2005 jospt - popliteus complex rehabilitation

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Anatomy, Function, and Rehabilitation othe Popliteus Musculotendinous Comple John Nyland, PT, EdD, SCS, ATC, FACSM1

Narusha Lachman, PhD 2

Yavuz Kocabey, MD 3

Joseph Brosky, PT, MS, SCS 4

Remziye Altun, MD 5

David Caborn, MD 6

We present a clinical commentary of existing evidence regarding popliteus musculotendinouscomplex anatomy, biomechanics, muscle activation, and kinesthesia as they relate to functionalknee joint rehabilitation. The popliteus appears to act as a dynamic guidance system formonitoring and controlling subtle transverse- and frontal-plane knee joint movements, controllinganterior-posterior lateral meniscus movement, unlocking and internally rotating the knee joint(tibia) during flexion initiation, assisting with 3-dimensional dynamic lower extremity posturalstability during single-leg stance, preventing forward femoral dislocation on the tibia duringflexed-knee stance, and providing for postural equilibrium adjustments during standing. Thesefunctions may be most important during mid-range knee flexion when capsuloligamentousstructures are unable to function optimally. Because the popliteus musculotendinous complex hasattachments that approximate the borders of both collateral ligaments, it has the potential forproviding instantaneous 3-dimensional kinesthetic feedback of both medial and lateraltibiofemoral joint compartment function. Enhanced popliteus function as a kinesthetic knee jointmonitor acting in synergy with dynamic hip muscular control of femoral internal rotation andadduction, and ankle subtalar muscular control of tibial abduction-external rotation or adduction-internal rotation, may help to prevent athletic knee joint injuries and facilitate recovery duringrehabilitation by assisting the primary sagittal plane dynamic knee joint stabilization provided bythe quadriceps femoris, hamstrings, and gastrocnemius. J Orthop Sports Phys Ther 2005;35:165 -179.

Key Words: knee, lateral meniscus, lower extremity

W eight acceptance during walking commonly involves

tibial internal rotation as the knee joint flexes. 29,34,58

Concurrently, the knee joint generally undergoes a

small but important amount of abduction.29,35

In conjunction with these kinematics the resultant line of

force during walking is located primarily in the medial joint compart-

1 Assistant Professor, Division of Sports Medicine, Department of Orthopaedic Surgery, University of Louisville, Louisville, KY; Frazier Rehabilitation Institute, Louisville, KY.2 Assistant Professor, Department of Human Biology, Tecnikon Natal, Durban, South Africa.3 Research Fellow, Division of Sports Medicine, Department of Orthopaedic Surgery, University of Louisville, Louisville, KY.4 Associate Professor, Bellarmine University, Louisville, KY.5 Visiting Professor, Hospital of Sanliurfa, Sanliurfa, Turkey.6 Professor, Division of Sports Medicine, Department of Orthopaedic Surgery, University of Louisville,Louisville, KY.Address correspondence to John Nyland, Division of Sports Medicine, Department of Orthopaedic Surgery, University of Louisville, 210 East Gray Street, Suite 1003, Louisville, KY 40202. E-mail: john.nyland@louisville .edu

ment in the nonimpairedknee. 18,44 Appropriate lateral me-niscus orientation is essential toavoid impingement as the knee joint flexes and the tibia internally rotates during weight acceptanceor as the knee joint extends andthe tibia externally rotates duringpropulsion.

Common weight-bearing mecha-nisms of noncontact posterolateral

knee joint injury are either a di-rect varus force, while the tibia isexternally rotated, or a suddenforced knee hyperextension withthe tibia internally rotated. 6,8,56

Clinical signs of posterolateralknee joint injury may be subtleand are often masked by the moreextensive symptoms associated withanterior cruciate ligament (ACL)or posterior cruciate ligament (PCL) injury. 31 Combined injury of the popliteus muscle-tendoncomplex (PMTC) and lateral (fibu-lar) collateral ligament (LCL) re-sults in serious posterolateral kneeinstability, which, if unrecognized,contributes to postsurgical cruciateligament reconstruction failure orchronic knee instability. 17,27,30,70

According to Last, 32 popliteusactivation primarily internally ro-tates the knee and its tendinousbands retract the posterior arch of the lateral meniscus. Lateral me-niscus movement guidance by the

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ligaments of Humphrey and Wrisberg and the menis-cal fibers of the popliteus helps to prevent meniscalinjury. 32 The PMTC contributes to both static anddynamic posterolateral knee joint stabilization. 32,62,66

During concentric activation, the popliteus internally rotates the tibia on the femur. During eccentricactivation, it serves as a secondary restraint to tibialexternal rotation on the femur. 50 Higgins 23 proposedthat the popliteus either caused the tibia to internally rotate on the fixed femur or it assisted with femoralexternal rotation on the fixed tibia during weight bearing. Based on the work of Versalius, 68 modelingtechniques were used by Furst 12 and Fuss 13 to con-firm that in the sagittal plane the popliteus muscle isnot a knee flexor, but instead provides a smallextensor function through the flexion-extensionrange of motion (only conceivably serving a flexorfunction at hyperextension angles of greater than orequal to 30) in addition to its transverse-plane roleas a tibial internal rotator or a femoral external

rotator. These findings suggest that the popliteusserves a more essential functional role in the transverse plane.

Because knee joint injury frequently displays somecomponent of transverse-plane rotation and thepopliteus muscle has been described as an important,primary, dynamic, transverse-plane, rotatory knee joint stabilizer, 1,2,32,50 improving our understanding of its function in relation to other posterolateral knee joint structures would be beneficial. The purpose of this clinical commentary is to summarize existingevidence regarding PMTC anatomy, biomechanics,muscle activation characteristics, and kinesthesia, andrelate these findings to functional rehabilitation.Functional rehabilitation is operationally defined asthe use of therapeutic exercises to simulate the weight-bearing and nonweight-bearing componentsof specific daily activities in a manner that replicates3-dimensional lower extremity function within joint ranges and velocities that facilitate the desired physi-ological results (improved neuromuscular responsive-ness and connective tissue integrity).

POPLITEUS ANATOMY AND BIOMECHANICSDuring weight bearing, the tibiofemoral joint has

distinctly differing functions at its medial and lateralcompartments. The arthrological characteristics cre-ated by the longer and larger medial femoral condyledictates a preeminence for compression load control,primarily during sagittal plane motion, while theshorter and smaller lateral femoral condyle dictates apreeminence for tensile load control, primarily froma transverse- and frontal-planemovement perspec-tive.18,44 Both osseous and capsuloligamentous struc-tures within each tibiofemoral joint compartment support these functions. Increased tibial size, gener-ally flatter and broader shaped capsuloligamentous

structures,18,32,47

and a less mobile, larger meniscus at

the medial tibiofemoral joint are suggestive of aprimary compressive loading function. 32,42,47 In con-trast, the smaller lateral tibiofemoral joint has moreropelike primary capsuloligamentous structures, sug-gesting a predominant tensile loading function, 18,44

and a highly mobile meniscus, suggesting the pres-ence of more varied rotatory loads. 32,42

The popliteus originates from the lateral femoralcondyle near the LCL and inserts along the proximal10 to 12 cm of the posteromedial tibial surface,forming the floor of the popliteal fossa. Some of itsdistal fibers are interconnected with fascial fibersattached to the distal region of the medial (tibial)collateral ligament (MCL). By attaching into theirtendon at an angle oblique to the resultant line of pull, popliteus muscle fibers enable uniform forcedistribution over a greater area. 2 Popliteus architec-ture assessments by Wickiewicz et al 72 and Lieber, 36

however, suggest that the ratio of cross-sectional area

to muscle fiber length of the popliteus only enablesforce production over a relatively short distance.Higgins 23 suggested that the horizontal groove cre-ated by the popliteus tendon along the lateral femo-ral condyle was formed by a bowstring effect frompopliteus muscle activation during mid-range kneeflexion.

To appreciate how the PMTC (Figure 1), lateralmeniscus, arcuate ligament, posterior capsule, andthe ligaments of Wrisberg and Humphrey contributeto knee joint stability it is important to understandthe intricacy of their attachments. Watanabe et al 71

identified 7 variants for anatomic popliteus attach-

FIGURE 1. Popliteus muscle, tendon, and popliteomeniscal fas-cicles (LCL, lateral collateral ligament).

LCL

Popliteus

Popliteomeniscalfascicles

TendonMuscle

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FIGURE 2. Popliteus musculotendinous complex (PMTC) and ad-joining structures. (LCL, lateral collateral ligament; PFL,popliteofibular ligament; MCL, medial collateral ligament; PCL,posterior cruciate ligament.)

ments to the fibular head in addition to the primary popliteus tendon attachment to the proximal fifth of the popliteal sulcus of the lateral femoral condyle.The PMTC has major attachments to the lateralfemoral condyle, the fibula, and the posterior hornof the lateral meniscus, and smaller attachments tothe arcuate ligament complex, the oblique poplitealligament, the ligaments of Wrisberg and Humphrey,and the PCL (Figure 2). 25 Two or 3 (anteroinferior,posteroinferior, and posterosuperior) clearly delin-eated but highly variable popliteomeniscal fascicleattachments blend into the lateral meniscus to helpcontrol its motion (Figure 3). 26,56,57,63,71 Variations inpopliteomeniscal fascicular attachments are believedto reflect differences in embryonic knee joint devel-opment. 23,71 Tria et al 65 in a dissection of 40cadaveric knees reported that 82.5% of the kneesthey evaluated failed to display any major attachment between the popliteus tendon and the lateral menis-cus. They reported that only 7 specimens (17.5%)

displayed a strong dual attachment to both the lateral

femoral condyle and the lateral meniscus, 15 speci-mens (37.5%) had both an attachment to the lateralfemoral condyle and a filmy, almost translucent attachment to the lateral meniscus, and 18 specimens(45%) had an isolated popliteus tendon insertion tothe lateral femoral condyle, with no connection tothe lateral meniscus. These results suggest that thecapacity for the PMTC to directly influence lateralmeniscus movement is highly variable between individuals.

At the popliteus musculotendinous junction thereare 2 popliteofibular ligament (PFL) divisions that course laterally and distally, attaching on theposteromedial aspect of the fibular styloid (Figure 4).In addition to providing noncontractile restraint totibial external rotation, the PFL serves as a pulley,helping to tether the tendon during popliteus activa-tion. 60 Fuss13 reported that the PFL is under maxi-mum tension during flexion, possibly taking over the

noncontractile knee joint stabilization function of theLCL, which is not taut in most flexion positions.During in vitro biomechanical testing, Maynard et

al40 reported a maximum load at failure of approxi-mately 425 N for the PFL compared to 750 N for theLCL. Because a mean force of greater than 400 N wasneeded to achieve PFL failure in cadaveric knees of individuals greater than 70 years of age, they con-cluded that it was an important noncontractile stabi-lizing structure. Krudwig et al 28 reported that 50 N of PMTC tension produced increases of 4 to 5 of tibialinternal rotation as the knee neared full extensionand increases of up to 12 at 90 of knee flexion.During cyclic biomechanical testing following sequen-tial PFL and LCL transection, they reported gradually

FIGURE 3. Popliteomeniscal fascicles.

MCL

Medialmeniscus

LCL

PCL

Ligament ofWrisberg

Ligament ofHumphrey

PFL

Popliteus

Lateralmeniscus

Popliteustendon

Popliteomeniscal

fascicles

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FIGURE 4. Popliteofibular ligament (PFL) divisions.

increased tibial external rotation and a lateral shift of the position of neutral tibial alignment. 28 By addition-ally cutting the popliteus tendon, substantially greaterexternal rotation and a more externally rotatedneutral tibial rotation position were noted. Using 10cadaveric knee specimens, Harner et al 17 reportedthat the addition of a 44-N force to the popliteusmuscle reduced PCL forces by 9% and 36% at 90and 30 of knee flexion, respectively. Consideringthese results, progressive PCL deficiency should beanticipated, following isolated and untreatedposterolateral capsuloligamentous, or PMTC injury.

Krudwig et al28

suggested that isolated posterolateralcapsuloligamentous injury should be reconstructed toprotect the PCL from overstress. Veltri et al 67 re-ported that cutting the PFL, after having cut the LCL with the popliteus tendon intact, produced only smalladditional external tibial rotation increases (0.9 versus 1.9). However, when the PFL was cut last,after the LCL and the popliteus tendon had beencut, 7 to 10 increases in tibial external rotation were reported. They concluded that both thepopliteal tendon and the PFL were important toprevent excessive tibial external rotation and poste-rior translation. 67 Shahane et al 60 reported that isolated popliteus muscle sectioning did not causesignificant posterolateral knee joint instability; how-ever, PFL sectioning produced 3 and 9 increases intibial external rotation at 60 and 90 of knee flexion,respectively, in addition to increased posterior transla-tion. They concluded that the PFL was the primary noncontractile restraint to tibial external rotation andthe LCL was the secondary restraint.

Recently, Pasque et al 50 suggested that the order of tissue transection influenced the results reported by Shahane et al. 60 When controlling for cutting order,Pasque et al 50 reported that isolated PFL sectioning

did not produce increased tibial external rotation

between 30 and 12 of knee flexion, while isolatedPFL sectioning and sectioning of the femoral attach-ment of the popliteus tendon produced a small(5-6) increase in external rotation in that samerange (30-12) of knee flexion. Even when the otherligamentous structures were cut first, cutting the PFLproduced only negligible tibial external rotation in-creases. Pasque et al 50 emphasized that because theorientation of each noncontractile, posterolateralcapsuloligamentous knee joint component changes with progressive flexion, the PMTC, LCL, and otherposterolateral structures must function together as a3-dimensional load-sharing unit to resist tibial exter-nal rotation and varus loading. In a similarbiomechanical study that controlled for the order of tissue cutting, Gollehon et al 15 reported that the LCLand the popliteus-arcuate ligament complex func-tioned together as the principal noncontractile struc-tures that prevent tibial varus and external rotation at all knee flexion angles. Nielsen et al 46 reported that the LCL and the posterolateral part of the knee joint capsule resisted tibial varus and external rotation, with the former having a greater role preventingtibial varus and the later having a greater rolepreventing excessive tibial external rotation.

During the initial 30 of knee flexion, the LCLprovides a greater contribution to resisting tibial varus and the PMTC provides a greater contributionto resisting tibial external rotation and posteriortranslation. 45 As the posterolateral knee joint capsuleslackens with increasing knee flexion, it contributesless to resisting tibial external rotation (transverse

plane), varus rotation (frontal plane), and posteriortranslation. Nielsen et al 45 also reported that thepopliteal tendon provided maximal resistance to ex-cessive tibial external rotation between 20 and 130of knee flexion and to excessive tibial varus rotationbetween 0 and 90 of knee flexion. Due to theinfluence of knee joint angle on capsuloligamentoustightness, the contractile component of the PMTCsubsumes a greater dynamic responsibility for provid-ing knee joint stability as knee flexion angles in-crease. Pasque et al 50 recommended that surgicalinterventions should address each of theseposterolateral capsuloligamentous structures individu-ally because the absence of load sharing between allcomponents may lead to residual instability andunacceptably high loads.

Wang et al 70 and others 43 have reported that current popliteus tendon surgical techniques tend torestore only static or noncontractile function. Ide-ally, surgical PMTC repair should produce improveddynamic function in addition to a slight tenodesiseffect on adjacent capsuloligamentous tissues. Im-proving our understanding of PMTC function may aid the development of knee injury prevention condi-tioning programs and functional rehabilitation ap-

proaches for patients who display posterolateral knee

Popliteus

Soleus

PFL



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joint instability either in isolation or in combination with cruciate ligament injury.

POPLITEUS MUSCLE FUNCTIONElectromyographic study of popliteus activation

requires the use of intramuscular electrodes. Thissection will review electromyographic investigations

performed during nonweight-bearing and weight-bearing activities. The use of categorical popliteusactivity grading in 2 of these studies 2,39 and thelimited use, 2,39 or absence, 1,10,55 of concurrent kine-matic assessment in many of these studies makes it difficult to derive definitive conclusions.

Activation During Non-Weight BearingIn testing right-side popliteus activity in combina-

tion with electrogoniometric measurements,Basmajian and Lovejoy 2 reported popliteus activationlevels during isometric knee extensor or flexor activa-tion at differing knee flexion angles (0, 5, 20, 45,and 60), with the tibia either in full internalrotation, full external rotation, or in neutral align-ment. Popliteus activation levels were reported as apercentage of the maximal values produced by eachsubject during testing. During seated isometric kneeextension with the tibia maintained in full internalrotation, the greatest popliteus muscle activationlevels were observed between 60 and 20 of kneeflexion, and decreased as full extension was reached. With subjects positioned in prone, beginning with theknee in full extension, popliteus activation markedly

increased over the initial 20 of knee flexion whenthe lower leg was maintained in a full internalrotation position. Activation levels gradually de-creased as 90 of knee flexion was reached. 2 Duringboth knee extension and flexion isometric contrac-tion, popliteus activation remained constant with low amplitudes when tested with the tibia in full externalrotation positions. 2

Mann and Hagy 39 categorized popliteus activity collected in synchrony with a motion picture, usinga 1-to-4 categorical rating scale (1, slight; 2, moder-ate; 3, marked; 4, very marked). In their study,subjects performed a series of 6 tasks in a consistent order: (1) internal and external lower leg rotation insitting, (2) seated knee extension and flexion withneutral lower leg rotation, (3) internal and externallower extremity rotation of the nonweight-bearing,lower extremity with the knee extended duringcontralateral stance on a small box, (4) internal andexternal lower extremity rotation of the weight-bearing lower extremity with the knee extendedduring unilateral stance, (5) squatting, and (6) nor-mal pace walking, followed by walking with internally or externally rotated lower legs. 39 They reported 4+popliteus activity during seated, nonweight-bearing

lower-leg internal rotation, and trace activity with

external rotation. During seated nonweight-bearingflexion/extension, 4+ popliteus activity was reachedonly near full extension. During unilateral stance, 4+popliteus activation amplitudes were also observed forthe nonweight-bearing lower extremity during inter-nal rotation of the lower extremity with the kneeextended. 39 Repeated studies using quantitative intra-muscular electromyographic techniques and concur-rent segmental 3-dimensional kinematic and kineticassessments are needed during the performance of functionally relevant tasks.

In evaluating 4 patients with anterolateral kneerotatory instability, Peterson et al 53 reported in-creased popliteus activity during volitional knee joint pivot shift tests. In evaluating the popliteus activity of 10 patients with posterolateral knee instability, who were capable of volitional tibial subluxation, Shino et al61 reported that the biceps femoris muscle createdthe major tibiofemoral joint subluxation force andthe popliteus created the major joint reduction force.

They concluded that popliteus activation was thedynamic key to the treatment of posterolateral knee joint instability.

Activation During Weight BearingIn a detailed biomechanical analysis of transverse-

plane knee joint muscle moment arms, using 17cadaveric hemipelvis specimens, Buford et al 3 identi-fied a mechanical advantage for tibial external rota-tors over internal rotators throughout the flexion-extension range of motion. The external rotationmoment arms of the long and short heads of biceps

femoris peaked near full external rotation. Themoment arms for tibial internal rotators, thesemimembranosus and semitendinosus, peaked near10 of internal rotation, while the gracilis andsartorius moment arms remained constant through-out the internal-external rotation range of motion. Asa tibial internal rotator, the popliteus displayed asmall moment arm that peaked near neutraltransverse-plane alignment. All other transverse-planetibial rotators displayed maximum moment armlengths with the knee flexed 70 to 90. In contrast,the popliteus displayed its maximum moment arm at 30 to 50 of flexion, essentially when the LCL, PFL,and ITB 14 were no longer capable of providingoptimal noncontractile knee joint postural control.

Using intramuscular electrodes, Prado Reis andFerraz de Carvalho 55 reported that the popliteus wasmost active during standing, when the ACL and PCLbecame uncrossed and relaxed during relative inter-nal tibial rotation, and particularly with the kneeflexed between 30 and 50. This relaxed cruciateligament position brings the knee joint to a criticalpoint of poor noncontractile tissue contributions to joint stability. At this interval, popliteus muscle activa-tion serves as a dynamic knee joint guidance substi-

tute for the action of crossed and tensed cruciate


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ligaments. A 30 to 50 knee joint flexion alignment correlates with the position commonly assumed withsudden stopping during running and cutting activi-ties.33,37,48

Barnett and Richardson 1 observed consistent popliteus activation when subjects assumed a crouch-ing or knee-bent standing posture. This activation was believed to assist the PCL with preventing ante-rior femoral dislocation on the fixed tibia. Similaractivation was not noted during standing on ex-tended knees when dislocation was not threateningthe joint. 1 Del Torto 10 observed that the popliteusdisplayed 2 primary activation phases during walkingand stair climbing: (1) to produce tibial internalrotation during swing phase (a concentric action),and (2) to brake or halt tibial external rotationduring stance phase (an eccentric action). Withsubjects in standing with their knees completely extended, and then with their knees flexed to 30,Basmajian and Lovejoy 2 reported constant right-sidepopliteus muscle activation when subjects rotatedtheir right shoulder anteriorly (presumably rotatingtheir body to the left), regardless of whether the feet were in a neutral, toed-in, or toed-out alignment. When the left shoulder was rotated anteriorly (pre-sumably rotating the body to the right), with the feet positioned in a toed-out alignment, right-sidepopliteus activity increased. With the feet in neutralor toed-in alignment, right-side popliteus activity lev-els were greatest with the subjects knees flexed.Right-side popliteus activity levels were consistently greater during left-shoulder rotation than during

right-shoulder rotation, with the feet in the samepositions. Using a 1-to-4 categorical rating of left-sidepopliteus activity, Prado Reis and Ferraz deCarvalho 55 reported increased popliteus activity whensubjects performed anterior-posterior weight shifting,or when they experienced loss of standing balance.Prado Reis and Ferraz de Carvalho 55 confirmed thefindings of Basmajian and Lovejoy 2 with increasedpopliteus activation when the trunk was rotatedtoward the side of the examined muscle duringstanding, particularly when the femur tended toexternally rotate while the tibia was maintained ininternal rotation. This movement created a compositeinternal rotation of the lower leg at the knee joint, 2presumably with concentric popliteus muscle activa-tion. Mann and Hagy 39 reported that, during unilat-eral stance with the knee extended, 4+ popliteusactivation amplitudes were observed at the weight-bearing lower extremity during external trunk rota-tion and medial hip rotation. 39 With internal trunkrotation and lateral hip rotation, 2+ popliteus activity was observed during unilateral stance at the weight-bearing extremity at maximum rotation. 39 Squattingdown and returning to an upright position produced3+ popliteus activity during the entire movement

cycle.

During level walking at 1.2 to 3.2 km/h, withnormal, toed-in, or toed-out gait patterns, Basmajianand Lovejoy 2 reported that popliteus activity wasgreatest at heel contact, and between foot-flat andtoe-off, regardless of gait pattern. Mann and Hagy 39

reported greatest popliteus activity during the early part of stance phase (0%-12%), presumably as aresponse to increased subtalar joint forces as the tibiainternally rotated on the femur, and at the end of swing phase. Perry 52 reported that popliteus activa-tion occurs during all walking gait-cycle phases, ex-cept during initial swing and midswing, withconsiderable variability between subjects. The largest amplitude popliteus activity she reported, based onpercentage of maximal manual muscle test values,occurred during terminal swing, the loading re-sponse, and preswing. 52 The variability in popliteusactivation levels reported by Perry 52 suggests that aprimarily sagittal plane locomotion pattern per-

formed at walking velocity may not provide the most relevant environment for studying a muscle that conceivably displays greater importance during move-ments that challenge frontal- and transverse-planeknee joint function. In combining electromyographicand kinematic techniques during level and downhill walking, with and without an 18.14-kg (40-lb) back-pack, Davis et al7 reported more than doubledpopliteus activity at midstance with only a slight increase in knee flexion during weighted downhill(23.5) walking compared to level (16.5) walking.Increased popliteus activity at midstance during

weighted downhill walking was believed to be inresponse to increased weight bearing on a flexedknee. 7 The finding of Davis et al 7 that popliteusdisplayed considerable activation during midstance with weighted downhill walking, as compared tostanding or level walking, suggests that it may also beconsiderably active during the forceful loads associ-ated with other activities, such as running downhill.Considering the findings of Buford et al, 3 becausethe popliteus muscle displays a maximum transverse-plane moment arm at 30 to 50 of knee flexion when noncontractile knee joint stabilizers do not provide optimal knee joint postural control, it may also serve an important function during the perfor-mance of athletic movements, such as running direc-tional changes.

Given the coupled movement of tibial externalrotation/posterior translation and tibial internalrotation/anterior translation, 11 the popliteus is ideally positioned to assist with 3-dimensional dynamic knee joint control by monitoring and controlling tibialexternal rotation and, consequently, posterior transla-tion during eccentric function (thereby protectingthe PCL), and by producing tibial internal rotationand posterior translation during concentric function

(thereby protecting the ACL).



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TABLE 1. Absolute and relative muscle spindle density of select lower extremity musculature.69

MuscleSpindles/Muscle

Weight (g)

Composite RelativeMuscle-

SpindleDensityRatio

(Popliteus:FunctionalMuscle Group)

Direct tibial internal rotators 7.85:1.18Semimembranosus 0.60Semitendinosus 1.40Sartorius 1.20Gracilis 1.50

Tibial internal rotators via the subtalar joint 7.85:2.99Extensor digitorum longus 3.73Fibularis (peroneus) longus 1.88Fibularis (peroneus) brevis 3.37

Tibial external rotators via the subtalar joint 7.85:2.16Tibialis anterior 2.02Tibialis posterior 1.64Flexor hallucis longus 1.70Flexor digitorum longus 2.94Extensor hallucis longus 3.73

Soleus (medial calcaneal insertion) 0.94Knee extensors 7.85:0.83Vastus medialis 0.80Vastus intermedius 0.90Vastus lateralis 0.70Rectus femoris 0.90

Femoral external rotators 7.85:2.45Gluteus maximus 0.80Gemellus inferior 3.40Gemellus superior 3.90Piriformis 3.50Quadratus femoris 1.90Sartorius 1.20

MUSCULOTENDINOUS KINESTHESIAThe term musculotendinous kinesthesia refers to

the capacity for musculotendinous structures to con-tribute to proprioception through the activation of muscle spindles and golgi tendon organs. Becausethe PMTC has connective tissue attachments that approximate the borders of both the MCL and LCL,it is ideally positioned for providing instantaneous3-dimensional kinesthetic feedback, helping to moni-tor medial and lateral tibiofemoral joint compart-ment function. For example, during a runningdirectional change excessive MCL tensile stress andincreased lateral tibiofemoral compartment compres-sion may produce immediate PMTC activation tofacilitate tibiofemoral joint internal rotation and ma-neuver the lateral meniscus to a functionally effectiveand protected position. Concurrently, eccentricpopliteus activation during excessive LCL tensilestresses and increased medial compartment compres-sion associated with knee flexion and internal rota-tion at the end of weight acceptance during gait may provide the primary kinesthetic cues to the centralnervous system (CNS) to facilitate a knee joint extensor/external rotation response to prevent knee

joint injury.

Based on the extensive work of Voss, 69 Peck et al 51

proposed that in the extremities, smaller muscles withhigh muscle spindle concentrations, arranged inparallel with larger, less spindle-dense muscles, func-tion primarily as kinesthetic monitors. The examplethey cited was a 3.71:0.67 relative muscle-spindledensity ratio (RMSD) (muscle spindles per gram of muscle weight) between the human plantaris and thetriceps surae muscles. Comparisons between thepopliteus muscle and other muscles that provide at least 1 of its functions (tibial internal rotation,modulation of tibial external rotation via eccentricactivation, femoral external rotation, knee extension)are presented in Table 1. With consideration forthese muscle spindle densities in combination, apreeminent sensory feedback role is suggested for thepopliteus to provide kinesthetic feedback to the CNSduring transverse-plane knee joint movements. 51,69

A predominant kinesthetic function for thepopliteus is supported by several comparative basicscience studies. 16,41,54 In a muscle spindle study of the cat knee joint, McIntyre et al 41 confirmed thepresence of slowly adapting popliteus muscle spindlesthat discharged tonically when the knee joint was

positioned in intermediate flexed positions or during


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rotation and external tibial rotation, and frontal-plane hip joint adduction and knee joint abductionare associated with genu valgus. Transverse-planeexternal femoral rotation and internal tibial rotation,and frontal-plane hip joint abduction and knee joint adduction are associated with genu varus. Therefore,dynamic frontal-plane hip joint control via the abduc-tor (gluteus medius) and adductor musculature may also influence knee joint position. From this perspec-tive, there may be considerable normal variability inhow subjects of differing lower extremity posturalalignments achieve 3-dimensional dynamic lower ex-tremity postural stability during the performance of functional movements, particularly during sudden,single lower extremity loading. The key should be totrain patients to perform tasks such as these using atechnique that is natural for them, while avoiding thetissue stresses associated with moving too far into agenu varus or valgus posture, or too fast if they havepoor 3-dimensional dynamic lower extremity posturalstability.

Delp et al, 9 using a 3-dimensional computer modelbased on the cadaveric moment arm measurementsof several hip muscles at varying hip flexion angles,reported that, in general, hip internal rotation mo-ment arms increased and hip external rotation mo-ment arms decreased with increasing hip flexion. Thegluteus maximus had the greatest capacity for provid-ing a hip external rotation moment, particularly forthe posterior fibers up to approximately 50 of hipflexion. 9 This finding suggests the need for furtherstudy of the capacity for training the hip extensor

and external rotator musculature (particularly gluteusmaximus) in synergy with knee and ankle joint musculature to facilitate enhanced long-axis femoraland tibial motion control during athletic movements.Synergistic gluteus maximus (sagittal- and transverse-plane) and gluteus medius (frontal-plane) functionduring single lower extremity loading, as previously described, may be essential to attaining effective3-dimensional dynamic lower extremity postural sta-bility when quadriceps femoris and hamstring musclegroup function is suboptimal due to excessive genu valgus.

In designing exercises to improve 3-dimensionaldynamic lower extremity postural stability, the clini-cian should consider postural differences betweenpatients, common, single, lower extremity loadingpathomechanics, hip, knee, and ankle joint positionsfor optimal muscle moment arm lengths, the inter-play between global and local proprioceptive mecha-nisms, and the concept of rehabilitating movementsthat facilitate the development of synergistic lowerextremity muscle function. Considering the role of the PMTC as a kinesthetic monitor, we provide aprogressive functional rehabilitation strategy to im-prove integrated 3-dimensional dynamic lower ex-

tremity postural stability.

FIGURE 6. (A) Initiation of popliteus musculotendinous complex(PMTC) exercise with resistance band attached to the forefoot of thenonweight-bearing lower extremity. (B) The foot on the nonweight-bearing side moves behind the stance lower extremity viaipsilateral hip external rotation and knee flexion. (C) The foot on thenonweight-bearing side continues to move behind the stance lowerextremity with increasing internal tibial rotation. (D) Completion of the concentric muscle action phase of the PMTC exercise. Return to

start position provides eccentric muscle action.

A

B

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We suggest a multiplanar task progression that provides a PMTC training stimulus during bothintegrated nonweight-bearing and weight-bearinglower extremity function. An example of a non weight-bearing exercise is standing in unilateral kneeflexion, in conjunction with tibial internal rotationand hip external rotation, performed either actively or with a resistive band (Figure 6A-D). To makeefficient use of elastic resistance, quick concentricactivation should be followed by a slower eccentricactivation.

For weight-bearing exercises we have the patient begin with 1 foot positioned on a 5.1- to 15.2-cm (2-to 6-in) step with skidproof surfaces (creating ap-proximately 20 to 40 knee flexion at the stanceleg), the patient performs a series of stepping tasks.The task is initiated by moving the lower extremity that is off of the step forward (Figure 7A) to a

FIGURE 7. (A) Initiation of stepping task on a 5.1-cm (2-in) stepwith the nonweight-bearing lower extremity actively rotated poste-rior and lateral to the stance lower extremity. (B) First of 3successive quick loading-unloading steps (crossover diagonal). (C)Return to begin push-off for the next quick loading-unloading step.(D) Second of 3 successive quick loading-unloading steps (forward).(E) Last of 3 successive quick loading-unloading steps (side diago-nal).

crossover foot plant position to the right (crossoverdiagonal) (Figure 7B). After quickly returning to thestarting position (Figure 7C), the next forward move-ment ends with the foot planted straight ahead(Figure 7D). After returning to the starting position,the final forward movement in 1 standard cycle,places the foot to the left (side diagonal) (Figure7E). Following completion of this series, the lowerextremities should switch positions. Conceivably, theon-step lower extremity functions in a manner similarto that of athletic movement stance phase, particu-larly at midstance, while the off-step lower extremity experiences sudden loads that replicate initial andterminal stance phase transitions, leading into swingphase. Task speed can be progressively increased andspontaneous responses can be achieved by having thepatient respond to random cues to direct the plant foot to the right, straight ahead, or to the left.Concurrent use of different size and weight balls for

catching and tossing may increase task specificity andserve as a distractor to better assess the patients trueability to maintain appropriate, well-controlled,3-dimensional dynamic lower extremity postural sta-bility. Visual denial using a blindfold may also beuseful to further challenge these capabilities.

When the patient is able to maintain a level pelvis with minimal trunk lean and hip adduction (frontalplane), and controlled femoral and tibial rotation(transverse plane) within mid-range hip and kneeflexion (sagittal plane) suggestive of adequate3-dimensional dynamic lower extremity postural sta-

bility on the step, the same progression can beperformed using variable-sized unstable surfaces (Fig-ure 8). The final component of this task series uses aseries of 3 cones and a zig-zag hopping progression. While standing to the side of the first cone (Figure9A), the patient hops off 1 foot (example of involvedright side) and either lands on the opposite foot fora concentric muscle activation bias (Figure 9B) or onthe same foot (Figure 9C) for a concentric toeccentric muscle activation bias (sudden stop). Toemphasize stretch-shortening cycle function, theconcentric-eccentric muscle activation bias movement can be followed by a quick hop to the opposite side,landing on the same foot (Figure 9D). The finalphase of this movement involves the performance of a series of 3 hops using alternating lower extremitiesfollowed by a series of 3 hops using the same lowerextremity (Figure 9E) to further challenge thestretch-shortening cycle. Both the patient and theclinician cri tique the appropriateness of 3-dimensional dynamic lower extremity postural sta-bility during both tasks, but especially during thehopping task. An example of qualitative criteriabasedon frontal- and sagittal-plane observation 38 of single- or double-leg hop performance by the clini-

cian is presented in Table 2. Information gathered

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FIGURE 8. Stepping task performed on an unstable surface (DynaDisc; Exertools Inc, Novato, CA).

from this type of assessment can be used as (1) aninjury prevention screening tool, (2) to identify functional movement deficiencies early in rehabilita-tion, and (3) to record the patients performancebehavior at the end of the intervention. As with thestepping task, verbal or visual cues can be used toadd spontaneity to the activity (including suddenstopping-starting and retromovements) to furtherchallenge 3-dimensional dynamic lower extremity pos-

tural stability.

No studies to date have evaluated the influence of knee joint postural alignment on popliteus activity or3-dimensional knee joint kinematics during weight-bearing activities. In general, with a genu varus/internal tibial torsion knee joint alignment, theposterolateral capsuloligamentous structures (LCLand PFL) and the iliotibial band (ITB) 14,18,44 wouldtend to be preloaded, while the posteromedialcapsuloligamentous structures, including the MCL, would tend to be preloaded with a genu valgus/external tibial torsion knee joint postural align-ment. 18,44 Femoral external rotation during early stance phase (among individuals with a genu varus/internal tibial torsion) and via femoral internal rota-tion (among individuals with a genu valgus/externaltibial torsion) may enable more effective mainte-nance of naturally balanced knee joint capsuloligamentous and popliteus musculotendinouslength-tension relationships. These examples repre-sent opposite ends of a postural continuum that may

FIGURE 9. (A) Starting position for the single-leg hopping task (tobegin on either right or left foot following cue). (B) Contralateral (leftfoot, noninvolved side) landing following right (involved) lowerextremity quick diagonal hop (right lower extremity concentricmuscle action bias). (C) Ipsilateral (right foot, involved side) landingfrom right (involved) lower extremity quick diagonal hop (rightlower extremity eccentric muscle action bias). Progression from thismovement occurs in Figure 9D. (D) Right foot (involved side) quickdiagonal hop to ipsilateral landing (stretch-shortening cycle bias). (E)Completion of series of 3 consecutive right lower extremity quickdiagonal hops beginning with initial right (involved) foot take-off.

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TABLE 2. Sample qualitative scoring form to grade frontal and sagittal plane body alignment during single- or double-leg hop or jumplandings.

Frontal Plane Sagittal Plane

Eye and Head Alignment

Head centered, eyeslooking forward

Head to one side,eyes looking for-ward

Head to one side,eyes looking downat feet

Head up, eyes look-ing forward

Head slightly down,eyes looking downat feet

Head down, eyeslooking down at feet

2 1 0 2 1 0Trunk Alignment

Well centered trunk Slight trunk leanduring landing

Excessive trunk leanduring landing

Slightly flexed,chest over knees

Excessively flexed,collapse with land-ing

Extended, not usinghip extensors

2 1 0 2 1 0

Arm Alignment

Symmetrical withslight, controlledarm-swing (abduc-tion), with low guard

Symmetrical withmoderate, con-trolled arm-swing(abduction), withlow guard

Asymmetrical withpoorly controlledarm swing (abduc-tion) with highguard

Symmetrical withslight, controlledarm-swing (flexion)with low guard

Symmetrical withmoderate controlledarm-swing (flexion)with low guard

Asymmetrical withpoorly controlledarm-swing (flexion)with high guard

2 1 0 2 1 0

Hip-Thigh Alignment

Symmetrical withalignment over feetwithout excessiveadduction or abduc-tion during con-trolled, soft landing

Symmetrical withmoderate adductionor abduction duringcontrolled, softlanding

Asymmetrical ad-duction or abduc-tion, knees touch orflare outward (ex-treme coxa varus orvalgus) during apoorly controlledlanding

Symmetrical withmoderate hip flex-ion during con-trolled soft landing

Symmetrical withexcessive hip flex-ion during con-trolled soft landing

Asymmetrical orwith excessive orminimal hip flexionduring poorly con-trolled landing

2 1 0 2 1 0

Knee-Leg Alignment

Symmetrical align-

ment over feet with-out visible wobbleor sway during con-trolled, soft landing

Symmetrical abduc-

tion or adduction,slight wobble orsway during con-trolled, soft landing

Asymmetrical ab-

duction or adduc-tion, knees touch orflare outward (ex-treme genu valgusor varus) noted dur-ing a poorly con-trolled landing

Symmetrical with

moderate knee flex-ion during con-trolled soft landing

Symmetrical with

excessive knee flex-ion during con-trolled soft landing

Asymmetrical or

with excessive orminimal knee flexionduring poorly con-trolled landing

2 1 0 2 1 0

Ankle-Foot Alignment

Symmetrical withfeet aligned withtoes pointing forwardor slightly toed outduring controlled,soft landing

Symmetrical withfeet moderatelytoed out or toed induring controlled,soft landing

Asymmetrical withone or both feet,extremely toed outor toed in, or a sec-ondary hop duringa poorly controlledlanding

Symmetrical withmoderate ankledorsiflexion duringcontrolled soft land-ing

Symmetrical withexcessive ankledorsiflexion duringcontrolled soft land-ing

Asymmetrical orwith excessive orminimal ankledorsiflexion duringpoorly controlledlanding

2 1 0 2 1 0

Total Frontal Plane Score = /12 Total Sagittal Plane Score = /12

Overall Qualitative Jump Landing Score = /24 = %

substantially affect a patients capacity for performingcertain athletic movements. Fortunately, more subtlerepresentations predominate, better enabling theclinician to effectively facilitate safer athletic move-ment patterns. Performance variability among

patients is to be expected, necessitating that the

clinician assist the patient to achieve his or her ownindividualized level of optimal 3-dimensional dynamiclower extremity postural stability.

Females more commonly display genu valgus/external tibial torsion, coxa varus/adduction, and

genu recurvatum postural alignments than males,37,49



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