrough the spine and pelvis for thelocal, stabilizing muscles. Examples ofthese local system exercises are shown inFigures 1-7. The local system is activat-

Figure 4. Prone bridge—hip extension.

Figure 5. Side bridge.

ed with low resistances and slow move-ments that prolong the low-intensityisometric contraction of these specificstabilizing muscles (51, 61). Becausemost isolation exercises of the localmusctilature, including the TrA, are innonfunctional positions, exercise train-ing may need ro shift to more function-

al positions and activities. The globalsystem, consisting of more type II fibersthat create movement of the spine, maybe emphasized through exercises thatinvolve more dynamic eccentric andconcentric movement of the spinethrough a full range of motion. Exam-ples of these global system exercises are

Figures. Side bridge abduction.


Figure 7. Long lever crunch.

Figure 8. T rotation.

Figure 9. Twist on ball.


Figure 10. Cable wood chop.

Figure 11 Cable reverse wood chop.

seen in Figures 8-14. The activity ofglobal muscles, which tend to shorten ortighten, will differ from that of the sag-ging local system (51). Rapid movementand higher resistances also will recruitthese global muscles, especially the rec-tus abdominis (51). These types of exer-cises not only emphasize the global sys-tem, but also create an environment forche local system to begin to stabilize thespine in varying, mu lei planar move-ments. Training the core for an emphasisin strength would Include high load, lowrepetition casks, while endurance en-hancement requires longer, less demand-

ing exercises (46). Beyond our knowl-edge of basic muscle physiology andadaptation, there is little research on thespecific types of core exercises to he usedand their effects on the abilicy of themusculature to stabilize the spine invarying planes of motion. With the lim-ited research in this area, specificity tothe individual's history and goals, alongwith progression, should not be over-looked.

Neural SystemThe stability of che lumbar spine is notdependent solely on chc basic mor-

phology of che passive and active sys-tems, but also the correct functioningof the neuromuscular system (52). Thepatterns of recruitment and relativeonset times between muscles are mod-ulated by the CNS, ensuring optimalmovement control, muscle perfor-mance of the core, and control of reac-tive forces produced by the limb move-ments (10, 35). Training and exercisecan lead to great increases in maximaldynamic strength through neuraladaptations in all musculature, so theneuromuscular system then can specif-ically compensate and improve dynam-

April 2007' Strength and Conditioning Journal

Figure 12. Skier crunch.

Figure 13. Overfiead press functional progression.

Figure 14. Two arm/single arm chest press functional progression.

ic stability of the spine (31, 51). Morefocus may be directed toward coordi-nation and timely muscle activation ofthe deeper local system to enhancespine stability, rather than just towardimproving strength and range of mo-tion (17, 26). There are no currentguidelines to accomplish these adapta-tions, cmpha.sizing another importantarea for future research. The reflex re-sponse of the stabilizing musculatureto applied or produced force combineswith the mechanical stability level todetermine the kinematic response ofthe trunk (16). It also seems that someunexpected loading scenarios onto the

trunk may be coo fast and/or with toohigh of a magnitude for the reflex re-sponse to control intersegmental dis-placement effectively and safely (16).In these scenarios, it may be more im-portant to consider that ic is not thestrength of the stabilizing muscula-ture, but the speed with which themuscles contract in reaction to theforces that are capable of displacing thespine (51). Potvin and O'Brien showedchat trunk muscle cocontraction in-creased firing during lateral bend con-tractions as the agonist trunk musclesfatigued (55). The investigators pro-posed that the fatigue produced by the

exertions compromised neural coordi-nation and chat chc increased cocon-traction served to maintain stability ofthe spine. As with other l̂ orms of train-ing, the neural component, as well asoptimal gains in musculature adapta-tion, should be considered for future-research.

It also should be noted here thatmuch emphasis has been placed onthe ability of the TrA and iniilciflili(core stabilization system) to activateprior to the limbs and global systemmuscles in order to stabilize the spineagainst gross movement patterns. De-


spite this emphasis, these differingmusculatures may not work in isola-lion, but may work together to stabi-lize and move the spine. The TrA hasbeen shown to activate independentlyof other core musculature to decrease.ictivation during lumbopelvic move-ment (38, 63). So, other musculaturewill activate to work together for sta-ble spine maintenance in dynamicmultiplanar movements. Beneficialprogression in exercises for enhancingspinal stability, then, may involve theentire spinal musculature and itsmotor control under various loadingconditions (17).

Until research begins to shed light uponoptimal progression during core train-ing, progression may be programmedaccording to the known frequency, in-censicy, time, and type (FITT) principlethat commonly is used in select trainingprotocols. As with most training proto-cols, the FITT principle is utilized andmanipulated to meet specific needs ofthe individual. This manipulationwould seemingly be no different in coretraining practices. Common sense withknowledge of genera! neuromuscularadaptations can be used to manipulatethese select variables, though alwayscon.sidering the safety of the individualfirst. Frequency can be adjusted, as withany other muscle being trained, to em-phasize overload, recovery, and specifici-ty. Incensicy can be progressed in exer-cises training the local and globalmusculatures by simply increasing theinstability of che environment, foot po-sitioning, and life progression (Table 4,Figures 13 and 14) or by increasing re-sistance by wearing a weight vest duringexercises. Time or duration of exercisestends to fluctuate, depending upon theparticular exercise used. I he local mus-culature exercises that require little to nomovement typically require durations of30-45 seconds, utilizing assumptionsbased on their type I fiber composition.md stabilization duties. At the pointwhere holding an exercise for45 secondsis no longer challenging, progressions

Table 4Functional Training Progressions

Stance

1.2 feetathip or shoulder width

2.Staggered stance

3.Single-leg

4. Repeat 1-3 on stability device

can be made with intensity, thus de-creasing the ability to hold the exercisefor a given duration. As the individualadapts to the training progression, ad-justments again can be made throughany of the other previously noted vari-ables. Research examining the alterationand progression of all training variableswith the core is at its genesis and is ofgreat importance to the advancement ofthe literature in this area. Movement orglobal system exercises may utilize num-bered repetitions, which will be adjustedand progressed according to the specificneeds of the individual. Again, the typeof exercises selected fall under che prin-ciple of specificity, always consideringthe exercise history, current level of fit-ness, and performance goals of the per-son involved in the individually de-signed training protocol.

Further, as core training progresses,consideration always should be placedon synergistic relationships within thehuman body, inckiding the ability ofthe 2 systems of the core musculatureto work together (17). Local and glob-al musculatures work together to cre-ate dynamically stable and functional-ly efficient multiplanar movements ofche spinal column. Argument could bemade that because, ideally, both sys-tems work together, training shouldbegin to utilize this relacionship andprogress from there to maximize itsfunctional transition to a specific out-come or function. Even as progressionaims to challenge the core musculaturein environments similar to those ofcompetition or of life, it may be wiseto begin slowly, using the specificity

Lifts

1. Both arms at the same time

2. Alternating

3.Single-arm

4. Repeat 1-3 with rotation

and FI IT principles. In addition. It Iscommonly agreed that most individu-als overtrain the global musculaturebefore optimal development of thelocal system has been accomplished.Overtraining of the global muscula-ture before sufficiently training thelocal musculature is thought to createa situation where force is being pro-duced by the global muscles chat can-not be controlled and handled by thelocal musculature. Anecdotally, thissituation has been exemplified as a fastsports car with a poor braking system.The car and Its associated engine de-signed to reach high speeds representsthe overtrained global system, whereasthe poor braking system represents theundercrained local system. Evenchough the sports car can reach accel-erated velocities, the brakes are unableto properly slow it down, and thus acrash occurs. In real life, the subse-quent crash refers to maladaptivemovement of che body and eventualinjury to the spine. Despite the effec-tiveness of such analogies, however,there is a lack of research to confirmthe theories behind the interaction ofthe different systems of the core mus-culature. As with all forms of training,progression and specificity of the coremusculature must be considered, andfucure research must seek to determinespecific training's effect on perfor-mance to accomplish safe and optimalperformance outcomes.

Functional TrainingWith che distinctions made betweentraining specifically for local and globalsystems and having established the im-


portance of proper, specific progression,one final area chat is often confused andmisunderstood will be addressed. Specif-ically, some confusion with core trainingarises with the mislabeling of certain ex-ercises as "core exercises." The generaldefinition of the word "core", as can befound in an ordinary dictionary, is "thecentral or most important part of some-thing." For instance, many weight train-ing protocols have a set of core exercises,such as the squat. In this case, core de-scribes the central, fundamental, or basiclifts that are needed to build upon in thatparticular area of training. In this exam-ple, core does not mean that the lifts arespecifically training the local and globalmusculature of the lumbar spine (our de-finition of core). This distinction mustbe made, because certain central or fun-damental exercises, such as the squat, arelabeled as core exercises. Ihe squat willrequire the activation of the core muscu-lature, both local and global systems, toensure proper spinal stability during themovement. However, the same can besaid of very simple tasks as well, such asbending over to pick up a small object(46). Picking up a small object is notconsidered a core exercise, even choughthe core musculature is activated. So, be-cause the core musculature is used in anymovement that requires segmental stabi-lization and protection of the spine, con-fusion may arise if we are not careful inusing the label of "core exercise."

1 his distinction muse be made, especial-ly with the rush of stability trainingequipment that is fiooding the market.Confusion occurs because certain de-vices and exercise protocols are adver-tised Co crain che core musculature.When performing exercises on unstablesurfaces, the core musculature will bechallenged, but the intent of the exerciseis to train the neuromuscular system byprogressively challenging balance, sta-bility of the limbs, coordination, preci-sion, skill acquisition, and propriocep-tion (7, 26, 62). These exercises also maybe considered functional exercises whenthey are specific to the demands of an

individual's sport or activity. The exer-cise does not have to be on an unstablesurface, a Swiss ball, or any other pieceof stability equipment to be consideredfunctional. On the one hand, for exam-ple, a controlled seated machine rowingexercise would be functional for a rower,because it is more specific to the athlete'sgoals and the seated environment ofcompetition. On the other hand, unsta-ble environments pose very functionallybased (task- and goal-specific) exercisesfor athletes and nonathletes alike, be-cause most sports and even activities ofdaily living require force production andacceptance in multiplanar, dynamicallyunstable environments. For instance, afunctional training mencality may pro-mote an offensive lineman to train attimes with a standing cable chest presson a single leg, because many times of-fensive linemen are required to exertupper-body force on an opponent dur-ing a game while in a single-leg environ-ment. This cype of exercise, like thesquat, is not considered a core exercisespecifically designed to train che localand global musculature of the spine. It isa functional, sport-specific performanceenhancement exercise that is individual-ized to the athlete. The core must be uti-lized in th[s movement, but the intent Isto create SLU environmenc co train theneuromuscular system to stabilize dy-namically, to produce force propriocep-tivety, and to manage force exerted onthe global movement system, which willminimize the force transferred onto thelocal stabilization system and the spine.In current theory, once the stability ofthe inner and global core musculatureshave been examined and have beentrained, then a progressive protocol maybe added to develop the enhanced capa-bilities of the limb musculature insport-specific training. Again, consider-ation may need to be made as towhether or not this approach is the bestin utilizing spinal stabilization in sport-specific environments. In addition, fu-ture research should examine the effec-tiveness of placing the individual in anenvironmenc that is overly unstable.

such as standing on a wobble-board.Questions concerned with the amountof force production loss, the amount ofneuromuscular stimulation, and icstransfer from these extremely unstableenvironments to performance should beanswered by future research in theseareas.

A commonly seen progression in func-tional lift and stance progressions isshown in Table 4, with examples in Fig-ures 13 and 14. Progression is required,because as the instability of the lift or en-vironment increases, so do the demandsplaced upon che stabilization muscula-ture (2, 21). These functional progres-sions seek to gradually place individualsin functional environments and plat-forms that will be required of them dur-ing life or sport. Advances in the stanceand lift progressions can be utilized inconjunction during training. For exam-ple, the most stable dumbbell overheadpre.ss exercise would be wich 2 feet at hipor shoulder width while pressing botharms at the same time. The most unstableformat of the sanic exercise would be asingle-leg, contralateral, single-arm presswith rotation. The importance of train-ing in these environments can be seen,because there is a decrease in force pro-duction as the environment becomesmore unstable (3). Given that productionand maintenance of force decreases withincreasingly unstable environments, suchas the offensive lineman blocking in a sin-gle-leg or staggered stance while applyingforce with a single arm, it may seem prac-tical and optimal to progressively trainthe individual's ability to produce andmaintain force in these naturalistic orfunctional environments. With the pro-gressive nature of functional training andits common confusion with core train-ing, careful attention may be needed inthe labeling of exercises to ensure the dis-semination of terminology and fucure re-search in these areas.

Future ResearchWith a growing underscanding of specif-ic labeling needs and the function of che


core musculature, research should beginto examine the effect of varying trainingprograms on core strength, core en-durance, and neuromuscular adapta-tions. Furthermore, researchers couldbegin to examine core training's func-tional application to specific perfor-mance variables or how specific core-ex-ercise training can be applied toperformance or sport-specific training.Limited research has been conducted in(his area. Stan ton, Reaburn, andHumphries compared core-training ef-fects on running economy (62). After 6weeks of Swiss ball training, despite sig-nificantly improving core musculature.strength and endurance, subjects didnot dcmonscrate any significant changesin running economy. The authors donote that these results may be applicableonly to the specific population that wasused for this study (male athletes, 15.5 ±1.4 years of age). In addition, despiteparticipants getting stronger and scor-ing higher on the Sahrmann test with noimprovement in running economy, onemay question the effectiveness of uni-planar core training on multiplanar per-formance. With the lack of research re-garding the application of core trainingon performance, further studies shouldexamine specific and varied trainingprotocols' effects on performance. Fu-lure research also should continue tovalidate core assessment techniques,with consideration of the specificity ofthe individual's history, goals, training,and prescription. Core assessment andtraining also should be investigated withathletic and normal populations forstandardization and application, in ad-dition to low back pain populations,wich emphasis on continued validationof protocols utilizing inflatable biofeed-back transducers to measure corestrength and endurance. It is hoped thatwith a more Informed understanding ofche research behind the core, profession-als can eliminate confusion over its defi-nition, varied terminology, and func-tional progressive applications, and willbe able to coordinate future research en-deavors. The main conclusion with the

core and its application to the strengthand conditioning disciplines Is that re-search is limited. •

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Faries

Mark Faries is a master's student in Exer-cise Physiology at Baylor University.

Greenwood

Mike Greenwood isa professor in the De-partment of Health, Human Performance,and Recreation at Baylor University. Hecurrently serves as an Executive CouncilMember of the NSCA-CC.

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