the role of instability with resistance

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Journal of Strength and Conditioning Research, 2006, 20(3), 716-722 2006 National Strength & Conditioning Association Brief ReviewT H E R O L E OF IN ST A BIL IT Y W I T H R E S IS T A N C ET R A I N I N GDAVID G. BEHMI AND KENNETH G. ANDERSON^'School of Hum an Kinetics andRecreation, Memorial University of Newfoundland, St. John's, Newfoundland,Canad a; ^School of Hum an Kinetics, University of British Colum bia, Vancouver, British Columbia, Canada.

ABSTRACT. Behm, D.G., andKG. Anderson. The role of insta-bility with resistance training. J. Strength Cond. Res. 20(3):716-722. 2006.There aremany instances in daily life and sport inwhich force must be exerted when an individual performing thetask is in an unstable condition. Instability can decrease theexternally-measured force o utput of a muscle while m aintaininghigh m uscle activation. The high muscle activation of limbs andtrunk when unstable can be attributed to the increased stabili-zation functions. T he increased stress associated with instabilityhas been postulated topromote greater neuromuscular adapta-tions, such as decreased cocontractions, improved coordination,and confidence in performing a skill. In addition, high muscleactivation with less stress on joints andmuscles could also bebeneficial for general musculoskeletal health and rehabilitation.However, the lower force output may be detrimental to absolutestrength gains when resistance training. Furthermore, otherstudies have reported increased cocontractions with unstabletraining. The positive effects of instability resistance training onsports performance have yet tobe quantified. The examinationof the literature suggests that when implementing a resistancetraining program for musculoskeletal health or rehabilitation,both stable an d u nstable exercises should be included to ensurean emphasis on both higher force (stable) and balance (unstable)stressors tothe neuromuscular system.KEY WORDS, balance, strength, muscle activation, cocontrac-tions, trunk musclesINTRODUCTION

esistance training involving balls, platforms,y andother devices to induce varying degrees ofinstability has recently enjoyed a surge in pop-V ' ularity. Balls have been used by entertainersand circus performers for many years. It is un-clear when they first began tobe used as a training andrehabilitation tool, butphysical therap ists have been us-ing "Physio B alls" since before W orld War II.W ith theupsurge of interest in neuromuscular training generatedby researchers such as Sherrington (43, 44), physicaltherapists began to integrate the use of balls into therapy.Physical therapists, especially the Germans and Swiss(consequently, the term "Swiss balls") were especially ac-tive in using balls for sports training and therapy. Morerecently, the rehabilitation literature has reported thesuccessful application of balance training to reduce theincidence of ankle sprain s in a group of volleyball players(53). This decrease in ankle injury incidence may be re-lated to the improved discrimination of ankle inversionmovements found with wobble board training (55). Simi-larly, the use of T ai Chi has been reported to improveknee joint proprioception (51) and functional b alance (17)in elderly individuals. The combination of resistancetraining and balance stressors may be an efficient meansof improving ba lance and strength.

Proponents of instability resistance training deducethat the greater instability of the unstable platform andhuman body interface will stress theneuromuscular sys-tem toa greater extent than traditional resistance train-ing methods using more stable benches and fioors. Stress,according to Selye's (42) adaptation curve, isessential inforcing the body to adapt tonew stimuli. The advantageof an unstable training environment would be based onthe importance of neuromuscular adaptations with in-creases in strength. Strength gains can be attributed toboth increases in muscle cross-sectional area and improvements in neuromuscular coordination (6). It hasbeen reported that neural adaptations play the most im-portant role in strength gains in the early stages of aresistance training program (6). Rutherford and Jones(39) suggested that the specific neural adaptation occur-ring with training wasnot increased recruitment or activation of motor units but an improved coordination ofagonist, antagonists, synergists, and stabilizers. Thusthe inherently greater instability of an unstable platformand body interface should challenge the neuromuscularsystem to a greater extent than under stable conditionspossibly enhancing strength gains attributed to neuraladaptations. B ase or platform instability can be inducedby sitting, lying, kneeling, or stan ding on balls (i.e., Swissballs or Physio Balls), "Dyna-Discs" (rubberized inflateddiscs), wobble and rocker boards, foam rollers, low-den-sity mats, andother similar devices. Instability can alsobe produced witb unstable loads, such as partially filledcontainers of water or sand, andflexible tubing. Similar-ly, some authors advise the use of free weights over ma-chines for improved train ing re sults (49), because th e bal-ance and control of free weights force the individual tostress and coordinate more synergist, stabilizing, and an-tagonist muscle groups. The rationale underlying destabilizing train ing environm ents would lead one to concludetha t unstable environments should provide a more variedand effective training stimulus. On the other hand, therare a variety of disadvantages associated with instabilityresistance training that may outweigh the advantageswhich will be discussed later in the review.

Resistance training is not only practiced by competi-tive atbletes, but also pursued for general health and re-habilitation. Tbis brief review will attempt toprovide information regarding some of the effects of instability re-sistance training on force output, trunk and limb muscleactivation, cocontractions, coordination, and o ther factorsand the application of instability training to sport, healthand rehabilitation.TRAINING SPECIFICITYAccording to the concept of training specificity (6), because not all forces are produced under stable conditions

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INSTABILITY RESISTANCE TRAINING 717(i.e., shooting a puck while balancing on a single skateblade in hockey, performing a routine on a balance beamin gymnastics, changing direction rapidly by pivoting on1 foot on uneven natural turf in football, soccer, fieldhockey, or other sports), then training must attempt toclosely mimic the demands of the sport or occupation. Forinstance, Behm et al. (8) showed significant correlations(p < 0.005) between hockey skating performance andstatic balance tests, with the highest correlation betweenbalance and the skating ability of bockey players underthe age of 19 years (r = 0.65). In addition, college baseballpitchers with weaker vestibular input are reported tohave higher levels of pitching errors (30).

Whereas most sports involve dynamic balance, insta-bility resistance training is typically performed underfairly stationary conditions. Whetber possible improve-ments in static balance or stability will transfer effective-ly to dynamic stability is still debatable. Shimada et al.(45) reported that walking (dynamic) balance did not cor-relate with standing (static) balance. Nonetheless, a num-ber of papers bave shown feedforward (37) or proactiveadjustments (28) with prior experience or knowledge offorthcoming perturbations resulting in a lower occurrenceof balance disruptions. Except in these studies, the train-ing and testing involved the same perturbations, whereaswith typical static instability devices such as Swiss balls,BOSU ("both sides up") domes, wobble boards, and Dyna-Discs, tbe training dynamics do not specifically matcb theathletic performance. Thus, it is debatable whether insta-bility resistance training may enhance sport performance.Willardson (56) states that "the optimal method to pro-mote increases in balance, proprioception and core sta-bility for any given sport is to practice the skill itself onthe same surface on which the skill is performed in com-petition." Unfortunately, this is not always possible, forexample, for some outdoor sports (i.e., football, baseball)during the winter season in northern climates or sportsthat utilize ice surfaces when the arenas are closed in thewarmer seasons. Thus, alternative challenges to the ath-lete's balance may be necessary.

Moreover, tbere is some evidence in the literature thatmay illustrate other positive adaptations associated withthe effect of instability resistance training on musculo-skeletal health. These adaptations, which include trunkstrengthening, changes in muscle function, limb strengthgains, and cocontractions, are discussed in the followingsections.TRUNK STRENGTHENING WITH INSTABILITYRESISTANCE EXERCISESImprovements in core stability (torso or trunk strength)have been postulated in the popular media to be en-hanced with instability training. Nevertheless, confiictingfindings are evident in the literature when balls are usedto enhance trunk or abdominal musculature. Siff (46)found tbat the wider range of movement that is availablewith the use of a ball (with an optimal starting positionfrom a few degrees of active trunk extension) is preferableto similar actions performed in most circuit traininggyms. Cosio-Lima et al. (14) illustrated greater gains intorso balance and trunk electromyographic (EMG) activ-ity after 5 weeks of Physio Ball training compared to tra-ditional floor exercises. In contrast, Stanforth et al. (48)stated that training with the "Resistaball" was compara-

ble to traditional floor work for training the back and ab-dominal muscles. Further evidence supporting the hy-pothesis of improved core or trunk strength has only re-cently begun to emerge in the literature.The strengthening of trunk or core stabilizing musclesis an important consideration for activities of daily living(ADL), sports performance, and the rehabilitation of lowback pain (LBP). A strong and stable trunk (core) pro-vides a solid foundation for the torques generated by tbelimbs. However, increased back strength is not necessar-ily associated with the prevention of LBP. Some studieshave reported no advantage of trunk strengthening (33)or lumbar muscularity (41) in the prevention of LBP. Yet,increased back strength may provide some protectionfrom LBP when greater forces are needed for the task(11). It has also been proposed tbat the spine may becomeunstable because of weak trunk stabilizer muscles (47).However, a lack of back muscle endurance is strongly as-sociated with LBP (36). Overall, there is general agree-ment that resistance exercise is beneflcial in the rehabil-itation of LBP (1).

A commonly-prescribed adaptation to trunk strength-ening rehabilitation exercises is the use of unstable sur-faces. Swiss balls or Physio Balls are often advocated topromote proper posture while seated in order to preventLBP (35). It has been proposed that the demands of anunstable surface will cause an increase in muscle acti-vation in order to complete the exercise in a controlledmanner (19). Behm et al. (9) illustrated that instabilitywith trunk strengthening exercises increased the activa-tion of the lower abdominal muscles. In the same study,shoulder and chest presses were performed with stableand unstable bases. Although there was no effect of in-stability on the shoulder press, the unstable chest presshad either signiflcantly greater or tendencies towardsgreater activation of the upper lumbar erector spinae,lumbosacral erector spinae, and lower abdominal mus-cles. Trunk stabilizer activation during a chest press withan unstable base exceeded activation with a stable baseby 37-54% (9). Increased trunk muscle activation can alsobe achieved whether the instability is derived from theplatform or the limbs when performing bench presses (18)or push-ups (21). Increased trunk stabilizer activationwith an unstable base concurs with the flndings of Aro-koski et al. (5) and Vera-Garcia et al. (52). Unfortunately,Arokoski et al. (5) applied an unstable base to only 2 ofthe 15 exercises they employed. In addition, the majorityof the activities they chose created greater stress on backrather than abdominal musculature. Vera-Garcia et al.(52) examined only curl-ups and found increased abdom-inal muscle activity with labile surfaces. Anderson andBehm (4) had subjects perform squats under differing de-grees of stability. The higher degrees of instability re-sulted in approximately 20-30% greater activation oftrunk stabilizing muscles. However, tbe submaximal re-sistance (maximum resistance was 60% of body mass)was moved at a relatively slow pace (1 second down, 1second transition, 1 second up) and thus the recruitmentpatterns would differ considerably from high-powersports such as football, rugby, hockey, and others. There-fore, based on these flndings, unstable platforms or resis-tance may be used during specific trunk strengtheningexercises or in conjunction with limb strengthening activ-ities to augment activation of the trunk musculature.

The instability-induced greater trunk activations in


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718 BEHM AND ANDERSONtbe aforementioned studies were never compared to tbegreater loads tbat can be accommodated witb stabletraining. For example, it is not known wbetber trunk ac-tivation levels are bigber wben performing a 3, 5, or 10repetition maximum (RM) squat or deadlift, as comparedto tbe lower loads exerted witb unstable squats or dead-lifts or witb unstable calistbenic-style trunk-strengtben-ing activities. In order to complete a 3-5RM squat ordeadlift, considerable activation of tbe tr un k is necessaryto protect tbe verte brae. Conversely, most members of tbepopulation wbo are primarily concerned witb musculo-skeletal bealtb or rebabilitation would not be interestedin attempting sucb bigb-load, intense exercises. Wbereascompetitive atbletes may be able to bigbly activate tbeirtrunks witb bigb-load, relatively stable resistance exer-cises, individuals more interested in bealtb and rebabil-itation can acbieve bigber trunk activation witb lowerloads using unstable conditions.TRUNK STRENGTHENING WITH UNILATERALEXERCISESFurtber modifications in addition to instability platformsmay be instituted witb limb resistance training exercisesto stress tbe trunk musculature. Traditional resistancetraining exercises aremore often bilateral using eitber abarbell or a pair of dumbbells. Conversely, numerousADL and sport actions are unilateral (31). Examples ofunilateral sport actions would include most racquetsports and throwing actions. Accordingly, for some ADLand sports, unilateral exercises may be more beneficialtban bilateral actions by adbering to tbe concept of train-ing specificity (40). Unilateral resistance exercises mayalso bave tbe additional bonus of stimulating tbe t runkstabilizers to a greater extent. Ratber tban implementingan unstable base, unilateral resisted actions would pro-vide a disruptive moment arm (torque) to tbe body, pro-viding anotb er type of uns table condition. Recently, Bebmet al. (9) reported greater trun k activation w itb unilateralsboulder and cbest press actions. Unilateral dumbbellpresses exbibited greater activation of tbe lumbosacraland upper lumbar erector spinae witb botb sboulder andcbest presses. However, lower abdominal muscle activa-tion was only significantly greater witb tbe unilateraldumbbell cbest press. It is common for individuals totrain witb 2 alternately moving dumbbells. However, tbemass of tbe contralateral dumbbell would provide a coun-terbalance, diminisbing tbe destabilizing moment arm oftbe unilateral movements. Tberefore, in order to morebigbly activate tbe trunk stabilizers wbile training tbeupper limbs, only 1 dumbbell sbould be bandied duringtbe action. Tbe advantage of tbis activity is tbat bigb re-sistive forces can still be applied w bile providing a gr eatercballenge to trunk muscle activation.EFFECT OF INSTABILITY ON MUSCLEFUNCTIONSTypically, tbe ability to exert force or power is depressedunder conditions of instability. Bebm et al. (7) found de-creases in force output of approximately 70% and 20%wben performing leg extensions and plan tar fiexor con-tractions, respectively, wbile seated on an unstable ball.Similarly, Kornecki and Zscborlicb (25) observed 20-40 %decreases wben exerting muscular power against an un-stable pendulumlike device. Wben comparing stable and

unstable cbest presses, Anderson and Bebm (3) sbowedtbat isometric cbest press forces were depressed by 60%under unstable conditions, altbougb muscle EMG activitywas not significantly altered. During tbe cbest press,tbere was no significant difference between unstable andstable conditions in tbe EMG activity of tbe pectoralismajor, anterior deltoid, triceps bracbii, latissimus dorsi,or rectus abdominus. Tbe similar extent of muscle acti-vation accompanied by decreased force witb instabilitysuggested tbat tbe motive forces of tbe muscles (tbeirability to apply external force) were transferred intogreater stabilizing forces. Tbus, altbougb externally-mea-sured forces are im paired by instability, muscle activationcan bemaintained or increased because of tbe increasedreliance on stabilization functions. In anotber study fromour laboratory, muscle activation measured by tbe interpolated twitcb tecbnique was recorded witb single- anddouble-leg extensions and squats (10). Tbe bigbest acti-vation levels were found with tbe squats and lowest witbtbe single-leg extensions. Tbe contractions of multiplelower-body muscle groups during tbe squats may baveenbanced quadriceps activation. In addition, greater lev-els of activation may have been necessary to cope witbtbe stabilization necessary for bilateral and multi-artic-ular contractions (squats). Tbese findings would benefitmusculoskeletal rebabilitation because bigb muscle acti-vation can bemaintained wbile using lower-intensity re-sistance. Tbe use of beavy weigbts under stable condi-tions to activate bigb-tbresbold motor units increases tbecbance of injuring tbe recovering muscle tissue. Currentresearcb in our laboratory is exploring wbetber longerterm instability resistance training can modify tbe extentof stabilization functions inorder to improve motive forces.

Furtbermore, coordination, force, and performancecould be bampered under unstable conditions by an in-crease in tbe stiffness of tbe joints performing tb e action.Carpenter et al. (13) indicated tbat a stiffening strategywas adopted wben individuals were presented witb atbreat of instability. Similarly, Adkin et al. (2) reportedtbat when subjects received a postural threat (fear of fall-ing), the magnitude and rate of voluntary movementswere reduced. In addition, participants reported in-creased anxiety and arousal as well as decreased confi-dence. Thus, one might argue that a program that couldimprove stability or balance could subsequently improveforce output, coordination, and confidence, all factors tbatare intricately involved in successful atbletic perfor-mance. An improvement in tbese same factors would ben-efit elderly individuals wbo migbt otberwise remainbousebound in tbe winter because of a lack of confidencein tbeir balance and strengtb.However, new movement patterns, and especiallymovement patterns performed wben unstable, are generally learned at a low velocity, wbereas most sports areconducted at bigb velocities, resulting in a contradictionof tra in ing specificity (56). Furtbermore, tbe specific prac-tice of a sport may be sufficient to ameliorate factors as-sociated witb stability. For example, triatbletes bavebeen reported to be more stable and less dependent onvision for postural control tban controls (34). Gymnastsare reported to be more efficient at integrating and reweigbting proprioceptive inputs (54). Because somesports may provide a balance training impetus, tbe pos-sibility of a limited transfe r of instab ility resistanc e trai n-


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INSTABILITY RESISTANCE TRAINING 719ing effects may not be signif icant in active sport part ici-p a n t s .EFFECT OF INSTABILITY RESISTANCE TRAININGON LIMB STRENGTH GAINSBehm et al. (7) reported th at force outpu t of leg extensorswas only 29.5% of a stable maximum voluntary contrac-tion (MVC) whereas unstable plantar flexors were 79.8%of a stable MVC when performing an open kinetic chainexercise (limb contracts with the trunk stationary or inthis case seated) with either an unstable (Swiss ball) orstable (chair) seat. As previously reported, Anderson andBehm (3) found force deficits of approximately 60% whenperforming an isometric chest press action with an un-stable base (Swiss ball). On the one hand, these deficitsmight promote the essential point of instability training:that because forces have been demonstrated tobe lowerwith unstable conditions, training in that environment isof utmost necessity to ensure action-specific strength ad-aptations. Conversely, overload tension on the muscle isessential for fostering strength training adaptations (6,50). Anumber of authors have stated that training pro-grams to promote general and maximal strength needrepetitions that provide a resistance intensity in therange of 40-120% of IR M or MVC (26, 49, 50). Averyunstable environment, as provided with the leg extensionprotocol, would not provide sufficient overload resistance(29.5%) to promote quadriceps strength adaptations.W hereas the plantar-fiexors protocol also had significant-ly less force than the stable condition, thehigher inten-sity of the contraction could still supply a n overload stress(79.8% of stable MVC) on the muscle with a limited num -ber of contractions.On the other hand, closed kinetic chain exercises (dis-tal portion of limb is stationary while trunk is in motion)have shown greater degrees of limb activation. SoleusEMG activity was approximately 30-40% greater whensquats were performed on an unstable platform (4). Quad-riceps activity was only 5-15% greater with the unstablesquat (4). Because the soleus may have more postural re-sponsibilities than thequadriceps, it would be logical toexpect greater soleus activity when unstable. Kornecki etal . (24) found t ha t c ontributions of stabilizing m uscles in-creased on average by 40% when the handle changedfrom stable to unstable during pushing m ovements. Theyshowed that theprocess of muscular stabilization of theinvestigated jo int caused, on average, 30% drops in force,velocity, and power. Instability-induced muscular stabi-lization of the wrist joint caused a significant increase inthe EMG contributions of tbe stabilizing muscles and avisible drop in tbe contributions of the muscles that re-alized motor functions, which in turn brought about a sig-nificant loss of maximum force, velocity, and power pro-duced against an external object.A number of other authors have examined the func-tion of limb stabilizing muscles. It was found that theshort and long heads of the biceps have similar functionsas anterior stabilizers of the glenohumeral joint, andtheir roles in stabilization increases as joint stability de-creases (22). The stab ilizing function of scapular stabiliz-ers while performing push-ups on miniature trampolineswas also examined (27). The researchers found no signif-icant difference instabilizer EMG activity between stableand unstable conditions; however, they acknowledged

that the degree of stability induced by the miniaturetrampolines was likely insufficient to elicit an unstableplatform.If the goal of the individual is to build limb musclestrength, not all training should be performed under veryunstable conditions. Particularly, unstable open kineticchain exercises for the lower limbs should not be empha-sized in favor of more action-specific closed kinetic chainactivities such as squats and lunges. A combination ofunstable and stable resistance training should provide amelding of balance and strength improvements especiallyfor the general population.EFFECT OF INSTABILITY RESISTANCE TRAININGON COCONTRACTIONSIn the same Behm et al. (7) study previously mentioned,the unstable plantar fiexors and leg extensor conditionsexperienced 30.7% and 40.2% greater antagonist activitythan the stable conditions respectively. The role of theantagonist in this case may have been attem pting to con-trol the position of the limb when producing force. Simi-larly, subjects who had to counteract a predictable unsta -ble upper limb force during 2 blocks of 144 trials eachincreased their accuracy by increasing muscle cocontrac-tions (32). Engelhorn (16) also reported increased antag-onist activity as subjects mastered a learned task (29).Antagonist activity has been reported to be greater whenuncertainty exists in the required task (15, 29). Increasedantagonist activity may also be present to increase jointstiffness (23) to promote stability (20). Whereas increasedantagonist activity could be utilized to improve motorcontrol and balance, it would also contribute to a greaterdecrement in force with theunstable conditions bypro-viding greater resistance tothe intended motion. Contin-ued training, though, can result in lower coactivation lev-els in certain types of work (38). Carolan and Cafarelli(12) demonstrated a decrease in coactivation associatedwith a resistance training program of the leg extensors.The use of instability resistance training to improve bal-ance and stability and decrease movement uncertaintymight be hypothesized todecrease cocontractions, whichin term s of energy conservation would improve movementefficiency.PRACTICAL APPLICATIONSUn stable conditions can lead to decreases in tbe force out-put of the limb and increases in antagonist activity.Greater degrees of instability exacerbate these changes.In light of these findings, the use of instability resistancedevices as a resistance training modality for peripheralor limb strength gains should be employed when the de-gree of instability is light tomoderate, allowing anover-load force or resistance to be developed. For example, ifan individual is in a position in which he or she cannotstay upright (attempting to stand or perform a squat ma-neuver on a Swiss ball or Physio Ball), the amount ofresistance that can be applied to the muscle will be di-minished because the focus is on balance (extreme insta-bility). On the other hand, performing contractions whileseated on a ball with 1 or 2 feet on the fioor (moderate tolight instability) requires less focus tomaintain balance,and hence more concentration and resources can be applied to moving greater resistance. However, although theresistive challenge to a limb under very unstable condi-


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720 BEHM AND ANDERSONTABLE 1. A 2-week resistance training program involving acombination of stable and unstable resistance exercises. The ac-ronym "BOSU" pertains to the p hrase "both sides up" and refersto a dome-shaped instability device. Dyna-Discs are smaller in-flated discs placed under each foot. Swiss balls are large ballswith typical diam eters of 55-75 cm.*

Strength Microcycle 1Exercise Strength Microcycle 2ExerciseShoulderflexion(scissors) on BOSU Up right rowingDB chest press on Swiss ball Bench pressBent-over rowing on BOSU Seated rowingBiceps curls on Dyna-Discs Preac her curlsFront lunges (stable) Squa t on BOSUSide lunges (stable)

* DB = dumbbell.

tions may be less than necessary to develop strength ad-aptations, torso musc ulature may be under greater stress.With unstable conditions, a relatively small resistivetorque on the distal portion of a limb can result in sub-stantia l motive torque by the torso. Perhaps the greatestcontribution of instability train ing m ay be to improve corestability rather than limb strength. In addition, the pre-liminary purpose of the insta bility need no t be significantstrength gains, but an attempt to improve balance, sta-bility and proprioceptive capabilities.Overall, the trunk stabilizer muscles are more highlyactivated by unstable than by stable exercises. In addi-tion, resistance exercises using a single arm (unilateral)will also cause greater activation of the contralateral side

trunk stabilizers. Therefore, it is recommended that fostrengthening or increasing the endurance of the trunkstabilizers, the exercise should involve a destabilizingcomponent. The lack of stability may originate from thebase or platform upon which the exercise is performed(i.e., ball or wobble board) or by placing body segm ents oresistance outside the base of support of the body (i.e.unilateral dumbbell resisted movements). However, imust be recognized tha t when a n individual is attem ptingto exert forces under unstahle conditions, the maximumforces achieved under stable conditions are not possiblbecause of the greater muscle stabilization functionsFurthermore, the number of RMs would also need to beadjusted to compensate for the unstable platform. Tbusit is recommended that, when an instability resistanctraining program is instituted, exercises be performed under stable conditions as well to ensure higher tensions onthe muscles. This combination of stable and unstable exercises for similar muscle groups could be organized w ithin a single training session or alternated over weekly sessions. Table 1 provides an example of a 2-week programalternating stable and unstable exercises.Finally, the henefits of instability resistance trainingmay be more pronounced for those individuals pursuingprimarily health and rehabilitation benefits and not participating in challenging athletic activities or trainingwith free weights involving high loads (Table 2). It is noknown at this time whether instahility resistance training provides greater benefits to active athletes for balance, trunk muscle activation, and coordination when implemented in conjunction with a traditional resistanc

TABLE 2. This table provides a general des cription of the benefits of specific instability re sistan ce and balan ce train ing effecton training for rehabilitation, general musculoskeletal health and sports performance.Instability resistance andbalance training effects Rehabilitation Musculoskeletalhealth Sportsperformance

T trunk activation when compared tosimilar intensity stable activitiesT limb muscle activation w hen com-pared to similar intensity stableclosed kinetic chain exercises (i.e.,squats)I limb muscle activation when com-pared to similar stable open kineticchain conditionsT cocontractions with acute exposureto instability

T agonist stabilization functions

i force and power outputT static balance

Action specificity

loads used to prevent injury

T cocontractions increase jointprotection

t agonist stabilization may in-crease joint protection

ttLimited or unknown applica-tion to dynamic balance

tNear-maximal or maximal activation can be achievedwith high loads

Unknown whether chronic instability training will re-duce cocontractionstUnknown whether chronic instability training will transfer agonist stabilizer to motive functionstSpeciflc sport practice mayprovide sufficient dynamicbalance training effect

* Signiflcant beneflt.t Minimal beneflt.i No beneflt.


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INSTABILITY RESISTANCE TRAINING 721TABLE 3. Instability resistance training literature.

What are some of the questions that remain unanswered?1. Can static instability training contribute to greater dynamicbalance?2. Will static instability resistance training provide greater forc-es or power during unstable dynamic conditions than tradi-tional resistance training (i.e., alter stabilizing to motivefunctions)?3. Because of the instability-induced decrease in force output,should balance training be performed independently of resis-tance training?4. Can the levels of trunk activation achieved with unstable ex-ercises be equaled or increased with the greater resistancepossible with stable resistance training (i.e., squats and dead-lifts)?5. Can instability resistance training reduce the extent ofco-contractions?6. For exercise prescription purposes, is it possible to generallyquantify the differences in muscle activation intensity orforce loss with unstable correlates of stable exercises?7. Can training on unstable platforms improve performance inactivities that receive stability perturbations from othersources (i.e., external forces such as encountered in footballand rugby?

training program. There are many questions still to beanswered regarding this area of training (see Table 3).REFERENCES

1. ABENHAIM, L., M. ROSSIGNOL, J.P. VALAT, M. NOE DIN, B.AvouAC, F. BLOTMAN, J. CHARLOT, L. DREISER, E. LEGRAND,S. ROZENBERG, AND P.VAU TRAVERS. Therole of activity in thetherapeut ic management of back pain. Report of the Interna-tional Paris Task Force onPain. Spine 25:1S-33S. 2000.

2. ADKIN, A.L., J.S. FRANK, M.G. CARPENTER, AND G.W. PKYSAR.Fear of falling modifies anticipatory postural control. Exp.Brain Res. 143:160-170. 2002.3. ANDERSON, K . , AND D.G. BEHM. Maintenance of EMG activityand loss of force output with instability. J. Strength Cond. Res.18:637-640. 2004.4. ANDERSON, K., AND D.G. BEHM. Trunk muscle activity increas-es with unstable squat movements. Can. J. Appl. Physiol 30'33-45. 2005.

5. AROKOSKI, J.P., T. VALTA, O. AIRAKSINEN, AND M. KANKAAN-PAA. Back and abdominal muscle function during stabilizationexercises. Arch. Phys. M ed. Rehabil. 82:1089-1098. 2001.6. BEHM, D.G. Neuromuscular implications and applications ofresistance training. J. Strength Cond. Res. 9:264-274. 1995.7. B E H M , D.G., K. ANDERSO N, AND R.S. C UR NE W. Muscle force andactivation under stable and unstable conditions. J. StrengthCond. Res. 16:416-422. 2002.8. B E H M , D.G., D. B UT T ON, K. POWER, K. ANDERSON, AND M.

CONNORS. Relationship between hockey skating speed and se-lected performance measures. J. Strength Cond. Res. 19:326-331. 2005.9. B E H M , D.G., A. LEONARD, W. Y O U N G , A. BONSEY, AND S.MACKINNON. Trunk muscle EMGactivity with unstable andunilatera l exercises. J. Strength Cond. Res. 19:193-201. 2005.

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the role of instability with resistance

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