30 Strength and Conditioning Journal June 2000

National Strength & Conditioning AssociationVolume 22, Number 3, pages 3037

The Benefits and Controversy of the Parallel Squat in Strength Trainingand Rehabilitation

Jennifer A. Neitzel, PT, ATC, CSCSGundersen-Lutheran Sports Medicine, La Crosse, Wisconsin

George J. Davies, MEd, PT, SCS, ATC, CSCSUniversity of WisconsinLa Crosse, Wisconsin

Keywords: parallel squat; anterior cruciate ligament; patello-femoral syndrome; rehabilitation; strength training; closed kinetic chain.

THE SQUAT HOLDS AN UNPAR-alleled position of eminence instrength training and conditioningas well as in rehabilitation. Noother lifting movement, with theexception of the 2 olympic-stylelifts (the snatch and the clean andjerk), places as much stress andstrain on the musculoskeletal sys-tem as does the squat (31). Be-tween these lifting styles, there is1 major difference. Squatting per-mits the lifter to maintain a me-chanically strong lifting basethroughout the full range of move-ment, and the knees are not sub-jected to the danger of stress andstrain as they are in snatchingand cleaning. This article will per-form a brief biomechanical analy-sis of the squat, including descrip-tion of the muscle activity involvedin its execution. It will also de-scribe the importance and contro-

versy of the squat in sport and inlower-extremity (LE) rehabilita-tion, particularly in patients whohave patellofemoral syndrome(PFS) or anterior cruciate ligament(ACL) deficiencies or who arepostACL reconstruction status.

Although the LEs are primari-ly responsible for movement dur-ing the parallel squat, the upperextremity (UE) and trunk are in-volved in stabilization. In descent,the UE and trunk are maintainedin the starting position as muchas possible. The trunk will have atendency to flex, and this move-ment should be controlled so thatstress on the lumbar spine is keptto a minimum (21).

In a study by Tibero (48) andOShea (31), a detailed biome-chanical analysis of the LE duringthe squat is described. The de-scending phase demonstrates

knee flexion, internal rotation ofthe tibia and femur, subtalar joint(STJ) pronation, ankle dorsiflex-ion, talus adduction, and cal-caneal eversion. When rising froma squat, knee extension, externalrotation of the tibia and femur,STJ supination, ankle plantarflexion, talus abduction, and cal-caneal inversion occur. Inclosedkinetic chain (CKC) activi-ty, the synchronous actions of theknee and STJ are interdependentmotions, and the rotation of thelower leg is an obligatory actionthat is necessary for normal kine-matics of both joints (48).Steindler (44) describes this mus-cular pattern as the concurrentshift. Biarticular muscle actionsof the LE during the parallel-squatexercise are also quite distinctive;the same muscle undergoes si-multaneous eccentric contraction

June 2000 Strength and Conditioning Journal 31

(lengthening of the muscle) at onejoint and concentric contraction(shortening of the muscle) at theother. For example, the ham-strings concentrically contract asthe hip extends and work eccen-trically as the knee extends.Therefore, the muscles change lit-tle in overall length through thelarge-joint excursions. The mus-cles of the LE also undergo oppo-site modes of contraction duringthe ascending, as compared withthe descending, phase of thesquat. For illustration, during thelowering phase of the squat, thequadriceps control knee flexion byacting eccentrically, then workconcentrically in extension to re-turn the body to an erect posture.

Strength and conditioningspecialists can utilize their under-standing of the muscle activitypatterns and biomechanics thatoccur during the squat to assistwith proper PFS rehabilitation.Ninos et al. (27) analyzed the mus-cle activity during the parallelsquat from the 0 to 60 range ofmotion (ROM) in the knee. Theyfound that the peak activity levelsin the vastus medialis oblique(VMO), vastus lateralis (VL), andbiceps femoris muscle groups oc-curred from 50 to 60 during theascending and descending phas-es, respectively (27, 51). No signif-icant changes in muscle activitywere found to occur in the ham-strings during either the descend-ing or ascending phases of thesquat. Wilk et al. (51) found thatthe greatest level of quadriceps ac-tivity during the full squat oc-curred from 88 to 102 of kneeflexion. A plausible explanationfor the variation of findings in thestudies by Wilk et al. (51) andNinos et al. (27) is the type ofsquat (full versus parallel) used inthe research designs. Ohkoshi etal. (29) confirmed increasedquadriceps activity with increased

knee flexion during a static squat;they also found increased ham-string activity with increasedangle of trunk flexion (Table 1).

Ninos et al. (27) and Signorileet al. (40) found that muscle activ-ity patterns can be affected bychanges in knee flexion and thedirection of movement but not byLE axial rotation. Anderson et al.(1) found that the VMO/VL ratioscould not be increased by widen-ing the squat stance. These find-ings refute the belief that varyingthe position of the feet during theparallel squat can target specificmuscles of the quadriceps (7, 20,25, 30, 33, 34). They further indi-cate that patients or athletesshould place their feet in a posi-tion of comfort that also providesthe greatest stability and safety forthe lift.

The knee angle also needsconsideration when rehabilitatingthe patellofemoral joint (PFJ) dur-ing CKC and openkinetic chain(OKC) exercises. CKC exercises re-quire the distal segment be fixedto a supporting surface (eitherstationary or moving), whereas inOKC exercises, the distal segment

is free to move. Upon analysis ofthe PFJ during the CKC squat ex-ercise as the knee moves from 0 to90 flexion, PFJ stress is steadilyincreased (29; Figure 1).

Steinkamp et al. (46) demon-strated that PFJ reaction forceand stresses were significantlygreater in the leg press, a CKC ex-ercise similar to a squat, than dur-ing leg extension (OKC) exercisesat 60 and 90 of knee flexion. Theyalso found that PFJ reaction forceand stresses were significantlygreater with OKC exercises thanin the squat at 0 and 30 of flex-ion. Steinkamp et al. (46) conclud-ed that increased knee flexionduring CKC exercises results inPFJ stress comparable to OKC ex-ercises that approach full knee ex-tension. This demonstrates an in-verse relationship of PFJ reactionforce with CKC and OKC exercis-es. Steinkamp et al. (46) recom-mend limiting the knee from 0 to45 flexion to decrease PFJ stressduring squats.

Despite the increase in PFJstresses with increasing knee flex-ion, Van Kampen and Huiskes(49) and Ninos et al. (27) found

Table 1Muscle Tension of Quadriceps and Hamstrings in Relation to Percentage of Body Weight (SD)

Flexion angle (degrees) Muscle tension (N)Knee Trunk Quadriceps Hamstrings30 0 0.53 0.30 0.04 0.03

15 0.44 0.22 0.25 0.3030 0.38 0.25 0.33 0.3060 0.29 0.2 0.60 0.4490 0.39 0.35 0.62 0.45

60 0 1.91 0.90 0.15 0.3215 1.91 0.68 0.15 0.2430 1.37 0.81 0.29 0.5160 1.17 0.68 0.35 0.3990 1.02 0.65 0.32 0.37

Table data is as reported in Ohkoshi et al. (29).

32 Strength and Conditioning Journal June 2000

that patellar tracking is not affect-ed by exercise type (OKC versusCKC) with knee flexion greaterthan 25 to 30, respectively. Theyfound that from 0 to approximate-ly 30 of knee flexion in CKC exer-cises there is less lateral patellartracking and less PFJ stress, pos-sibly due to the effects of internaltibia and femoral rotation andsynergistic muscular activity ofthe LE (27, 49). It has been foundthat most functional activities areperformed between 0 to 40 ofknee ROM; and this is also wherethe patella is least stable (27).Ninos et al. (27) and Steinkamp etal. (46) conclude that in the func-tional ROM, the squat allows forbetter positioning and less PFJ ir-ritation when performing activi-ties to strengthen the VMO. How-ever, when exercising from 30 to90 ROM, OKC may be a betterchoice than CKC exercises be-cause there may be less PFJstress and more VMO electromyo-graphic activity (27).

Stiene et al. (47) compared theeffect of OKC and CKC exercise(including the parallel squat) onquadriceps muscle performanceand perceived function in patientswith PFS. Statistical analysisshowed that both groups had sig-nificant improvement in peaktorque at all speeds of OKC testingbut that only the CKC groupshowed significant improvementin CKC testing and perceivedfunctional status. They concludedthat CKC training, including thesquat exercise, might be more ef-fective than OKC exercise inrestoring function in athletes andpatients with PFS.

In the past decade, the squathas become the standard of carefor not only patients with PFS dis-orders but also for the rehabilita-tion of patients with ACL-deficientand ACL-reconstructed knees (10,38). Closedkinetic chain exercis-es such as the squat appear tohave gained popularity over moretraditionally used OKC exercises

because many clinicians believethat CKC exercises are safer andmore functional (4, 8, 9, 13, 14,16, 35, 38, 39, 41). There report-edly are decreased shear forces atthe knee joint in the parallel squatas compared with OKC exercises(3, 4, 7, 12, 13, 27, 29, 32, 35, 36,50, 52). The squat decreases theshear on the ACL by 3 mecha-nisms.

First, the squat involves co-contraction of the hamstringswith the quadriceps femoris,whereas OKC knee extension re-sults in isolated quadriceps con-traction unless there is an artifi-cial activation of the hamstrings(29). Renstrom et al. (36),Solomonow et al. (42), and Lutz etal. (22) have studied the effect ofthe hamstrings as a counterforceto the anterior pull of the quadri-ceps. When the hamstrings areactivated, the knee stiffens anddecreases anterior shear associat-ed with isolated quadriceps con-traction in the end range of exten-sion (24, 29, 36, 41, 42). Duringsquatting, the hamstrings arethought to produce a greater mag-nitude of contraction becausethey stabilize the pelvis and thetrunk as well as the knee (17, 29).Second, the squat is a weight-bearing exercise, which causes in-creases in joint compression andwhich also decreases tension andshear on the ACL (15, 2224, 27,28, 39, 50, 52). Lastly, contractionof the gastrocnemius muscle aidsin stabilization of the tibia andhas an indirect effect that resultsin a reduced amount of anteriorshear forces at the knee (29; Table2).

There is debate whether these3 mechanisms of decreasingstress at the tibiofemoral joint areenough to reduce the shear forcesthat occur during the squat. Someauthors believe that the squat ex-ercise can increase knee ligamen-

Figure 1. Mean ( SD) of patellofemoral joint stress at 4 flexion angles (46).

June 2000 Strength and Conditioning Journal 33

tous laxity (18, 19, 37, 43, 52).Klein (18) is the only researcher todemonstrate that such is the case.Using undescribed standard or-thopedic tests, Klein (18) con-cluded that the full-squat exercisewas responsible for increased liga-mentous laxity in the medial, lat-eral, and anterior cruciate liga-ments. Meyers (26), using areplicate of Kleins knee liga-menttesting device, concludedthat the full- and parallel-squatexercises did not produce signifi-cant differences on lateral or me-dial collateral ligament stretch.Chandler et al. (5) concluded thatanterior or posterior knee stabilitydid not change with 8 weeks of theparallel- or full-squat training asmeasured by the KT-1000 kneeligament arthrometer.

Some research has demon-strated a decreased amount ofstrain to the tibiofemoral jointduring the squat as comparedwith OKC exercises in subjectswith a partially torn (13) or a com-pletely ruptured ACL (15, 23, 29,52). Henning et al. (13) found thatCKC exercises such as the parallelsquat are safer than OKC exercis-es secondary to reduced shearingon the partially torn ACL. Henninget al. (13) estimated ACL strain as

a percentage of the strain that oc-curred during a 36.3-kg Lach-mans test, reporting 21% strainduring a single-leg parallel squat.They found that the strain of legextension exercises, from 22 to 0of range of motion was 79% for thewoman subject and 12% for theman subject; leg extension from36 to 0 was 7% in the womansubject (Table 3).

Fitzgerald (9) questions the effi-cacy of this particular study byHenning et al. (13), stating that the

strain behavior measurementstaken from subjects with partiallytorn ACLs are most likely differentthan those from subjects with in-tact ACLs. Therefore, inferencesmade by Henning et al. (13) maynot be applicable to the strain onan intact ACL graft. Also, the largedegree of variability in the mea-surements between the 2 subjects,combined with a small sample size,reduces the meaningfulness of anyconclusions made from these mea-surements (9).

Table 3Estimated Percentage of Anterior Cruciate LigamentStrain During Various Activities in Subjects With ACL

Activity ACL strain (%)Single-leg parallel squat 21Walking (FWB) 36Jogging on floor 89Stationary cycling 7Leg extension (from 220 of flexion)

Women 79Men 12

Leg extension (360 of flexion) 7

All data are as reported by Henning et al. (13) and are forwomen except as noted. ACL strain is a percentage ofstrain occurring during a 36.3-kg Lachmans test.

ACL = anterior cruciate ligament; FWB = full weight bear-ing.

Table 2Maximal Tibiofemoral Joint Kinetics During OpenKinetic Chain (Knee Extension)

and ClosedKinetic Chain (Leg Press) Exercises

ExerciseParameter Parallel squat Leg press Knee extensionMaximal compressive force (N) 6139 1709 5762 1508* 4598 2547*

Direction descent descent ascentMaximal posterior shear force (N) 1783 634 1667 556 1178 +- 594*

Direction ascent ascent descentMaximal anterior shear force (N) NA NA 248 259*

Direction NA NA ascentMaximal extension torque (N - m) 150 40 160 41 200 120*

Direction descent ascent ascent

Data in table as reported by Wilk et al. (51).

* = Values were significantly different (p 0.05) from the parallel squat values.

34 Strength and Conditioning Journal June 2000

Little in vivo research hasbeen performed to date on intactACLs (3, 5, 45). Beynnon et al. (2)found no significant difference inACL strain characteristics be-tween OKC and CKC exercisesfrom 30 to 90 of flexion on intactACLs. Beynnon et al. (3) alsofound no difference in ACL strainbetween the parallel squat andOKC exercises from 0 to 90 flex-ion. Beynnon et al. (3) furtherdemonstrated that using a sportcord as resistance during thesquat does not produce a signifi-cant increase in ACL strain (2;Table 4). ACL strain values werecalculated based on a referencethat differentiated between theunstrained (nonload-bearing), andstrained (load-bearing) conditionsof this ligament.

Markolf et al. (24) and Shoe-maker and Markolf (39) have stud-ied the shear forces in ACL-defi-cient cadaver knees. Openkineticchain exercise always resulted inan increased amount of anteriortibial displacement than did CKCexercise, regardless of position oramount of force applied. Theyfound that CKC movement was,however, more effective in limitinganterior displacement at low levelsof applied force (50 N). When high-er levels of applied force (200 N)were utilized, the effects of jointcongruency were overcome, andgreater amounts of anterior tibialshear occurred.

The majority of researchershave not found either CKC or OKCexercises to be superior in reduc-ing anterior shear of the tibia butinstead found a more preferredROM for each exercise type. Jenk-ins et al. (16) measured tibialtranslation in subjects with ACLdeficiencies in vivo and found4.68-mm translation when per-forming OKC exercises and 1.26-mm translation for CKC exercisesat 30 with a 400-lb force on the

leg press machine. At the 60 po-sition, the differences were 1.23mm for OKC and 0.6 mm (poste-rior movement) for CKC exercises.When one considers the 3-mm fig-ure that Daniel et al. (6) has pro-posed as the difference betweenthe ACL-intact and ACL-deficientknee, it is clear that the 30 OKCposition may need to be avoided inthe patient with a healing ACL

graft. Openkinetic chain exerciseat 60 and the CKC squat exerciseat 30 have similar values andmay also need to be considered aspositions that cause anterior dis-placement, leading to possiblecreep of the healing ACL graft (16).The 60 position in the leg pressexercise appears to prevent anteri-or tibial displacement (16).

Lutz and colleagues (22) calcu-

Figure 2. Electromyographic (EMG) activity of the quadriceps and hamstrings during the closedkinetic chain (CKC) leg press and openkinetic chain(OKC) leg extension exercise and during the OKC exercise in the seat-ed position (22).

Table 4Maximal Percentage (Max %) of Anterior Cruciate Ligament Strain

Values During Open and ClosedKinetic Chain Exercises

Activity Maximum strain on ACL (%)Squatting (CKC)no resistance 3.6Squatting (CKC)with sport cord 4Knee extension (OKC) 3.8Knee extension (OKC)with 45 N weight 3.8Isometric knee extension15 of

knee flexion, 30 N*M torque 4.4Isometric knee extension30 of

knee flexion, 30 N*M torque 2Isometric knee extension60 of

knee flexion, 30 N*M torque 0Isometric knee extension90 of

knee flexion, 30 N*M torque 0

All data are as reported by Beynnon et al. (2, 3).

ACL = anterior cruciate ligament, CKC = closedkinetic chain exer-cise, OKC = openkinetic chain exercise.

June 2000 Strength and Conditioning Journal 35

lated that the squat exercise pro-duced a posterior-directed force of500 N on the tibia between 30 and90 of knee flexion in ACL-intactknees. For isometric contraction ofthe quadriceps, an anterior-di-rected force was reported to act onthe tibia, increasing in magnitudeas the knee was extended (Figure2). From these predictions, the in-vestigators recommended exercis-es that involve squatting instead ofisolated, isometric quadricepsmuscle contractions to rehabili-tate a knee during graft healing.

Despite the various benefits ofthe parallel squat, there are somelimitations of this exercise thatshould be noted. In particular, Lutzet al. (22) found through an elec-tromyogram study that only 63 21% maximum volitional contrac-tion (MVC) of the hamstrings and59 31% MVC of the quadricepswas noted during the partial squat.This is compared with 82 15%MVC for the quadriceps and 68 14% MVC for the hamstrings dur-ing leg extension exercises. Gra-ham et al. (11) found similar re-sults. These findings stress theimportance of using both CKC andOKC exercises for completestrengthening and rehabilitation ofthe LEs. See Table 5 for data.

In conclusion, with proper un-

derstanding of the biomechanicsand muscle activity patterns dur-ing a parallel squat, use ofstrength training and conditioningprinciples can be maximized, andproper rehabilitation can be ob-tained. Several studies have foundthe parallel squat advantageousfor people with PFS during the be-ginning ROM (030 or 45) ascompared with OKC exercises.The squat has also been shown toprovide decreased anterior shearof the tibia in individuals who areACL deficient. There is no evidenceto date that demonstrates anybenefits of the squat for a newlyreconstructed ACL, but inferencescan be made by comparing studieson individuals who have intactACLs with those on individualswho are ACL deficient. The ques-tion of debate, however, is which ofthese 2 subject groups is best forcomparison of the effects on anewly reconstructed ACL. Lastly,it should be noted that for maxi-mizing hamstring-strength gains,OKC exercises are far more supe-rior to the parallel squat. s

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36 Strength and Conditioning Journal June 2000

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Jennifer Neitzel is a physicaltherapist/certified athletic trainerat Merrill Physical Therapy inMerrill, Wisconsin. She also is theHead Varsity Gymnastics Coachat Merrill High School.

George J. Davies is the Clinicaland Research Services Director atGundersen Lutheran Sports Med-icine in Onalaska, Wisconsin. Heis also a professor in the PhysicalTherapy Department at the Uni-versity of WisconsinLa Crosse, LaCrosse, Wisconsin.

Neitzel

Davies