1. Research Report
Patellar Kinematics, Part I:
The Influence of Vastus Muscle
Activity in Subjects With and Without
Patellofemoral Pain
Background and Purpose. Reduced motor unit activity of the vastus
medialis muscle relative to the vastus lateralis muscle has been impli-
cated as a cause of lateral patellar subluxation. The purpose of this
study was to assess the influence of vastus muscle motor unit activity on
patellar kinematics. Subjects. Twenty-three women (mean age 26.8
years, SD 8.5, range 14 – 46) with a diagnosis of patellofemoral pain
and 12 women (mean age 29.1 years, SD 5.0, range 24 –38) without
patellofemoral pain participated. Only female subjects were studied
because of potential biomechanical differences between sexes.
Methods. Patellar kinematics (kinematic magnetic resonance imaging)
and vastus muscle electromyographic (EMG) activity using indwelling
electrodes were measured during resisted knee extension. Measure-
ments of medial and lateral patellar displacement and tilt obtained
from magnetic resonance images were correlated with normalized
vastus lateralis:vastus medialis oblique muscle and vastus lateralis:vastus
medialis longus muscle EMG ratios at 45, 36, 27, 18, 9, and 0 degrees
of knee flexion using a stepwise regression procedure. Results. The
vastus lateralis:vastus medialis longus muscle EMG ratio contributed to
the prediction of lateral patellar displacement at 27 degrees of knee
flexion (r -.48), with increased vastus medialis longus muscle activity
being associated with greater lateral patellar displacement. A similar
inverse relationship was evident with lateral patellar tilt at 36, 27, 18,
and 9 degrees of knee flexion. Conclusion and Discussion. These
results suggest that increased motor unit activity of the vastus medialis
muscle appears to be associated with abnormal patellar kinematics in
women, but it is not necessarily a cause of abnormal patellar kinemat-
ics. [Powers CM. Patellar kinematics, part I: the influence of vastus
muscle activity in subjects with and without patellofemoral pain. Phys
Ther. 2000;80:956 –964.]
Key Words: Electromyography, Magnetic resonance imaging, Patellar kinematics, Patellofemoral joint,
Quadriceps femoris muscle.
Christopher M Powers
956 Physical Therapy . Volume 80 . Number 10 . October 2000

2. P
atellar instability is commonly thought to be importantly, whether it is predictive of abnormal patello-
the result of unequal activity of the various femoral joint function.
components of the quadriceps femoris mus-
cle.1–5 More specifically, lateral patellar sublux- With the advent of kinematic magnetic resonance imag-
ation has been attributed to reduced motor unit activity ing (KMRI), quantification of patellar kinematics
of the vastus medialis muscle.6 – 8 Lieb and Perry9 sepa- throughout an arc of resisted knee extension is possi-
rated the vastus medialis muscle of cadavers into 2 ble.16,17 This diagnostic technique has a distinct advan-
functional components based on fiber orientation, with tage over imaging procedures that do not allow for knee
the proximal longitudinal fibers being termed the vastus movement because contributions of the extensor mech-
medialis longus muscle (VML) and the distal oblique anism to patellofemoral joint kinematics can be
fibers being designated the vastus medialis oblique mus- assessed.18
cle (VMO). As a result of its more horizontal fiber
orientation, they considered the VMO to be the primary The purpose of this investigation was to assess the
medial stabilizer of the patella. This premise has formed influence of vastus muscle activity (as determined
the theoretical basis for exercises for patellofemoral pain through EMG) on patellar tracking patterns in subjects
(PFP) because improving VMO force is thought by some with and without PFP. I hypothesized that lateral dis-
authors3,10,11 to be essential in overcoming the lateral placement and lateral tilting of the patella would be
pull of the much larger vastus lateralis muscle (VL). associated decreased vastus medialis muscle activity rel-
ative to the VL.
Despite the large emphasis on the VMO in the treatment
of PFP, assessment of VMO force production in vivo is Method
not possible. In lieu of this limitation, electromyography
(EMG) has been used to establish the activity patterns of Subjects
the vastus muscles with the rationale that decreased Twenty-three women with a diagnosis of PFP and 12
activity of the VMO relative to the VL is indicative of women without PFP participated in this study. Only
compromised medial patellar stability. Numerous female subjects were studied because of potential biome-
researchers7,8,12–15 have compared the EMG activity of chanical differences between sexes. Both groups were
the VMO with that of the VL. There is no general similar in age, height, and weight (Tab. 1). Age, height,
consensus, however, as to whether reduced motor unit and weight were found to be normally distributed within
activity of the VMO exists in people with PFP or, more each group and when data from both groups were
CM Powers, PT, PhD, is Director, Musculoskeletal Biomechanics Research Laboratory, and Assistant Professor, Department of Biokinesiology and
Physical Therapy, University of Southern California, 1540 E Alcazar St, CHP-155, Los Angeles, CA 90033 (USA) (powers@hsc.usc.edu).
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

3. Table 1.
Subject Characteristics
Subjects With Patellofemoral Pain Subjects Without Patellofemoral Pain
(n 23) (n 12)
X SD Range X SD Range Pa
Age (y) 26.8 8.5 14–46 29.1 5.0 24–38 .38
Height (cm) 165.6 7.2 151.3–177.1 168.4 8.0 153.6–183.5 .29
Weight (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.
combined. No attempt was made to match each subject and a 7-mm section thickness with an interslice spacing
specifically for age, height, and weight, as there is no of 0.5 mm.16 Acquisition time was 6 seconds to obtain 6
evidence in the literature to suggest that individuals of images (ie, 1 image per second).
different ages, heights, and weights will demonstrate
differences in patellar kinematics. All imaging was performed using a specially constructed,
nonferromagnetic positioning device† that permitted
The subjects with PFP were patients of the Southern bilateral knee extension against resistance (in the prone
California Orthopaedic Institute who were deemed to be position) from 45 degrees of flexion to full extension
appropriate candidates by the treating physician. Prior (Fig. 1). The device was designed to allow uninhibited
to participation, all subjects with PFP were screened to movement of the patellofemoral joint and natural rota-
rule out ligamentous instability, internal derangement, tion of the lower extremities. I believe that these design
and patellar tendinitis. Each subject’s pain originated features are important because patellar tracking may be
from the patellofemoral joint, and only patients with influenced by tibial rotation.20
histories relating to nontraumatic events were accepted.
In addition, pain had to be readily reproducible with at Resistance was provided through a pulley system with a
least 2 of the following activities: stair ascent or descent, constant 30.5-cm lever arm. The design of the device was
squatting, kneeling, prolonged sitting, or isometric such that the application of the force was always perpen-
quadriceps femoris muscle contraction.2,19 Subjects were dicular to the tibia to ensure a constant (isotonic) torque
excluded from the study if they reported previous knee throughout the entire range of motion.16 Weights con-
surgery or a history compatible with acute traumatic structed of nonmagnetic, 316L series stainless steel‡
patellar dislocation. supplied the resistive force for this maneuver. These
plates were placed on a movable carriage that was
Individuals comprising the comparison group were attached to the pulley apparatus (Fig. 1).
recruited by word of mouth and were either employees
of Rancho Los Amigos Medical Center (Downey, Calif) Electromyography. Indwelling, fine-wire electrodes
or students from the University of Southern California. were used to record the intensity of vastus muscle
Subjects had to have no history or diagnosis of knee activity. The electrodes were bipolar in configuration
pathology or trauma and they had to be free of any and were made of nylon-insulated 50- m wire (nickel-
current knee pain. In addition, these subjects did not chromium alloy). The wires were passed through the
report pain with any of the activities listed earlier. cannula of a 25-gauge hypodermic needle with the distal
ends staggered and folded over the needle tip as
Instrumentation described by Basmajian and DeLuca.21
Kinematic magnetic resonance imaging. Kinematic mag- After insertion into the muscle, the wires were secured to
netic resonance imaging (KMRI) of the patellofemoral a plate that also contained a ground electrode and the
joint was assessed with the transmit and receive quadra- signals were fed directly into a differential amplifier/FM
ture body coil of a 1.5T magnetic resonance system* radio transmitter unit.§ The differential amplifier had a
using a fast-spoiled GRASS pulse sequence.16 Axial-plane common mode rejection ratio of 60 dB. The EMG
imaging was performed using the following parameters: signals were then telemetered from the transmitter to
time to repeat 6.5 milliseconds, time to echo 2.1 the receiver unit where the signal was band-pass filtered
milliseconds, number of excitations 1.0, matrix size
256 128, field of view 38 cm, flip angle 30 degrees,
†
Captain Plastic, PO Box 27493, Seattle, WA 98125.
‡
Esco Corp, 6415 E Corvette St, Los Angeles, CA 90242.
§
* General Electric Medical Systems, 3200 N Grandview Ave, Waukesha, WI 54601. Biosentry Telemetry Inc, 20270 Earl St, Torrance, CA 90503.
958 . Powers Physical Therapy . Volume 80 . Number 10 . October 2000

4. imaging at 45, 36, 27, 18, 9, and 0 degrees of knee
flexion. Approximation of this rate was made by the
principal investigator (CMP) with the use of a stopwatch.
Once the subject was able to reproduce the desired rate
of motion in a smooth and even manner, imaging
commenced. Subjects were instructed to initiate exten-
sion upon verbal command and continue until full
extension had been reached. Imaging was done at 3
different image planes to assess the entire excursion of
the patella in relation to the trochlear groove (ie, 3 slices
were obtained for each angle of knee flexion). These
procedures were repeated if the rate of knee extension
was too fast or too slow, or not performed in a smooth
manner. In addition, assessment was repeated if 6 ade-
Figure 1.
Subject set-up on the nonferromagnetic positioning device used for
quate images were not obtained. An adequate image was
imaging. A pulley system consisting of a one foot lever arm (back- one in which the medial and lateral borders of the
ground) and a movable carriage (foreground) allowed resisted knee midsection of the patella, the trochlear groove, and the
extension from 45 degrees to full extension. This device was designed to posterior femoral condyles were well defined. Visualiza-
permit uninhibited movement of the patella and natural rotation of the tion of these landmarks was necessary for subsequent
lower extremity. Velcro straps were used to secure the subject’s thigh
and tibia to the apparatus. Reprinted by permission of Lippincott
analysis.
Williams & Wilkins from Powers CM, Shellock FG, Beering TV, et al.
Effect of bracing on patellar kinematics in patients with patellofemoral Electromyography. Following KMRI, all subjects under-
joint pain. Med Sci Sports Exerc. 1999;31:1714 –1720. went EMG analysis at the Pathokinesiology Laboratory,
Rancho Los Amigos Medical Center. This analysis typi-
(150 –1,000 Hz) and amplified to a gain of 1,000. The cally occurred within 24 hours of the KMRI evaluation.
raw signal was sampled and digitized by a DEC 11/23
data acquisition computer. Each analog channel was Sterilized, fine-wire electrodes were inserted into the
sampled at 2,500 Hz. mid-belly of the VMO, VML, and VL, with electrode
placement being confirmed by mild electrical stimula-
Procedure tion. To allow for comparison of EMG intensity between
This study involved 2 different testing sessions: KMRI to subjects and muscles and to control for the variability of
determine patellar kinematics and EMG evaluation to electrode placement, EMG data were normalized to the
assess the vastus muscle activity pattern. The EMG and EMG data acquired during a maximal isometric knee
KMRI data could not be collected simultaneously in our extension effort. This was done on a LIDO dynamome-
study due to magnetic interference. Prior to testing, all ter** with the subject seated and the knee flexed to 60
procedures were explained to each subject and written degrees. This position was used because women without
informed consent was obtained. musculoskeletal impairment are thought to generate the
greatest extensor torque in this position and because this
Kinematic magnetic resonance imaging. All imaging was position is supposed to provide greater patellar stabili-
performed at Tower Imaging Center in west Los Ange- zation within the trochlear groove.2,19 It would appear,
les, Calif. Subjects were placed prone on the position- therefore, that positioning subjects in 60 degrees of knee
ing device with care taken to allow for natural lower- flexion would minimize quadriceps femoris muscle inhi-
extremity rotation. After this position was achieved, bition resulting from the pain associated with patellar
Velcro straps# were used to secure the subjects’ thigh instability.
and tibia to the positioning device. Resistance on the
device was then set at 15% of body weight. Vastus muscle activity then was recorded during active
knee extension using the positioning device described
After familiarization with the knee extension apparatus, previously for the KMRI. Procedures for subject position-
subjects were instructed to practice extending their ing, setting of the device resistance, and familiarization
knees at a rate of approximately 9°/s. This rate ensured practice were identical to those reported earlier. To
6 evenly spaced images throughout the 45-degree arc of ensure the same rate of knee extension during KMRI,
motion (including the 45° position) and permitted signals from an electric goniometer positioned at the
axis 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.
Physical Therapy . Volume 80 . Number 10 . October 2000 Powers . 959

5. Figure 4.
Method used to assess patellar tilt. Patellar tilt was defined as the angle
formed by lines joining the maximum width of the patella (AB) and the
posterior femoral condyles (BC). All tilt measurements were reported in
degrees. Reprinted by permission of Lippincott Williams & Wilkins from
Figure 2. Powers CM, Shellock FG, Beering TV, et al. Effect of bracing on patellar
Experimental set-up used to assess vastus muscle activity using the kinematics in patients with patellofemoral joint pain. Med Sci Sports
magnetic resonance imaging positioning device. Subject positioning Exerc. 1999;31:1714 –1720.
and resistance were the same as reported for kinematic magnetic
resonance imaging (KMRI). In order to ensure the same rate of knee
extension during the KMRI assessment, electrical signals from an electric Following the knee extension trials, the maximal isomet-
goniometer positioned at the axis of rotation of the knee were fed into an ric muscle test on the LIDO dynamometer was repeated,
oscilloscope to provide visual feedback (background).
with the maximal EMG activity being recorded. This was
done in an effort to ensure that the intramuscular
electrodes had not been displaced during the testing
procedure. At no time during the course of this study
was electrode displacement observed.
Data Management
Kinematic magnetic resonance imaging. Prior to analy-
sis, all images were screened to ascertain the midsection
of the patella (maximum patellar width) at each angle of
knee flexion. Once this was determined, measurements
for these images were obtained. Only images containing
a midpatella slice were analyzed.
To accurately assess patellofemoral joint relationships at
the various degrees of knee flexion, measures that were
independent of the shape of the patella and the anterior
Figure 3. femoral condyles were used.16 This was done to avoid
Method used to measure medial and lateral displacement using the
bisect offset measurement. This was determined by drawing a line measurement variability resulting from the continually
connecting the posterior femoral condyles (AB) and then projecting a changing contour of these structures when viewed at
perpendicular line anteriorly through the deepest portion of the trochlear different angles of knee flexion and to allow assessment
groove (CD) to a point where it bisected the patellar width line (EF) (left). of patellar orientation when the intercondylar groove
To obtain data when the trochlear groove was flattened, the perpendic-
ular line was projected anteriorly from the bisection of the posterior
was not well visualized. All measurements were made
condylar line (right). The bisect offset was reported as the percentage of with a custom-made, computer-assisted program and
patellar width lateral to the midline. Reprinted by permission of Lippin- included assessment of medial and lateral patellar dis-
cott Williams & Wilkins from Powers CM, Shellock FG, Beering TV, et al. placement, medial and lateral patellar tilt, and the sulcus
Effect of bracing on patellar kinematics in patients with patellofemoral angle.
joint pain. Med Sci Sports Exerc. 1999;31:1714 –1720.
Medial and lateral patellar displacement were deter-
mined by the “bisect offset” measurement as described
to provide visual feedback (Fig. 2). Once the requested by Stanford et al22 and modified by Brossmann et al.23
rate of knee extension could be consistently achieved, 6 The bisect offset was measured by drawing a line con-
seconds of EMG activity was recorded while performing necting the posterior femoral condyles and then project-
this maneuver. Data were collected during 5 trials. ing a perpendicular line anteriorly through the deepest
point (apex) of the trochlear groove (Fig. 3). This line
960 . Powers Physical Therapy . Volume 80 . Number 10 . October 2000

6. intersected with the patellar width line, which connected Table 2.
Electromyography Trial Ratio Reliability: Intraclass Correlation
the widest points of the patella. The perpendicular line
Coefficients (ICC 1)25 (Averaged Across All Angles of Knee Flexion)a
was projected anteriorly from the bisection of the poste-
rior condylar line to obtain data when the trochlear
No. of Averaged
groove was flattened (Fig. 3).16 All bisect offset data Measurements VL:VMO VL:VML
represented the extent of the patella lying lateral to the
projected perpendicular line and were expressed as a 1 .47 .54
percentage of patellar width. 2 .48 .62
3 .61 .64
4 .62 .61
Medial and lateral patellar tilt were measured using a 5 .59 .63
modification of the technique described by Sasaki and a
VL vastus lateralis muscle, VMO vastus medialis oblique muscle,
Yagi.24 The patellar tilt angle was reported as the angle VML vastus medialis longus muscle.
formed by the lines joining the maximum width of the
patella and the line joining the posterior femoral con-
dyles (Fig. 4). All tilt measurements were reported in
Reliability of the EMG measurements (VL:VMO and
degrees.
VL:VML ratios) and MRI measurements (bisect offset
and patellar tilt) was assessed using ICCs. Multiple
Electromyography. Digitally acquired EMG data were
one-way analyses of variance (ANOVAs) for repeated
full-wave rectified and integrated over 0.25-second inter-
measures were used to compare EMG ratios between
vals. Intensities were reported as a percentage of the
sessions at each designated angle of knee flexion. The
EMG data collected during the maximum isometric
mean squares between subjects and the mean squares
muscle test.
within subjects were substituted into the ICC 1 equa-
tion described by Bartko.25 This analysis was repeated for
Intensity of VL, VMO, and VML contraction was assessed
each measurement to obtain correlation coefficients for
at points in the range of motion that corresponded to
each of the number of averaged values (ie, ICCs were
the angular position at which the magnetic resonance
calculated for the data obtained by averaging 2 measure-
images were obtained. These data were further analyzed
ments for both sessions and were compared with ICCs
to obtain VL:VMO and VL:VML ratios.
calculated for data obtained by averaging 3, 4, and 5
measurements). I determined that averaging data
Reliability of KMRI and EMG data. Because MRI and
obtained from 3 EMG trials produced the most consis-
EMG data were not collected simultaneously, I believed
tent results (Tab. 2). Overall, moderate reliability in
that it was particularly important to assess reliability of
obtaining the VL:VMO and VL:VML ratios was evident
these measures in order to compare data between testing
across all angles of knee flexion (ICC values of .61 and
sessions. In addition, determination of the number of
.64, respectively).
trials to be averaged for consistent data was necessary. To
assess the reproducibility of the measurements, 7 sub-
To determine whether EMG ratios varied between
jects without PFP underwent repeated testing. All repeat
groups or angles of knee flexion, a 2 6 (group
testing took place within 24 hours of the initial testing
angle) ANOVA for repeated measures on one variable
session, using the same procedures outlined earlier. The
(angle) was performed. This analysis was performed for
day-to-day reliability of KMRI data, assessed using the
each EMG ratio. Main effects were reported if there were
procedures and measurements described earlier, was
no interactions. If an interaction was found, the individ-
previously reported to have intraclass correlation coeffi-
ual main effects were analyzed separately.
cients (ICCs) ranging from .79 to .85.16 Intraobserver
measurement error was determined to be 3.4% for the
A regression analysis was performed to determine
bisect offset measurement and 2.9 degrees for patellar tilt.
whether the VL:VMO ratio or the VL:VML ratio was
predictive of patellar tilt or displacement. This analysis
Data Analysis
was repeated for patellar tilt and displacement at each
All statistical procedures were performed with BMDP
angle of knee flexion. Because the presence of PFP
statistical software.†† Prior to analysis, descriptive statis-
could potentially influence the relationship between the
tics were calculated for all variables, and normality of
variables, I deemed it necessary to account for this factor
distribution was assessed using the Wilk-Shapiro test.
by including the grouping variable in all regression
Based on the analysis of distribution, all data were
equations. This type of analysis was used in an effort to
analyzed using parametric tests. All significance levels
ensure that an overall relationship between the EMG
were set at P .05.
ratios and patellar kinematics could be ascertained
regardless of a diagnosis of PFP.
††
SPSS Inc, 444 N Michigan Ave, Chicago, IL 60611.
Physical Therapy . Volume 80 . Number 10 . October 2000 Powers . 961

7. Figure 5. Figure 6.
Comparison of the vastus lateralis:vastus medialis oblique muscle (VL: Comparison of the vastus lateralis:vastus medialis longus muscle (VL:
VMO) electromyographic ratio between the subjects with patellofemoral VML) electromyographic ratio between the subjects with patellofemoral
pain (PFP) and the subjects without PFP from 45 to 0 degrees of knee pain (PFP) and the subjects without PFP from 45 to 0 degrees of knee
flexion. Error bars indicate one standard deviation. flexion. Error bars indicate one standard deviation.
Table 3. and the subjects without PFP (no group effect or inter-
Pearson Correlation Coefficients for Bisect Offset (After Controlling for action) (Fig. 6). When averaged across all angles of knee
Group) flexion, the mean VL:VML ratio for the subjects with PFP
was .78 compared with .94 for the subjects without PFP.
Knee Flexion Angle (°)
Independent
Variablea 45 36 27 18 9 0 Patellar Kinematics
For results and discussion concerning the comparison of
VL:VMO ratio .03 .10 .04 .14 .23 .13
VL:VML ratio .02 .22 .48b .25 .31 .30
patellar kinematic data between groups, the reader is
referred to the article by Powers titled “Patellar Kinemat-
a
VL vastus lateralis muscle, VMO vastus medialis oblique muscle, ics, Part II: The Influence of the Depth of the Trochlear
VML vastus medialis longus muscle.
b
Significant predictor of bisect offset (P .05). Groove in Subjects With and Without Patellofemoral
Pain” in this issue.
Table 4. Relationship Between EMG Ratios and Patellar Kinematics
Pearson Correlation Coefficients for Patellar Tilt (After Controlling for
In general, the Pearson correlation coefficients ranged
Group)
from .48 to .37 for both bisect offset (Tab. 3) and
patellar tilt (Tab. 4). The VL:VML ratio was found to be
Knee Flexion Angle (°)
Independent the only predictor of patellar tilt at 36 degrees (r .40,
Variablea 45 36 27 18 9 0 R2 .16), 27 degrees (r .48, R2 .24; Fig. 7), 18
VL:VMO ratio .06 .14 .17 .02 .22 .13 degrees (r .42, R2 .18), and 9 degrees of flexion
2
VL:VML ratio .23 .40b .48b .42b .45b .37 (r .45, R .20). Similarly, the only EMG predictor of
a
VL vastus lateralis muscle, VMO vastus medialis oblique muscle,
the bisect offset measurement was the VL:VML ratio at
VML vastus medialis longus muscle. 27 degrees of flexion (r .48), which accounted for
b
Significant predictor of patellar tilt (P .05). 24% (R2) of the variability (Fig. 8).
Discussion
Results The EMG data obtained from the comparison group
were relatively consistent, with the VL:VMO and
Electromyography VL:VML ratios averaging 1.17 and 0.94, respectively,
There was no difference in the VL:VMO ratio between across all knee flexion angles. This finding is consistent
the subjects with PFP and the subjects without PFP (no with those of previous studies7,19,26 in which the activity
group effect or interaction) (Fig. 5). When averaged of the VMO relative to the VL in subjects without pain is
across all knee flexion angles, the mean VL:VMO ratio approximately 1:1. The EMG data obtained from the
was 1.85 for the subjects with PFP compared with 1.17 for subjects with PFP, however, showed much greater vari-
the subjects without PFP. Similarly, there was no differ- ability. The lack of statistical significance in the VL:VMO
ence in the VL:VML ratio between the subjects with PFP and VL:VML EMG ratios between groups may have been
962 . Powers Physical Therapy . Volume 80 . Number 10 . October 2000

8. Figure 7. Figure 8.
Relationship between the vastus lateralis:vastus medialis longus muscle Relationship between the vastus lateralis:vastus medialis longus muscle
(VL:VML) electromyographic (EMG) ratio and patellar tilt for the subjects (VL:VML) electromyographic (EMG) ratio and bisect offset (percentage
with patellofemoral pain (PFP) and the subjects without PFP at 27 of patella lateral to midline) for the subjects with patellofemoral pain
degrees of knee flexion (r .48; F 10.9; df 2,33; P .05). (PFP) and the subjects without PFP at 27 degrees of knee flexion
(r .48; F 4.6; df 2,33; P .05).
the result of a type II statistical error due to the high
variability of the subjects with PFP throughout the range flexion. In contrast, the VL:VML ratio was a predictor of
of motion, variability that was 2 to 3 times greater than patellar tilt at 36, 27, 18, and 9 degrees of flexion, as well
for the comparison group. A post hoc power analysis as of bisect offset at 27 degrees of flexion. All correla-
revealed that the number of subjects in each group was tions involving the VL:VML ratios were negative, how-
adequate to test the null hypothesis (no group effect), as ever, indicating an inverse relationship between EMG
the statistical power to detect a 60% change was found to activity and patellar motion. For example, subjects with
be greater than 0.90. lower VL:VML ratios (increased VML activity relative to
the VL) were found to have greater degrees of lateral
The EMG ratio data obtained from the subjects with PFP patellar displacement and tilt, whereas subjects with
suggests that the motor unit activity of the VML and that higher VL:VML ratios (decreased VML activity relative to
of the VMO were different during the knee extension the VL) had less severe abnormalities. These results do
maneuver. This difference was reflected by the observa- not support the original hypothesis that decreased activ-
tion that the VL:VML ratio remained consistent ity of the vastus medialis muscle is a cause of patellar
throughout knee extension, whereas the activity of the malalignment. To the contrary, increased motor unit
VMO (with respect to the VL) became more pro- activity of the vastus medialis muscle appeared to be in
nounced at terminal knee extension. This pattern of response to meeting the increased demand of providing
EMG activity may be related to the fact that the VML is patellar stability. The fact that VML activity was increas-
primarily a knee extensor (as a result of a more longitu- ing as the patella demonstrated greater malalignment
dinal fiber orientation), whereas the VMO is much less was suggestive of an active, but inadequate, effort to
efficient in this role because of its oblique fiber arrange- center the patella within the trochlear groove.
ment.9 Because knee extension was the primary move-
ment performed, I believe it is logical that the VML A premise behind the use of EMG biofeedback to
would be recruited more readily to accomplish this task. evaluate VMO activity is, in my opinion, that diminished
However, the fact that the EMG activity of the VMO VMO EMG activity is indicative of abnormal patello-
became more pronounced at the end-range of extension femoral joint function. The finding that VMO activity
emphasizes the function of this structure in providing could not be shown to be predictive of patellar kinemat-
medial patellar stability, as it is at this point in the range ics illustrates the limitations associated with the use of
of movement where maximum lateral displacement typ- EMG ratios as indictors of patellofemoral joint pathome-
ically occurs.2 Differences in the observed EMG activity chanics. Although normalized EMG data are useful in
of these 2 portions of the vastus medialis muscle com- measuring relative levels of activity between muscles
pared with the VL suggests that this muscle may have (ie, intensity of effort), such information is not indicative
varied roles with respect to patellofemoral joint of muscular strength or “muscular balance,” as is com-
mechanics. monly assumed.27 Without considering factors such as
muscle length, cross-sectional area, and angle of inser-
Regression analysis of the EMG and KMRI variables tion of the various muscle fibers, it would appear that
revealed that the VL:VMO EMG ratio was not predictive EMG has limited use in determining the effective muscle
of patellar motion at any point in the range of knee force.27
Physical Therapy . Volume 80 . Number 10 . October 2000 Powers . 963

9. The correlation coefficients in this study had R2 values 7 Souza DR, Gross MT. Comparison of vastus medialis obliquus:vastus
that were small (ranging from .16 to .24), indicating that lateralis muscle integrated electromyographic ratios between healthy
subjects and patients with patellofemoral pain. Phys Ther. 1991;71:
only a small percentage of the variance in patellar 310 –316.
kinematics could be explained by the EMG ratios.
8 Wise HH, Fiebert IM, Kates JL. EMG biofeedback as treatment for
Although the inherent variability in EMG data combined
patellofemoral pain syndrome. J Orthop Sports Phys Ther. 1984;6:95–103.
with inability to precisely control the speed of knee
extension could have contributed to the low r values, 9 Lieb FJ, Perry J. Quadriceps function: an anatomical and mechanical
study using amputated limbs. J Bone Joint Surg Am. 1968;50:1535–1548.
further research should be directed toward identifying
additional factors that can improve the predictability of 10 LeVeau BF, Rogers C. Selective training of the vastus medialis
muscle using EMG biofeedback. Phys Ther. 1980;60:1410 –1415.
patellofemoral joint kinematics.
11 McConnell J. The management of chondromalacia patellae: a long
As a result of the limitations imposed by the size of the term solution. Australian Journal of Physiotherapy. 1986;32:215–223.
MRI bore, the loading condition used in this study 12 Boucher JP, King MA, Lefebvre R, Pepin A. Quadriceps femoris
(non–weight bearing) was not consistent with the load- muscle activity in patellofemoral pain syndrome. Am J Sports Med.
1992;20:527–532.
ing condition that would be evident with running or
climbing stairs or with our inability to take all of our 13 MacIntyre DL, Robertson DG. Quadriceps muscle activity in women
measurements simultaneously (EMG and KMRI). runners with and without patellofemoral pain syndrome. Arch Phys Med
Rehabil. 1992;73:10 –14.
Although there have been no studies that have exam-
ined the differences in patellar tracking patterns 14 Moller BN, Krebs B, Tidemand-Dal C, Aaris K. Isometric contrac-
between weight-bearing and non–weight-bearing activi- tions in the patellofemoral pain syndrome: an electromyographic
study. Arch Orthop Trauma Surg. 1986;105:24 –27.
ties, an argument can be made that the non–weight-
bearing knee extension maneuver does not simulate 15 Wild JJ Jr, Franklin TD, Woods GW. Patellar pain and quadriceps
rehabilitation: an EMG study. Am J Sports Med. 1982;10:12–15.
most functional tasks. Therefore, care must be taken in
interpreting the results of this study until differences in 16 Powers CM, Shellock FG, Pfaff M. Quantification of patellar track-
patellar kinematics can be established between various ing using kinematic MRI. J Magn Reson Imaging. 1998;8:724 –732.
loading conditions. 17 Shellock FG, Mink JH, Deutsch AL, Pressman BD. Kinematic
magnetic resonance imaging of the joints: techniques and clinical
applications. J Magn Reson Imaging. 1991;7:104 –135.
Conclusion
The results of this study showed an inverse relationship 18 Shellock FG, Mink JH, Deutsch AL, Foo TK. Kinematic MR imaging
between the VL:VML EMG ratio and lateral patellar tilt of the patellofemoral joint: comparison of passive positioning and
active movement techniques. Radiology. 1992;184:574 –577.
at 36, 27, 18, and 9 degrees of knee flexion and lateral
patellar displacement at 27 degrees of knee flexion. 19 Powers CM, Landel R, Perry J. Timing and intensity of vastus muscle
activity during functional activities in subjects with and without patel-
Although increased activity of the vastus medialis muscle lofemoral pain. Phys Ther. 1996;76:946 –955.
relative to the VL may be a response to patellar malalign-
20 van Kampen A, Huiskes R. The three-dimensional tracking pattern
ment, decreased activity does not appear to be associated
of the human patella. J Orthop Res. 1990;8:372–382.
with abnormal patellar tracking. The premise that lateral
patellar displacement and tilt are the result of dimin- 21 Basmajian JV, DeLuca CJ. Muscles Alive: Their Functions Revealed by
Electromyography. 5th ed. Baltimore, Md: Williams & Wilkins; 1985.
ished activity of the vastus medialis muscle is not sup-
ported by this study. 22 Stanford W, Phelan J, Kathol MH, et al. Patellofemoral joint
motion: evaluation by ultrafast computed tomography. Skeletal Radiol.
1988;17:487– 492.
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964 . Powers Physical Therapy . Volume 80 . Number 10 . October 2000