A three-dimensional biomechanical analysisof the squat during varying stance widthsRAFAEL F. ESCAMILLA, GLENN S. FLEISIG, TRACY M. LOWRY, STEVEN W. BA.RRENTINE, andJAMES R. ANDREWS

Michael W. Krzyzewski Huiman Performance Laboratory. Division of Orthopaedic Surgery, Duke University MedicalCenter, Durham, NC 27710; and American Sports Medicine Institute. Birmingham, AL 35205

ABSTRACT

ESCAMILLA, R. F.. G. S. ELEISIG. T. M. LOWRY, S. W. BARRENTINE, and J. R. ANDREWS. A three-dimensional biomechanicalanalysis of the squat during varying stance widths. Mfed. Si. Sports Exerc.. Vol. 33, No. 6. 2001, pp. 984-998. Purpose: The purposeof this study was to quantify biomechanical parameters employing two-dimensional i2-DI and three-dimensional i3-D) analyses whileperforming the squat with varying stance widths. MIethods: Two 60-Hz cameras recorded 39 lifters during a national powerli'tingchamnpionship. Stance width was normalized by shoulder width (SW), and three stance groups were defined: 1) narrow stance squat(NSl. 07 l09 SW: 2 j medium stance squat (MS), 142 + 12% SW- and 3) wide stance squat 'WS), 169 2Ci; SW. Results: Mostbiomechanical differences among the three stance groups and between 2-D and 3-D analyses occurred between the NS and WS.Compared wi,h the NS at 45' and 90' knee flexion angle (KF). the hips flexed 6-.11 rnore and the thighs were 7-12 more horizontalduring the MS and WS. Compared with the NS at 900 and maximum KF. the shaniks were 5-9' more vertical and the feet 'aere turnedout 6- more during the WS. No significant differences occurred in trunk positions. Hip and thigh angles were 3-- 13' less in 2-Dcompared with 3-D analyses. Ankle plantar flexor (10-51 N'm), knee extensor (359-573 Nm). and hip extensor (275-5-77 N-m) net

mnuscle mrnoments wvere generated for the NS. whereas ankle dorsiflexor (34-284 N n ). knee extensor (a 47-756 N-m;., and hip extensor(382-628 N-m) net muscle moments were generated for the MS and WS. Significant differences in ankle and knee moment armsbetween 2-D and 3-D analyses were 7-9 cm during the NS, 12-14 cm during the MS, and 16-18 cm during the WS. Conclusions:Ankle plantar flexor net muscie moments were generated during the NS. ankle dorsiflexor net muscte moments were produced duringthe MS and WS. and knee and hip moments were greater during the WS compared with the NS. A 3-D biomechanical analysis of thesquat is more accurate than a 2-D biomechanical analysis, especially during the WS. Key Words: POWERLIFTING. WEIGHT-LIFTING. JOINT MOMENTS, JOINT MOMENT ARMS, JOINT ANGLES, SEGMENT ANGL.ES. KINEMATICS, KINETICS.

MIECHANICAL WORK

T he squat, which measures lower body and trunkstrength, is the first of three lifts in powerliftingcompetition. The starting and ending position for the

powerlifting squat is when the lifter is in the upright positionwith the knees and hips near full extension. After the"squat" command is given by the head judge, the lifterdescends until an imaginay line from the top of the kneesto the hips moves below a parallel position relative to theground. and in a continuous motion the lifter ascends backto the upright position. According to the American DrugFree Powerliftin2 Association (ADFPA) rules at the time ofthis study, causes for disqualification include failure to waitfor the referee's "squat" command at the beginning of thelift, not descending to the below parallel position, position-ing the bar on the back greater than 5 cm below the acro-mion, any stopping or downward movement of the bar oncethe ascent begins, any shifting of the feet throughout the lift,and failure to wait for the 'rack' command by the head

0195-913110 1 /3306-0984/$3.00/0MEDICINE & SCIENCE IN SPORTS & EXERCISE,Copyright ( 2001 by the American College of Sports Medicine

Received for publication Januarv 2000.Accepted for publication September 2000.

judge at the completion of the lift. All squat trials analyzedin the current study were in accordance with these rules.

Strength athletes. such as Armerican football players andpowerlifters. often employ the barbell squat in their trainingor rehabilitation regimens. These athletes use the squat toenhance hip. thigh, and back strengath. Although varyingsquat stance widths and foot angles are employed in trainingaccording to an athlete's goals and preferences. the efficacyof one stance over another is unclear. Only a few studieshave quantified stance widths or foot angles during the squat(8,10.2-0,25.29). Escamilla et al. (8) reported a preferredmean stance (inside heel to inside heel distance) of 40 _ 8cm and a preferred mean forefoot abduction of 22 I 11from 10 male powerlifters and bodybuilders performing thesquat. In a follow-up study, Escamilla et al. (10)O examinedthe effects of defined narrow and wide stance widths onknee biomechanics. McCaw and Melrose t20) examined theeffects of defined narrow and wide stance widths on iowerextremity muscle activity. Both Signorile et al. (29) andNinos et al. (25) examined the effects of turning the feet inor out on lower extremity muscle activity. However, none ofthese aforementioned studies have examined how varyingstance widths affects joint and segment angles and jointmoments and moment arms.

984

Because the squat is considered a closed kinetic chainexercise (31,37), it can also be employed in knee rehabili-tation programs, such as after anterior cruciate ligament(ACL) reconstruction. Studies have shown that the squat isan effective exercise during ACL rehabilitation (31.37). Themoderate to high hamstring activity that has been reportedduring the squat (8,31) may help protect the ACL duringknee rehabilitation. However, the role of varying stancewidths during the squat is unclear in knee rehabilitation. Itis hvpothesized that knee and hip moments will increase asstance width increases.

Although there are several studies that have quanti-fied joint. moments during the barbell squat (2,8,12,16,17,22.26,28.34,35), none of these studies examined the ef-fects of stance width on joint moments. Similarly, there areonly a few studies that have quantified select joint andsegment angles during the barbell squat (16,17,21,28), andnone of these studies examined the effects of stance widthon joint and segment angles. In addition, one limitation tomost barbell squat studies that quantified joint and segmentsangles and joint moments is that a two-dimensional (2-D)analysis was employed using a single camera to record asagittal view of the lifter. Although trunk movementsthrough spinal and hip flexion and extension occur primarilyin the sagittal plane, flexion and extension movements at theankle and knee occur in the sagittal plane only if the feet arepositioned in that plane (i.e., pointing straight ahead). Thisis because the ankles and knees primarily function as hingejoints during the squat, and thus move in the direction thefeet point. Therefore. the lower extremities will move out ofa sagittal plane as the feet turn outwards and the stancewidens. These will cause erroneous measurements of lowerextremity joint and segment angles and ankle and kneemoments and moment arms (9). These errors are minimalwhen the feet are pointing straight ahead, but considerableerrors occur as the feet turn out to a greater extent and thestance widens (9). Therefore, the purpose of this study wasto compare joint and segment angles and ankle, knee, andhip moments and moment arms between 2-D and 3-D anal-yses while performing the squat with varying stance widths.It was hypothesized that the number of significant differ-ences in joint kinematic and kinetics between 2--D and 3-Danalyses will increase as stance width widens and that 3-Djoint kinematics and kinetics will be significantly differentamong varying stance widths,

MATERIALS AND METHODSSubjects. Thirty-nine male powerlifters served as sub-

jects, with a mean mass of 91.0 t 25.2 kg, a mean height of174.9 7.0 cm, a mean age of 45.7 t 5.2 yr, and a meanload lifted of 225.4 - 58.0 kg. All subjects wore a one piecelifting suit. All subjects participated in a national powerlift-ing masters' championship that was sanctioned by the AD-FPA. To participate in masters' level powerlifting compe-tition, all lifters had to be at least 40 yr old. All subjectssigned a human consent form giving their approval to bevideotaped and participate in this study.

ANALYSIS OF THE SQUAT DURING VARYING STANCES

Data collection. Two synchronized Sony HVM 200video cameras were used to collect 60-Hz video data. Onecamera faced the subject's left side while the other camerafaced the subject's right side, with each camera's opticalaxis forming a 45 angle to the sagittal plane of the lifter.The cameras were positioned approximately 14 in apart andfaced perpendicular to each other, with each camera approx-imatelylO m from the subject. To minimize the effects ofdigitizing error, the cameras were positioned so that thelifter-barbell system was as large as possible within theviewing area of the cameras.

Just before a subject initiated their lift, an external lightsource was activated in both camera views to help matchvideo frames when viewing the videotapes. Before and justafter the subjects were videotaped, a 2 X 1.5 X I m 3-Dcalibration frame (Peak Perfornance Technologies, Inc.,Englewood, CO), surveyed with a measurement tolerance of0.5 cm, was positioned and videotaped in the same volumeoccupied by the lifter-barbell system. The calibration framewas comprised of 24 spherical balls of known spatial coor-dinates, with the x- and z-axes positioned parallel to theground, and the y-axis pointing vertical.

Data analysis. In powerlifting competition, lifters aregiven three attempts during the squat to maximize theamount of weight they can lift. A lifter's first attempt isusually submaximal, whereas their second and third at-tempts are near the maximal weight thev are capable oflifting. Therefore, only second and third attempts that weresuccessfully completed (i.e., ruled a "good lift" by a panelof three judges) were analyzed. Twenty-two of the 39 liftsanalyzed were third attempts. The 17 second-attempt liftswere used because the third attempts were unsuccessful dueto the lifter attempting a weight that was beyond their onerepetition maximum (I RM). Therefore, it was thought thatall lifts analyzed were vely near each lifter's I RM.

Previously, the squat has been divided into three descentand three ascent phases (16,17,21): 1) starting position to450 knee flexion angle (KF); 2) 45"KF to 901KF; 3) 901KFto maximum KF; 4) maximum KF to 90'KF; 5) 90'KF to450 KF; and 6) 4Y5KF to lift completion. The end of each ofthe first five phases was chosen for kinematic and kineticanalyses: I) 450 KF Jduring descent; 2) 90'KF during de-scent; 3) maximumi iKFE; 41 90QKF during ascent. and 5)450 KE during- ascent. Kinematic and kinetic analyses werealso performed at minimum bar velocity. which alwaysoccurred between 900 and 450KF during the ascent. Becausethe starting and ending positions of the squat are similarwith the knees and&hips near full extension, kinematic andkinetic analyses Were not performed at these positions.

A 3-D video analysis system (Peak Performance Tech-nologies, fnc,) Was used to manually digitize data for all 39subjects. A 15-point spatial model was created, comprisedof the top of the head and centers of the left and rightmid-toes, ankles, knees, hips, shoulders. hands, and end ofbar. All points were seen in each camera view. Each of these15 points was digitized in every video field (60 Hz). whichwas adequate due to the slow movement of the lift (3,17).Digitizing began five video fields (0.17 s) before the start of

Medicine & Science in Sports & Exercise,6 985

the descent and ended five video fields after the end of theascent.

A fourth-order, zero-lag Butterworth digital filter wasused to smooth the raw data with a cutoff frequency of 5 Hz.A cutoff frequency between 3 and 5 Hz has been demon-strated to be adequate during lifting 1-RM loads involvingslow movements (9). Bv using the direct linear transforma-tion method (l33 ), 3-D coordinate data were derived from the2-D digitized images from each camera view. An averageresultant mean square calibration error of 0.3 cm producedan average volurne error of 0.121%.

The origin of the 3-D orthogonal axis system was firsttranslated to the right ankle joint and rotated so that thepositive x-axis pointed to the left ankle joint, the positivez-axis pointed anteriorly in the direction the lifter was fac-ing, and the y-axis pointed in the vertical direction (9). Thevertical positions of the digitized left and right ankles werewithirn I cm of each other. This axis system was initiallyused to calculate all joint moments, monnent arms, andangles. Muscle moment arms were not quantified in thisstudv. Because hip flexion and extension during the squatoccur primarily in the y-z sagittal plane about the x-axis, hipmoments were calculated about the x-axis and hip momentarms were calculated in the z-axis direction. Ankle and kneemoment arms were also calculated in the z-axis direction.which equates to a 2-1D analysis using one camera to recorda sagittal view of the lifter. These 2-D data were comparedwith 3-D data from the 3-D analysis. To calculate the actualankle and knee moment arms from a 3-D analysis. the axessystem was translated to each ankle joint center and rotatedso that the positive z-axis pointed fromn the ankle joint centerto the mid-toes, the y-axis pointed vertical, and the x-axiswas orthogonal to the y- and z-axes (9). Hence. for bothsides of the body, arkie and knee mnoments were calculatedabout the x-axis. and ankle and knee moment arms werecalculated in the z-axis direction. In addition, movement ofknees relative to the ankles were measured in the z-axis (i.e..in the direction of the longitudinal axis of the foot) todetermine how far forward the knees translated over the feetduring the squat. with left and right side measurementsaveraged.

Linear and anguiar displacements and velocities werecalculated for both the left and right sides of the body. andthen averaged t9). Relative knee and hip anglies and absolutetrunk, thigh, and shank angles were defined in accordancewith previous lifting studies (3.9). Trunk, thigh. and shankangles were measuared relative to the x-z horizontal plane(i.e., from a right horizontal relative to a sagittal view of thelifter's right side). Knee angles were measured relative tothigh and leg segments, whereas hip angles were measuredrelative to trunk and thigh segments. As long as the trunk isrigid and straight. this relative angle approximated the truehip angle. From qualitative analyses of the squat, unlike thedeadlift, the trunk typically remains rigid and straight. Kneeand hip angle measurements were expressed as 0 at fullknee and hip extension by subtracting relative angle mea-surements fromr 180'. To compare joint and segment angledifferences between 2-D and 3-D analyses, hip, knee, thigh,

986 Officiat Joumat of the American College of Sports Medicine

and shank angles from 3-D analyses were projected onto a2-D sagittal plane (9). Foot angle was defined as the angleformed between the foot segment and the y-z sagittal plane.Stance width was defined as the linear distance between theleft and right ankle joint cernters, whereas hand width wasdefined as the linear distance between the left and right handcenters.

During the I RM squat (21). the barbell initially accel-erates at the beginning of the ascent to a first peak velocity.then decelerates to a minimum velocity, accelerates again toa second peak velocity. and finally decelerates until the endof the ascent. Therefore, the squat ascent was divided intofour lifting phases ,9): a) acceleration phase maximumKF to first peak bar velocity: b) sticking region -- first peakbar velocity to minimum bar velocity; c) maximum strengthregion -* minimum bar velocity to second peak bar veloc-ity; and d) deceleration phase second peak bar velocity tolift completion.

Because segment and barbell accelerations are verv smallwhile lifting maximum or near maximum loads. joint: mo-ments can accurately be calculated using quasi-static models(16.17,21.22,26). Lander et al. (17 found that joint mo-ments varied less than 1I% between quasi-static and dynamicanalyzes during the squat exercise with near maximumloads. Hip, knee, and ankle moments and moment armswere calculated for left and right sides and then averaged(9). Body segment center of mnasses and weights were cal-culated bv using appropriate anthropometric data (7), andeach lifter's knowvn mass and segment lengths. The geomet-ric center of the barbell represented the center of mass of thebarbell (COMb.,). Position coordinates for x. y, and z werecalculated for both COM,hA,r and the center of rnass of thesvstem (CONM Ystem), which included both barbell and bodymasses. Joint moments and moment arms were calculatedrelative to both barbell weight and system weight (9). Thesystem weight used to calculate joint moments was the sumof the barbell weight and the weight of body segments abovethe joint in which the moments were calculated (9). Anklemoment arms (MAankle) were calculated as the distance inthe z-axis direction from the ankle joints to COMi,,ar orCOMsy,steni, with the system comprising barbell and bodymasses minus the mass of the feet. Ankle morments werecalculated as the product of MAanki, and barbell weight andthe product of MAankie and system weight. Knee momentarms (MAknlee) were calculated as the distance in the z-axisdirection from the knee joints to COMba, or COM,,,tem, withthe system comprising barbell and bodv masses minus themasses of the legs and feet. Knee moments were calculatedas the product of MAkn.e and barbell weight. and the productof MAkle, and s'stem weight. Hip moment arms (MAh,P)were calculated as the distance in the z-axis direction fromthe hip joints to COMba, or COM,vstew. with the systemcomprising barbell and body masses mninus the masses of thethighs. legs and feet. Hip moments were calculated as theproduct of MA1 ,i, and barbell weight, and the product ofMAhip and system weight.

Because bar motion primarily' occurred in the verticaldirection, vertical bar displacement was calculated from

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maximum KF to lift completion and normalized by bodyheight. Mechanical work, which was calculated relative toboth barbell weight and system weight, was the product ofsystem or barbell weight and total vertical displacement ofCOMba, or COMsvstem, The svstem weight used to calculatemechanical work was the sum of barbell weight and bodyweight minus the weight of the feet.

To compare stance widths relative to individual size dif-ferences, stance width was normalized and expressed as apercent of each subject's shoulder width (distance betweendigitized shoulder joint centers). The 39 normalized stanceswere ranked from lowest to highest (87-196% shoulderwidth) and divided into defined narrow, medium, and widestance groups. The 13 lowest normalized stances (87-118%shoulder width) were assigned to the narrow stance (NS)group; the middle 13 normalized stances (121-153% shoul-der width) were assigned to the medium stance (MS) group;and the highest 13 normalized stances (158-196% shoulderwidth) were assigned to the wide stance (WS) group. Toassess kinematic and kinetic differences among the threestance groups, a three-way, mixed-factor multiple analysisof variance (MANOVA) was employed (P < 0.05). Therepeated factors in the MANOVA consisted of 2-D versus3-D comparisons and descent versus ascent comparisons.Stance was the between subjects factor in the MANOVA,which consisted of narrow, medium, and wide stances. Asimple linear regression was used to assess the relationship(P < 0.05) between normalized stance width and joint andsegment angles.

RESULTSA representative graph of joint and segment angles as a

function of time is shown in Figure 1. During the squat, theknees and hips flex and extend together with similar mag-

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nitudes and shapes. Thigh angles follow the same generalpattern as knee and hip angles. Shank angles have similarshapes and magnitudes as trunk angles. The minimum barvelocity shown in Figure I corresponds to the minimum barvelocity in Figure 2 (occurring at approximately 2.15 s),which shows a representative graph of vertical bar velocityduring the squat. The acceleration and deceleration phaseswere the shortest in duration, whereas the sticking andmaximum strength regions were the longest in duration.Typical differences between 2-D and 3-D joint angle anal-yses for the NS and WS are shown in Figure 3. Represen-tative graphs for hip, knee, and ankle angular velocities asfunctions of knee and hip angles are shown in Figures 4 and5. Shapes and magnitudes of hip and knee angular velocitiesas functions of hip and knee angles were similar during thesquat motion. In addition, hip, knee, and ankle angularvelocities as a function of knee angles were very similar tohip, knee, and ankle angular velocities as a function of hipangles. Ankle angular velocities remained low and fairlyconstant throughout the descent and ascent phases. Employ-ing different stance widths did not affect the general shapesand magnitudes seen in the representative graphs from Fig-ures 1, 2, 4, and 5.

Significant positive correlations (r 0.5-0.85) werefound between normalized stance width and joint and seg-ment angles, with joint angles increasing as normalizedstance width increased. The highest correlations were foundbetween normalized stance width and thigh angles (r =0.75-0.85). A representative graph between normalizedstance width and thigh angles is shown in Figure 6. Low tomoderate correlations (r = 0.5- 0.7) were observed betweennormalized stance width and hip and shank angles, whereasnonsignificant correlations (r = 0.15-0.35) were observedbetween normalized stance width and trunk angle.

la,

i t : :Ba

Ve 3 vyTlgeoo~ hn2 >~~~11

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FIGURE I-Representa-tive graph for joint andsegment angles.

ANALYSIS OF THE SQUAT DURING VARYING STANCES

0.0 0.S 1.0 i.5 2.0 2.5 3.0 3.5

TrW I

Medicine & Science in Sports & Exerciseg 987

Ascent Phase

MinBar

Velocity

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Time (s)

There were no significant differences in kinematic andkinetic measurements between left and right sides of thebody. Subject characteristics among the NS, MS, and WSare shown in Table 1. Among the three stance groups therewere no significant differences in age. body height, bodyweight, barbell load, and hand width. Joint and segmentangles among the three stance groups are shown in Table 2.with significant differences found in hip, thigh. and shankmeasurements. Most significant differences occurred be-tween NS and WS groups, with no differences observedbetween MS and WS groups. In the MS and WS groups at450, 900. and maximum KF, the hips flexed approximately

i 83 -

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203

FIGURE -Representativegraph for joint angles be-tween 2-D and 3-D analysesfor narrow and wide stancesquats.

320 -

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49

10 more compared with the NS. Compared with the NS, theshanks were approximately 80 more vertical. the thighs wereapproximately 10 more horizontal, and the feet were turnedout approximately 60 more in the WS. There were nosignificant differences in trunk positions among the stancegroups. At 45KF, there was significantly less hip flexionand forward trunk tilt in the descent phase compared withthe ascent phase. There was significantly greater hip flexion(12-14') and forward trunk tilt (7-l00) at minimum barvelocitv during the ascent (Table 2) compared with hip andtrunk angles at corresponding KF arngies during the descent,in which the hips flexed 67 70, 68 10^, and 69 12',

Narro% Stance - 2-D Analysis

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7 1

Wide Stance - 34 A.a,- ,2-DAnaly,is

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40 .

10

80

604

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3Time { s,

988 Official Journal of the American College of Sports Medicine

i 2 ;Time (s)

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0.75

0.50

FIGURE 2-Representative graph for verti-cal bar velocity. AP, acceleration phase; SR,sticking region; MSR, maximum strength re-gion; and DP, deceleration phase.

0.25

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U

0

-0.50

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I

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FIGURE 4-Representative graph for hip,! 8 gf @ knee, and ankle angular velocities as a func-

tion of knee angle.-120-I40

--- -,-,-

0 20 40 60 80 100 120 100 80 60 40 20 0Descent Phase Ascnt Phase

Kne Angle (deg)

-4_ Knee Angular Velocitv

e Hip Angsslar Velocity-- Anikle AngtsAw Velocity

respectivelv, and trunk angles were 65 50, 64 4, and the ankle joints the knees translated forward over the feet66 + 5.0 respectively, during the NS, MS, and WS. There 21.7 + 4.4 cm during the NS, 18.0 - 2.6 cm during the MS.were no significant differences found among the three and 16.0 4.6 cm during the WS. There was significantlystance groups at minimum bar velocity. Comparing joint greater forward knee translation over the feet during the NSand segment angles between 2-D and 3-D analyses, the NS compared with the MS and WS.showed the fewest number of significant differences, The joint moments and moment arms expressed in Tableswhereas the WS showed the greatest number of significant 8-10 are relative to barbell or system loads. Positive mo-differences (Table 3). Hip and thigh angles were 3-13' less ment arms are anterior to joint, producing positive hipin 2-D analyses compared with 3-D analyses. flexor, knee extensor, and ankle dorsiflexor system mo-

There were no significant differences in temporal, work, ments. Hip extensor, knee flexor, and ankle plantar flexorand bar velocity comparisons among the three stance groups resultant muscle moments are needed to counteract these(Tables 4-6). On the average. it took 4.60 1.20 s to system moments. Negative moment arms are posterior tocomplete the I RM squat, with a descent time of 1.92 + joint, producing negative hip extensor, knee flexor. and0.51 s and an ascent time of 2.68 1.04 s. Significant ankle plantar flexor system moments. Hip flexor, knee ex-differences among the three stance groups in peak hip, knee, tensor, and ankle dorsiflexor resultant muscle moments areand ankle angular velocities, and corresponding knee and needed to counteract these system moments. Joint momentship angles, are shown in Table 7. The only significant and moment arms were significantly different among thedifferences found among the three stance comparisons were three stance groups (Table 8). with most differences occur-greater hip angles in the MS and WS compared with the NS ring between the NS and WS. From Table 8. most signifi-at peak hip, knee, and ankle angular velocities. Relative to cant differences in moments and moment arms involved the

40 0

80~~~~~~~~~~~~~~~~~~~

80

FIGURE S--Representative graph for hip,10 00 knee, an ankle angular velocities as a function-120 o0of hip angle..140-

i 2 40 60 80 10 120 100 80 60 40 20 8Descern Phae Ascent Pha

*lio _Anglo (deg)

-X- Knee Angular Velocity| Hip Angular Velocity

Ankle Angular Velocity

ANALYSIS OF THE SQUAT DURING VARYING STANCES Medicine & Science in Sports & Exercise#, 989

140

ia 125

115-, * * 0 ~~~Thigh 93.856 -(0.19i1Stance)/- * r 0.81 1; SEE 3,.95; p < 0.001

105 i -- -_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _80 100 128 140 160 180 200 220

Stance Width 1% shoulder width)

FIGURE 6-Representative graph of simple linear regression betweennormalized stance width and thigh angle at 45 knee flexion during thesquat descent.

ankle, whereas significant differences in knee and hip mo-ments and moment arms occurred only at 450KF. Ankleplantar flexor resultant muscle moments were generatedexclusively during the NS, whereas ankle dorsiflexor result-ant muscle moments were generated exclusively during theMS and WS. Peak ankle moments and moment arms oc-curred at maximum KF during the NS and at 45KF duringthe MS and WS. Peak knee moments and moment armsoccurred at maximum KF, whereas peak hip moments andmoment arms occurred at minimum bar velocity. The onlysignificant differences between the squat descent and ascentoccurred at 455KF. in which hip moments and moment armswere significantly greater during ihe ascent compared withthe descent. Ankle, knee. and hip moment anns and ankleand hip moments were not significantly different betweenCOMbar and COMS,,t,. However, knee moments weresignificantly greater in system load compared with barbellload for all three stance groups (Table 9). Significant dif-ferences in ankle and knee moments and moment armsbetween 2-D and 3-D analyses are shown in Table 10 for theNS and WS. During the NS, ankle moment arns were 7-8cm less in a 3-D analysis compared with a 2-D analysis,whereas knee moment arms were 9-10 cm greater in a 3-Danalysis compared with a 2-D analysis. This same tread inankle and knee moment arms occurred during the MS andWS. with a 12.-14 cm difference between 2-D and 3-Danalyses during the MS and a 16-18 cm difference betweer2-D and 3-D analvses during the WS. During the NS, ankle

plantar flexor resultant muscle moments were significantlygreater in 2-D analyses compared with 3-D analyses. Duringthe WS, ankle resultant muscle moments changed fromplantar flexor in 2-D analyses to dorsiflexor in 3-D analyses.In 2-D analyses knee moments were greater with an NScompared with a WS. Conversely. in 3-D analyses kneemoments were greater with a WS compared with a NS. Alldata in Tables 1-10 and Figures 1-6 are from 3-D analysesunless specified otherwise.

DISCUSSIONMany athletes and coaches believe that technique varia-

tions occur in the sauat as different stance widths are em-ployed. There are currently no known studies that havequantified joint angles, moments. and moment arms whileperforming the squat with varying stance widths. Therefore,the objective of this study was to compare squat kinematicsand kinetics between 2-D and 3-D analyses among threedefined stance groups. The results from the current studydemonstrate that kinematic and kinetic differences do occuramong the three stance groups and that 2-D kinematic andkinetic analyses produce erroneous results compared with3-D analyses, especially during the WS.

In the current study, linear and angular displacements andvelocities, as well as joint moments and moment arms, wereaveraged from the left and right sides of the body. Therewere no significant differences between bilateral measure-ments. which demonstrate the symmetrical nature of thesquat exercise. This implies that during the squat analyzingonly one side of the body may be adequate in calculatingjoint and segment angles, joint moments, and joint momentarms. The symmetrical nature of the deadlift, which issimilar to the squat, has previously been demonstrated byEscamilia et al. (9), who found no significant differences inkinematic and kinetic measurements between left and rightsides of the body.

Joint and segmental angles. Although trunk anglewas not significantly different among the three stancegroups at 45S, 90g, and maximum KF, greater hip flexion, amore horizontal thigh position. and a more vertical shankposition were observed in the WS and MS compared withthe NS. These chanees occurred in part because the NS hadapproximately 4-6 cm greater forward knee movement inthe direction of the toes compared with the MS and WS.

BA E 1. Subiect characteristics (mean S0M.Signilicant

Narrow Stance Medium Stance Wide Stance DifferencesAge (yr) 437 ,3.5 45 7 6.4 478- 4.Bodv height (cm' 176.7 _ 6.9 173.6 + 7.3 174.3 6.8Body mass (kg) 82.4 + 21.7 93.1 - 24.8 97.5 a 28.2Barbeil load (kg5 208.3- 54.9 229.2 64.4 2387 54Systemr load (kg) 290.7 74.2 322.4 86.7 336.2 _ 80.1Stance width (cVm 40.9 + 3.8 59.7 6.6 69.6 +.5 95 Stance width (% shoulder widthi 107 1 0 142 12 169 12Hand width (cm) 88.2 15.6 98.4 13.5 94.3 + 15.9

* Significant differences (P < 0.05).a Mediumn vs narrow: I medium vs wide; 'wide vs narrow.

990 Official Journai of the American College of Sports Medicine hftr,v/www.acsrn-rnsse.org

TABLE 2. Joint and segment angles (mean :- SD).

Narrow Stance Medium StanceDescent phase

45s KFHipi') 4717 55+62*Trunk () 70 + 4*- 68 + 4-Thigh (0)117 - 5 124 +4Shank1; 71 3 73 4

90' KFHip) 0) 92t 8 98 6runk( 60- 4- 60 6

Thlign 153 5 160 +5Shank fi 61 5 66 4

Maximumr KFHip ('I 107. 0 109 8runk(j 58 5 61 6

T.hiah ') 166 5 171 5ShanrkL 58 5 64 -4Knee (j) 106 8 102 7

Ascert phase90' KF

Hip(0 97+7 1006Trunk ! 56-8 61 6Thigh) 0) 154 4 162 +5Shank '(i 63 5 66 5

Minimum bar velocityHip (C) 81 - 7 80 - 11Tunk (') 56 4 57 t.9

Thiah ') 138 5 137 6Shanh () 69 -5 72 +4Knee (j 67 9 59 8

45' KFHip) 52 9 63 9"7runk 66+ 6* 62 6**'high() 118 4 126 5Shank () 71 A 74 4

Foo Angle( 20 5 23 3Significant differences (P < 0.05).Medium vs narrow; 'medium vs wide: 'wide vs narrow.

Significant differences (P < 0.05) at 45' KF and 90' KF between descent and ascent phases.

Greater forward knee movements during the squat havebeen shown to increase knee shear forces (2), which impliesthat employing a MS or WS may be more effective than anNS in minimizing knee shear forces.

Trunk. knee, hip, and thigh angle patterns and magnitudesfrom Figure I are similar to data from several other studieswhich quantified joint and segment angles during the squat( 1,12,16,1-,21,26.35). Averaged amnong all three stancegroups, nm;nimal bar velocity occurred at 62 IO'KF,which is slightly lower than the 75 6 KF reported byMcLaughlin et al. (21). Averaged among all three stancegroups. hip, trunk, thigh, and shank angles at minimum barvelocity were 81 9G, 57 6, 139* 6', and 70 5PQ *respectively, in the current study, and 110 1 ll 11, 39 I 1 iP,150 + 2c, and 74 + 50, respectivelv. from McLaughlin et al.(21). Although subjects from the current study and subjectsfrom McLaughlin et al. (21) all participated in a nationalpowerlifting competition involving 1-RM4 liffing, subjectsfrom McLaughlin et al. (21) had 13 more knee fl&exion. 129'more hip flexion, and 18 more forward trunk tilt at mini-mum bar velocity. This implies that, compared with thesubjects in M'cLaughlin et al. (21), the subjects in the currentstudy obtained minimum bar velocity later in the ascent andmaintained a more upright trunk Dosition. Some of thesejoint position differences between the current studv andMcLaughlin et al. (21) are probably due to methodologicalANALYSIS OF THE SQUAT DURING VARYING STANCES

differences, because McLaughlin et al. employed a 2-Danalysis whereas the current study employed a 3-D analysis.However, because trunk positions between 2-D and 3-Danalyses should be similar due to the trunk moving in asagittal plane, the observed joint position differences be-tween these two studies are also probably due to techniquedifferences.

All previous studies that quantified joint and segmentangles employed a 2-D sagittal plane analysis(16.17,21.26,35). It was hvpothesized that hip, thigh, andshank angles would show a greater number of significantdifferences, between 2t-D and 3-D analyses during a WScomparedt with an NS. During the WS, the feet are typicallyturned6out to a greater degree compared with an NS. The

ore; the feet turn out, the greater the lower extremitiesdeviateWfrom sagittal plane movements, and the greater thedifferences will be between 2-D and 3-.D analyses. Thesignificantly greater foot angles in the WS compared withthe-NS contributed to several significant differences be-tween 2-1l and 3-D analyses in the WS, but fewer significantdifferences were observed between 2-D and 3-D analyses inthe NS. This is supported by Figure 3, which shows typicaldifferences in joint angles between 2-D and 3-D analyses forthe NS and WS. During the NS, 2-D and 3-D joint angleanalyses were nearly identical. whereas larger differencesbetween 2-D and 3-D analyses were observed during the

Medicine & Science in Sports & Exercise. 991

SignificantDifterences

*a,

-'

Wide Stance

55 1 770 5"

126 _471 * 4

101 8962 + 6

165 t 769 + 6

110 t 762 _ 5

173 - 567 . 799 10

104 760 7

166 - 868 5

82 956 - 6

140 + 671 .- 660 - 12

63 1363 7"

.28 - 772 + 626 _o 4

TABLE 3. Comparisons of lower extremity joint and segment angles (mean SD) between 2-D and 3-D analysesNarrow Stance Medium Stance

2-0 3-D 2-0Descent phase

45 KFHip(') 40+7* 477*Knee (1 42 44 1'Thigh ( 114 3' 117 5'Shank(f 72 3 71+3

90' KFHipf) 88 9* 92 8*Kneel') 88 2* 90 1VThigh (') 151: 5 153 5Shank ) 63 4 61 5

Maximum KFHip (') 103 13' 107 _10*Kneef, 105+t9 106 8Thigh' 166 5 166 5Shanki') 58 5 58 5

Ascent phase90' KF

Hip(') 92 10* 977*Knee( 88 5 90 2Tnigh('} 153 4 154 4Shank(*) 64 4 63 5

Minimum bar velocityHip)') 77 81 7'Kneel') 66 - 9 67 9Thigh ( 137 5 138 5Shank (i) 700 S 69 5

45' KFHip,') 46 9* 52 9*Knee) 43 + 2' 45 1*Thigh( 115 _ 3* 118 4'Shanki 73 + 3 71 4

Significant differences (P < 0.05) between 2-D and 3-0 analyses.

WS. However, the relatively small 65 difference in footangle between the NS and WS may have contributed toseveral WS comparisons between 2-D and 3-D analyses notbeing significantly different (Table 3). In examining 2-Dand 3-D joint and segment angle comparisons during sumo(wide stance) and conventional (narrow stance) style dead-lifts. Escamilla et al. (9) generally found no significantdifferences between 2-D and 3-D analyses during the con-ventional deadlift. whereas all 2-D versus 3-D comparisonswere significantly different in the sumo deadlift. However,mean foot angles were 14 + 6G for the conventional deadliftand 42 8G for the suino deadlift. Furthermore. a compar-isor between deadlift stance widths (9) and squat stancewidths from the current study revealed that the NS had a33% greater stance width than the conventional deadlift,whereas the suamo deadlift had an 11 % greater stance widththan the WS. These data imply that a 2-D analysis may beadequate to calculate joint and segment angles when the footangle is relatively small (i.e., 0-1 5c) and a narrow stance isemployed, but significant errors can occur from 2-D anal-vses as foot angles and stance widths increase.

Mechanical work. Although there were no significantdifferences in vertical bar distance and mechanical workamong the three stance groups (Table 4), the overall me-chanical work on the svstem (1444 366 J) was signifi-cantlv greater than the total mechanical work on the bar(1 107 278 J). This difference between bar mechanicalwork and system mechanical work implies that the totalenergy expenditure during the squat is underestimated if

992 Official Joumai of the American College of Sports Medicine

45 6*42 4*

118 975- 5

91 +8*87 + 5

154 567 4

104 - 13*105 I 11169 865 t3

90 10l88+7

157 5*67 + 4

72 14*57 -8

133 7*75 4'

55 11'43 3

121 5'77 4*

Wide Stance3-0 2-D 3-0

55 7*45_ 2*

124 473 4*

98 6'89: 1

160 5'66 4

109 _ 8'102 717'1 564 +4

100 6*90 2

162 +5'66 5

80 11'59 8

1 37 - 6*72 _4'

63 945 :

126: 574 4'

42 7'42 4'

115 + 4*74 4'

92 11 *87 6

159 4*70 6

106 X 12'13+ 12'1 70 7'68 _7

96:a,89 6

162 : 9'70 _ 6

74 _ 1158 - 14

133: 7'75 7*

54 14*43- 3

119 6*77 6i

55 7'45- 1

126 t 4*71 +4'

101 9*88-3

165 7*69 - 6

99 _ 10'173 5*67 : 7

104 ~ 7'88 +3

166 8*68 6

82 9*60: -1264^. 6 7'1 + 6*

63 + 13*45- I

125:-7'72 6*

only the bar mechanical work is calculated, because energyexpenditure during the squat increases "inearly as mechan-ical work increases (5). Bar and system mechanical workvalues in the current study were approximately 10% greaterthan bar and system mechanical work values reported byEscamilla et al. (9) during the conventional style deadlift,which is performed similar to the squat. Because the sub-jects in Escamilla et al. (9) lifted a mean load of 222 34kg, which is nearly identical to the mean load lifted in thecurrent study, the 1.0% difference in mechanical work be-tween these two studies is from a 109-c greater vertical bardistance in the current study. High energy expenditures havebeen reported during both the deadlift (4,9) and squat (5),which suggests that these types of multi-muscle. multi-jointexercises are more effective in energy expenditure and mus-cle development compared with single-joint, single-muscleexercises. Several studies have shown moderate to high

TABLE 4. Temporal and work comparisons (mear -- SD).Narrow Stance Medium Stance Wide Stance

Total lift time is) 4.43 -0.60 4.69 1 1.3c 4.68 1.48Total descent time Is) 1,95 0.55 2.02 0.47 ' .80 0.50Total ascent time (s) 2.48 0 C.50 2.6? :1.36 2.88 -1 1Total vertical bar distance (% -it) 29.2 1.7 28.5 - 2.2 28.6 -2.6Total vertical system. distance i(% Ht) 27.0 1.9 26.7 2.3 26.2 +. 21Total vertical bar distance (cm) 51.6 4.1 49.6 5.3 49.8 -4.9Total vertical system distance (cm. 47.8 - 4.2 46.5 z. 5.4 45.7 3.8Total mechanical work on bar (J) 1058 302 1110 312 1154 :228Total mechanical work on svstem iJ) 1371 : 394 1468 +. 406 1493 + 309For all parameters there were no significant differences among stance comparisons.

http:Ilwww.acsm-msse.org

TABLE 5. Select events (mean - SD).Narrow Stance Medium Stance Wide Stance

Descent phasePeak bar veiocitv

Velocity (ms 1) 0.527 0.140 0.508 0.097 0.559 0.156Time occurred (% descent time) 29.5 - 10.8 22.66 10.4 30.9 11.0Vertical bar position (% descent vertical bar distance) 27.4 11.7 23.2 11.0 29.5 14.1

Ascent phaseFirst peak bar velocity

Velocity (m-s 1) 0.380 0.,102 0.365 0.085 0.367 0.055lime occuirred (% ascent time) 13.8 8.5 18.4 10.7 19.3 10.9Vertical bar position (% ascent vertical bar distance) 10.0 _ 3.3 13.6 6.1 15.9 '0.4

Minimum bar velocityVelocity (ms 1) 0.086 t 0.069 0.00 0.097 0.058 0.089Time occurred (% ascent time) 47.8 9.8 50.7 6.8 47.2 + 8.5Vertical bar position 1% ascent vertical bar distance) 42.0 5.9 43 4 8 8.4 41.6 -2.0

Second peak bar velocityVelocity (ms-') 0.464 t 0.116 0.457 0.111 0.434 0.130Time occurred (% ascent time) 86.5 9 '.2 85.6 - 6.5 84.4 10.7Vertical bar position (% ascent vertical bar distance) 84.6 + 114 84.6 + 4.4 82.3 9.5

For all parameters there were no significant difterences among stance comparisons.0% ascent time or vertical bar distance is the beginning of the ascent, and 100% ascent time or vertical bar distance is the end of the ascent.

muscle activitv during the squat from the quadriceps, ham-strings, giuteus maximus, thigh adductors. abdominals, ob-liques, and erector spinae (6,8,14,17,20,25,29-31,34-36).These are the largest and most powerful muscles in the bodyand generate a high force production and energy expendi-ture when active.

Selected events and lifting phases. Mean peak ver-tical bar velocity during the squat descent (0.531 + 0.132111-s) was slightly greater than mean peak bar velocityduring the squat ascent (0.452 0 Q I 17 mr-s-) and occurredat approximately 27% of both the descent time and thedescent vertical bar distance. Mean peak vertical bar veloc-ity durin,g the descent was nearly identical to the high skilledsquat group from McLaughlin et al. (21) but approximately15% lower than the less skilled squat group from McLaugh-lin et al. (21). This implies that higher-skilled lifters lowerthe bar at a slower rate compared with lesser-skilled lifters.This is important because several studies have reportedsignificantly greater tibiofemoral shear and compressiveforces during a fast squat cadence compared with a slowsquat cadence (1.6.11). This occurs because faster descentrates require greater deceleration forces from the knee andhip extensors in order to slow down and stop the weight atthe end of the descent.

The vertical bar velocity curve shown in Figure 2 is thesame pattern reported by McLaughlin et al. (21). Of the 39lifters in the current study. 28 lifters achieved maximumvertical bar velocity at their second peak vertical bar veloc-ity, whereas the remaining 11 lifters achieved maximumvertical bar velocity at their first peak vertical bar velocity.However, for all stance comparisons, the mean second peakbar velocity was not significantly different from the mean

first peak bar velocity (Table 5). These data are similar tothe I RM squat data reported by McLaughlin et al. (21), inwhich most lifters also reached their maximum vertical barvelocity at their second peak vertical bar velocity. Both theacceleration and deceleration phases of the squat comprised15-25% of the ascent time, whereas the sticking region andmaximum strength region comprised 30-40% of the ascenttime. These values are similar to data reported by McLaugh-lin et al. (21). These results imply that during the squatapproximately twice as much time is spent in the stickingregion and maximum strength region compared with theacceleration and deceleration phases.

The end of the sticking region (i.e., minimum bar veloc-ity) has previously been reported as the "sticking point"(21), which occurred at approximately 60-650 KF and 80-85"' hip angle (Table 2). The "sticking point" appears to bethe most difficult part of the lift, and is often where pow-erlifters fail in their attempt for a successful lift. Becauseknee and hip moments and moment arms generated by thesystem weight generally decrease during the ascent as theknees and hips extend (16,22,35), a mechanical disadvan-tage is believed to occur namong knee and hip muscle ex-tensor moments duunng the sticking region, being greatestnear the sticking point. The sticking point phenomena mavin part be due to mechanical principles of skeletal muscle,such as to the length-force relationship and muscle momentarm lengths. It is well known that as a muscle contractsconcentrically and shortens its abilitv to generate forcediminishes. Because the product of muscle force and musclemoment arm:determines the net muscle moment generatedatf a join t,la decrease: in::both of these variables or a dispro-

portionately decrease: in one variable with respect to the

TABLE 6. Lifting phases (mean _ SD).Narrow Stance

Acceleration phase (% ascent time) 13. -8 8.5Sticking region (% ascent time) 33.9 t 9.7Maximum strength region (% ascent time) 38.8 --12.8Deceleration phase (/0 ascent time) 13.5 9.2

For all parameters there were no significant differences among stance comparisons.% ascent time is the total percentage of the ascent represented by each phase or region.

ANALYSIS OF THE SQUAT DURING VARYING STANCES

Medium Stance18.4 10.732.3 m 12.335.0 6.114.4 - 6.5

Wide Slance19.3 + 10.927.9 + 12.237.2 - 10.715.6 10.7

Medicine & Science in Sports & Exercise(,, 993

TABLE 7. Peak hip, knee, and ankle angular velocities and corresponding hip and knee angles.

Descent phasePeak hip angular veiocitv ".shKnee angie at peak hip anoular velocity (1Hip angle at peak hip angular velocity (}Peak knee angtilar velocity (' sKnee angle at peak knee angular velocity ()Hip angie at peak knee angular velocity (01

Peak angie angular velocity ('-s -')Knee anlgle at peak ankle anguiar velociht (0)Hip angie at peak ankle angular velocity i

Ascent phasePeak hip angular velocity ( 's Knee angle at peak hip anguilar veiocity (0Hip angle at peak hip angular velocity (0)

Peak knee angular velocity l"s -'Knee angle at peak knee angular velocity (0)HiP angle at peak knee angular veiocity ')Peak ankle angular velocity ("5s'Knee angie at peak ankleHip angle at peak ankie angular velocity (0i

0 Significant differences (P < 0.051.aMedium vs narrow: b medium vs wide,C wide vs narrow.

other will cause a decrease in the net muscle moment. Thenet hip extensor moment generated by the gluteus maximus.hamstrings, and ischial fibers of adductor magnus has beenshown to be maximumn at 90o hip angle, decreasing progres-sive1v as the hips extend (23). This decrease in the net hipextensor moment with hip extension contributes to the stick-ing point. Interestingly. in contrast to hip extensor mnoments,the hip extensor morment arms for both the hamstrings andgluteus maximus have their smallest values at 90' hip angle(24). increasing progressively as the hips extend. This im-plies that the length-force relationship and muscle forcesfromn the gluteus maximus and hamstrings have a muchgreater influence on hip extensor moments compared withthese muscle's respective moment arms. It has been previ-ouslv demonstrated that changes in muscle forces affectmuscle moments to a much greater extent than changes inmuscle moment arms (18 e, All moment arms from the hipextensors have been shown to initially increase as the hipsextend from 90' hip angle (24). The moment arms of theadductor magnus increase until approximately 75G hip an-(le. and then progressively decrease with further hip exten-sion (24). Because the adductor magnus is a uni-articularmuscle, a decrease in this muscle's ability to generate forcedue to muscle shortening, combined with its decreasingmoment arms as the sticking point is approached, produce adecrease in the net hip extensor moment near the stickingpoint. Although the moment arms of the uni-articular glu-teus maximus increase progressively with hip extension(24). a disproportional decrease in muscle force as thismuscles shortens may produce a decrease in the net hipextensor moment near the sticking point. Because the ham-strings are bi-articular. during the ascent these muscleslengthen at the knees and shorten at the hips, which helpsmaintain a more optimal force-length relationship during thesquat. However, hip extension causes the hamstrings to

994 Official Journa, of the American College of Sports Medicine

lengthen to a greater extend than knee tlexion causes thehamstrings to shorten (19,32). Therefore, it is common nearthe sticking point for lifters to slightly increase forwardtrunk tilt and hip flexion, as shown in Figure 1 at minimumbar velocity. so as to increase the length of the hamstringsand other hip extensors, thus increasing these muscle'sability to generate force. The inclined for-ward trunk tilt alsoallows a greater contribution from. the powerful back mus-cles. Although hip moment arms from the hamnstrings in-crease as the hips extend fromn 90t to 35 ' (24), knee momentarms from the hamstrings peak near 50-60'KF (15,27).which increase knee flexor moments near the sticking point.The gastrocnemius also generates knee flexor mroments dur-ing the squat as they contract to cause ankle plantar flexionduring the ascent. These increased knee flexor momentsgenerated by the hamstrings and gastrocnernius. which op-pose the knee extensor moments generated by the quadri-ceps, contribute to the sticking point. In addition. kneeextensor moments generated by the quadriceps decrease asthe knees extend toward the sticking point. This decrease inknee extensor moments as the knees extend is primarily dueto a decrease in quadriceps force. because patellar tendonmoment arms change only a few millimeters frorm full kneeflexion to full knee extension (13,15j. Interestingilv. the12-140 greater hip flexion and 7-.00 greater forward trunktilt observed at the sticking point compared with the sameKF position during the squat descent implies that the stick-ing region causes an asymmetrical pattern to occur betweenthe squat descent and ascent. This explains why at 45eKFthere was significanthy greater hip flexioon and forward trunktilt during the ascent compared with the descent (Table 2).

Joint moments and moment arns. Although sev-eral studies have quantified joint moments during the squat(2,8,12 ,16,17,12,26,28,31,34.35). there are no known stud-ies that have quantified hip, knee, and ankle momernt arms

http:,'v/wwv.acsm-rmsse.org

Narrow Stance Wide StanceSignificantDifferences

-106 t 2450 1450 e 15

-123 t 2437 t 1139 11

-40 843 _ 1945 t 19

121 t 2440 1245 12

130 3236 - 1342 12

42 = 1352 + 2759 - 25

Medium Slance

-104 2245 1356 14

-112 + 1936 C 948 1 '0

-30 '259+ 1570 19

104 2443 + 1658 --44

109 -: 2949 2163 18

33 t 665 - 1780 18

-. 0 _- 2146 - 1458 14

-20 _ 2334 - 1246_ 11

-3 5_ 1I54 2573 _ 25

I.02 2436 1 I52 = 13

"I n - 3>45 . 1660 -a 24

35 964 -- 2080 23

TABLE 8. Joint moments and moment arms (mean - SD) relative to system load._ , ... - .. -- - --- --

Narrow StanceDescent phase

Moment arms at 450 KF fcm)AnkleKneeHip

Moments at 45' KF (N-m)AnkleKneeHip

Moment arms at 90 KF (cmiAnkleKneeHip

Moments at 900 KF (N-m)AnkieKneeHip

Moment arms at maximum KF (cm)AnkleKneeHip

Moments at maximum KF (N-m)AnkieKneeHip

Ascent pnaseMoment arms at 90 KF (cm)

AnkleKneeHip

Moments at 90 KF (N-m)AnkleKneeHip

Moment arms at minimum bar velocity (cm)AnkleKneeHip

Moments at minimum bar velocity (N m)AnkleKneeHip

Moment arms at 450 KF (cm)AnkleKneeHip

Moments at 450 KF (N-m)AnkleKneeHip

0.5 t 2.1-13.8 4.3

1.1 . 3.0

10 t 52-382 + 145

275 + 71 '

1.0 2.3-18.9 5.2

.9.4 3.9

34 66-517 + 169

505 t 169

1.7t 2.6-21.0 t 5.7

19.9 t 4.2

51 t 72-573 179

514 159

1.3 2.318.45.421.1 t 3.5

40C 68--504 - 187

547- 169

0.5 2.5-14.5 5.6

22.0 3.5

22 73-393 + 161

577 r 195

0.5 t 2.8-12.9 4.1

13.9 2.8**

16 79359 t 141355 8 59**

Medium Stance

-4.2 2.7--16.7 4.2

13.5 + 2.0*i

-136 t 122-521 + 248

382 92**

-2.2 t 2.6-19.7+ 5.0

17.3 3.8

-73 99-605 t 247

487 t 123

-1.4 t 2.6-20.4 + 3.9

17.8+ 3.7

-47 100-627 t 233

507 t 158

- 1.0 t 2.5-18.4 3.9

I17.3 3.6

-34 t 108--564 + 217

498 170

-2.9 2.6-15.2 4.3

21.0 + 5.1

-90 t 100-475 236

595 t 188

-3.6 2.1-14.2 3.7

18.8 3.7

-112 81-447 - 221

549 t 211

Wide Stance

-79 5.0-21.6 5.3

12 4 3.3*

-284 t 236-698 299

383 t 161*0

-7.0 5.1-22.6 t 5.0

16.3 3.5

-254 233-723 t 267

479- 134

-6.6 + 5.0-23.7 t 4.0

16.7 3.6

-242 t 228- 756 235

499 158

-6.0 t 5.5-22.1 3.8

17.6 t 4.2

-224 t 242-709 244

516 - 143

-6.1 =5.3- 19.2+ 6.1

21.2 4.0

-227 224-623 t 274

628 + 167

-6.7+ 4.4-18.5 4.6

17.5 t 4.6*-

- 243 t 201-605 + 254

526 + 197-

SignificantDifferences

fib,c

.a,c

C

* a,c

ohs,

*6.6

*ac

i Significant differences (P < 0.05).'Medium vs narrow; 'medium vs wide; wide vs narrow.00 Significant differences (P < 0.05) at 450 KF and 9Q0KF between descent and ascent phases.Positive moment arms are anterior to joint, producing positive hip flexor, knee extensor, and ankle dorsiflexor system moments. Hip extensor, knee flexor. and ankie plantarflexor resultant muscle moments are needed to counteract these system moments. Negative moment arms are posterior to joint, producing negative nip extensor. kneeflexor, and ankle plantar flexor system moments. Hip flexor, knee extensor, and ankle dorsiflexor resultant muscle moments are needed to counteract these systemmoments.

during the squat. Positive ankle moment arms during the NSproduced dorsiflexor system moments (Table 8) that mustbe counterbalanced by plantar flexor resultant muscle mo-ments. In contrast, negative ankle moment arms during theMS and WS generated ankle plantar flexor system moments(Table 8) that must be counterbalanced by dorsiflexor re-sultant muscle moments. This implies that the ankle plantarflexors may be recruited to a greater extent during the NScompared with the MS and WS. whereas the ankle dorsi-flexors may be recruited to a greater extent during the MSand WS compared with the NS. Escamilla et al. (8), whosesubjects had a stance width similar to the NS group, reportedlow to moderate gastrocnemius activity during the 12 RM

ANALYSIS OF THE SQUAT DURING VARYING STANCES

squat. In addition, gastrocnemius activity has been reportedto be 10-15% greater in the NS squat compared with theWS squat (10). Ankle moment arms peaked at maximumKF, which is consistent with squat data from Escamilia et al.(8) and Isear et al.(14), which show peak gastrocnemiusactivity near maximum KF.

Several studies have shown moderate to high activityfrom the quadriceps during the squat (8,20,25,29-31,34,35).with peak activity occurring near maximum KF(8,14,25,30.31,35). Moderate to high knee extensor musclemoments imply overall knee extensor activity, especiallyduring the MS and WS, which produced significantlygreater knee moments compared with the NS. However, two

Medicine & Science in Sports & Exerciset 995

studies have reported no significant differences in quadri-ceps activity between the NS and WS (10,20).

Moderate to high hip extensor muscle moments suggestoverall hip extensor activity. especially in the MS and WSgroups. which at 450 KF produced significantly greater hipmoments compared with the NS. At minimum bar velocity(approximately 60KF) peak hip extensor muscle momentswere generated in all stance groups. Escamilla et al. (8)reported peak hamstring activity (approximately 50% of amaximum voluntary isometric contraction) near 50OKF.whereas other studies have reported peak hamstring activityduring the squat between 10 and 600 KF (14,25.31). Thehamstrings, gluteus maximus, and ischial fibers of the ad-ductor magnus have all shown significantly greater activityin the WS squat compared with the NS stance (10,20).Compared with the squat descent at corresponding KF an-gles, the significantly greater hip flexion and forward trunktilt observed during the squat ascent at minimum bar veloc-ity and 45'KF caused significantly greater hip moments andmoment arms.

It is difficult to compare ankle, knee, and hip momentsamong squat studies in the literature because methodologiesand loads lifted (20-270 kg range) varied greatly. In quan-tifying ankle, knee, and hip moments, some studies used asingle camera (2-D) and no force platform (2.12,22,26),some studies used a single camera and one force platform(16.17,28.34,35), whereas some studies used multiple cam-eras (3-D) and one force platforrm. (8,31). In addition, somestudies had subjects squat with one foot on a force platform(8,31), some studies had subjects squat with both feet on aforce platform (16,17.34,35x, some studies quantified jointmoments relative to barbell weight t2,26), and other studies

TABLE 9. Mean (mean + SD) knee moments (N m) between barbell ioad andsystem load.

Relative to Bar Load Relative to System LoadDescent phase

Narrow stance45' KF --249 102' -382 145'90' KF -324 117' --517t169'Maximum KF --366 129' -573 t 179*

Medium stance45' KF -341 168* -521 - 248'90' KF -393 t 178' --605 247'Maximum KF -411 174* -627t 233*

Wide stance45' KF -468 t 210' -698 t 299'90' KF -477- -191' -723 267'Maximum KF -498 164' -756 t 235*

Ascent phaseNarrow stance

90' KF --310 t 133* -504 -187'Minimum bar veiocitv -226 t1-.3' --393 + 161'45' KF ---223 1 l00* -359 t 141'

Medium stance90' KF -362 160' -564 t 217*Minimum bar velocity -288 160' -475 1236'45 KF -270 149* -447 t 22-1

Wide stance90 KF -462 168* -709 t 244*Minimum bar veiocity -386 - 196' - 623 - 274'45' KF -384 178' -605 t 254'

'Significant differences (P < 0.05).All moments are knee flexor moments generated by barbell or system loads. Kneeextensor resultant muscle moments are needed to counteract these barbell orsystem moments.

996 Official Journal of the American Coliege of Sports Medicine

quantified joint moments relative to system weight(8,12,16,17,22,28.31,34.35). Peak ankle moments havebeen reported between 50 and 300 N-m (16,17,22,26).peak knee moments between 100 and 500 N-m(2.8,12,16,17,22.26,28.31,34.35), and peak hip momentsbetween 150 and 600 N-m (1 6,17,22,34,35). Hence, ankle,knee, and hip moment magnitudes in the current study aresimilar to corresponding moment magnitudes in the squatliterature.

One of the most important findings from the current studyis the numerous significant differences observed in mo-ments and moment arns between 2-D and 3-D analyses.Most of the above squat studies that quantified joint mo-ments employed a single camera to record a sagittal view ofthe lifter, thus performing a 2-D analysis. Although a 2-Danalysis may be appropriate in calculating hip and spinalmoments and moment arms during squat, because the trunkmoves primarily in the sagittal plane. it is only appropriatein calculating ankle and knee moments and moment arms iflower extremity movements occur primarily in the sagittalplane. However, as the stance widens and the feet turn out,greater errors in 2-D moment and moment arm calculationswill occur, because lower extremitv movements move out ofthe sagittal plane. Escamilla et al. i9) reported only- a few cmdifferences in ankle and knee moment arm calculationsbetween 2-D and 3-D analyses during the conventionaldeadlift exercise. in which a very narrow stance was emn-ployed (32 _ 8 cm, 80 _ 16% shoulder width) with the feetonly slightly tumed out (14 = 60). However, these authorsreported 20-25 cm differences in ankle and klnee momentarms between 2-D and 3-D analvses during the sumo dead-lift, in which a very wide stance was employed (70 11 cm,188 + 37-/% shoulder width) with the feet turned out 42 8'. Although 2-D analyses in the current study were signif-icantlv different than 3-D analyses for all three stancegroups, errors in moment arm. magnitudes were twice asgreat in the WS compared with the NS. This occurredbecause the WS had a significantly greater stance width andfoot angle compared with the NS.

When circumstances do not permit multiple cameras to beemployed during the squat, a 3-D analysis can still beperformed on some kinematic and kinetic variables by in-putting 2-D data from a single camera into appropriatederived -nathematical equations. For example, ankle mo-ment arms derived fromn a 3-D analysis car be calculatedgiven ankle moment arms from a 2-D analysis by employingthe following equation: MA 3 r - cos 0(MA,D) (0.5S- ,,.h-)sin 0, where MA2 D is the ankle moment arm measured froma 2-D analysis, MAOD is the predicted ankle moment armfrom a 3-D analysis, B is the angle measured between thesagittal plane and the longitudinal axis of the foot. and Swidthis the stance width measured between ankles. This equationis easily derived by utilizing elementary trigonometry. Thevalues predicted for MA3D by emnploying this equation werenearly identical to the actual measured values of MA3 D fromthe 3-D analysis. A 2-D analysis from a single camera willyield correct ankle moments and moment arms only whenthe feet are pointing straight ahead (i.e., B = 0"), and will

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TABLE 10. Comparisons of ankle and knee joint moments and moment arms (mean t SD) relative to system load between 2-D and 3-D analyses.Narrow Stance Wide Stance

2-D 3-D 2^0 3-0

Descent phaseMoment arms at 45' KF (cm;

Ankie 7.1 + 2.0* 0.5 2.1* 7.9 2.0* -7.9 5.0*Knee - 4.5 + 2.4 13.8 4.3* - 3.3 2.9' -21.6 5,3*

Moments at 45* KF (N-inlAnkie 207 80* 10 52' 257 -78 -284 236'Knee -133 + 86* -382 t 145* -104 - 99* -698 299'

Moment arms at 90 KF (cm)Ankle 8.8 +2.2* 1.0 t 2.3* 9.0 2.4' -7.0 + 5.1'Knee -8.7 + 2.2* -18.9 5.2' -4.0 3.3' -22.6 + 5.0

Moments at 90' KF (N-mjAnkle 254 98' 34 t 66' 291 77' -254 + 233*Knee -248t 86' -517 169* -119+. 89* -723 + 267'

Moment arms at maximum KF (cm)Ankle 9.5 2.3* 17 - 2.6* 9.4 + 2.1* -6.6 5.0'Knee -10.5 :2.7* -21.0 5.7' --4.9 - 3.1* -23.7 4.0'

Moments at maximum KF iN-m)Ankle 272 _ 98' 5t 72' 305 80* -242 t 228*Knee -297t 97* -573179* -146 75* -756 235'

Ascent phaseMoment arms at 90' KF (cm)

Ankle 9.1 2.0* 1.3: 2.3 10.1 2.4' -6.0 5.5'Knee -8.2 t 2.4* -18.4 t 5.4' -3.A 2.3* -22.1 3,8*

Moments at 90' KF (N-m)Ankle 260: 86* 40 + 68* 325 78* -224 242'Knee -236 t 104* -504 187* -101 t 67* -709 244*

Moment arms at minimum bar velocity (cm)Ankle 8.3 + 2.4* 0.5 + 2.5' 10.0 2.8* -6.1 + 5.3*Knee -4.9 t 2.8' -14.5 t 5.6* -0.9 4.1* --19.2 t 6.1"

Moments at minimum bar velocity (N-miAnkle 241 102' 22 t 73* 323 t 100* -227 t 224'Knee -137 89* -393 t 161 -24 35' -623 +273'

Moment arms at 45' KF (cm)Ankie 8.3 :3.2* 0.5 t 2.8' 9.3 :2.1' --6.7 + 4.4*Knee -- 3.6 2.3' -12.9 4.1' -0.4 3.3* -18.5 t 4.6'

Moments at 45' KF (N-m)Ankle 235 101' 16 79* 305 98* -243 t201Knee -107 77 -359 t 141 - 12 t 122' -605 254'

'Significant differences (P < 0.05) between 2-D and 3-D analyses for the NS and WS.Positive moment arms are anterior to joint, producing positive hip flexor, knee extensor, and ankle dorsiflexor system moments. Hip extensor, knee fiexor, and ankle piantarflexor resultant muscle moments are needed to counteract these system moments. Negative moment arms are posterior to joint, producing negative hip extensor, kneeflexor, and ankle plantar fiexor system moments. Hip flexor, knee extensor, and ankle dorsiflexor resultant muscle moments are needed to counteract these systemmoments.

produce only small errors when the feet are slightly turnedout with an NS employed. For exaimple, if during the NSsquat MA,D measured 9 cm, stance width was 35 cm, andthe foot angle was 10', MA3D would yield a value ofapproximately 6 cm, which is only a few cm different thanMAID. However, if during the WS squat MA2 D measuredthe same 9 cm, but stance width was 70 cm and foot angleIwas 45', MA 3D would yield a value of approximately - 18cm, a difference of 27 cm. Also, the difference in themoment arm sign implies a change from a plantar flexor netmuscle moment to a dorsiflexor net muscle moment. Theemployment of a 3-D analysis is clearly more paramountduring the WS squat with a large foot angle compared withthe NS squat with a small foot angle.

When peak mean joint moments from the squat literatureinvolving 2-D analyses are normalized by body heig ainsystem weight, their normalized peak mean values are verysimilar to the normalized mean peak 2-D moments calcu-lated in the current study. For example, from Table 10 thepeak mean 2-D knee moment calculated for the NS was 297N-m. When this peak knee moment is normalized by theproduct of body height (1.77 m, Table 1) and system loadANALYSIS OF THE SQUAT DURING VARYING STANCES

(2852 N, Table 1) and expressed as a percentage, the nor-malized mean peak knee moment for the NS is between 5.5and 6%. Compared with other squat studies that reportedpeak Mean knee 0 moments using a 2-D analysis, Ariel (2)found a. :normalized:g, peak meanh knee moment of approxi-mately 55%, Similarly. Laider et al. (17i and Wretenberg

aLet a.(35) found pek mean knee moments of approximately6-6-5%, which are similar to the nonnalized peak meanknee moment in the current study. As seen in Table 10. kneeimoments and moment arms are significantly underestimatedin -D0 analyses compared with 3-D analyses.

Thet 0knee moment significant differences observed be-tween barbell 7 load and system load implies that knee mo-ment contributions from bodv segments should not be dis-counted when calculating the actual joint moments thatoccur urri}nsg0lifting.t is interesting that hip and anklemoments were not significantly different between barbellload and system load. There are several reasons why thisoccurred. First, knee moment arms were generally greaterand hip and ankle moment arms were generally less for thesystem loads compared with the barbell loads. Therefore,these smaller system hip and ankle moment arms would

Medicine & Science in Sports & Exercisea 997

produce a relative decrease in hip and ankle svstem mo-ments. Second, the system load was greater for knee mo-ments than hip moments, because there is greater body massabove the knee joints compared w-ith the hip joints. Third,the relative small ankle moment arms compared with theknee and hip moment arms produced relatively large stan-dard deviations for both ankle moments and moment arms(Table 8).

REFERENCESI. ANDREWS. J. G., J G. HAY. and C. L. VAUTGHAN. Knee shear forces

during a squat exercise using a barbeli and a weight machine. In:Biomechanics VIhf-B. X. Matsui and K. Kobavashi (Eds.). Cham-paign, IL: Human Kinetics, 1983, Pp. 923-927.

2. ARIEL. B. G. Biomechanical analysis of the knee joint during deepknee bends with heavy loads. In: Biomechanics IV, R. Nelson andC. Morehouse (Eds.). Baltimore: University Park Press, 1974, pp.44-52.

3. BROWN. E. W.. and K. ABANI. Kinematics and kinetics of the deadlift in adolescent power lifters. Med. Set. Sports Exerc. 17:554-566, 1985.

4. BROWN, S. P., J. M. CLEMONS. Q. HE. and S. Lit'. Prediction of theoxygen cost of the deadlift exercise. J. Sports Sel. 12:371-375,1994.

5. BYRD. R., K. PIERCE, R. GENTRY, and M. SWISHER. Predicting thecaloric cost of the parallel back squat in women. J. Strength Cond.Res. 10:184-185. 1996.

6. DAIIL.KVIST, N. 1J., P. MAYO. and B. B. SEEnHoM. Forces duringsquatting and rising from a deep squat. Engl. Med. Ii (2):69-76,1982.

7. DEMPSTER, W. T. Space requirements of the seated operator(WADC Technical Report). Wright-Patterson Air Force Base,Ohio. 1955, pp. 55-159.

8. ESCANMILLA. R. F., G. S. FLEISIG. N. ZHENG. S. W. BARRENTINE, K. E.WILK, and J. R. ANDREWS. Biomechanics of the knee during closedkinetic chain and open kinetic chain exercises. Med. Sci. SportsExerc, 30:556-569. 1998.

9. ESCAMILLA. R. F.. A. C. FRANCISCO, G. S. FLEISIG, et al. A three-dimensional biomechanical analvsis of sumo and conventionalstyle deadlifts. Med. Sci. Sports Exere. 32:1265-`275. 200O.

10. ESCAMILLA. R. F., N. ZHENG. G. S. FLEISIG. et: al. The effects oftechnique variations on knee biomechanics during the squat andleg press. Med. Sci. Sports Exerc. 29:S156, 1997.

1]. HATTIN. H. C., M. R. PIERRYNOWSKI. and K. A. BAiL. Effect of load,cadence, and fatigue on tibio-femoral joint force during a halfsquat. Med. Sci. Sports Exerc. 21:613-61&, 1989.

12. HAY, J. G., J. G. ANDREWS. C. L. VAAUGHAN, and K. UEYA. Load,speed, and equipment effects in strength-training exercises. In:Biomechanics VII1-B, H. Masui and K. Kobayashi (Eds.). Cham-paign, IL: Human Kinetics Publishers, 1983, pp. 939-950.

13. HERZOG, W., and L. J. READ. Lines of action and moment arms ofthe major force-carrying structures crossing the human knee joint.J. Anat. 1}82(Pt 2):213-230, 1993.

14. ISEAR, J. A., Jr., J. C. ERICKSON, and T. W. WORRELL, EMG analysisof lower extremity muscle recruitment patterns during an unloadedsquat. Med. Sci. Sports Exerc. 29:532-539. 1997.

15 KELLIS, E., and V. BALT-zoPouLos. In vivo determination of thepatelia tendon and hamstrings moment arms in adult males usingvideofluoroscopy during submaximal knee extension and fiexion.Clin. Biomech. 14:118- 24, 1999.

16. LANDER, J. E., B. T. BATES, and P. DEVITA. Biomechanics of thesquat exercise using a modified center of mass bar. MAed. Sci.Sports Exere. 18:469-478, 1986.

17. LANDER. J. E._ R. L. SIMoNToN, and J. K. GIACOBBE. The effective-ness of weight-belts during the squat exercise. Med. Sci. SportsExerc. 22:1 17-1 26, 1990.

18. LIEBER, R. L., and J. L. BOAKES. Muscle force and moment armcontributions to torque production in frog hindlimb. Am. J.Physiol. 254(6 Pt 1):C769-772. 1988.

The authors extend a special thanks to Andy Demonia and Chris-tian Welch for all their help in collecting the data and Abidemi BoluAjiboye, Herbert Bohnet, and Brian Pullin for all their assistance inmanually digitizing the data. Also, we would like to extend a specialthanks to Tom and Ellen Trevorah, powerlifting meet directors, for alltheir support throughout this project.

Address for correspondence: Rafae. Escamilla, Ph.D., C.S.C.S..Duke University Medical Center, P.O. Box 3435, Durham, NC 27710;E-mail: rescamil@duke.edu.

19. LIEBER, R. L., and C. G. BRowN. Sarcomere length-joint anglerelationships of seven frog hindlimb muscles. Acta Anat. 145:289-295, 1992.

20. MCCAW. S. T., and D. R. MELROSE. Stance width and bar loadeffects on leg muscle activity during the parallel squat. Med. Sci.Sports Exere. 3}(3):428-436. 1999.

21. MCLAUGHLIN, T. M., C. J. DILLMAN. and T. J. LAR3sNER. A kinematicmodel of performance irn the parallel squat by champion power-lifters. Med. Sci. Sports Exerc. 9:128-133, 1977.

22. MCLAUGHLIN. T. M., T. J. L.ARDNER. and C. J. DILLMAN. Kinetics ofthe parallel squat. Res. Q. 49:175-189. 1978.

23. NEvEmm, G. On hip and lumbar biomechanics: a studv of joint load andmuscular activity. Scand. J. Rehabil. Med Suppl. 10: 1-35, 1984.

24. NEMETH, G.. anid H. OHLSEN. In vivo moment arm iengths for hipextensor muscles at different angles of hip fiexion. J. Biomech.18:129-140. 1985.

25. NINOS, J. C., J. J. IRRGANG. R. B- RDFTI and J. R. WEISS. Electro-myographic analysis of the squat performed in self-selected lowerextremity neutral rotation and 30 degrees of lower extremityturn-out from the self-selected neutral position. [. Orthop. SportsPhys. Ther. 25:307-315. 1997.

26. NISELL, R., and J. EKHOLM. Joint load during the parallei souat inpowerlifting and force analysis of in vivo bilateral quadricepstendon rupture. Scand J. Sports Sci. 8:63-73. 1986.

27. OCONNOR. J. J. Can muscle co-contraction protect knee ligamentsafter injury or repair? J. Bone Joint Surg. (Br.) 75-B.1():41-48.1993.

28. RUSSELL, P. J.. and S. J. PHILLiPS. A preliminary comparison offront and back squat exercises. Res. Q. 60:201-208. 1989.

29. SIGNORILE, J. F.. K. KwIA iTKOWSKI, J. F. CARUSO, and B. ROBERTSON.Effect of foot position on the electromryographical activit of thesuperficial quadriceps muscles during the parallel squat and kneeextension. J. Strength Cond. Res. 9:182 187. 1995.

30. SIGNoRiLE, J. F.. B. WErER. B. RouL. T. F. C.p.Uso. 1. LoAE_rrY.N, andA. C. PERRY. An electromyographicai comparison of the squat and kneeextension exercises. J. Strength Cond Res. 8:178-183, 1994.

31. STUART, M. 21.. D. A. MEGLAN, G. E. Liurz, E. S. GROWNEY, andK. N. AN. Comparison of intersegmnental tibiofemoral joint forcesand muscle activity during various closed kinetic chain exercises.Am. J. Sports Med. 24:792-799, 1996.

32. VISSER, J. J., J. E. HOOGKAIMER, M. F. BOBBERT, and P. A. HUIJ1NG.Length. and moment arm of human leg muscles as a function ofknee and hip-joint angles. Eur. J. Appl. PhYsioi. 61(5-6):453-460, 1990.

33. WOOD, G. A., and R. N. MARSHiALL. The accuracy of DIT extrap-olation in three-dimensional film analysis. J. Biomech. 19:781-785, 1986.

34. WRETENBERG, P.. Y. FENG, and U. P. ARBORELIUS. High-and low-bar squatting techniques during weight-training. Med. Sci. SportsExerc. 28:218-224. 1996.

35. WRETENBERG, P., Y. FENG, F. LINDBERG. and U. P. ARBORELILS.Joint moments of force and quadriceps activity during squattingexercise. Scand. J. Med. Sci. Sports. 3:244-250, 1993.

36. WRIGHT, G. A., T. H. DELONG, and G. GEHLSEN. Electromryographicactivity of the hamstrings during performance of the leg curls,stiff-leg deadlift, and back squat movements. J. Strength Cond.Res. 13:168-174, 1999.

37. YACK, H. J., C. E. COLLINS, and T. J. WHIELDON. Comparison ofclosed and open kinetic chain exercise in the anterior cruciateligament-deficient knee. Am. J. Sporfs Med. 21:49-54. 1993.

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TITLE: A three-dimensional biochemical analysis of the squatduring varying stance widths

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