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journal of orthopaedic & sports physical therapy | volume 44 | number 10 | october 2014 | 785
[ research report ]
Running is a popular form of exercise, with approximately 36
million people engaging in this activity in the United States
alone.36 Despite the positive health eects associated with
running, a high incidence of lower extremity running injuries
has been reported (19%-79%).21,40,43,44 Among these injuries, half
occur at the knee joint, with patellofemo-
ral pain (PFP) being the most common
diagnosis.40,43
It has been proposed that PFP is the
result of elevated patellofemoral joint
(PFJ) stress.10,16 As stress is defined as
force per unit area, elevated PFJ stress
could occur as a result of an increase in
the PFJ reaction force and/or a decrease
in contact area between the patella and
the trochlear groove of the femur. In turn,
an increase in the PFJ reaction force
would occur with an increase in the knee
extensor moment and/or knee flexion
angle.4,16,23 Additionally, PFJ contact area
increases with knee flexion and decreases
with knee extension.32,37
To reduce risk of PFP in runners,
modification of the foot strike pattern
has been proposed. Specifically, convert-
ing from a rearfoot to a forefoot or mid-
foot strike pattern has been promoted as
a means to reduce the peak impact force,
loading rate, and knee extensor mo-
ment.5,8,12,18,19,38 In a case series, Cheung
and Davis12 reported that individuals with
PFP exhibited improvements in pain af-
ter transitioning to a nonrearfoot strike
pattern. Furthermore, barefoot running,
which typically results in a forefoot or
midfoot strike pattern,5,25,46 has been re-
ported to decrease peak PFJ stress by 12%
in asymptomatic runners.6 Although cur-
rent literature supports the use of modi-
fying foot strike pattern to reduce PFJ
loading, several studies have reported
that adopting a forefoot and/or midfoot
strike pattern leads to increased loading
STUDY DESIGN: Cross-sectional, repeated-
measures.
OBJECTIVES: To examine the association
between sagittal plane trunk posture and patel-
lofemoral joint (PFJ) stress, and to determine
whether modifying sagittal plane trunk posture
influences PFJ stress during running.
BACKGROUND: Patellofemoral pain is the most
common injury among runners and is thought to
be the result of elevated PFJ stress. While sagittal
plane trunk posture has been shown to influence
tibiofemoral joint mechanics, no study has exam-
ined the influence of trunk posture on PFJ kinetics.
METHODS: Twenty-four asymptomatic
recreational runners (12 women, 12 men) ran
overground at a speed of 3.4 m/s under 3 trunk-
posture conditions: self-selected, flexed, and
extended. Trunk and knee kinematics, ground reac-
tion forces, and electromyographic signals from
selected lower extremity muscles were obtained. A
previously described PFJ biomechanical model was
used to quantify PFJ stress.
RESULTS: The mean SD trunk flexion angles
under the self-selected, flexed, and extended
running conditions were 7.3° 3.6°, 14.1° 4.8°,
and 4.0° 3.9°, respectively. A significant inverse
relationship was observed between mean trunk
flexion angle and peak PFJ stress during the self-
selected condition (r = –0.60, P = .002). Peak PFJ
stress was significantly lower in the flexed condi-
tion (mean SD, 20.2 3.4 MPa; P<.001) and
significantly higher in the extended condition (23.1
3.4 MPa; P<.001) compared to the self-selected
condition (21.5 3.2 MPa).
CONCLUSION: Sagittal plane trunk posture
has a significant influence on PFJ kinetics dur-
ing running. Incorporation of a forward trunk
lean may be an eective strategy to reduce PFJ
stress during running. J Orthop Sports Phys
Ther 2014;44(10):785-792. Epub 25 August 2014.
doi:10.2519/jospt.2014.5249
KEY WORDS: anterior knee pain,
chondromalacia, patella
1Division of Biokinesiology and Physical Therapy, University of Southern California, Los Angeles, CA. 2Department of Radiology and Biomedical Imaging, University of California
San Francisco, San Francisco, CA. This study was approved by the Health Science Institutional Review Board of the University of Southern California. This study was partially
funded by the International Society of Biomechanics Dissertation Grant. The authors certify that they have no aliations with or financial involvement in any organization or
entity with a direct financial interest in the subject matter or materials discussed in the article. Address correspondence to Dr Hsiang-Ling Teng, University of California San
Francisco, Department of Radiology and Biomedical Imaging, 185 Berry Street, Suite 350, San Francisco, CA 94107. E-mail: Hsiang-Ling.Teng@ucsf.edu Copyright ©2014
Journal of Orthopaedic & Sports Physical Therapy®
HSIANG-LING TENG, PT, PhD1,2 • CHRISTOPHER M. POWERS, PT, PhD1
Sagittal Plane Trunk Posture
Influences Patellofemoral Joint
Stress During Running
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[ research report ]
at the ankle plantar flexors.2,5,8,19,31,35,46
Recent studies suggest that sagittal
plane trunk posture may be associated
with tibiofemoral joint biomechanics
during weight-bearing activities. For ex-
ample, a forward trunk lean posture has
been found to be associated with lower
knee extensor moments during walking,
stair ascent, and single-leg-hop land-
ing.3,24,30 Based on these findings, modi-
fying sagittal plane trunk posture may
provide an alternative means to reduce
PFJ stress during running.
Using a previously described biome-
chanical PFJ model,9,10,13,20 the purpose
of the current study was 3-fold. First, we
sought to examine the association be-
tween sagittal plane trunk posture and
PFJ stress using a self-selected trunk
posture. Second, we evaluated the eects
of modifying sagittal plane trunk posture
on PFJ stress during running. A tertia-
ry purpose of this study was to identify
patellofemoral and tibiofemoral kine-
matics and kinetics that may explain the
changes in PFJ stress while running with
dierent trunk postures. Based on exist-
ing literature evaluating the influence of
trunk posture on tibiofemoral joint kinet-
ics and kinematics, we hypothesized that
an individual’s self-selected trunk flexion
angle would be inversely associated with
the peak PFJ stress during the stance
phase of running. We also hypothesized
that, compared to a self-selected trunk
posture, a more flexed trunk posture
would result in a decrease in peak PFJ
stress during running and, conversely,
a more extended trunk posture would
result in an increase in peak PFJ stress.
Understanding the association between
sagittal plane trunk posture and PFJ
stress during running may advance the
understanding of the etiology of PFP in
runners and facilitate the development of
running techniques to reduce PFJ load-
ing in this population.
METHODS
Participants
Twenty-four recreational run-
ners between the ages of 18 and 39
years participated in this study (12
men, 12 women) (TABLE 1). Participants
were natural rearfoot strikers, which was
verified using sagittal plane images from
high-speed video (sampling rate, 125 Hz).
Potential participants were excluded if
they reported any of the following: (1)
current lower extremity or low back pain,
(2) previous history of lower extremity or
low back surgery, and (3) lower extremity
or low back pathology that caused pain
or discomfort during running within 6
months prior to participation.
Instrumentation
Three-dimensional trunk and lower ex-
tremity kinematics were collected using
an 11-camera motion-capture system
(Qualisys AB, Göteborg, Sweden) at a
sampling rate of 250 Hz. Ground reac-
tion forces were obtained at a rate of 1500
Hz using a single force plate (Advanced
Mechanical Technology, Inc, Watertown,
MA). Electromyographic (EMG) signals
of selected lower extremity muscles were
collected at a sampling rate of 1500 Hz
using a wireless EMG system (Telemyo
DTS; Noraxon USA Inc, Scottsdale,
AZ) and Ag/AgCl surface electrodes
(Norotrode 20; Myotronics-Noromed,
Inc, Kent, WA). The EMG system had a
dierential input impedance of greater
than 100 MΩ, a common-mode rejection
ratio greater than 100 dB, and a baseline
noise of less than 1 µV root-mean-square.
Marker, ground reaction force, and EMG
data were collected and synchronized us-
ing motion-capture software (Track Man-
ager Version 2.8; Qualisys AB).
Procedures
Data were collected at the Jacquelin Perry
Musculoskeletal Biomechanics Research
Laboratory at the University of Southern
California. Prior to participation, partici-
pants were informed as to the objectives,
procedures, and potential risks of par-
ticipation in the study and provided in-
formed consent as approved by the Health
Science Institutional Review Board of the
University of Southern California.
Participants wore shorts, tank tops,
and their personal running shoes during
the evaluation. Data were obtained from
each participant’s dominant leg. Leg
dominance was determined by asking the
participants which leg they preferred to
use when kicking a ball.
Participants were first instrument-
ed with EMG electrodes. Electromyo-
graphic signals were recorded from the
knee flexor muscles (medial and lateral
hamstrings and gastrocnemius), and data
were used to account for muscle cocon-
traction in our biomechanical model.
The electrodes of the medial and lateral
hamstrings were placed midway between
the ischial tuberosity and the medial and
lateral sides of the popliteal fossa, respec-
tively.34 The electrodes for the medial
and lateral gastrocnemius were placed
at one third of the distance between the
medial and lateral sides of the popliteal
fossa, respectively, and the Achilles ten-
don insertion, starting from the popliteal
fossa.34 Prior to placement of the EMG
electrodes, the skin was shaved, abraded,
and cleaned with isopropyl alcohol to re-
duce electrical impedance.
Following electrode placement, partic-
ipants were asked to warm up by jogging
TABLE 1 Participant Demographics*
*Values are mean SD.
Men (n = 12) Women (n = 12)
Age, y 28.1 7.2 26.5 6.4
Height, m 1.74 0.08 1.66 0.08
Weight, kg 70.5 7. 0 62.7 6.6
Running distance, km/wk 19.3 10.1 22.9 11.1
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journal of orthopaedic & sports physical therapy | volume 44 | number 10 | october 2014 | 787
at a self-selected speed on a treadmill
for 5 minutes. After the warm-up, EMG
signals were collected during a maximal
voluntary isometric contraction (MVIC).
The MVIC test for the medial and lateral
hamstrings was performed with partici-
pants in a seated position, with their hip
and knee joints at 85° and 60° of flexion,
respectively.1 A strap was secured around
the distal tibia just superior to the lat-
eral malleolus to resist knee flexion. The
MVIC test for the medial and lateral
gastrocnemius was performed in stand-
ing. Participants stood on the tested leg
and raised their heel against resistance,
which was applied by a fixed bar across
the shoulders. The ankle of the tested
leg was at 15° of plantar flexion, and the
knee was fully extended during testing.1
During each MVIC test, participants
were instructed to produce a maximum
eort. Three trials of a 5-second MVIC
were obtained from each muscle, with a
40-second break between trials.7,3 9
Prior to the running trials, 21 anatom-
ical markers (reflective 14-mm diameter)
were placed on the following bony land-
marks: end of second toes, first and fifth
metatarsal heads, medial and lateral mal-
leoli, medial and lateral epicondyles of
femurs, greater trochanters, iliac crests,
L5-S1 junction, and acromioclavicular
joints. In addition, tracking marker clus-
ters mounted on semi-rigid plastic plates
were placed on the lateral surfaces of the
participant’s thighs, shanks, and heel
counters of the shoes. A standing calibra-
tion trial was first obtained to define the
segmental coordinate systems and joint
axes. After the calibration trial, anatomi-
cal markers were removed, except for
those at the iliac crests, L5-S1 junction,
and acromioclavicular joints. The track-
ing markers remained on the participant
throughout the entire data-collection
session.
Participants were instructed to run
at a controlled speed of 3.4 m/s along a
14-m runway using 3 dierent trunk pos-
tures: self-selected, flexed, and extended
(FIGURE 1). Participants first ran using
their self-selected trunk posture. During
the flexed condition, participants were
instructed to increase their trunk flexion
angle within a range in which they felt
comfortable when running. Similarly,
participants were asked to decrease their
trunk flexion angle during the extended
condition.
The order of the flexed and extended
conditions was randomized for each par-
ticipant. Practice trials were permitted
to allow participants to become familiar
with the running speed and various trunk
postures. Five successful running trials
were obtained for each trunk condition.
A trial was counted as successful when
the foot of the dominant leg fell within
the borders of the force plate from initial
contact to toe-o and the running speed
was within 5% of the target velocity.
Data Analysis
Kinematic and kinetic data were pro-
cessed and analyzed using Visual3D
software (C-Motion, Inc, Germantown,
MD). Marker trajectory data were low-
pass filtered at 12 Hz, using a fourth-or-
der Butterworth filter. The trunk segment
was defined by markers placed bilaterally
on the iliac crests and acromioclavicular
joints.29 The pelvis and trunk segments
were modeled as cylinders, and the lower
extremity segments were modeled as
frusta of cones. The local orthogonal
coordinate systems of the trunk, pelvis,
thigh, shank, and foot segments were de-
rived from the standing calibration trial.
Joint kinematics were calculated using a
Cardan rotation sequence in an order of
flexion/extension, abduction/adduction,
and internal/external rotation. The trunk
angle was calculated as the orientation of
the trunk segment relative to the global
coordinate system (global vertical axis).
Knee kinematics were calculated as the
motion of the shank relative to the thigh.
The net knee joint moment was com-
puted using inverse-dynamics equations.
Moment data were expressed as internal
(muscle) moments and normalized to
each participant’s body mass.
Electromyographic data were pro-
cessed using MATLAB software (The
MathWorks, Inc, Natick, MA). Raw EMG
signals were band-pass filtered (20-450
Hz, fourth-order Butterworth),15,28 recti-
fied, and smoothed using a 10-Hz low-
pass filter (fourth-order Butterworth).11
The smoothed EMG data during the
stance phase of running were normalized
to the average EMG intensity recorded
from the middle 3 seconds of the MVIC
trials. The stance phase was defined when
the vertical ground reaction force exceed-
ed 30 N.
FIGURE 1. Trunk posture and lower extremity biomechanics were obtained during 3 trunk conditions: (A) extended,
(B) self-selected, and (C) flexed.
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[ research report ]
A previously described sagittal plane
biomechanical model was used to quan-
tify PFJ reaction force and stress (FIGURE
2).9,10,13,20 Input variables for the model al-
gorithm consisted of participant-specific
kinematics and kinetics (ie, knee flexion
angle and adjusted knee extensor mo-
ment) and data from previous literature
(ie, PFJ contact area,32 quadriceps ef-
fective lever arm,41 and relationship be-
tween quadriceps force and PFJ reaction
force42).
To account for cocontraction at the
knee joint during running, an estimation
of knee flexor moment was required. The
knee flexor moment was calculated us-
ing SIMM modeling software (Motion
Analysis Corporation, Santa Rosa, CA).
Using SIMM, a generic lower extremity
musculoskeletal model was created with
6 musculotendon actuators: semitendi-
nosus, semimembranosus, biceps femoris
long and short heads, and medial and lat-
eral gastrocnemius.14 The SIMM model
also contained information about peak
isometric muscle force, optimal muscle
fiber length, pennation angle, and tendon
slack length.14,17,27,45
The input variables of the SIMM
model were participant-specific lower
extremity kinematics and normalized
EMG of the knee flexors. Lower extrem-
ity kinematic data were used to deter-
mine individual muscle tendon lengths
and contraction velocities for the Hill-
type muscle model in SIMM. Normalized
EMG data were used to represent the lev-
el of muscle activation. Muscle activation
of the semitendinosus was assumed to be
the same as that of the semimembra-
nosus, and the biceps femoris long and
short heads were assumed to have the
same activation.26 Torque produced by
each knee flexor muscle was computed,
added together, and normalized to body
weight to represent knee flexor moment.
To obtain a more accurate assessment of
the knee extensor moment during run-
ning, the knee flexor moment calculated
by the SIMM model was added to the net
knee extensor moment, as quantified us-
ing the inverse-dynamics equations. This
resulted in an adjusted knee extensor
moment that accounted for antagonist
muscle activation.
The first step of the model algorithm
was to approximate the quadriceps force.
First, the eective lever arm of the quad-
riceps was determined at each degree of
knee flexion by fitting a nonlinear equa-
tion to the data of van Eijden et al.41 Next,
the quadriceps force was calculated by
dividing the adjusted knee extensor mo-
ment calculated during running by the
eective lever arm.
The second step of the algorithm was
to estimate the PFJ reaction force. This
was accomplished by multiplying the
quadriceps force by a ratio reported by
van Eijden et al,42 which defined the re-
lationship between quadriceps force and
PFJ reaction force as a function of knee
flexion angle. The third step of the algo-
rithm was to calculate PFJ stress. The
PFJ reaction force obtained in the second
step was divided by the PFJ contact area,
which was determined using a second-
order polynomial curve fitted to data of
Powers et al.32 The model outputs were
PFJ stress and reaction force as a func-
tion of the gait cycle.
The primary variables of interest
were the mean trunk flexion angle and
peak PFJ stress during the stance phase
of running. The secondary variables of
interest included PFJ reaction force, PFJ
contact area, adjusted knee extensor mo-
ment, and knee flexion angle. Each of
these variables was analyzed at the time
of peak PFJ stress. All variables were
calculated for each stride and averaged
over 5 successful strides for each trunk
condition.
Statistical Analysis
A Pearson product-moment correla-
tion was used to examine the associa-
tion between mean sagittal plane trunk
posture and peak PFJ stress during the
self-selected condition. Separate repeat-
ed-measures, 1-way analyses of variance
(ANOVAs) were used to assess dier-
ences in each variable of interest among
the 3 trunk conditions. For all significant
ANOVA tests, post hoc Bonferroni tests
were employed. All statistical analyses
were performed using PASW Statistics
software (SPSS Inc, Chicago, IL). The
level of statistical significance was set at
.05.
RESULTS
Self-Selected Trunk Posture
and PFJ Stress
Results of Pearson correlation
indicated a significant inverse cor-
relation between mean trunk flexion
Net knee joint moment Knee flexor moment
Quadriceps eective
lever arm†
Relationship between
QF and PFJ RF‡
PFJ stress
Adjusted knee
extensor moment
PFJ RF
Knee flexion angle
QF
PFJ contact area*
FIGURE 2. Flow chart of patellofemoral joint model. *Data from Powers et al.32 †Data from van Eijden et al.41 ‡Data
from van Eijden et al.42 Abbreviations: PFJ, patellofemoral joint; QF, quadriceps forcce; RF, reaction force.
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journal of orthopaedic & sports physical therapy | volume 44 | number 10 | october 2014 | 789
angle and peak PFJ stress for the self-
selected condition (r = –0.60, P = .002)
(FIGURE 3).
Trunk Kinematics
Sagittal plane trunk posture during the
stance phase of running for the 3 trunk
conditions is presented in FIGURE 4. The
ANOVA comparing average trunk flex-
ion angle across the 3 trunk conditions
indicated a significant dierence across
conditions (P<.001). Post hoc analysis re-
vealed that, compared to the self-selected
condition (mean SD, 7.3° 3.6°), there
was a significant increase in mean trunk
flexion angle during the flexed condition
(14.1° 4.8°, P<.001) and a significant
decrease in mean trunk flexion angle dur-
ing the extended condition (4.0° 3.9°,
P<.001).
PFJ Biomechanics
Patellofemoral joint stress during the
stance phase of running for the 3 trunk
conditions is presented in FIGURE 5. The
ANOVA comparing peak PFJ stress
across the 3 trunk conditions indicated
a significant dierence across conditions
(P<.001) (TABLE 2). Post hoc analysis re-
vealed that peak PFJ stress was signifi-
cantly lower during the flexed condition
(mean SD, 20.2 3.4 MPa; P<.001)
and significantly higher during the ex-
tended condition (23.1 3.4 MPa,
P<.001) when compared to the self-se-
lected condition (21.5 3.2 MPa).
The ANOVA comparing PFJ reaction
force at the time of peak PFJ stress across
the 3 trunk conditions also indicated a
significant dierence across conditions
(P<.001) (TABLE 2). Post hoc analysis re-
vealed that the PFJ reaction force at the
time of peak stress was significantly lower
during the flexed condition (mean SD,
71.0 11.1 N/kg; P<.001) and signifi-
cantly higher during the extended con-
dition (81.3 12.7 N/kg, P<.001) when
compared to the self-selected condition
(75.0 10.2 N/kg).
The ANOVA comparing PFJ contact
area at the time of peak stress across the
3 trunk conditions indicated a significant
dierence across conditions (P = .001)
(TABLE 2). Post hoc analysis revealed that
the PFJ contact area at the time of peak
stress was significantly larger during the
flexed (mean SD, 232.5 4.2 mm2; P
= .048) and extended (233.5 4.5 mm2,
P = .001) conditions when compared to
the self-selected condition (231.6 4.4
mm2).
Tibiofemoral Joint Biomechanics
The ANOVA comparing adjusted knee
extensor moment at the time of peak PFJ
stress across the 3 trunk conditions was
significant (P<.001) (TABLE 2). Post hoc
analysis revealed that the adjusted knee
extensor moment at the time of peak
stress was significantly lower during the
flexed condition (mean SD, 3.29
0.34 Nm/kg; P<.001) and significantly
higher during the extended condition
(3.70 0.32 Nm/kg, P<.001) when com-
pared to the self-selected condition (3.54
0.31 Nm/kg).
The ANOVA comparing knee flexion
angle at the time of peak stress across
the 3 trunk conditions also was signifi-
cant (P<.001) (TABLE 2). Post hoc analysis
revealed that the knee flexion angle at
the time of peak stress was significantly
higher during the flexed (mean SD,
44.5° 3.7°; P = .046) and extended
(45.5° 4.6°, P = .001) conditions when
compared to the self-selected condition
(43.6° 3.5°).
DISCUSSION
The findings of the current
study support the hypothesis that
an individual’s self-selected sagit-
tal plane trunk posture is associated with
peak PFJ stress during running. Specifi-
cally, individuals who ran with a more
flexed trunk posture exhibited lower peak
PFJ stress. In contrast, individuals who
ran with a more upright trunk posture
exhibited higher peak PFJ stress. Fur-
thermore, our findings support the prem-
0
0246810121416
5
10
15
20
25
30
35
Mean Trunk Flexion Angle, deg
Peak PFJ Stress, MPa
r = –0.60
P = .002
Women Men
FIGURE 3. Association between peak PFJ stress
and mean trunk flexion angle during the self-
selected trunk-posture condition. Abbreviation: PFJ,
patellofemoral joint.
0
0102040
30 50 7060 80 90 100
2
6
4
8
10
12
14
16
18
Stance Phase, %
Trunk Flexion Angle, deg
Flexed ExtendedSelf-selected
FIGURE 4. Sagittal plane trunk posture during the
stance phase of running under 3 trunk conditions:
extended, flexed, and self-selected. The shaded area
represents 1 SD for the self-selected condition.
0
0102040
30 50 7060 80 90 100
5
10
15
20
25
Stance Phase, %
PFJ Stress, MPa
Flexed ExtendedSelf-selected
FIGURE 5. Patellofemoral joint stress during the
stance phase of running under 3 trunk conditions:
extended, flexed, and self-selected. The shaded area
represents 1 SD for the self-selected condition.
Abbreviation: PFJ, patellofemoral joint.
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[ research report ]
ise that modifying sagittal plane trunk
posture can result in significant changes
in PFJ stress during running. On average,
a 6.8° increase in the mean trunk flexion
angle resulted in a 6.0% decrease in peak
PFJ stress, whereas a 3.3° decrease in
mean trunk flexion angle led to a 7.4%
increase in peak PFJ stress.
The changes in PFJ stress during the
dierent trunk postures were primarily
driven by changes in PFJ reaction force
as opposed to PFJ contact area. When
compared to the self-selected condi-
tion, the PFJ reaction force at the time of
peak PFJ stress decreased by 5.3% in the
flexed condition and increased by 8.4%
in the extended condition. Conversely,
the changes in PFJ contact area across
the dierent trunk conditions were less
than 1%. In contrast to findings by Len-
hart et al,23 who reported that a change
in knee flexion was the most important
predictor of PFJ loading during run-
ning, the observed changes in PFJ reac-
tion force in the current study appeared
to be influenced to a greater extent by
the adjusted knee extensor moment as
opposed to the knee flexion angle. This
was reflected by the fact that adjusted
knee extensor moment at the time of
peak PFJ stress decreased by 7.1% in the
flexed condition and increased by 4.5% in
the extended condition. In contrast, the
changes in knee flexion angle were less
than 2° across all conditions.
The finding that an increase in the
forward trunk lean resulted in a decrease
in the knee extensor moment is in agree-
ment with previous studies.3,24,30 Asay et
al3 reported that a 6.3° increase in the
trunk flexion angle was associated with a
35.2% lower peak knee extensor moment
during stair ascent. In addition, Ober-
länder et al30 reported that a 6° greater
forward trunk lean resulted in a 15% re-
duction in peak knee extensor moment
during hop landing.
The results of the current study have
several clinical implications. First, the
observed inverse correlation between
trunk flexion angle and PFJ stress (r
= –0.60) suggests that running with a
relatively extended trunk posture may
be a contributing factor with respect to
the development of PFP. However, lon-
gitudinal studies are needed to verify
this hypothesis. Second, incorporating a
forward-lean trunk posture during run-
ning could be used as a strategy to reduce
PFJ loading in runners. For example, our
data suggest that increasing one’s natural
trunk flexion angle by approximately 7°
could lead to a 6.0% reduction in peak
PFJ stress (1.3 MPa). Although the eects
of increased trunk forward lean on PFP
symptoms were not examined in the cur-
rent study, Powers et al33 reported that a
1-MPa decrease in PFJ stress during fast
walking corresponded to a 56% decrease
in pain in individuals with PFP. Given the
repetitive nature of running, a small re-
duction in PFJ stress per step could result
in meaningful reductions in cumulative
PFJ loading. Further studies are needed
to examine the ecacy of forward trunk
lean on PFP in symptomatic runners who
presented with a more upright trunk
posture.
Recent studies have advocated chang-
ing the foot strike pattern and/or in-
creasing step rate to reduce PFJ loading.
Kulmala et al22 reported that a forefoot
strike pattern resulted in a 14.6% reduc-
tion in peak PFJ stress compared to a
rearfoot strike pattern. Bonacci et al6 re-
ported that barefoot running led to a 12%
reduction in peak PFJ stress compared to
shod running. In addition, Lenhart et al23
found that increasing step rate by 10%
resulted in a 14% decrease in peak PFJ
reaction force.
The findings of the current study sug-
gest that incorporating a forward-lean
trunk can be used as an alternative strat-
egy to reduce PFJ loading as opposed
to the aforementioned running modifi-
cations. For example, a 10° increase in
sagittal plane trunk flexion was found to
lead to a similar percent of reduction in
PFJ loading (13.4% decrease in peak PFJ
stress and 13.8% decrease in PFJ reaction
force) without changing the foot strike
pattern. This is important, as barefoot
running as well as adopting a forefoot or
midfoot strike pattern has been shown
to increase the mechanical demand of
the ankle plantar flexors.2,5,8,19,31,35,46 Fur-
thermore, post hoc analysis revealed that
there was no significant dierence in
ankle plantar flexor moment at the time
of peak PFJ stress across the 3 trunk
conditions. We propose that adopting a
forward-lean trunk posture during run-
ning may be a preferable strategy to re-
duce PFJ stress without increasing the
mechanical demand on ankle plantar
flexors.
The change in trunk flexion angle in
the flexed condition was achieved, at least
in part, by an increase in hip flexion. Post
hoc analysis revealed a small but signifi-
cant increase in hip flexion angle at the
TABLE 2
Comparison of Patellofemoral
and Tibiofemoral Joint Biomechanics
at the Time of Peak Patellofemoral Joint
Stress During Flexed, Self-Selected,
and Extended Trunk Conditions*
Abbreviation: PFJ, patellofemoral joint.
*Values are mean SD.
†Significantly dierent from self-selected trunk condition (P<.05).
Flexed Self-Selected Extended
PFJ stress, MPa 20.2 3.4†21.5 3.2 23.1 3.4†
PFJ reaction force, N/kg 71.0 11.1†75.0 10.2 81.3 12.7†
PFJ contact area, mm2232.5 4.2†231.6 4.4 233.5 4.5†
Adjusted knee extensor moment, Nm/kg 3.29 0.34†3.54 0.31 3.70 0.3 2†
Knee flexion angle, deg 44.5 3.7†43.6 3.5 45.5 4.6†
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time of peak PFJ stress in the flexed con-
dition (mean SD, 33.5° 7.0°; P<.001)
compared to the self-selected condition
(29.9° 5.9°). No significant dier-
ence in hip flexion angle was observed
between the extended (29.6° 6.5°, P =
1.0) and self-selected conditions. As such,
it is reasonable to assume that utilization
of a forward trunk lean during running
may result in an increased demand on the
hip extensors.
Several limitations need to be con-
sidered when interpreting the results of
this study. First, a planar (2-dimensional)
model was used to estimate PFJ stress. As
such, our approach did not account for
joint motions and forces in the frontal
and transverse planes. However, previ-
ous studies using this modeling approach
have been able to discriminate between
individuals with and without PFP.10 Sec-
ond, only healthy individuals were ex-
amined in this study. Caution should be
taken when generalizing the results to
various patient populations. Third, we
did not obtain running performance or
comfort data as part of the study. It is un-
clear how altering trunk posture would
aect oxygen consumption or comfort
levels during bouts of prolonged running.
CONCLUSION
An individual’s self-selected
sagittal plane trunk posture was
inversely associated with PFJ stress
during running. Specifically, an upright
trunk posture was found to be associated
with higher peak PFJ stress. In addition,
a 6.8° increase in sagittal plane trunk
flexion resulted in a significant reduction
in PFJ stress. The change in PFJ stress
primarily was due to changes in the PFJ
reaction force, which was driven by a
reduction in the adjusted knee exten-
sor moment. Based on our findings, we
propose that an upright trunk posture
during running may predispose an indi-
vidual to a higher risk of PFP. In addition,
incorporating a forward-lean trunk dur-
ing running may be an alternative means
to reduce PFJ stress, as opposed to run-
ning modifications such as changing foot
strike pattern, barefoot running, and in-
creasing step rate. t
KEY POINTS
FINDINGS: A more upright trunk posture
during running was found to be as-
sociated with higher peak PFJ stress. A
relatively small increase in sagittal plane
trunk flexion posture led to significant
reduction in peak PFJ stress.
IMPLICATIONS: An upright trunk posture
during running may expose an individu-
al to a higher risk of PFP. Incorporating
a slightly forward-leaning trunk posture
during running may be an alternative
means to reducing PFJ stress.
CAUTION: Only healthy individuals were
examined in this study. Caution should
be taken when generalizing the results
to various patient populations.
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