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A Preliminary Comparison of Front and Back SquatExercisesPamela J. Russell a & Sally J. Phillips ba Department of Physical Education , University of Maryland , College Park , MD , 20742 ,USAb Department of Physical Education , USAPublished online: 08 Feb 2013.

To cite this article: Pamela J. Russell & Sally J. Phillips (1989) A Preliminary Comparison of Front and Back Squat Exercises,Research Quarterly for Exercise and Sport, 60:3, 201-208, DOI: 10.1080/02701367.1989.10607441

To link to this article: http://dx.doi.org/10.1080/02701367.1989.10607441

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RUSSBlL AND PHILLIPS

RBSBARCH QUARTBRLY

FOR ExmlClSB AND SPORT

1989, VOL.6O,~o.3,pp.201-208

A Preliminary Comparison of Frontand Back SquatExercises

PAMELA J. RUSSELL AND SALLY J. PHILLIPSUniversity of Maryland

The purpose ofthis study was to compare the knee extensordemands and low back injury risks ofthe front and back squatexercises. Highly strength-trained college-aged males (n = 8),who performed each type ofsquat (Load =75% offront squat onerepetition maximum), were filmed (50 fps) from the sagittal view.The body was modeled as a five link system. Film data weredigitized and reduced through Newtonian mechanics to obtainjoint forces and muscle moments. Mean and individual subjectdata results were examined. The maximum knee extensor momentcomparison indicated similar knee extensor demands, so eithersquat exercisecould be used to develop knee extensor strength.Both exercises had similar low back injury risks for four subjects,but sizable maximum trunk extensor moment and maximumlumbar compressive and shear force differences existed betweenthe squat types for the other subjects. The latter data revealed thatwith the influence oftrunk inclination either exercisehad thegreatest low back injury risk (i,e., with greater trunk inclination:greater trunk extensor demands and lumbar shear forces, butsmaller lumbar compressive forces). For these four subjects lowback injury risk was influenced more bytrunk inclination thansquat exercise type.

Key words: biomechanics, squat. low back injury

Athletes may be more susceptible to back pain since 15%of all sports-related injuries involve the back (Williams,

1980). There is an even greater risk of low back injury forathletes who lift weights since some lifts directly overload thespine (Williams, 1980). However, the risk oflow back injuryin weightlifting has received little documented attention.With the increasing number of athletes who weightlift, thislack of information takes on even greater significance. Forparticipants at a greater risk of injury (i.e., heavyload, fewrepetition lifters versus light load, many repetition lifters), an

awareness of injury risk would certainly be valuable.Investigations of the competitive Olympic and power

weightlifting events, where performers lift maximal loads,have focused on descriptive kinematics and kinetics to distin­guish among performers and evaluate technique (Campbell,Pond, & Trenbeath, 1979; Garhammer 1978, 1979, 1980,1981, 1982, 1985; McLaughlin, Dillman, & Lardner, 1977;McLaughlin, Lardner, & Dillman, 1978). Investigations ofthe noncompetitive general weightlifting exercises, whereperformers lift submaximalloads, have just begun to addressinjury risk (Cappozzo, Felici, Figura, & Gazzani, 1985; Lan­der, Bates, & DeVita 1986; Madsen & McLaughlin, 1984).

With the exception of the competitive Olympic snatchand clean and jerk lifts, the parallel squat is believed to placethe moststresson the musculoskeletal system (O'Shea, 1985).Based on a kinetic investigation of the parallel squat as apowerlifting event performed with maximal loads, McLaug­lin et al. (1978) recommended that caution be used duringsquat exercises, particularly ifindividuals had weak or injuredtrunk extensor musculature. Even when the parallel squat hasbeen performed as a general weightlifting exercise with sub­maximal loads, large magnitudecompressive (6704 N--6980N)and shear forces (3070 N-3219 N) have been reported at theL5/S 1joint (Lander et al., 1986). Large trunk extensor forces(i.e., 30% - 50% of the maximum isometric force of 4800 ±1800 N) and compressive loads (i.e., 6-10 times body weight)at the lumbar level also have been reported during perform­

ance of the parallel half-squat by track athletes involved in asport-specific weightlifting program (Cappozzo et al., 1985).Thus, there is no doubt that potentially large trunk extensormoments and high force values are generated within thelumbar region during these movements. Consequently, there

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is a risk of low back musculoskeletal injuryduring perform­anceof theparallelsquatasapowerliftingeventor asa generalweightlifting exercise.

Despite the risk of injury, the parallel squat has beentermed"the king of all weightliftingexercises,"for its abilityto "maximizeathleticpotential" (O'Shea, 1985,p. 4). Alongwithother physiologicalbenefits,the parallelsquatenhancesmajormuscledevelopmentwithinthelowerbody. in termsofknee extensor development, the front parallel squat, a com­monvariationof theback parallelsquat is consideredinvalu­able (O'Shea, 1985). In addition, compared to the backparallelsquat, the front parallel squat is consideredsaferandless strenuous on the lower back (Klein & Allman, 1985).Thus, the purpose of this study was to compare the kneeextensordemandsand low back musculoskeletal injuryrisksof the front and back parallel squat exercises.

Dueto a differencein thelocationof theweightedbar, thefront squat was hypothesized to have the greatest low backinjurypotential.In the frontsquat,the weightedbar is locatedin frontof thebody's line of gravity (i.e., in frontof the neck,partiallysupportedby the shouldersand/or thechest), whichcreates a longer resistance moment arm than the locationofthe weightedbar in the back squat, slightlybehindthebody'slineof gravity (i.e., behind the neck and atop the shoulders).Even a smalldifferencein the resistancemomentarm lengthwouldhavea considerableeffect on trunk extensordemandssince the resistance is of such large magnitude (lift loads =86.36 kg-I36.36 kg). Thus, by virtue of the weighted barlocation,greater demands may be placedon the trunkexten­sors during the front squat resultingin a greaterpotential forlowbackinjury.Inaddition,thefrontandbacksquatexerciseswerehypothesized to havesimilarkneeextensordemandsdueto similaritiesin lower body motion (i.e., flexion thenexten­sion of the ankle, knee, and hip joints to lower then raise theweightedbaras it remainsstationarywithrespectto thebody).

calculated for each subject as 75% of his front squat onerepetition maximum (IRM) (see Table 1). The lift load re­mainedconstantforboth types of squatexercise.Theorderofsquat exercise type performance was randomized for eachsubject.

Data Collection

After signingan informedconsentdocument,each sub­jectparticipatedinaseriesofwarmupsquatexercises(Load=50% to 60% of front squat lRM). Following the warmup,black circular joint markerswere placed on the right lateralside of the body at the following anatomical locations: fifthmetatarsolphalangeal (MP) joint, malleolusof theankle,kneejoint centeraxis,greater trochanterof the femur,and superioriliac crest. A joint marker was also placed at the end of theweightedbar.Allsubjectswererequiredtoweara weightbelt.

The sagittal view of the movement was filmed at 50frames/second (Cps) witha 16 mm Photosonicscamera.Dur­ingthefilmingsession,eachsubjectperformed3consecutiverepetitionsof one type of squat exercise, followedby a 3-5minute rest period and then 3 consecutiverepetitionsof thesecondtypeof squatexercise.All subjectswere instructed toperformeachliftat theirnormalspeed. Foreach type of squatexercise, the subject's second repetition was filmed. If thesecond repetition appeared to contain excessive bar move­ment in the transverseand/or frontalplanes, the third repeti­tion was filmed.

Variables

The vertical velocity of the weighted bar defmed themovement, the movement began when the weighted bar'sverticalvelocitybecamenegative(i.e., the subject loweredit

Table 1Subject Characteristics

Case Height Weight Lift •RelativeMethods (m) (kg) (kg) Load

1 1.78 112.73 102.27 90.73

Subject Characteristics 2 1.74 114.55 136.64 119.043 1.79 103.64 102.27 98.68

The participants in this study were eight college-age 4 1.64 86.36 106.82 123.69

maleswithat least3yearsof strengthdevelopmentweightlift-5 1.70 80.91 86.36 106.746 1.52 99.09 93.18 94.04

ing experience (see Table 1). One subject with 12 years of 7 1.57 111.36 131.82 118.37experience and a national bodybuilding competitionrecord 8 1.52 90.91 93.18 102.50was considered highly skilled. All subjects were competent M 1.66 99.94 106.53 106.72

performers inboththefrontandbacksquatexercisesandwere SO 0.10 11.99 17.06 11.59

free from performancealteration injuries (particularly backNote. All values based on n • 8.and knee injuries).Prior to data collection, the lift load was·Values reported as a percentage of bodyweight.

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toward the ground), and the movement ended when uponreachingan uprightposture the subjectloweredthe weightedbar slightlyand its verticalvelocitychangedfrompositive tonegative.

Four dependent variables were selected for comparisonof the front and back squat exercises:

1) Maximumknee extensormoment;indicatorof kneeextensor demands.

2) Maximumtrunkextensormoment; indieatoroftrunkextensordemands.

3) Maximum normalized compressive force at thelumbar level; indicatorof vertebralcolumn stress.

4) Maximum normalized shear force at the lumbarlevel; indicatorof vertebralcolumn stress.

The magnitudes of the maximum muscle moment andjointreactionforcesasopposedto thesevaluesover timewereassumed to be indicators of injury and maximal. muscledemands.The authors were concerned with injury potentialand maximalmuscledemandsduringa singleliftwitha heavyload as opposed to numerous lifts (i.e., an entire set ofrepetitions carried out until failure). If the latter had beenstudied, examinationof momentand force values over timemay have been important.However,during a single lift withaheavyloadasuddenmusculoskeletal injuryand themaximalmusculardemandswereassumedtobeassociatedwithinstan­taneoushigh or maximumforce and moment values.Devot­ing specificattention to maximalvalues is not uncommon inweightlifting investigations (Cappozzo et al., 1985; Hall,1985; Lander et al., 1986).

Model andassumptions

For thepurposesof data reductionthebodywasmodeledas a five link system (see Figure 1). The advantage of thismodelwas the divisionof the trunk segmentinto twolinks, apelvic link (i.e., from the hip to the approximateL3/lA joint)and a trunklink (i.e., fromtheapproximateL3/lA joint to theshoulder).The superioriliaccrestwasusedas thedemarcationbetween the two links (Gray, 1977, p. 241). Other squatexercise investigationshave used four link models, with thetrunklinkdefinedbetweenthehipandshoulderjoints. Intheseinvestigations trunkextensormomentsweredefinedas thoseacting at the hip (McLaughlin et al., 1978), or hip joint datawere used to estimate muscular forces at the lumbar level(Lander et al., 1986). The five link model permits directcalculationof both trunkand hip extensor moments.

In addition to the normal assumptions of a link modelapproach(Miller& Nelson,1973), thefollowing assumptionswere accepted:

1) The weighted bar was assumed to be fixed at theshoulderand the joint marker at the end of the weightedbarapproximated the shoulder joint center. This common as-

Figure 1. Five link rigid body model of theweighted bar/lifter system.

sumptionwasnecessarytodefmethe trunklink(McLaughlinet al., 1978), sinceduringboth squatexercisesthe locationofthe weighted bar obscured the sagittal plane view of theshoulderjoint center.If therewasanyerrorbetweentheexactlocationof the shoulderjoint center and the weightedbar itwas considered constant among subjects and across squatexercise types.

2) Thebar, treatedas a point mass,wasconsideredto act"with" thetrunksegment Thusa newcenterof massposition,momentof inertia value, and radius of gyrationwere calcu­lated for the trunk link. These new valuesaccountedfor thesegment lengths, positions, masses, and moment of inertiavaluesof thearms,head/necksegments,and theweightedbar.

3)Themovementwasassumedtobe bilaterallysymmet­ricaland to occur in only the sagittalplane with the MPjointfixed. The effects of lateral bending in the frontal plane ortwisting in the transverse plane were not included in theanalysis. Although movementin these planes may have ex­isted, the effects were considered small given the sagittal

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Maximum trunk extensor moments

1 469.9 477.52· 435.4 363.53 509.6 293.54 306.2 351.35 355.4 380.76 547.8 635.47 690.9 800.58 696.8 525.6M 501.51 478.50SD 132.81 170.94

The meanmaximum trunkextensormomentcomparisonrevealed a slightly greater mean moment during the frontversus the back squat (see Table 3). Subjects 1,2,3, and 4exhibited sizable trunk extensor moment differences(SEMD=150.0Nm) between the front and back squats.Thegreatest trunk extensor momentswere during the front squatfor subjects 2 and 3 and during the back squat for subjects 1and 4.

To supplement the maximum trunk extensor momentcomparison,the segmentaltrunkangleat the timeof themax­imum trunkextensor momentwas examined.The segmental

plane nature of the task in conjunction with the skill of thesubjects. This assumption has been used and validated byotherinvestigators(Cappozzoet al., 1985;McLaughlinet al.,1978).In addition, all filmed trials were screened for frontaland/or transverse plane movement and omitted if excessivemovementexisted in either plane.

Datareduction

A motionanalyzer was used to project the film imageontoa digitizin$platenfordata reduction.Theframingrateofallanalyzed trials was verified (50 Cps). A Numonicselectronicgraphiccalculatorwasused todigitizetworeferentpointsandthe joint marker position data in each frame of the definedmovement.

Thedigitizedposition-timedata, smoothedwitha secondorder recursiveButterworthfilter (frequencycut-off = 6 Hz),were differentiated using first finite differences to obtainsegmental kinematics. Joint reaction forces and musclemoments at the end of each link segment were obtainedthroughNewtonianequationsof motion.The normalorienta­tionof theCartesiancoordinatereferentsystemwasrotatedtothe position of the trunk link and the resultant lumbar forcewas decomposed to obtain lumbar compressive and shearforces(see Figure I), which were normalizedto systemmass(i.e., subject mass plus weightedbar mass).

Data Analysis

Case

Table 2Maximum Knee Extensor Moments (Nm)

Squat Exercise TypeFront Back

Mean comparisons were made for each of the four de­pendentvariables.Foreach variableindividualsubjectdiffer­encesbetweenthe frontand back squatswerejudged in termsof the standarderror of the mean difference (SEMD)for thatvariable.Sizabledifferenceswerethosethateitherapproachedor exceeded the SEMD.

Note. All values based on n - 8.·Highly skilled subject

Table 3Maximum Trunk Extensor Moments and

Trunk Inclination at the Time of theMaximum Trunk Extensor Moment

Results

Trunk ExtensorMoment (Nm)

Case Front Back

TrunkAngle (deg)

Front Back

Note. All values are based on n =8.·Highly skilled subject

Maximum kneeextensor moments

The front squat mean maximumknee extensor momentwasslightlygreater than the back squatmean maximumkneeextensormoment(seeTable 2). Subjects3, 7, and 8 exhibitedsizable differences (SEMD=119.1Nm) between their frontand back squat knee extensor moments. The greatest kneeextensor moments were during the front squat for subjects 3and 8, and during the back squat for subject 7.

12·345678MSD

406.5784.0758.4427.1329.7354.9469.6297.6478.48188.88

587.9556.6493.2519.8289.2368.2479.9262.3444.64122.80

62.348.039.863.050.961.356.575.157.1110.85

54.451.045.852.766.359.461.971.057.81

8.40

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trunk angle means were very similar (Front squat =57.11;Back squat =57.81). For 6 of 8 (75%) subjects, includingthe highly skilled subject, the squat exercise type whichdemonstrated more trunk flexion at the time of the maximumtrunk extensor moment also demonstrated a greater max­imum trunk extensor moment.

Normalized maximum compressive andshearforces at the lumbarlevel

The normalized maximum lumbar compressive forcecomparison revealed a larger mean compressive force duringthe back (111.36% of system mass; 229% of body weight)versus the front squat (105.62% of system mass; 218% ofbody weight) (see Table 4). Sizable lumbarcompressive forcedifferences (SEMD=13.65% of system mass) were exhibitedby subjects 1,2,3, and 5. For 75% of these subjects (I, 3, &5) the greatest lumbar compressive forces were during theback squat.

The segmental trunk angle at the time of the maximumlumbar compressive force was examined to supplement thecompressive force comparison. The mean back squat segmen­tal trunk angle was slightly greater than that of the front squat(see Table 4). For7 of8 (87.5%) subjects, including the highlyskilled subject, the squat exercise type which demonstratedthe greatest lumbar compressive force demonstrated the leasttrunk flexion:

A slightly larger normalized mean maximum lumbarshear force existed during the front (69.13% of system mass;

Table 4Normalized Maximal Lumbar Compressive Force

and Trunk Angle at the Timeof the Maximal Compressive Force

143% of body weight) versus the back squat (67.34% ofsystem mass; 139% ofbody weight) (see Table 5). Subjects I,2,3, and 4 displayed sizable lumbar shear force differences(SEMD = 18.79% of system mass).

To supplement the maximum lumbar shear force com­parison, the segmental trunk angle at the time ofthe maximumlumbar shear force was examined. The segmental trunk anglemeans were very similar (see Table 5). All of the subjectsdisplayed more trunk flexion during the squat exercise typewhere they experienced the greatest lumbar shear force.

Discussion

One major concern of this study was to compare the kneeextensor demands of the front and back parallel squat exer­cises. The maximum knee extensor moment mean compari­son showed that the front squat elicited slightly greater kneeextensordemands than the backsquat (see Table 2). However,the mean difference was very small (23 Nm) given the kneeextensor moment standard deviation of each squat exercisetype. In addition, of the 3 subjects exhibiting sizable kneeextensor moment differences only 2 (3 & 8) had the greatestknee extensormoments during the front squat. Thus, given thelack of a large mean difference and the lack of consistentsupport for the mean relationship by subjects with sizablemoment differences, the knee extensor demands of both squat

Table 5Normalized Maximal Lumbar Shear Force and

Trunk Angle at the Time of the Maximum Shear Force

Trunk Extensor Trunk Trunk Extensor TrunkMoment (Nm) Angle (deg) Moment (Nm) Angle (deg)

Case- Front Back Front Back Case Front Back Front Back

111.27 141.52 64.61 69.62 1 58.34 74.95 62.28 49.022* 109.42 95.69 83.43 79.90 2* 97.49 67.86 47.98 50.623 91.23 102.45 43.93 70.41 3 101.03 73.09 39.84 48.394 108.40 101.53 72.73 52.67 4 66.64 86.62 53.51 42.865 90.35 103.71 51.48 57.91 5 64.30 58.29 54.89 58.726 108.89 118.01 61.36 59.90 6 55.93 61.90 61.29 59.387 102.92 107.85 67.15 71.43 7 70.03 67.53 48.12 59.738 122.47 120.13 75.14 71.03 8 39.29 48.50 74.97 71.03M 105.62 111.36 64.98 66.10 M 69.13 67.34 55.34 54.97SD 10.67 14.75 12.80 8.93 SD 20.82 11.52 10.82 8.93

Note. All values based on n .. 8.Reported values were normalized to system mass, so the

unit is percent of system mass.*Highly skilled subject

Note. All values based on n .. 8.Reported values were normalized to system mass, so the

unit is percent of system mass.*Highly skilled subject

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exercises may actually be more similar than different. Sincethe lower body motions of the front and back squats appearedvery similar, this result was not surprising. Thus, for kneeextensor strength development purposes the front squat didnot appear superior to the back squat.

The second major concern of this study was to determinewhich type ofsquatexercise, front or back, presenteda greaterrisk of low back musculoskeletal injury. In injury risk analy­ses slight mean or within subject differences may provemeaningful and should not go unrecognized. For instance, ifa slight difference were neglected in this study, both squatexercises could be promoted as having the same injury riskswhen they may actually have different injury risks (i.e., TypeI statistical error). Since weightlifting participants would useeither exercise, their chance of injury would increase. How­ever, ifa slightdifference were recognized, the squatexerciseswould be promoted as having different injury risks eventhough they may actually have the same injury risks (i.e., TypeII statistical error). Since weightlifters would use the exerciseleast likely to promote injury, the chance of injury woulddecrease. The latter approach was cautious, but more appro­priate for an injury risk analysis.

Using this cautious approach, meaningful differenceswere not revealed by the injury risk indicator variable meancomparisons (see Tables 3, 4, & 5), but were displayed bywithin subject data comparisons. Generally, subjects 1,2,3,and 4 consistently displayed sizable differences betweensquat exercise types on each injury risk indicator variable.Considering the magnitude of their injury risk indicator vari­able differences, coupled with their consistent demonstrationof these differences, special attention was given to their dataas opposed to mean relationships. The other subjects didperform consistently, but since they did not exhibit sizablesquat exercise type differences on the injury risk indicatorvariables, their data was not investigated further.

The maximum trunk extensor moment comparisonshowed different relationships across squat exercise types.For subjects 2 and 3, the front squat elicited the greatest trunkextensor moments while for subjects 1 and 4 the back squatelicited the greatest trunk extensor moments (see Table 3). Tofurther investigate these different momentrelationships, trunkinclination at the time of the maximum trunk extensor mo­ment was examined. Others had concluded that a more erecttrunk posture (i.e., less trunk flexion) minimized trunk exten­sor moments (Lander et al., 1986; McLaughlin et al., 1978).All 4 of these subjects were more erect during the squatexercise type where they had the smallest trunk extensormoments. Subject 8, the most erect during both squat exercisetypes, had the smallest trunk extensor moments. On thecontrary, subjects 2 and 3, the least erect during the frontsquat, had the largest trunk extensor moments during the frontsquat. Since increased trunk flexion would increase the resis-

tance moment arm, greater trunk extensor demands would beexpected. Thus, the data from these subjects supported theconclusions of other researchers: a more erect trunk postureminimized trunk extensordemands. In addition, trunk inclina­tion appeared to have a stronger influence upon trunk extensordemands than squat exercise type.

Trunk inclination also has been considered an importantdeterminant of lumbar compressive load (Cappozzo et al.,1985). Since subjects 1, 3, and 5 exhibited the greatestcompressive force during the back squat while subject 2exhibited the greatest compressive force during the frontsquat, trunk inclination at the time of the maximum lumbarcompressive force was examined. All of the aforementionedsubjects were more erect during the squat exercise type wherethey experienced the greatest lumbar compressive force. Inaddition, across squat exercise types, subjects 3 and 5, whohad the most trunk flexion, had the least compressive force.Since the lumbar compressive force was the resultant forcevector component projected along the trunk link, greatercompressive force may have been expected with less trunkflexion (i.e., a decrease in the angle between the resultant forcevector and the trunk). However, Cappozzo et al. (1985)suggested that with less trunk flexion "spinal loading" de­creased. (Note: The exact defmition of spinal loading wasunclear.) If the compressive force at the lumbar level wasconsidered to be the spinal load, then the results of this studydiffered: with less trunk flexion the lumbarcompressive forceincreased. Thus, trunk inclination also appeared to have astrong influence on compressive load, but lumbar compres­sive loads did exhibit a tendency to be greatest during the backsquat.

Since the lumbar shear force was the resultant forcevector component projected perpendicular to the trunk link, italso would be influenced by trunk inclination. These datasupported this notion, that during the front squat, subjects (2& 3) with the most trunk flexion had the greatest shear forces,while during the back squat, subjects (1 & 4) with the mosttrunk flexion had the greatest shear forces. In addition, subject8, who had the least trunk flexion in both squat exercise types,had the smallest shear forces. As trunk flexion increased,lumbar shear force increased. Thus, not only did trunk incli­nation appear to influence lumbar shear force, it appeared tohave a stronger influence than squat exercise type.

All of the injury risk variables appeared to be stronglyinfluenced by trunk inclination. However, other variablessuch as the relative magnitude of the lift load (Chaffm &Anderson, 1984; Anderson, Chaffin, Herrin, & Matthews,1985) may have influenced the results of the subjects whoconsistently displayed sizable injury risk indicator variabledifferences. Heavier loads should have been associated withlarger moments and forces. Subjects 2 and 4 lifted the heaviestloads (see Table 1), but only subject 2 exhibited one of the

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largest trunkextensormoments(seeTable 3)and someof thelargest lumbar forces (i.e., in termsof absolute magnitudes).Noneof the absolutemagnitudesof theseforcesexceededthesafety action limit=3400 N (i.e., someindividualsat risk asset forthby the NationalInstituteof OccupationalSafetyandHealth, (1981). Subjects 1 and 3 lifted some of the lightestloads,butneithersubjectexhibitedthesmallesttrunkextensormomentsor the smallest lumbar forces. Also, subject7, wholiftedoneof theheaviestloads,exhibiteda largekneeextensormomentbutnot large injuryrisk indicatormomentsor forces.Thus, relative lift load magnitude did not appear to consis­tently influence the magnitude of the injury risk indicatorvariablesfor the subjectsconsistentlydisplayingsizabledif­ferences.

Other unmeasured variables may have influenced themagnitudesof theinjuryriskindicatorvariables.Forexample,the effect of additionallumbar/abdominal support providedby the weightbeltwas unknown.Perhaps the added support,usedmoreeffectivelyby somesubjects,affectedthe momentandforce magnitudes.In addition, the intraabdominal pres­sure (lAP) forceeffect (i.e.,decreasedtrunkextensormuscu­lar force [Chaffm& Anderson, 1984]and lumbar compres­siveforce[Andersonetal., 1985;Andersson, 1985])werenotincludedin theanalysis.IfsomesubjectsexertedgreaterlAPforces during one or both squat exercise types, their trunkextensor moment and lumbar compressiveforce differencesmayhavebeengreateror thesemomentandforcemagnitudesmayhavebeensmaller.Differencesinmuscleactivationlevel(Burke, 1981) and muscle contraction velocity (Hill, 1938)also mayhaveinfluencedtrunkextensormomentmagnitudes.Thus,inadditiontotheapparentinfluenceof trunkinclinationand the potential influence of relative lift load magnitude,injury risk indicator variables may have displayeddifferentrelationshipsacross squatexercise types becauseof unmeas­ured influences from the aforementioned variables.

The results for the subjects who consistentlydisplayedsizable, differences showed that, across injury risk indicatorvariables,trunk inclinationappearedto havea strongerinflu­ence than squat exercise type. The one exception may havebeen the results of the compressiveforce comparisonwherethe back squat appeared to elicit the greatest compressiveforces.Regardless,upperbody techniqueappearedtoplayanimportant role in the magnitudes of the variables used toassess injury risk. Two techniques seemed apparent In thefJI'St technique the back squat minimized trunk extensordemands and lumbar shear forces, but the front squat mini­mizedlumbarcompressiveforces(Subjects3& 5).Subject2,the highly skilled subject, also demonstrated this technique,excepthis back squatminimizedlumbarcompressiveforces.For these subjects, the back squat presented less risk of lowback musculoskeletal injury. In the second technique, thefront squat minimized trunk extensor demands and lumbar

shear forces (subjects1 & 4). This techniquealso minimizedlumbarcompressive forcesduring the front squat for subjectI, butnotforsubject4.Thus,thefrontsquatpresentedlessriskof low back musculoskeletal injury for these two subjects.Perhaps even a third technique was used for both squatexercisesby the subjects who did not display sizable injuryriskindicatorvariabledifferences. Acombinedlookatstrengthdevelopment and injury risk showed that only subject 3displayedsizabledifferencesinbothareas.For thissubjectthefrontsquatprovidedsuperiorkneeextensorstrengthdevelop­ment, but also a greater risk of low back musculoskeletalinjury.

The purpose of this study was to compare the kneeextensordemandsand low back musculoskeletal injuryrisksof the front and back squat exercises. The knee extensordemandsof both squatexercisesappearedmore similarthandifferent. Thus, both squat exercises may be used inter­changeably for knee extensor strength development. Bothsquat exercisesalso presented the same low back injuryriskfor 4 subjects. However, the data from the other 4 subjectsconsistently displayed sizable injury risk indicator variabledifferencesbetweenthe frontandback squats,Thesedata re­vealedthatwiththeinfluenceof trunkinclination,eithersquatexercisecould present greater potential for low back injury.With greater trunk inclination there appeared to be greatertrunk extensor demandsand larger lumbar shear forces, butsmaller lumbar compressive forces. Therefore, low backinjuryrisk appeared" to be influencedmore by trunk inclina­tion than squat exercise type.

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Acknowledgements

Computer time for this project was supported through theComputerScienceCenterat theUniversity of Maryland. Theinitialresultsof thisprojectwerepresentedat the IntemationalSocietyofBiomechanicsin SportsConference, Halifax,NovaScotia,Canada,July 1986.

Pamela Russell is a doctoral candidate and graduate assis­tant in theDepartment ofPhysicalEducation at the Univer­sityofMaryland, College Park,MD 20742. SallyJ. Phillipsis an associate professorin the samedepartment.

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