patellar kinematics, part i

Patellar Kinematics, Part I:The Influence of Vastus MuscleActivity in Subjects With and WithoutPatellofemoral Pain

Background and Purpose. Reduced motor unit activity of the vastusmedialis muscle relative to the vastus lateralis muscle has been impli-cated as a cause of lateral patellar subluxation. The purpose of thisstudy was to assess the influence of vastus muscle motor unit activity onpatellar kinematics. Subjects. Twenty-three women (mean age526.8years, SD58.5, range514–46) with a diagnosis of patellofemoral painand 12 women (mean age529.1 years, SD55.0, range524–38) withoutpatellofemoral pain participated. Only female subjects were studiedbecause of potential biomechanical differences between sexes.Methods. Patellar kinematics (kinematic magnetic resonance imaging)and vastus muscle electromyographic (EMG) activity using indwellingelectrodes were measured during resisted knee extension. Measure-ments of medial and lateral patellar displacement and tilt obtainedfrom magnetic resonance images were correlated with normalizedvastus lateralis:vastus medialis oblique muscle and vastus lateralis:vastusmedialis longus muscle EMG ratios at 45, 36, 27, 18, 9, and 0 degreesof knee flexion using a stepwise regression procedure. Results. Thevastus lateralis:vastus medialis longus muscle EMG ratio contributed tothe prediction of lateral patellar displacement at 27 degrees of kneeflexion (r 5-.48), with increased vastus medialis longus muscle activitybeing associated with greater lateral patellar displacement. A similarinverse relationship was evident with lateral patellar tilt at 36, 27, 18,and 9 degrees of knee flexion. Conclusion and Discussion. Theseresults suggest that increased motor unit activity of the vastus medialismuscle appears to be associated with abnormal patellar kinematics inwomen, but it is not necessarily a cause of abnormal patellar kinemat-ics. [Powers CM. Patellar kinematics, part I: the influence of vastusmuscle activity in subjects with and without patellofemoral pain. PhysTher. 2000;80:956–964.]

Key Words: Electromyography, Magnetic resonance imaging, Patellar kinematics, Patellofemoral joint,

Quadriceps femoris muscle.

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Patellar instability is commonly thought to bethe result of unequal activity of the variouscomponents of the quadriceps femoris mus-cle.1–5 More specifically, lateral patellar sublux-

ation has been attributed to reduced motor unit activityof the vastus medialis muscle.6–8 Lieb and Perry9 sepa-rated the vastus medialis muscle of cadavers into 2functional components based on fiber orientation, withthe proximal longitudinal fibers being termed the vastusmedialis longus muscle (VML) and the distal obliquefibers being designated the vastus medialis oblique mus-cle (VMO). As a result of its more horizontal fiberorientation, they considered the VMO to be the primarymedial stabilizer of the patella. This premise has formedthe theoretical basis for exercises for patellofemoral pain(PFP) because improving VMO force is thought by someauthors3,10,11 to be essential in overcoming the lateralpull of the much larger vastus lateralis muscle (VL).

Despite the large emphasis on the VMO in the treatmentof PFP, assessment of VMO force production in vivo isnot possible. In lieu of this limitation, electromyography(EMG) has been used to establish the activity patterns ofthe vastus muscles with the rationale that decreasedactivity of the VMO relative to the VL is indicative ofcompromised medial patellar stability. Numerousresearchers7,8,12–15 have compared the EMG activity ofthe VMO with that of the VL. There is no generalconsensus, however, as to whether reduced motor unitactivity of the VMO exists in people with PFP or, more

importantly, whether it is predictive of abnormal patello-femoral joint function.

With the advent of kinematic magnetic resonance imag-ing (KMRI), quantification of patellar kinematicsthroughout an arc of resisted knee extension is possi-ble.16,17 This diagnostic technique has a distinct advan-tage over imaging procedures that do not allow for kneemovement because contributions of the extensor mech-anism to patellofemoral joint kinematics can beassessed.18

The purpose of this investigation was to assess theinfluence of vastus muscle activity (as determinedthrough EMG) on patellar tracking patterns in subjectswith and without PFP. I hypothesized that lateral dis-placement and lateral tilting of the patella would beassociated decreased vastus medialis muscle activity rel-ative to the VL.

Method

SubjectsTwenty-three women with a diagnosis of PFP and 12women without PFP participated in this study. Onlyfemale subjects were studied because of potential biome-chanical differences between sexes. Both groups weresimilar in age, height, and weight (Tab. 1). Age, height,and weight were found to be normally distributed withineach group and when data from both groups were

CM Powers, PT, PhD, is Director, Musculoskeletal Biomechanics Research Laboratory, and Assistant Professor, Department of Biokinesiology andPhysical Therapy, University of Southern California, 1540 E Alcazar St, CHP-155, Los Angeles, CA 90033 (USA) ([email protected]).

Dr Powers provided concept/research design, writing, data collection and analysis, subjects, project management, and fund procurement.

This study was approved for human subjects by the Los Amigos Research and Education Institute Inc of Rancho Los Amigos Medical Center,Downey, Calif.

This study was partially funded by a grant from the Foundation for Physical Therapy.

This article was submitted November 5, 1998, and was accepted May 29, 2000.

Physical Therapy . Volume 80 . Number 10 . October 2000 Powers . 957

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combined. No attempt was made to match each subjectspecifically for age, height, and weight, as there is noevidence in the literature to suggest that individuals ofdifferent ages, heights, and weights will demonstratedifferences in patellar kinematics.

The subjects with PFP were patients of the SouthernCalifornia Orthopaedic Institute who were deemed to beappropriate candidates by the treating physician. Priorto participation, all subjects with PFP were screened torule out ligamentous instability, internal derangement,and patellar tendinitis. Each subject’s pain originatedfrom the patellofemoral joint, and only patients withhistories relating to nontraumatic events were accepted.In addition, pain had to be readily reproducible with atleast 2 of the following activities: stair ascent or descent,squatting, kneeling, prolonged sitting, or isometricquadriceps femoris muscle contraction.2,19 Subjects wereexcluded from the study if they reported previous kneesurgery or a history compatible with acute traumaticpatellar dislocation.

Individuals comprising the comparison group wererecruited by word of mouth and were either employeesof Rancho Los Amigos Medical Center (Downey, Calif)or students from the University of Southern California.Subjects had to have no history or diagnosis of kneepathology or trauma and they had to be free of anycurrent knee pain. In addition, these subjects did notreport pain with any of the activities listed earlier.

Instrumentation

Kinematic magnetic resonance imaging. Kinematic mag-netic resonance imaging (KMRI) of the patellofemoraljoint was assessed with the transmit and receive quadra-ture body coil of a 1.5T magnetic resonance system*using a fast-spoiled GRASS pulse sequence.16 Axial-planeimaging was performed using the following parameters:time to repeat56.5 milliseconds, time to echo52.1milliseconds, number of excitations51.0, matrix size5256 3 128, field of view538 cm, flip angle530 degrees,

and a 7-mm section thickness with an interslice spacingof 0.5 mm.16 Acquisition time was 6 seconds to obtain 6images (ie, 1 image per second).

All imaging was performed using a specially constructed,nonferromagnetic positioning device† that permittedbilateral knee extension against resistance (in the proneposition) from 45 degrees of flexion to full extension(Fig. 1). The device was designed to allow uninhibitedmovement of the patellofemoral joint and natural rota-tion of the lower extremities. I believe that these designfeatures are important because patellar tracking may beinfluenced by tibial rotation.20

Resistance was provided through a pulley system with aconstant 30.5-cm lever arm. The design of the device wassuch that the application of the force was always perpen-dicular to the tibia to ensure a constant (isotonic) torquethroughout the entire range of motion.16 Weights con-structed of nonmagnetic, 316L series stainless steel‡

supplied the resistive force for this maneuver. Theseplates were placed on a movable carriage that wasattached to the pulley apparatus (Fig. 1).

Electromyography. Indwelling, fine-wire electrodeswere used to record the intensity of vastus muscleactivity. The electrodes were bipolar in configurationand were made of nylon-insulated 50-mm wire (nickel-chromium alloy). The wires were passed through thecannula of a 25-gauge hypodermic needle with the distalends staggered and folded over the needle tip asdescribed by Basmajian and DeLuca.21

After insertion into the muscle, the wires were secured toa plate that also contained a ground electrode and thesignals were fed directly into a differential amplifier/FMradio transmitter unit.§ The differential amplifier had acommon mode rejection ratio of 60 dB. The EMGsignals were then telemetered from the transmitter tothe receiver unit where the signal was band-pass filtered

* General Electric Medical Systems, 3200 N Grandview Ave, Waukesha, WI 54601.

† Captain Plastic, PO Box 27493, Seattle, WA 98125.‡ Esco Corp, 6415 E Corvette St, Los Angeles, CA 90242.§ Biosentry Telemetry Inc, 20270 Earl St, Torrance, CA 90503.

Table 1.Subject Characteristics

Subjects With Patellofemoral Pain(n523)

Subjects Without Patellofemoral Pain(n512)

PaX SD Range X SD Range

Age (y) 26.8 8.5 14–46 29.1 5.0 24–38 .38Height (cm) 165.6 7.2 151.3–177.1 168.4 8.0 153.6–183.5 .29Weight (kg) 62.2 9.1 42.0–82.7 61.2 8.0 48.7–74.1 .76

a Probability values based on independent t tests.

958 . Powers Physical Therapy . Volume 80 . Number 10 . October 2000

(150–1,000 Hz) and amplified to a gain of 1,000. Theraw signal was sampled and digitized by a DEC 11/23data acquisition computer.\ Each analog channel wassampled at 2,500 Hz.

ProcedureThis study involved 2 different testing sessions: KMRI todetermine patellar kinematics and EMG evaluation toassess the vastus muscle activity pattern. The EMG andKMRI data could not be collected simultaneously in ourstudy due to magnetic interference. Prior to testing, allprocedures were explained to each subject and writteninformed consent was obtained.

Kinematic magnetic resonance imaging. All imaging wasperformed at Tower Imaging Center in west Los Ange-les, Calif. Subjects were placed prone on the position-ing device with care taken to allow for natural lower-extremity rotation. After this position was achieved,Velcro straps# were used to secure the subjects’ thighand tibia to the positioning device. Resistance on thedevice was then set at 15% of body weight.

After familiarization with the knee extension apparatus,subjects were instructed to practice extending theirknees at a rate of approximately 9°/s. This rate ensured6 evenly spaced images throughout the 45-degree arc ofmotion (including the 45° position) and permitted

imaging at 45, 36, 27, 18, 9, and 0 degrees of kneeflexion. Approximation of this rate was made by theprincipal investigator (CMP) with the use of a stopwatch.

Once the subject was able to reproduce the desired rateof motion in a smooth and even manner, imagingcommenced. Subjects were instructed to initiate exten-sion upon verbal command and continue until fullextension had been reached. Imaging was done at 3different image planes to assess the entire excursion ofthe patella in relation to the trochlear groove (ie, 3 sliceswere obtained for each angle of knee flexion). Theseprocedures were repeated if the rate of knee extensionwas too fast or too slow, or not performed in a smoothmanner. In addition, assessment was repeated if 6 ade-quate images were not obtained. An adequate image wasone in which the medial and lateral borders of themidsection of the patella, the trochlear groove, and theposterior femoral condyles were well defined. Visualiza-tion of these landmarks was necessary for subsequentanalysis.

Electromyography. Following KMRI, all subjects under-went EMG analysis at the Pathokinesiology Laboratory,Rancho Los Amigos Medical Center. This analysis typi-cally occurred within 24 hours of the KMRI evaluation.

Sterilized, fine-wire electrodes were inserted into themid-belly of the VMO, VML, and VL, with electrodeplacement being confirmed by mild electrical stimula-tion. To allow for comparison of EMG intensity betweensubjects and muscles and to control for the variability ofelectrode placement, EMG data were normalized to theEMG data acquired during a maximal isometric kneeextension effort. This was done on a LIDO dynamome-ter** with the subject seated and the knee flexed to 60degrees. This position was used because women withoutmusculoskeletal impairment are thought to generate thegreatest extensor torque in this position and because thisposition is supposed to provide greater patellar stabili-zation within the trochlear groove.2,19 It would appear,therefore, that positioning subjects in 60 degrees of kneeflexion would minimize quadriceps femoris muscle inhi-bition resulting from the pain associated with patellarinstability.

Vastus muscle activity then was recorded during activeknee extension using the positioning device describedpreviously for the KMRI. Procedures for subject position-ing, setting of the device resistance, and familiarizationpractice were identical to those reported earlier. Toensure the same rate of knee extension during KMRI,signals from an electric goniometer positioned at theaxis of rotation of the knee were fed into an oscilloscope

\ Digital Equipment Corp, 146 Main St, Maynard, MA 01754-2571.# Velcro USA Inc, PO Box 5218, 406 Brown Ave, Manchester, NH 03108. ** Loredan Biomedical Corp, PO Box 1154, Davis, CA 95617.

Figure 1.Subject set-up on the nonferromagnetic positioning device used forimaging. A pulley system consisting of a one foot lever arm (back-ground) and a movable carriage (foreground) allowed resisted kneeextension from 45 degrees to full extension. This device was designed topermit uninhibited movement of the patella and natural rotation of thelower extremity. Velcro straps were used to secure the subject’s thighand tibia to the apparatus. Reprinted by permission of LippincottWilliams & Wilkins from Powers CM, Shellock FG, Beering TV, et al.Effect of bracing on patellar kinematics in patients with patellofemoraljoint pain. Med Sci Sports Exerc. 1999;31:1714–1720.


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to provide visual feedback (Fig. 2). Once the requestedrate of knee extension could be consistently achieved, 6seconds of EMG activity was recorded while performingthis maneuver. Data were collected during 5 trials.

Following the knee extension trials, the maximal isomet-ric muscle test on the LIDO dynamometer was repeated,with the maximal EMG activity being recorded. This wasdone in an effort to ensure that the intramuscularelectrodes had not been displaced during the testingprocedure. At no time during the course of this studywas electrode displacement observed.

Data Management

Kinematic magnetic resonance imaging. Prior to analy-sis, all images were screened to ascertain the midsectionof the patella (maximum patellar width) at each angle ofknee flexion. Once this was determined, measurementsfor these images were obtained. Only images containinga midpatella slice were analyzed.

To accurately assess patellofemoral joint relationships atthe various degrees of knee flexion, measures that wereindependent of the shape of the patella and the anteriorfemoral condyles were used.16 This was done to avoidmeasurement variability resulting from the continuallychanging contour of these structures when viewed atdifferent angles of knee flexion and to allow assessmentof patellar orientation when the intercondylar groovewas not well visualized. All measurements were madewith a custom-made, computer-assisted program andincluded assessment of medial and lateral patellar dis-placement, medial and lateral patellar tilt, and the sulcusangle.

Medial and lateral patellar displacement were deter-mined by the “bisect offset” measurement as describedby Stanford et al22 and modified by Brossmann et al.23

The bisect offset was measured by drawing a line con-necting the posterior femoral condyles and then project-ing a perpendicular line anteriorly through the deepestpoint (apex) of the trochlear groove (Fig. 3). This line

Figure 2.Experimental set-up used to assess vastus muscle activity using themagnetic resonance imaging positioning device. Subject positioningand resistance were the same as reported for kinematic magneticresonance imaging (KMRI). In order to ensure the same rate of kneeextension during the KMRI assessment, electrical signals from an electricgoniometer positioned at the axis of rotation of the knee were fed into anoscilloscope to provide visual feedback (background).

Figure 3.Method used to measure medial and lateral displacement using thebisect offset measurement. This was determined by drawing a lineconnecting the posterior femoral condyles (AB) and then projecting aperpendicular line anteriorly through the deepest portion of the trochleargroove (CD) to a point where it bisected the patellar width line (EF) (left).To obtain data when the trochlear groove was flattened, the perpendic-ular line was projected anteriorly from the bisection of the posteriorcondylar line (right). The bisect offset was reported as the percentage ofpatellar width lateral to the midline. Reprinted by permission of Lippin-cott Williams & Wilkins from Powers CM, Shellock FG, Beering TV, et al.Effect of bracing on patellar kinematics in patients with patellofemoraljoint pain. Med Sci Sports Exerc. 1999;31:1714–1720.

Figure 4.Method used to assess patellar tilt. Patellar tilt was defined as the angleformed by lines joining the maximum width of the patella (AB) and theposterior femoral condyles (BC). All tilt measurements were reported indegrees. Reprinted by permission of Lippincott Williams & Wilkins fromPowers CM, Shellock FG, Beering TV, et al. Effect of bracing on patellarkinematics in patients with patellofemoral joint pain. Med Sci SportsExerc. 1999;31:1714–1720.


intersected with the patellar width line, which connectedthe widest points of the patella. The perpendicular linewas projected anteriorly from the bisection of the poste-rior condylar line to obtain data when the trochleargroove was flattened (Fig. 3).16 All bisect offset datarepresented the extent of the patella lying lateral to theprojected perpendicular line and were expressed as apercentage of patellar width.

Medial and lateral patellar tilt were measured using amodification of the technique described by Sasaki andYagi.24 The patellar tilt angle was reported as the angleformed by the lines joining the maximum width of thepatella and the line joining the posterior femoral con-dyles (Fig. 4). All tilt measurements were reported indegrees.

Electromyography. Digitally acquired EMG data werefull-wave rectified and integrated over 0.25-second inter-vals. Intensities were reported as a percentage of theEMG data collected during the maximum isometricmuscle test.

Intensity of VL, VMO, and VML contraction was assessedat points in the range of motion that corresponded tothe angular position at which the magnetic resonanceimages were obtained. These data were further analyzedto obtain VL:VMO and VL:VML ratios.

Reliability of KMRI and EMG data. Because MRI andEMG data were not collected simultaneously, I believedthat it was particularly important to assess reliability ofthese measures in order to compare data between testingsessions. In addition, determination of the number oftrials to be averaged for consistent data was necessary. Toassess the reproducibility of the measurements, 7 sub-jects without PFP underwent repeated testing. All repeattesting took place within 24 hours of the initial testingsession, using the same procedures outlined earlier. Theday-to-day reliability of KMRI data, assessed using theprocedures and measurements described earlier, waspreviously reported to have intraclass correlation coeffi-cients (ICCs) ranging from .79 to .85.16 Intraobservermeasurement error was determined to be 3.4% for thebisect offset measurement and 2.9 degrees for patellar tilt.

Data AnalysisAll statistical procedures were performed with BMDPstatistical software.†† Prior to analysis, descriptive statis-tics were calculated for all variables, and normality ofdistribution was assessed using the Wilk-Shapiro test.Based on the analysis of distribution, all data wereanalyzed using parametric tests. All significance levelswere set at P,.05.

Reliability of the EMG measurements (VL:VMO andVL:VML ratios) and MRI measurements (bisect offsetand patellar tilt) was assessed using ICCs. Multipleone-way analyses of variance (ANOVAs) for repeatedmeasures were used to compare EMG ratios betweensessions at each designated angle of knee flexion. Themean squares between subjects and the mean squareswithin subjects were substituted into the ICC21 equa-tion described by Bartko.25 This analysis was repeated foreach measurement to obtain correlation coefficients foreach of the number of averaged values (ie, ICCs werecalculated for the data obtained by averaging 2 measure-ments for both sessions and were compared with ICCscalculated for data obtained by averaging 3, 4, and 5measurements). I determined that averaging dataobtained from 3 EMG trials produced the most consis-tent results (Tab. 2). Overall, moderate reliability inobtaining the VL:VMO and VL:VML ratios was evidentacross all angles of knee flexion (ICC values of .61 and.64, respectively).

To determine whether EMG ratios varied betweengroups or angles of knee flexion, a 2 3 6 (group 3angle) ANOVA for repeated measures on one variable(angle) was performed. This analysis was performed foreach EMG ratio. Main effects were reported if there wereno interactions. If an interaction was found, the individ-ual main effects were analyzed separately.

A regression analysis was performed to determinewhether the VL:VMO ratio or the VL:VML ratio waspredictive of patellar tilt or displacement. This analysiswas repeated for patellar tilt and displacement at eachangle of knee flexion. Because the presence of PFPcould potentially influence the relationship between thevariables, I deemed it necessary to account for this factorby including the grouping variable in all regressionequations. This type of analysis was used in an effort toensure that an overall relationship between the EMGratios and patellar kinematics could be ascertainedregardless of a diagnosis of PFP.

†† SPSS Inc, 444 N Michigan Ave, Chicago, IL 60611.

Table 2.Electromyography Trial Ratio Reliability: Intraclass CorrelationCoefficients (ICC21)25 (Averaged Across All Angles of Knee Flexion)a

No. of AveragedMeasurements VL:VMO VL:VML

1 .47 .542 .48 .623 .61 .644 .62 .615 .59 .63

a VL5vastus lateralis muscle, VMO5vastus medialis oblique muscle,VML5vastus medialis longus muscle.


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Results

ElectromyographyThere was no difference in the VL:VMO ratio betweenthe subjects with PFP and the subjects without PFP (nogroup effect or interaction) (Fig. 5). When averagedacross all knee flexion angles, the mean VL:VMO ratiowas 1.85 for the subjects with PFP compared with 1.17 forthe subjects without PFP. Similarly, there was no differ-ence in the VL:VML ratio between the subjects with PFP

and the subjects without PFP (no group effect or inter-action) (Fig. 6). When averaged across all angles of kneeflexion, the mean VL:VML ratio for the subjects with PFPwas .78 compared with .94 for the subjects without PFP.

Patellar KinematicsFor results and discussion concerning the comparison ofpatellar kinematic data between groups, the reader isreferred to the article by Powers titled “Patellar Kinemat-ics, Part II: The Influence of the Depth of the TrochlearGroove in Subjects With and Without PatellofemoralPain” in this issue.

Relationship Between EMG Ratios and Patellar KinematicsIn general, the Pearson correlation coefficients rangedfrom 2.48 to .37 for both bisect offset (Tab. 3) andpatellar tilt (Tab. 4). The VL:VML ratio was found to bethe only predictor of patellar tilt at 36 degrees (r 52.40,R25.16), 27 degrees (r 52.48, R25.24; Fig. 7), 18degrees (r 52.42, R25.18), and 9 degrees of flexion(r 52.45, R25.20). Similarly, the only EMG predictor ofthe bisect offset measurement was the VL:VML ratio at27 degrees of flexion (r 52.48), which accounted for24% (R2) of the variability (Fig. 8).

DiscussionThe EMG data obtained from the comparison groupwere relatively consistent, with the VL:VMO andVL:VML ratios averaging 1.17 and 0.94, respectively,across all knee flexion angles. This finding is consistentwith those of previous studies7,19,26 in which the activityof the VMO relative to the VL in subjects without pain isapproximately 1:1. The EMG data obtained from thesubjects with PFP, however, showed much greater vari-ability. The lack of statistical significance in the VL:VMOand VL:VML EMG ratios between groups may have been

Figure 5.Comparison of the vastus lateralis:vastus medialis oblique muscle (VL:VMO) electromyographic ratio between the subjects with patellofemoralpain (PFP) and the subjects without PFP from 45 to 0 degrees of kneeflexion. Error bars indicate one standard deviation.

Figure 6.Comparison of the vastus lateralis:vastus medialis longus muscle (VL:VML) electromyographic ratio between the subjects with patellofemoralpain (PFP) and the subjects without PFP from 45 to 0 degrees of kneeflexion. Error bars indicate one standard deviation.

Table 3.Pearson Correlation Coefficients for Bisect Offset (After Controlling forGroup)

IndependentVariablea

Knee Flexion Angle (°)

45 36 27 18 9 0

VL:VMO ratio .03 .10 2.04 .14 2.23 .13VL:VML ratio 2.02 2.22 2.48b 2.25 2.31 .30

a VL5vastus lateralis muscle, VMO5vastus medialis oblique muscle,VML5vastus medialis longus muscle.b Significant predictor of bisect offset (P,.05).

Table 4.Pearson Correlation Coefficients for Patellar Tilt (After Controlling forGroup)

IndependentVariablea

Knee Flexion Angle (°)

45 36 27 18 9 0

VL:VMO ratio .06 .14 .17 .02 2.22 .13VL:VML ratio 2.23 2.40b 2.48b 2.42b 2.45b .37

a VL5vastus lateralis muscle, VMO5vastus medialis oblique muscle,VML5vastus medialis longus muscle.b Significant predictor of patellar tilt (P,.05).


the result of a type II statistical error due to the highvariability of the subjects with PFP throughout the rangeof motion, variability that was 2 to 3 times greater thanfor the comparison group. A post hoc power analysisrevealed that the number of subjects in each group wasadequate to test the null hypothesis (no group effect), asthe statistical power to detect a 60% change was found tobe greater than 0.90.

The EMG ratio data obtained from the subjects with PFPsuggests that the motor unit activity of the VML and thatof the VMO were different during the knee extensionmaneuver. This difference was reflected by the observa-tion that the VL:VML ratio remained consistentthroughout knee extension, whereas the activity of theVMO (with respect to the VL) became more pro-nounced at terminal knee extension. This pattern ofEMG activity may be related to the fact that the VML isprimarily a knee extensor (as a result of a more longitu-dinal fiber orientation), whereas the VMO is much lessefficient in this role because of its oblique fiber arrange-ment.9 Because knee extension was the primary move-ment performed, I believe it is logical that the VMLwould be recruited more readily to accomplish this task.However, the fact that the EMG activity of the VMObecame more pronounced at the end-range of extensionemphasizes the function of this structure in providingmedial patellar stability, as it is at this point in the rangeof movement where maximum lateral displacement typ-ically occurs.2 Differences in the observed EMG activityof these 2 portions of the vastus medialis muscle com-pared with the VL suggests that this muscle may havevaried roles with respect to patellofemoral jointmechanics.

Regression analysis of the EMG and KMRI variablesrevealed that the VL:VMO EMG ratio was not predictiveof patellar motion at any point in the range of knee

flexion. In contrast, the VL:VML ratio was a predictor ofpatellar tilt at 36, 27, 18, and 9 degrees of flexion, as wellas of bisect offset at 27 degrees of flexion. All correla-tions involving the VL:VML ratios were negative, how-ever, indicating an inverse relationship between EMGactivity and patellar motion. For example, subjects withlower VL:VML ratios (increased VML activity relative tothe VL) were found to have greater degrees of lateralpatellar displacement and tilt, whereas subjects withhigher VL:VML ratios (decreased VML activity relative tothe VL) had less severe abnormalities. These results donot support the original hypothesis that decreased activ-ity of the vastus medialis muscle is a cause of patellarmalalignment. To the contrary, increased motor unitactivity of the vastus medialis muscle appeared to be inresponse to meeting the increased demand of providingpatellar stability. The fact that VML activity was increas-ing as the patella demonstrated greater malalignmentwas suggestive of an active, but inadequate, effort tocenter the patella within the trochlear groove.

A premise behind the use of EMG biofeedback toevaluate VMO activity is, in my opinion, that diminishedVMO EMG activity is indicative of abnormal patello-femoral joint function. The finding that VMO activitycould not be shown to be predictive of patellar kinemat-ics illustrates the limitations associated with the use ofEMG ratios as indictors of patellofemoral joint pathome-chanics. Although normalized EMG data are useful inmeasuring relative levels of activity between muscles(ie, intensity of effort), such information is not indicativeof muscular strength or “muscular balance,” as is com-monly assumed.27 Without considering factors such asmuscle length, cross-sectional area, and angle of inser-tion of the various muscle fibers, it would appear thatEMG has limited use in determining the effective muscleforce.27

Figure 7.Relationship between the vastus lateralis:vastus medialis longus muscle(VL:VML) electromyographic (EMG) ratio and patellar tilt for the subjectswith patellofemoral pain (PFP) and the subjects without PFP at 27degrees of knee flexion (r 52.48; F510.9; df52,33; P,.05).

Figure 8.Relationship between the vastus lateralis:vastus medialis longus muscle(VL:VML) electromyographic (EMG) ratio and bisect offset (percentageof patella lateral to midline) for the subjects with patellofemoral pain(PFP) and the subjects without PFP at 27 degrees of knee flexion(r 52.48; F54.6; df52,33; P,.05).


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The correlation coefficients in this study had R2 valuesthat were small (ranging from .16 to .24), indicating thatonly a small percentage of the variance in patellarkinematics could be explained by the EMG ratios.Although the inherent variability in EMG data combinedwith inability to precisely control the speed of kneeextension could have contributed to the low r values,further research should be directed toward identifyingadditional factors that can improve the predictability ofpatellofemoral joint kinematics.

As a result of the limitations imposed by the size of theMRI bore, the loading condition used in this study(non–weight bearing) was not consistent with the load-ing condition that would be evident with running orclimbing stairs or with our inability to take all of ourmeasurements simultaneously (EMG and KMRI).Although there have been no studies that have exam-ined the differences in patellar tracking patternsbetween weight-bearing and non–weight-bearing activi-ties, an argument can be made that the non–weight-bearing knee extension maneuver does not simulatemost functional tasks. Therefore, care must be taken ininterpreting the results of this study until differences inpatellar kinematics can be established between variousloading conditions.

ConclusionThe results of this study showed an inverse relationshipbetween the VL:VML EMG ratio and lateral patellar tiltat 36, 27, 18, and 9 degrees of knee flexion and lateralpatellar displacement at 27 degrees of knee flexion.Although increased activity of the vastus medialis musclerelative to the VL may be a response to patellar malalign-ment, decreased activity does not appear to be associatedwith abnormal patellar tracking. The premise that lateralpatellar displacement and tilt are the result of dimin-ished activity of the vastus medialis muscle is not sup-ported by this study.

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