forms of resistance

Forms of VariableResistance TrainingD. Travis McMaster,1 John Cronin, PhD,1,2 and Michael McGuigan, PhD, CSCS1

1Edith Cowan University, Perth, Australia; and 2AUT University, Auckland, New Zealand

S U M M A R Y

RESISTANCE TRAINING METHODS

HAVE BEEN BROADLY CLASSIFIED

INTO 3 CATEGORIES: CONSTANT,

ACCOMMODATING, AND

VARIABLE RESISTANCE. VARIABLE

RESISTANCE TRAINING METHODS,

WHICH INCLUDES CAMS AND

LEVERS, CHAINS, AND

RUBBER-BASED RESISTANCE,

WILL BE THE FOCUS OF THIS

ARTICLE. THE KINEMATICS,

KINETICS, AND HUMAN STRENGTH

CURVE CHARACTERISTICS

ASSOCIATED WITH THESE 3

TYPES OF VARIABLE RESISTANCE

ARE DISCUSSED, GIVEN THAT

EACH RESISTANCE TYPE MAY

OFFER A UNIQUE SET OF ME-

CHANICAL STIMULI AND, HENCE,

MUSCULOSKELETAL

ADAPTATIONS. THE PRACTICAL

APPLICATIONS AND LIMITATIONS

ASSOCIATED WITH EACH FORM

OF VARIABLE RESISTANCE WILL

ALSO BE CONSIDERED.

INTRODUCTION

Resistance training, once used bya very small group of eliteathletes and weight lifting en-

thusiasts, has grown immensely inpopularity during the past 3 decadesand is currently practiced by a largenumber of people within society. Thereare many modes of resistance trainingand a multitude of variables that can bemanipulated to benefit the musculo-skeletal system. In terms of the modesof resistance that can be used to inducemusculoskeletal adaptation, 3 broadcategories typically are used: constantexternal resistance, accommodating

resistance, and variable resistance(VR). Constant external resistance isdefined as an unchanging external loadthroughout the range of motion and isthe most popular form of resistancetraining, because many believe it bettersimulates real-life activities and pro-vides a more natural coordination ofthe musculature (32,48,56). Accommo-dating resistance equipment is de-signed to exert speed controlled orisokinetic resistance throughout thefull range of motion, more recentlytermed semi-isokinetic resistance (60).Fluid-based resistance remains unde-fined, because it resembles both ac-commodating and VR. Hydraulic(liquid-controlled) and pneumatic(gas-controlled) equipment are 2 formsof fluid resistance. VR equipment isdesigned to change the external re-sistive load throughout an exercise’srange of motion; the different types ofVR equipment include rubber-basedresistance (RBR), chains, and cam andlever systems.

To assist in understanding some of theconcepts discussed in this article,a basic knowledge of human strengthcurves, kinematics, and kinetics andtheir importance in explaining theadaptive potential of an exercise,loading scheme, and resistance typeare required because it is the mechan-ical stimuli that dictate the hormonaland metabolic responses. Kinematicsdescribes the change in position of anobject via variables such as position,displacement, time, velocity, and ac-celeration (28). Kinetics describes theforces and their effects on the motionor kinematics of an object (51). Eachtype of resistance can offer a uniqueset of kinematics and kinetics and

therefore differential musculoskeletaladaptation.

Human strength curves are based onmovements about single joints using2-dimensional coordinate systems butalso have been extended to includemulti-joint movements with the useof 3-dimensional coordinate systems.Strength curves are classified into 3categories: ascending, descending, andbell-shaped, which are determined bythe force-angle (torque) relationshipwithin the musculoskeletal system(Figure 1) (24,40,66,67). Multi-jointstrength curves are calculated bysumming the torques of all involvedjoints in the exercise movement, whichprovides an overall measure (estimate)of the maximum muscular capability inthe system; for example, during thetraditional squat, a sum of torqueswould be determined by adding theindividual torques produced at ankle,knee, hip, and trunk, respectively. Thepercentage of force and torque pro-duced at each joint may very depend-ing on the mode of resistance, exercisemovement, velocity of movement, andthe amount of external resistance lifted;however, further investigation is re-quired to determine the exact contri-bution of the involved musculature andjoints (23). Strength curves have beendeveloped by theoretical and experi-mental means. Theoretical strengthcurves predict the torque capabilitiesof the musculoskeletal system throughcadaver dissection where the followingphysiological and biomechanical

KEY WORDS :

biomechanics; rubber bands; steel chains;cams and levers; kinematics; kinetics

VOLUME 31 | NUMBER 1 | FEBRUARY 2009 Copyright � National Strength and Conditioning Association50

Copyright © . N ational S trength and Conditioning A ssociation. Unauthorized reproduction of this article is prohibited

properties are measured: muscle cross-sectional area, fiber lengths, sarcomerelengths, the arrangement of thin andthick filaments, the origin and insertionlocation points, and correspondingangles (36). Experimental strengthcurves are easier to develop thantheoretical strength curves because livehuman subjects and direct strengthmeasuring devices, such as electromy-ography, force plates, in vivo forcetransducers, ultrasound, and isokineticdynamometers are used to calculatejoint torque capability (23,35,36,39,42).Strength can be measured during iso-metric, concentric, and eccentric con-tractions at varying speeds and loadingconditions; the shape of strengthcurves are similar when comparingisometric to concentric or eccentricisokinetic testing for a specific jointrange of motion (30).

Knowledge of the biomechanical rela-tionships and the subsequent benefitsand limitations of the different modesof resistance would seem of greatpractical benefit to the practitioner,strength and conditioning coach, andclinician. This framework provides thedirection for future discussion of VRtraining.

VR TRAINING

VR equipment is designed to alter theresistance placed on the musculoskel-etal system throughout the range ofmotion, in an attempt to match thevarious exercise strength curves (24).

Strength curves approximate the tor-que (relationship between force gener-ation and joint angle) productioncapabilities for specified exercisemovements (37). Strength can be de-fined as the maximal force and torque(rotational force) a musculoskeletallever system can generate at a givenvelocity (18,38). Muscular force gener-ation and torque production are depen-dent upon a number of physiological,biomechanical, and neural factors,including muscle cross-sectional area,muscle length, pennation angle, theradius of the internal and externalmoment arms, contraction speed, andthe size, number, and type of motorunits recruited (20,21,40).

Human strength curves of single jointmovements (see Figure 1) are generallyeasy to categorize, as movement isgenerated via a single muscle or groupof muscles (i.e., biceps brachii, brachia-lis, and brachioradialis muscles) withproximal insertion points causing rota-tion about a single axis (1 degreeof rotational freedom), such as flex-ion–extension, adduction–abduction,elevation–depression, or internal rotation–external rotation. The dumbbell armcurl can be used to explain the rela-tionship between internal forces actingwithin and external forces acting onthe musculoskeletal system. During thedumbbell arm curl, the biceps brachii,brachialis, and brachioradialis musclesmust produce a force and create atorque that is greater than that created

by the weight of the dumbbell to causea concentric contraction and flexion atthe elbow. Biomechanically, the bicepsbrachii, brachialis and brachioradialismuscles generate an internal force thatis transferred through their respectivetendons, creating torque about theelbow leading to an external endpointforce that is applied to an externalresistive load (e.g., dumbbell) leading torotation about the elbow joint (66).

Multi-joint movements are complexand more difficult to categorize be-cause movement occurs about multiplejoints (multiple degrees of rotationalfreedom) in multiple planes and mustbe represented by 3-dimensional co-ordinate systems. Torque capabilitiesduring multi-joint movements are in-fluenced by a number of physiologicaland biomechanical factors, includingthe architecture of the involved mus-culature and joints, the type(s) ofcoordinated muscle actions, and thelocation of origin and insertion points.The majority of sport movements, suchas running, kicking, and throwing,occur in sequential order, where move-ment is initiated by the larger proximalmuscles and segments, and then trans-ferred to smaller distal muscles andsegments along the kinetic chain. Thisphenomenon is known as the summa-tion of forces and or the summation ofspeed principle (10,28). During manymulti-joint movements, the accumula-tion of forces generated about eachjoint along the kinetic chain results inan assimilation of joint torques; exer-cise movement examples include thebench press, leg press, squat, deadlift,and power clean (10). When thesegments of these multi-joint move-ments approach full extension, themusculoskeletal lever system gainsa mechanical advantage and is able tobare larger external resistive forces;theoretically these movements wouldbe supplemented well by VR equip-ment that increases in a linear orcurvilinear fashion (66).

Exercises with ascending strengthcurves include the squat, deadlift,bench press, leg press, and shoulderpress. In these exercises, maximum

Figure 1. Three major strength curves: force production versus joint angle.

Strength and Conditioning Journal | www.nsca-lift.org 51


strength and force production capabil-ity occur near the apex of the lift (24).In a descending strength curve, max-imum strength is produced at the startof the lift, examples include bent-overand upright rows, pull-ups, chin-ups,and lat pull-downs. Single joint exercisemovements generally have bell-shapedstrength curves (e.g., elbow flexion andextension and knee flexion and exten-sion), where maximum strength occursaround the mid-phase of the lift (24).The reader needs to be cognizant ofthe fact that human strength curves aregeneralized to the samples studied, asbiomechanical and physiological dif-ferences between body segments andindividuals are complex and not alwaysaccounted for.

Current forms of VR equipment in-clude cams and levers, rubber-basedresistance (RBR), and chains. Theseforms of resistance are used through-out the sport science, strength andconditioning, rehabilitation, powerlift-ing, and weightlifting world. In theory,if the equipment is designed to matchthe different human strength curves,then the contracting muscles wouldmaximize force production throughoutthe range of motion and maximal gainsin strength would be achieved (22).Whether this is actually the case andthe practical relevance of VR trainingmodes are not well understood, whichprovides the focus of subsequentdiscussion.

CAMS AND LEVERS

Lever and cam systems are designed tochange the external moment arm (thelength or radius) of the correspondinglever or cam to approximate the body’schanging moment arm (leverage andmechanical advantage) during the lift,which forces the muscles to exert nearmaximal effort throughout the range ofmotion (59). Therefore, the systemattempts to provide resistance changesto match the musculoskeletal systemsability to produce torque at variousjoint angles along the movement path(26,32). Mechanical advantage isa product of the force–joint anglerelationship, where the force exertedby the muscles will vary with the

mechanical advantage of the jointsassociated with a specific movement(64). An ‘‘irregular-shaped cam’’ de-signed by Herz of Vienna in 1901,allowed for increased resistance atpoints where strength was greatestand decreased the resistance at pointswhere the strength was lowest, accom-modating for the musculoskeletalsystems mechanical advantage, sup-posedly leading to improvements instrength (37). This design was adaptedby Jones, the inventor of Nautilus, whoin his equipment design used a shell-shaped cam very similar to that ofHerz. Jones designed his first prototypein 1948 and then released the first camrun Nautilus machine onto the marketin 1970 (58). Several years later, in1972, Ariel designed the ‘‘dynamicvariable resistance’’ machine, whichused an external lever arm to matchthe musculoskeletal changing leversystem for numerous of exercise move-ments (2).

Kinematics and Kinetics of Cams andLevers. Cam and lever systems createa varying torque that opposes andcorresponds to the torque productionability of the different musculoskeletallever systems (38). In other words, themachines resistive torque attempts tomatch human torque capabilities foruniarticular movements, such as legextensions, leg curls, bicep curls, and

pullovers (37). The equation for torqueis as follows:

Torque ¼ ForceðNewtonsÞ3 Length

of Movement Arm ðMetersÞ

From this equation, it can be observedthat torque may be altered by eitherchanging the amount of external orinternal force acting on or withina body segment, or by changing thelength of these respective momentarms. The moment arm is the perpen-dicular distance from the fulcrum (axisof rotation) to the point of forceapplication (external load or tendonattachment). The length of the mo-ment arm is proportional to the jointangle between 2 longitudinal segmentsduring uniarticular movements, whichis known as a relative joint angle(shown in Figure 2).

Cam and lever systems have a fixedresistive load; therefore, the externalresistance is varied by altering thelength of the moment arm or radiusand hence the changing radius of an‘‘irregular shaped cam’’ (Figure 3). Acam system’s resistive torque increasesin proportion to the radius of the cam;therefore, the larger the radius, thegreater the resistance and vice versa(30). The same can be said for leversystems, where length changes in theeffective lever increase or decreasethe effective resistance throughoutthe exercise movement. Three notable

Figure 2. Moment arm versus relative elbow joint angle. The length of the momentarm shortens and lengthens in proportion to the decreasing and increasingjoint angles.

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Forms of Variable Resistance Training


cam shapes with varying momentarms have been designed by Nautilus,Universal, and other manufacturers, tomatch the machines resistive torqueto human torque capabilities forspecific exercises (Table 1).

The cams changing radius is designedto minimize the negative effects ofmomentum by matching externalresistance to the internal forceand torque generating capabilities(mechanical advantage) of the muscu-lature, theoretically causing the work-ing muscles to exert maximum forcethroughout the complete range ofmotion. Angular impulse is propor-tional to the amount of applied torqueproduced over time; therefore, angularimpulse should be greater when cam-based machines are used, as theworking muscles should be producing

near maximal force/torque values overa longer period of time than constantexternal resistance modes. Given thatmass remains constant, the net result ofan increase in angular impulse shouldbe an increase in average angularvelocity, which is a desirable traininggoal for many athletic movements.

Hay and Andrews (34) studied thebiomechanical effects of the Universalarm curl machine versus barbell armcurls and found that the machineprovided a VR more consistent withthe working muscles capacity to exertforce throughout the range of motionthan the barbell curl. They found thatthe barbell curl was inferior to theUniversal arm curl machine, in terms ofmatching the external resistance andthe muscular force capabilities of theelbow flexors at varying joint angles

(see Figure 4) (59). Johnson et al. (37)compared human torque capabilitiesand machine resistive torque by using 4Eagle cam-based resistance machinesthat included knee extension, kneeflexion, elbow extension, and elbowflexion. They found that the 4 ma-chines accommodated the subjectsfairly well by creating machine resistivetorque curves similar to human torquecurves, an example of which can beobserved in Figure 5. It must be notedthat as a result of individual variationsof size and strength, it is difficult toconstruct a machine that accommo-dates everyone’s unique anthropome-try. Regardless of this inherentlimitation, cam and lever systems havegrown in popularity and are currentlyused in many fitness and rehabilitationcenters worldwide. The benefits and

Figure 3. Changing radius of a pullover cam machine.

Table 1Human strength curves for various cam based exercise movements (adapted from Fleck and Kraemer [24])

Strength curve Ascending Descending Bell-shaped

Exercise example Deadlift Seated row Elbow flexion andextension

Chest press Lat pull-downs Knee flexion andextension

Squats

Mechanical definition Ability of muscle to produceforce and create torque islowest at the start of liftand increases throughoutthe ascent phase due toan increase in mechanicaladvantage.

Ability of muscle to produceforce and create torque isgreatest in the first quarterof the lift and progressivelydecreases throughout the lastthree-quarters of the lift asthe mechanical advantagediminishes throughout the lift.

Ability of muscle toproduce force andcreate torque ishighest during themiddle portions ofthe lift and lowest inthe first and lastquarter of the lift.



limitations of their use are nowdiscussed.

Practical Applications of Cams andLevers. As mentioned previously, themost notable benefit to using cams istheir ability to create a resistancesystem that matches the musculature’sability to generate force and producetorque throughout the entire range ofan exercise. Cam and lever equipmentis suitable for beginner and weakresistance trainers because it followsa fixed movement path and requiresless skill, decreased intermuscular co-ordination, and is less likely to causeinjury compared with other modes ofresistance because it is easier tomaintain control of the load(26,29,32,33). It allows for unilateral

and bilateral training options and nospotter is required. Furthermore, theuse of such cams is common in therehabilitation setting where single joint(i.e., rotation about a single joint axis)and multi-joint (i.e., rotation about 2 ormore joint axes) exercise movementsare used in combination for the re-habilitation of injuries (e.g., anteriorcruciate ligament reconstructions) andcertain pathologies, such as patellofe-moral pain syndrome (25,45).

Single joint movements (e.g., kneeextension and flexion) have the capa-bility to isolate specific muscle groups(e.g., the quadriceps and hamstrings),which can be beneficial when correct-ing muscle imbalances. Patellofemoralpain syndrome (patellar malalignment)is thought to be caused by a strength

imbalance between the vastus lateralisand vastus medialis, which can beremedied by using single-joint move-ments in combination with multi-jointmovements to increase vastus medialisstrength (16). In general, multi-jointmovements (i.e., traditional squats)produce greater compressive forcesand single joint movements (i.e., legextensions) produce greater shearingforces on the involved joints (e.g., knee)and surrounding musculature (43).Both forms of exercise should be usedin rehabilitation and training programsto provide varying training stimuli, interms of kinematic and kinetic profiles.Cam and lever resistance may also bebeneficial as a supplement to weight-bearing exercises in programs for osteo-porotic and osteoarthritic populations,as it may provide a safe-controlledform of resistance that would helpimprove bone density and limit deteri-oration according to the stress–strainrelationship between load and bonedensity (19,51,65). Some of the afore-mentioned benefits also are consideredlimiting factors in the development ofcertain kinematic and kinetic variables,which are discussed in the followingsection.

Limitations of Cams and Levers. Onemajor limitation is that the movementpattern of cam and lever equipmentcannot be manipulated by the lifter tosuit his/her mechanical and physio-logical needs; instead, the lifter isrequired to adapt to the equipment.During equipment design, torquechanges through the various anglesare based on sample averages (26,29).A problem arises in the fact that theequipment is designed for the averageperson and may not accommodatepeople that have extreme differencesin anthropometry. Force-angle curvesare affected by mechanical variables,such as limb length, point of tendonattachment, and velocity of movement(26). Attempts have been made tomatch the machines resistive torquecurves to the 3 types of human strengthcurves; however, the efficacy of such anapproach is debatable (11). For exam-ple, Harman (31) found that Nautilus

Figure 4. Comparison of 3 force-angle curves for elbow flexion (adapted fromSmith [58]).

Figure 5. Machine resistive torque (MRT) versus human torque capability (HTC) forknee flexion (adapted from Johnson et al. [36]).




machine resistive torque did not cor-respond to human torque curves forthe following 5 exercises: the chest fly,knee-flexion, knee-extension, elbowflexion, and the pullover, because de-sign flaws were visible; therefore, camshapes require much modification tomatch machine resistive torque pat-terns with human torque capabilities.

Nautilus cams are theoretically de-signed to match the human strengthcurves as the joint segment movesthrough a controlled path (32). Asa result, ballistic and explosive move-ments, which are key characteristics inalmost all sports, are difficult toperform on such equipment and,therefore, this form of resistance maynot be beneficial as a form of sport-specific training (29). Furthermore,because of the fixed movement pathof the exercise equipment, develop-ment of neuromuscular coordination ishindered, and the antagonist andsynergist muscles are less active (26).Other limitations include capital outlayfor equipment cost and thereafterequipment maintenance (29).

RUBBER-BASED RESISTANCE

RBR has been primarily used bypractitioners and clinicians to helppatients regain strength after injury(53). This form of resistance has alsobecome popular in fitness centers andfrequently prescribed by gym staff andpersonal trainers. Strength and condi-tioning coaches have also adopted andadapted RBR training in an attempt toimprove athletic performance. Bands,tubing, and bungies are polymer-basedproducts with varying compositiondepending on the type of polymer(i.e., thermoplastics or elastomers) usedduring production. The composition ofthese products affects the physical andmechanical properties, such as stiffness(stress–strain), density, yield and tensilestrengths (4). The aesthetic differencesbetween the various RBR products canbe observed in Figure 6 (53). The termRBR is used throughout this articlebecause the term elastic resistance canbe misleading, since it does not fullydescribe the exact type of resistancethat tubing, bungies, and bands

provide. That is, all of these productsare viscoelastic, exhibiting nonlinear orviscous properties in combination withlinear elastic properties. This can beobserved in the diagram below com-paring the tension and deformation ofvarious sized rubber bands (Figure 7).The nonlinear curves observed can bebest fitted with second order polyno-mials, rather than simple linear func-tions (64). For a detailed discussion ofviscoelasticity please refer to any goodbiomechanical text.

As can be observed from Figure 7,there is a relatively linear-elastic regionthat can be explained using Hooke’slaw if the resistive forces need to bequantified. That is the tension providedby the RBR is equal to the stiffnessconstant (k) multiplied by the defor-mation (d):

Tension ¼ kðstiffnessÞ3dðdeformationÞ

During the elastic region, deformationincreases in direct proportion to theamount of tension placed on the RBRproduct, and this linear relationshipcan exist for deformations of up to300% depending on the composition ofthe product (4,28).

The Hygenic Corporation was the firstmajor producer and distributor of RBRtraining products and currently has 7

resistance levels on the market, rangingfrom 1.5 to 10.75 Newtons in tension attwice the bands resting length (53,61).Today there are a number of manu-facturers that make a variety of bandswith different tensile properties. RBRtension can also be increased byshortening the resting length or byincreasing the number of bands (7).The various rubber band tensions andelongation percentages (changes indeformation) are depicted in Figure 8(53). RBR products were initiallydesigned for rehabilitating and restor-ing muscle and joint functions; today,they also are used for improvingconditioning and balance and buildingstrength in all individuals (61).

The way in which tension of RBR isdelivered is dependent on the setup ofthe apparatus (64). RBR can be set upto increase or decrease the resistiveload during the ascent phase of a liftand decrease or increase the resistiveload during the descent phase.Deformation-tension curves of RBRare closely related to the ascendingstrength curve; therefore, RBR shouldtheoretically supplement ascendingstrength curve exercises. The equip-ment can also be set up to help athletesovercome the initial sticking point inthe bench press and the squat, i.e., RBRcan be attached to the bar and/orathlete to offer a degree of unloading.

Figure 6. Various elastic resistance products.



For this to occur, the bands are attachedto the top of a power rack, to provideassistance when the active muscles inmovements, such as the bench pressand squat are at maximal lengths, wherethe mechanical advantage is least.

Kinematics and Kinetics of RBR. RBRis proportional to the amount ofdeformation multiplied by the stiffnessconstant during the elastic region ofthe deformation–tension curve. Rubberbands allow for variable resisted move-ment in a multiple of planes, such asthe sagittal, frontal, transverse, ora combination (oblique) of planeswhile maintaining consistent resistiveproperties in all planes, unlike gravitydependent modes. Most gravity-de-pendent modes of training (e.g., freeweights and chains) offer the greatestamount of resistance with movements

in the vertical (frontal and sagittal)plane only. A comparison between thedirection of external resistance of RBRand free weights is shown in Figure 9.

The curvilinear deformation–tensionproperties of RBR allow for increasedacceleration in the initial, less-resistedpositive phase of a lift; as a result,velocity of the exercise movementincreases. During the ascent phase ofan ascending strength curve lift, themusculoskeletal system gains a me-chanical advantage and force produc-tion decreases (6,17,24,66). This area isone in which the added band tensionhas the potential to increase musclestimulation, motor unit recruitment,and firing rates and in turn preventa decrease in muscular force produc-tion throughout the last quarter of thelift (17). The use of RBR in training hasbeen shown to increase strength in

recreational athletes by 10–30% over 6-to 12-week training periods; however,similar gains also have been reportedwith most other modes of resistancewhen used properly; therefore, furtherresearch is needed to determine whetherone mode is superior to the other (53).

A study by Damush and Damush (15)found that older adult women in-creased strength during an 8-weekRBR training program. The programconsisted of 2 training sessions perweek. Seven 1-set exercises wereperformed at each session, each exer-cise was performed until a level of 4was reached on the Borg PerceivedExertion Scale. The 7 exercises in-cluded seated lat. pulldown, seatedsingle leg press, seated chest press,seated single toe press, standing triceppress, standing bicep curls, and seatedleg extension. Strength improvementsoccurred in 3 major muscle groups;the latissimus dorsi (19.7 6 10.3%), thequadriceps (27.7 6 17.6%), and thepectoralis major (16.56 11.2%); wherethe subjects 3 repetition maximumswere measured on the seated lat pulldown, seated leg extension, and seatedchest press, respectively.

When bands are added to free weightexercises, such as the squat, dead-lift,and bench press, the added tensiontowards the end of the positive con-centric phase may trigger an increase inmuscular force generation and peakpower production (64). Wallace et al.(64) reported increases in peak force(16%) and peak power (24%) were

Figure 7. Deformation versus tension curves for 5 different types of RBR.

Figure 8. Tension of rubber bands at different percentages of elongation (adaptedfrom Page and Ellenbecker [52]).




greater during the back squat whenrubber bands were used in combina-tion with free weights compared withthe use of free weights only. Thesevariables (peak force and peak power)were recorded at a frequency of 500 Hzusing a Quattro Jump force plate andassessed with Quattro software. Itmay be speculated that RBR allowedthe lower-extremity muscles of thesubjects to produce greater peak forceat the position where the mechanicaladvantage of the segment lever systemwas greatest (near full extension) dueto the curvilinear tensile properties ofRBR and ascending strength curveproperties of the squat. Another studyby Cronin et al. (14) compared thekinematics, kinetics, and EMG ofballistic squats with and without rubberbungies and a nonballistic squat; allmovements were performed with a su-pine squat machine. They found thatwhen loads were equated that therewas greater EMG activity of the vastuslateralis during the eccentric contrac-tion of the rubber bungie supine squat,and greater peak velocities were foundfor both ballistic squats (14). Improve-ments in peak velocity, force, andpower in the lower extremities maybe beneficial to athletes participating incontact sports, such as rugby, alldisciplines of football, ice hockey, aswell explosive track and field events(sprinting, shot putt, hammer throw,high jump and long jump).

In summary, the addition of RBR toa constant external resisted movementalters the biomechanical profiles, asRBR products allow for greater

accelerations to be produced in theinitial phases of a movement comparedto a constant external resisted move-ment with an equivalent load. How-ever, the force required to elicitmovement will increase proportionallywith the displacement and changingdeformation–tension of the RBR; andthe greatest forces will be required atmaximal displacement or end range ofmovement. Thereafter, greater accel-erations occur earlier in the eccentricphase, which should result in increasedeccentric forces and potentially greaterSSC enhancement. It has been sug-gested that the benefits of RBR aremost apparent when combined withfree weights, so that the inherentshortcomings of either resistance typecompensate for one another (7,64).That is, free weights provide greatestmechanical overload at the beginningor inner range of the concentric phaseand the bands provide overload at theend or outer range of the concentricphase for ascending strength curvemovements. The opposite is true ofthe eccentric phase in terms of themechanical overload; therefore, thecombination of both resistance modesmay compliment each other through-out the entire concentric and eccentricphases for most multi-joint movements.

Practical Applications of RBR. RBR isprescribed and used in many rehabil-itation programs because it is a portableand a relatively inexpensive low-impact,momentum-controlled resistance usedfor multiplanar training unaffected bygravitational forces. It also promotes

neuromuscular coordination and sta-bilization, which are all importantfactors in postinjury rehabilitation(52). RBR training may be used tobenefit many different components ofthe musculoskeletal system, includingincreases in muscle mass, power andendurance, decreased body fat, andimproved balance, gait, and mobility(53). Exercises performed in the trans-verse plane would benefit from RBRbecause the resistance supplied by therubber bands is collinear to the move-ment. Rotator cuff rehabilitation pro-grams use rubber bands for this veryreason, as internal and external rota-tions about the shoulder are performedin the transverse plane. Many baseballpitchers use RBR training after re-constructive surgery of torn rotatorcuffs initially to increase strength of theinternal and external rotators and tothe increase range of motion about theglenohumeral joint (47,54,63). Reha-bilitation programs that use RBRtraining include stroke patients, osteo-arthritic patients, elderly populations,contracture patients, and other patho-logical groups. Through supplementedRBR training programs, stroke, osteo-arthritic, and elderly patients havedemonstrated improvements in strength,gait, balance, and function, leading toimproved quality of life and the pre-vention of falls (13,41,50,52,55). Manoret al. (44) also found that rubber bandswere a valid and reliable means to assessupper extremity strength in older adults.

It is inferred that RBR training can beused effectively in exercises with as-cending strength curves because of the

Figure 9. (a) Rubber based resistance (collinear to the movement) versus (b) free weight resistance (perpendicular to themovement).



similarities between the deformation–tension relationship of RBR and theforce-joint angle relationship of anascending strength curve. When com-bining bands with free weights, it isrecommended that RBR comprises20–35% the total resistance and freeweights comprise the remaining 65–80%, with a total load of 60% or 80–85% of the athletes 1-repetition max-imum (1RM) to improve peak powerand peak force outputs, respectively,for the back squat and possibly otherascending strength curve exercises(49,57,64). Anderson et al. (1) com-pared a free weight training group toa combined training group (free-weight plus RBR) with similar baselinemeasurements. They found that sup-plemented RBR training improve-ments during the back squat andbench press were significantly greaterthan that of the free weight-onlytraining group. 1RM performancewas 3 times greater for the back squat(16.476 5.67% kg vs. 6.846 4.42 % kg)and 2 times greater for the bench press(6.68 6 3.41% kg vs. 3.34 6 2.67% kg)with the supplemented RBR training(1). The back squat and bench presssupplemented with RBR may be help-ful in training athletes who couldbenefit from increased strength, aver-age and peak power, and peak force, asfound in studies by Anderson et al. (1)and Wallace et al. (64). These increasesmay translate into improvements inmaximum strength, vertical jump, bal-listic ability, and enhanced sport per-formance, but investigations have yetto fully demonstrate these adaptations.

RBR also can be used to magnify thestretch load and enhance the eccentricphase during plyometric training (seeFigure 10). In this exercise, the athletebegins the movement with the RBR onstretch, which increases the accelera-tion and subsequent medio-lateralground reaction forces to the athletewhen they step inwards (14). RBR isalso applied to other sport-specifictraining programs, including the vari-ous tennis strokes and catcher-specificmovements in baseball, but there is noempirical evidence related to the

efficacy of these programs (6,22). Forthe various tennis strokes, tubing maypromote the development of speed andstrength, an important combination indeveloping a superior tennis stroke.Initially, resistance in the tubing is low,allowing for an increase in limbvelocity and, as the athlete movesthrough the range of motion, resistanceprogressively increases, providingoverload to the involved musculature(6,7). If resistance is too low initially,dumbbells can be supplemented withrubber bands and tubing to provide anincrease in resistance (7). For catchers,once the ball is caught, it is importantto ascend quickly into the throwingposition. This is done by exploding outof the squat position while simulta-neously rotating the hips and feet intothe correct throwing stance. A harnessand RBR bands are attached to theathlete and as the athlete explodes outof the squat position resistance isincreased, training the athlete to gener-ate greater speed and power throughthe quadriceps, hamstrings, gluteals, hip,and torso musculature (22). It must benoted that adding RBR to sport-specificmovements may change the naturalcoordination of the movement andresult in performance decrement ifused as the main source of resistance

training; therefore, the use of RBR incombination with sport-specific freeweight and body weight movementsmay prove to be the most beneficialform of training.

Limitations of RBR. The lack of re-search and scientific evidence to sup-port the practical benefits of RBR isa major limitation to the efficacy ofRBR training. Some clinicians thinkthat the limitations of RBR trainingoutweigh the benefits, such as theinability to quantify resistive load andprescribe specific loading patterns, dueto the viscoelastic properties of rubber.It should also be noted that RBRproducts have stretch maximumsknown as yield and fracture points,where the rubber begins to break downand eventually fail, which may posea problem for movements with sig-nificant displacements, such as thesquat, standing shoulder press and pushpress (57).

It may be that the use of RBR is limitedto a specific group of ascendingstrength curve exercises and may notbe suitable for exercises with bell-shaped and descending strength curvesbecause it could be detrimental to thedevelopment of strength and otherkinematic and kinetic variables.

Figure 10. Bungie plyometric training.




However, this is dependent upon theequipment set-up and goal of themovement. For example, because ofcontradicting deformation–tension re-lationship of RBR and the force–jointangle relationship of the musculoskel-etal lever system, RBR may be in-adequate for certain resistance trainingexercises, because the contractingmuscle(s) would be improperly over-loaded (22,28,46,51). Wallace et al. (64)found that peak power during the backsquat decreased by 13% using RBR ata substituted 35% of the total loadwhen compared with RBR at 20%.Another study on the back squat byEbben and Jensen (17) found nosignificant differences for integratedelectromyography and mean groundreaction forces when using RBR ata substituted 10% of the total loadcompared to traditional constant ex-ternal resistance squats.

Given these findings, it may be thatsupplemented RBR training is bene-ficial to a very narrow range ofconstant resistance loads on theloading spectrum. Future researchshould be conducted on measuringthe kinematic and kinetic variablesduring varying intensities of RBRtraining for exercises with ascendingstrength curves. The long-term effi-cacy of RBR training on force andpower output needs to be investigatedand compared with other modes ofresistance training. Also, charts con-verting or equating the deformation–tension relationships of RBR bands tomass in pounds and kilograms shouldbe readily available to assist clinicians

and practitioners in prescribing spe-cific loading intensities (62).

CHAINS

Chain and RBR properties are similarin that resistance increases throughoutthe range of motion; however, oneresistance type increases in a linear(chains) and the other in a curvilinear(RBR) fashion (Figure 11). The 2 formsdiffer in terms of their physical andmechanical properties; rubber bandsare composed of hydrocarbon poly-mers and chains are composed of steel,which is a combination of iron andcarbon. RBR is dependent on thestress–strain or deformation–tensionrelationship, whereas chain resistanceis dependent on vertical displacementand gravitational force.

Chains can be added to free weights tovary the loading pattern (external re-sistance) and training stimulus. Chains,although an unconventional trainingtechnique, have become popularamong some high-level power andweightlifters (12,57). In a researchstudy on chains, Coker et al. (12)observed that time of applied force,initial acceleration, and the recruitmentand activity of stabilizing and synergistmuscles are increased via chain train-ing. Members of the powerliftingcommunity also have used chains intheir training because it is believed thatthis is an effective resistance mode fordeveloping speed, acceleration, andabsolute strength. However, little sci-entific evidence exists to support theseclaims, and future research is requiredto validate such contentions (57).

Kinematics and Kinetics Chains. Me-chanically, adding chains to the ends ofa barbell has a similar net effect as RBRin the vertical direction; as the barmoves upward, resistance progres-sively increases because the chainsare lifted off the ground, and as thebar is lowered the resistance decreases(9). Chain structure, density, length,and diameter are determinants of chainweight and must be known to prescribespecific loading intensities. Berninget al. (9) developed charts matchingchain link diameter and length to massthat could prove to be of practicalbenefit, as chain weight increases anddecreases in proportion to the numberof links that leave the floor. An exampleis displayed below in table form basedon chain sets (two chains of equaldiameter and length) (Table 2) (9).Chains should be a minimum of 2½meters in length, as some powerliftingand weightlifting movements havea large range of bar displacementsdepending on the exercise-lift andheight of the athlete-lifter. The addi-tional chain length is needed because itis important to keep a portion of thechain in contact with the floor at alltimes to prevent injury and limitexcessive oscillation and sway (9).The weight of the chain is calculatedby multiplying the mass (kg) of thechain links leaving the floor by gravity(~9.81 m/s2). Because of the gravita-tional dependent properties of chains,movements should be performed in thevertical plane for maximal chain re-sistance and optimal training benefits.

Figure 11. Resistive curves of; (a) rubber-based resistance (5 different bands), and (b) chain resistance (5 different chain widths).



The multi-joint movements in power-lifting (e.g., bench press, deadlift, andsquat) and specific phases (assistancelifts) of weightlifting (e.g., snatch, cleanand jerk) have ascending strengthcurves and are performed primarily inthe vertical planes (sagittal and frontal),as illustrated in Figure 12. Withascending strength curve exercises,the musculoskeletal system gains a me-chanical advantage as the workingmuscles extend the involved joint seg-ments. Varying the resistance through-out the lift alters the kinetic (e.g., force,work, and power) and kinematic (e.g.,time, velocity, and acceleration) varia-bles. Theoretically, adding chains topowerlifting movements and assistancelifts (e.g., first pull in the clean and thesnatch, and the jerk phase in the cleanand jerk) in weightlifting with ascend-ing strength curves, should allow forenhanced acceleration during the ini-tial phase of the lifts. At the base of thelift, the chains’ external resistance andhuman torque capabilities are low; andprogressively increase throughout therange of motion to match the muscu-loskeletal system’s increased ability toproduce force and create torque (9).During these multi-joint movementsthe musculoskeletal system’s ability tohandle greater external forces (loads)increases as the involved joints extendand reach a position of single jointconfiguration i.e. the involved jointshave reached full extension and theexternal load is applying a compressive

force (65). It has been proposed thatusing chains with powerlifting andvarious assistant weightlifting move-ments may promote the developmentof power, acceleration, motor control,stabilization and enhanced neurologi-cal adaptation (9,12,27). In terms ofpower development, this makes sense,as there is an initial increase inmovement velocity and latter increasein muscular force requirement, causedby the progressively increasing externalresistive load of the chains. However, itshould be remembered that most of theaforementioned claims are anecdotal.

Practical Applications of Chains. Theuse of chains alone and in combinationwith free weight training is becomingmore frequently used by strength andconditioning practitioners as a methodof training. It has been suggested thatchains improve strength and power,extend the duration of the accelerationphase and subsequently increase ve-locity during the positive (concentric)phase of the lift (e.g., bench press).Whether this is actually true is yet to bedetermined and the reader needs to becognizant of this limitation whenreading the literature in this area.One practitioner has suggested thatoptimal strength gains are achievedusing 1RM loads of 80–100% com-posed of a chain resistance of 10–15%of the total load with the remaining85–90% of the load comprised of freeweight resistance (5,57). On the basis of

the overload principle, muscular forceproduction may be increased towardsthe end range of the lift, as themusculoskeletal system must adapt tothe increasing external load of thechains (24,27). Athletes could possiblyincrease absolute strength, as supple-menting free-weights with chains willresult in a greater maximum loadcompared with free-weight only train-ing, due to the reduction and change inposition of the sticking point forascending strength curve movements.For example, if an athlete has a maxi-mum bench press of 100 kg, he/sheshould be able to bench press a totalload greater than that of his of hermaximum, due to the change (rise) inposition of the sticking point. Onepractitioner suggested using a lighterfree weight load (e.g., 90 kg) and attachadditional chain resistance (e.g., 15 kg)to the bar, resulting in a total apex loadgreater than the athletes 1RM (105 kg),possibly leading to increases in maxi-mal strength (17,57). As for optimalgains in power, another practitionersuggested that a lighter total resistancebetween 60 to 90 % of the athletes1RM be used, with 80–85% of thepercentage load coming from freeweight resistance and 15–20% fromchains; as this changes the kinetics ofa strength exercise into a power exer-cise allowing for greater accelerationthroughout the range of motion (5). Allof these practical benefits may poten-tially lead to a stronger more explosive

Table 2Chain mass, length and diameter

DiameterLength(cm)

cm in cm in cm in cm in cm in

10 4 50 20 100 40 150 59 200 79

Mm Inches Mass(kg)

kg lbs kg lbs kg lbs kg lbs kg lbs

6.4 ¼ 0.3 0.6 1.3 2.8 2.5 5.5 3.8 8.3 5.0 11.0

9.5 3/8 0.4 0.8 1.9 4.1 3.7 8.1 5.6 12.2 7.4 16.3

12.7 ½ 0.7 1.6 3.7 8.1 7.4 16.3 11.1 24.4 14.8 32.6

19.1 G 1.4 3.1 7.0 15.4 14.0 30.8 21.0 46.2 28.0 61.6

22.2 7/8 2.2 4.8 10.8 23.8 21.6 47.5 32.4 71.3 43.2 95.0

25.4 1 2.8 6.2 14.0 30.8 28.0 61.6 42.0 92.4 56.0 123.0




athlete and could possibly be applied tosport specific training programs. Ath-letes who must overcome large exter-nal resistances (e.g., American football,rugby, wrestling, and mixed martialarts) may benefit from strength andpower training programs supple-mented with chains.

Chains may be a useful supplement tofree-weight resistance in that they adda variable training stimulus to a trainingprogram, which would be otherwiseunavailable (57). The oscillating chainsprovide a predictable, but varyingmovement path, as a result of theapplied force. Practitioners claim thatthe oscillating chains may promoteimproved motor control, increasedactivation and recruitment of stabiliz-ing and synergist muscles, and en-hanced neurological adaptation; butthese claims are not scientificallysupported and may merely be a resultof performing free weight resistance(9,12). It may be speculated that chainsoffer a changing external resistanceproportional to the mechanical advan-tage gained by the musculoskeletallever system as the joints extend andflex throughout positive-concentricand negative-eccentric phases. How-ever, the benefits of training with

chains are not well documented andcurrent claims lack scientific support;therefore, some of the contentionsmust be viewed with caution.

Limitations of Chains. One distinctlimitation in chain training is the lackof scientific research performed in thisarea; most claims are anecdotal anduntil conclusive evidence is presented,the proposed benefits remain hypo-thetical. Two studies, one by Cokeret al. (12) and the other by Berninget al. (8), found that lifting with chainsversus lifting without chains did notcause any changes in the kinematic andkinetic variables of the lifter during thesnatch; which may have been due tothe a low chain load (5% of the totalload) used. In addition, only a limitednumber of kinematic and kinetic var-iables were measured in these studies,none of which truly reflected thetechnique of the lifters, giving sportsscientists another reason to be skepticalof chain training. Because chain loadvaries with position and, therefore,time, position- and time-dependentvariables should be considered infuture research (e.g., time to peak forceand peak power, position of peak force,peak power, and maximum velocity).

There is limited information on therelationship between chain diameterand length to weight ratios (9). Thelength, diameter, and density of thechain, as well as the selected exercise,segment length, and height of the lifter,dictates how much weight the lifterwill be moving throughout the range ofmotion (9). All of these factors must betaken into consideration when calcu-lating the exact load of the correspond-ing chain. In this process, the chance ofcalculation error would increase as thenumber of factors considered increases.The average external chain load andlength to weight ratios over the entirerange of motion should be measuredand calculated to allow practitioners toprescribe specific and precise loadingpatterns for the desired lift and in-dividual. These measures need to bevalidated and published in a standard-ized chart, as none currently exist. Thefollowing measures should be in-cluded; chain diameter to weight ratiosand chain length to weight ratiosranging from 1 to 250 centimeters inlength as the ranges in displacements(range of motion) vary largely fromexercise to exercise.

In terms of the strength curves, becausechain weight increases in a positive

Figure 12. The deadlift supplemented with chains, illustrating a collinear movement with respect to gravity.



linear fashion, exercises with descend-ing and bell-shaped strength curvesmay not be properly overloaded withchains. For optimal benefits, chaintraining may be limited to ascendingstrength curve exercises. The gapbetween scientific data and anecdotalclaims is a clear indication that futureresearch is required if chains are tobecome a valid mode of training forenhancing power, strength and athleticperformance (12).

CONCLUSION

After reviewing the different forms ofVR training, we hope that a betterunderstanding and appreciation of thekinematics and kinetics, practical ap-plications, and limitations has beenacquired. Cam and lever systems aredesigned to accommodate exerciseswith ascending, descending and bell-shaped strength curves purportedlyenabling greater specificity in termsof overloading the working muscles,but there is contradictory evidence inthe literature. The systems resistivetorque is designed to match humantorque capabilities, increasing resis-tance at points where strength isgreatest and decreasing the resistanceat points where the strength is lowest;but again there is much debate over thevalidity and effectiveness of cam andlever systems as a mode of resistancetraining. Cams and levers also are usedwidely in the rehabilitation setting tocorrect muscle imbalances and certainpathologies; where single joint-designed machines allow clients toisolate a specific group of muscles. Amajor limitation to cam and lever sys-tems is that the movement must travela fixed path and may not accommo-date for the biomechanical and phys-iological variations of the individual. Afixed movement path may also hinderdevelopment of intramuscular andintermuscular coordination. Even withthese limitations, cam and lever sys-tems can still be utilized effectively inresistance training programs.

RBR is viscoelastic and its tension isdetermined by its stiffness properties,where its curvilinear deformation-tension relationship is better fitted by

a second order polynomial, rather thana first order-linear function polynomial,as rubber is not purely elastic. RBRmay be used in a multiple of planes (e.g.sagittal, frontal, transverse, and obli-que), as the resistance is collinear toand directly opposes most exercisemovements. For this reason, RBR isused widely in the rehabilitation settingfor many pathologies (e.g., contrac-tures, patellofemoral pain, stroke,elderly, and osteoarthritis) and sportspecific training (e.g. sprint, rotatorcuff rehabilitation, racket sports).RBR provides a low-load resistancefor initial rehabilitation, and when usedin conjunction with constant externalresistance, it can gradually and pro-gressively increase strength in weakatrophied muscles. RBR used in com-bination with free weight resistance hasbeen beneficial in improving variouskinematic and kinetic variables overa narrow range of loading intensities.Limitations arise, as RBR has visco-elastic properties where tension in-creases in a curvilinear fashion andmaynot compensate exercises with de-scending and bell-shaped strengthcurves. The lack of scientific supportfor RBR training and the limitedresearch quantifying its viscoelasticproperties (deformation-tension rela-tionship), provides further constraintsto RBR training, hence the need forfuture research.

Chain resistance increases linearly withdisplacement and can be representedby a basic linear function (first orderpolynomial), as chain resistance isgravity dependent and determined bythe density, diameter and length of thechain; therefore, chain resistancewould appear best suited for trainingexercises with ascending strengthcurves, such as the squat, dead-lift,bench press and shoulder press. Vari-ous reputed practitioners have claimedimprovements in the kinematic andkinetic variables of the lifter, as well asincreased activation of stabilizingmuscles and enhanced neurologicaladaptations; but a lack of scientificevidence has led many to be skepticalof chain training. Limitations to chain

training also include the lack ofstandardization in regards to the typeand quality of steel used for resistancetraining. Another inherent limitation isthat chain weight to displacementratios have not been properly quanti-fied over a large displacement range.

Future biomechanical research is re-quired in the area of VR trainingmodes, in order to bridge the gapbetween the practitioner and scientist.Ariel (3) once said that ‘‘the equipmentshould adapt to the user rather thanthe user adapt to the equipment.’’Following along with this line of logicwhen implementing resistance trainingprograms, the kinematic and kineticprofiles associated with the variousmodes of resistance should be consid-ered and prescribed appropriately tomatch the varying human strengthcurves and mechanical advantage ofthe different musculoskeletal leversystems; as well as the sport andathlete specific training goals (2,3).

Travis

McMaster isa Master ofSports Sciencestudent specializ-ing in biome-chanics at EdithCowan Univer-sity, WesternAustralia.

John Cronin isan AssociateProfessor at EdithCowan University.

Michael

McGuigan isa Senior Lecturerat Edith CowanUniversity anda strength andconditioningspecialist.




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