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SPORTS HEALTHvol. XX • no. X
Patellofemoral pain syndrome (PFPS) is known as
“runner’s knee”13 and is defined as anterior knee pain
involving the patella, occurring with knee flexion and
extension motions.13,63 Several studies have reported various
factors that lead to PFPS, such as quadriceps,7,53,65 abnormal
hip biomechanics,62 inflexibility50,64 and malalignment17 of the
lower limbs, and altered neuromuscular control.1 Among
these, quadriceps tightness can lead to patellar alta and
patellar tilt,64,65,67 which result in quadriceps weakness and
muscle imbalance caused by pain due to increased
compression force of the patellofemoral joint and abnormal
movements of the patella during knee motion65,67; thus, the
progression of PFPS may be accelerated.53,67 Therefore, many
authors have focused on the restoration and measurement of
inflexible quadriceps in patients with patellofemoral pain
(PFP).25,44,53,65
Recent studies40,51,56,65 have reported that stretching exercises
are as effective as muscle strengthening exercises for improving
993777SPHXXX10.1177/1941738121993777Lee et alSports Health
research-article2021
Static and Dynamic Quadriceps Stretching
Exercises in Patients With Patellofemoral
Pain: A Randomized Controlled Trial
Jin Hyuck Lee, PT,†‡ Ki-Mo Jang, PhD, MD,‡§ Eunseon Kim, PT,‡ Hye Chang Rhim, MD,§
and Hyeong-Dong Kim, PhD, PT*†
Background: Limited data are available on the effect of stretching exercise in patients with patellofemoral pain (PFP) who
have inflexible quadriceps, which is one of the various causes of PFP syndrome. This study compares quadriceps flexibility,
strength, muscle activation time, and patient-reported outcomes after static and dynamic quadriceps stretching exercises in
patients with PFP who had inflexible quadriceps.
Hypothesis: Quadriceps flexibility and strength, muscle activation time, and patient-reported outcomes would improve
with dynamic quadriceps stretching as compared with static quadriceps stretching exercises.
Study Design: Randomized controlled trial.
Level of Evidence: Level 2.
Methods: Of the 44 patients included in the study, 20 performed static stretching and 24 performed dynamic stretching.
Quadriceps flexibility was assessed by measuring the knee flexion angle during knee flexion in the prone position (the Ely
test). Muscle strength and muscle activation time were measured using an isokinetic device. The patient-reported outcomes
were evaluated using the visual analogue scale for pain and anterior knee pain scale.
Results: No significant differences in quadriceps flexibility and strength, muscle activation time, and patient-reported
outcomes in the involved knees were found between the 2 groups (P values > 0.05).
Conclusion: Quadriceps flexibility and strength, muscle activation time, and patient-reported outcomes in patients with
PFP who had inflexible quadriceps showed no significant differences between the static and dynamic quadriceps stretching
exercise groups.
Clinical Relevance: Both static and dynamic stretching exercises may be effective for improving pain and function in
patients with PFP who have inflexible quadriceps.
Keywords: dynamic stretching; static stretching; quadriceps flexibility; muscle strength; muscle activation time
From †Department of Physical Therapy and School of Health and Environmental Science, College of Health Science, Korea University, Seoul, Republic of Korea, ‡Department
of Sports Medical Center, Korea University College of Medicine, Anam Hospital, Seoul, Republic of Korea, and §Korea University College of Medicine and School of Medicine,
Seoul, Republic of Korea
*Address correspondence to Hyeong-Dong Kim, PhD, PT, Department of Physical Therapy and School of Health and Environmental Science, College of Health Science, Korea
University, Seoul, 145, Republic of Korea (email: gnkfcc@naver.com).
This work was supported by the Korea Medical Device Development Fund grant funded by the Korea government (the Ministry of Science and ICT, the Ministry of Trade,
Industry and Energy, the Ministry of Health & Welfare, Republic of Korea, the Ministry of Food and Drug Safety) (Project Number: 202014X11-04).
Jin Hyuck Lee and Ki-Mo Jang are co–first authors and contributed equally to this work.
DOI: 10.1177/1941738121993777
© 2021 The Author(s)
Mon • Mon 2021Lee et al
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pain and function of muscles, including the quadriceps and hip
muscles, in patients with PFP; thus, stretching exercises were
recommended for patients with PFP.47,64,65 However, results in
terms of pain and muscle performance were reported to vary
among different types of stretching exercises such as static,
dynamic, or precontraction stretching.46,68,69 A recent study32
reported that dynamic hamstring stretching exercises are more
effective than static hamstring stretching for improving muscle
performance such as muscle activation time and pain in patients
with PFP who have inflexible hamstrings. Furthermore, only one
previous study44 measured the change in quadriceps flexibility
in patients with PFP after static and precontraction stretching
exercises. To date, therefore, no studies have compared the
flexibility and strength of the quadriceps muscles,
neuromuscular control, and patient-reported outcomes such as
those assessed using the visual analogue scale (VAS) for pain
and anterior knee pain scale (AKPS) between static and
dynamic quadriceps stretching exercises in patients with PFP
who have inflexible quadriceps.
Quadriceps flexibility and strength, neuromuscular control,
and patient-reported outcomes between static and dynamic
quadriceps stretching exercises in patients with PFP who had
inflexible quadriceps were compared. The hypothesis is that the
dynamic stretching group would have better improvement in all
the parameters than the static stretching group.
Methods
Enrollment of Patients
This prospective randomized study enrolled 116 patients with
anterior knee pain who were recruited consecutively between
January 2018 and August 2019, except patients with a history of
surgery. This study was approved by the institutional review
board (2017AN0830); all participants provided informed
consent; and the rights of the subjects were protected. All the
patients were confirmed to have normal ranges of the
parameters measured in the various radiological imaging
assessments,41 including the Insall-Salvati ratio (normal range,
0.8-1.2; with ratios >1.2 indicating patella alta and <0.8
indicating patella baja),51 congruence angle, and sulcus angle.
Physical examinations were performed by 2 orthopaedic knee
surgeons to find any definite patellofemoral pathologies. Any
disagreements were resolved by discussion to reach a
consensus. The inclusion criterion was inflexible quadriceps
with a knee flexion angle <125° during the Ely test,67 defined as
the knee flexion angle at which the quadriceps resist knee
flexion without hip flexion in the prone position. The exclusion
criteria were as follows (Figure 1): chondromalacia patella
(known as sick cartilage, which is pain caused by erosion,
fissuring, and subsequent tearing of the hyaline cartilage of the
articular surfaces of the bone),21 patellar dislocation, plica
syndrome, osteoarthritis, and tight hamstrings with normal
quadriceps flexibility, with the latter being an exclusion criterion
because it does not fall within the purposes of this study. Fifty-
two patients were randomized. Allocation to the 2 groups was
achieved with the use of a random-number table. Treatment
using a 6-month intervention protocol was performed by 1 of
the physical therapists who was blinded to the details of the
assessments and data analyses. The assessments and data
analyses were performed by an independent physical therapist
blinded to the rehabilitation protocol. Of the 116 patients
enrolled, 72 were excluded and 44 were analyzed in this study,
20 in the static stretching group and 24 in the dynamic
stretching group.
Patient-Reported Outcomes
The patient-reported outcomes were confirmed using the VAS
score for pain and the AKPS score.12 In the VAS, with scores
ranging from 0 to 10 points, 0 points means no pain and 10
points means the worst pain. The AKPS is also known as the
Kujala scale and contains 13 items, for a total score ranging
from 0 to 10030; higher scores indicate lower disability. Similar to
a previous report,12 a 1.5- to 2-point difference in VAS score and
an 8- to 10-point difference in AKPS score were defined as
minimal clinically important differences (MCIDs).
Quadriceps Flexibility Test (the Ely Test)
In a systematic review64 and several previous studies,41,47,67
quadriceps flexibility in patients with PFP was measured using
the knee flexion angle in the prone position. This test is called
the Ely test. According to a previous study by Peeler and
Anderson,48 0.83 is a high intraclass correlation coefficient (ICC)
in the Ely test.
Isokinetic Muscle Performance Test
Muscle strength and neuromuscular control were evaluated
using an isokinetic device (Biodex Multi-Joint System 4, Biodex
Medical Systems, Inc). The participants were seated in an
upright position in an isokinetic chair, with hips and knees
flexed at 90° and with 2 straps fixed across their chest. The
lateral femoral condyle of the knee joint was aligned with the
rotational axis of an isokinetic dynamometer. The participants
were allowed 5 submaximal flexion repetitions and extension
motions at 180 deg/s for warm-up before testing, followed by 5
maximal contractions at 180 deg/s after 1-minute rest periods.
Muscle strength was evaluated by peak torque, normalized to
body weight (peak torque/body weight, N·m·kg−1 × 100).34,35
Neuromuscular control is defined as the efferent motor response
to afferent sensory stimulation38,55; thus, in the present study, it
was measured as acceleration time (milliseconds),31,33,35 defined
as the muscle activation time required for attaining a preset
angular velocity (180 deg/s in our study) during muscle
contraction. Therefore, a slow acceleration time was associated
with delayed muscle activation responses and muscular
recruitment ability. Gravity and limb weight were corrected at
30° of knee extension by the Biodex Advantage software.
Quadriceps Stretching Interventions
1. Static stretching: In the prone and standing positions, the
involved leg maintained 90° of knee flexion, followed by
SPORTS HEALTHvol. XX • no. X
3
grasping of the foot using the ipsilateral hand, with the heel
of the involved leg slowly pressed on the buttock.49,58 For
the static stretching, 3 sets were repeated, with a holding
time of 15 seconds.
2. Dynamic stretching: In the prone position, the knee joint of
the involved leg was flexed maximally, followed by hip
extension and hyperextension of the hip joint.26 In the
standing position, the involved leg maintained 90° of knee
flexion, followed by continuing with the hip extension
motion actively.2,49 For the dynamic stretching, 3 sets were
performed, with 15 repetitions with a holding time of 1
second.
The holding time and number of sets of the static and
dynamic stretching exercises were similar, at 15 seconds and 3
sets, respectively. After the 6-month follow-up, the final
isokinetic test was performed in our institution after a 4-minute
recovery period for each type of stretching.
Conservative Rehabilitation Protocol
All the participants were treated and educated in our clinic with
the same rehabilitation protocol except for stretching exercises,
with a total of 6 visits once a month for 6 months.14,19 In
addition, the home exercise program recommended 3 sets twice
a day for each stretching with strengthening exercise, and
Figure 1. Study flow diagram.
Mon • Mon 2021Lee et al
4
compliance with the protocol was checked continuously during
clinic visits. The goals of the rehabilitation program were to
improve strength, proprioception, and neuromuscular control
and to normalize biomechanics. In the open kinetic chain
exercises, multidirectional straight leg raises, including flexion,
extension, abduction, and adduction, were performed to
improve quadriceps, hamstring, and hip muscle strengths. In
addition, short arc knee extensions with knee adduction in the
pain-free range were performed. However, if the patients had
no pain during the exercise, full-range knee flexion and
extension were permitted. In the closed kinetic chain exercises,
wall squat exercises, along with knee adduction and single-leg
squats, were performed in a range of <50°.15 Balance exercises,
with the eyes open and closed, were performed to improve
proprioception and neuromuscular control. A core and hip
muscle strengthening program was implemented, with all the
exercises.
Statistical Analyses
A priori power analysis was performed to determine the sample
size at an alpha level of 0.05 and a power of 0.8 to compare the
static and dynamic quadriceps stretching exercises in patients
with PFP who had inflexible quadriceps. On the basis of
previous reports,20,67 in the quadriceps muscle strengths in
patients with PFP, a difference of at least >10% was regarded as
clinically significant. A pilot study was performed on 5 knees
from each group to estimate the sample size (Cohen d effect
size: 0.993); thus, 34 patients were required to detect a
between-group difference in quadriceps strength of >10%. In
the present study, 20 patients who performed static stretching
exercises and 24 who performed dynamic stretching exercises
were evaluated. In the present study, the power was 0.802 for
detecting a significant difference between the strengths in the 2
groups after each stretching exercise.
The ICCs were calculated for 2 trials of flexibility and strength
of the quadriceps muscles to quantify the test-retest reliability.
ICCs were also calculated for the 2 trials of muscle activation
time. A previous study by Shrout and Fleiss60 suggested that ICC
values >0.75 and <0.4 indicate good and poor accuracies,
respectively. The Student t test was used to compare the
flexibility and strength of the quadriceps muscles, muscle
activation time, and patient-reported outcomes between static
and dynamic quadriceps stretching in the PFP patients with
inflexible quadriceps. The paired t test was used to compare all
variables in each group before and after quadriceps stretching.
The statistical analysis was performed using SPSS (SPSS 21.0,
IBM Corp). P values <0.05 were considered statistically
significant.
Results
The patients’ baseline demographic characteristics showed no
significant differences between the 2 groups (Table 1). In all the
participants, the test-retest reliabilities of the quadriceps flexibility
and strength were good, with ICCs of 0.75 and 0.80, respectively.
In addition, the test-retest reliability of muscle activation time was
acceptable for the quadriceps muscle (ICCs = 0.82).
No significant differences in quadriceps flexibility and strength,
muscle activation time, and patient-reported outcomes in the
involved knees were found between the 2 groups after each
quadriceps stretching exercise (P values > 0.05; Tables 2 and 3).
In the static stretching group, the quadriceps flexibility and
strength, muscle activation time, and patient-reported outcomes
in the involved knees significantly improved after the stretching
Table 1. Demographic data of the patients in the static and dynamic quadriceps stretching groups
Static Stretching
Group (n = 20)
Dynamic Stretching Group
(n = 24)
P
Sex (male/female), n 7/13 8/16
Age,a y 26.5 ± 6.8 28.1 ± 4.2 0.77
Height,a cm 173.4 ± 4.0 171.8 ± 4.3 0.48
Weight,a kg 68.9 ± 8.2 71.3 ± 5.4 0.82
Body mass index,a kg/m225.1 ± 3.2 26.7 ± 2.6 0.51
Duration of symptoms,a mo 3.8 ± 2.1 4.1 ± 1.6 0.32
Injured side (right/left), n 17/3 20/4
Sports and activity (low:high), n 4:16 7:17 0.66
Insall-Salvati ratio 1.16 1.13 0.90
aValues are expressed as mean ± standard deviation.
SPORTS HEALTHvol. XX • no. X
5
exercise as compared with before the exercise (quadriceps
flexibility: 109 ± 9.7 vs 128 ± 2.6, P = 0.00; strength: 94 ± 40 vs
165 ± 49.1, P = 0.00; muscle activation time: 65 ± 17 vs 52 ±
14.4, P = 0.007; VAS score: 5 ± 0.9 vs 3 ± 1.0, P = 0.00; and
AKPS score: 48 ± 6.1 vs 70 ± 6.8, P = 0.00; Figure 2A).
In the dynamic stretching group, quadriceps flexibility and
strength, muscle activation time, and patient-reported outcomes
significantly improved in the involved knees after the stretching
exercise (quadriceps flexibility: 104 ± 9.2 vs 126 ± 7.9, P = 0.00;
strength: 93 ± 37.6 vs 183 ± 46.5, P = 0.00; muscle activation
time: 67 ± 16.7 vs 50 ± 16.4, P = 0.00; VAS score: 5 ± 1.1 vs 2 ±
1.0, P = 0.00; and AKPS score: 51 ± 5.8 vs 74 ± 8.4, P = 0.00;
Figure 2B).
discussion
The primary finding of the present study was that in all the
parameters, including quadriceps flexibility and strength, muscle
activation time, and patient-reported outcomes, no significant
differences were found between the static and dynamic
Table 2. Quadriceps flexibility and strength between the patients in the static and dynamic quadriceps stretching groupsa
Preintervention Postintervention
Static
Stretching
Group
(n = 20)
Dynamic
Stretching
Group
(n = 24) P
Static
Stretching
Group
(n = 20)
Dynamic
Stretching
Group
(n = 24) P
Quadriceps flexibilityb109 ± 9.7 104 ± 9.2 0.13 128 ± 2.6 126 ± 7.9 0.35
MD (95% CI) 5.0 (−1.4 to 10.1) 1.0 (−1.9 to 5.5)
Quadriceps strengthc94 ± 40 93 ± 37.6 0.91 165 ± 49.1 183 ± 46.5 0.21
MD (95% CI) 1.0 (−22.3 to 24.9) −18 (−47.4 to 10.9)
MD, mean difference.
a Values are expressed as mean ± standard deviation.
b The measurement unit for quadriceps flexibility was degrees.
c The measurement unit for quadriceps strength was N·m·kg−1 × 100.
Table 3. Muscle activation time and functional outcomes between the patients in the static and dynamic quadriceps stretching
groupsa
Preintervention Postintervention
Static
Stretching
Group
(n = 20)
Dynamic
Stretching
Group
(n = 24) P
Static
Stretching
Group
(n = 20)
Dynamic
Stretching
Group
(n = 24) P
Quadriceps muscle activation
time
65 ± 17 67 ± 16.7 0.70 52 ± 14.4 50 ± 16.4 0.81
MD (95% CI) −2.0 (−12.3 to 8.3) 2.0 (−8.3 to 10.6)
VAS 5 ± 0.9 5 ± 1.1 0.67 3 ± 1.0 2 ± 1.0 0.12
MD (95% CI) 0 (−0.5 to 0.8) 1.0 (−1.1 to 1.1)
AKPS 48 ± 6.1 51 ± 5.8 0.12 70 ± 6.8 74 ± 8.4 0.07
MD (95% CI) −3.0 (−6.5 to 0.8) −4.0 (−11.7 to −3.0)
AKPS, anterior knee pain scale; MD, mean difference; VAS, visual analogue scale.
a
Values are expressed as mean ± standard deviation. The measurement unit for neuromuscular control was milliseconds.
Mon • Mon 2021Lee et al
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quadriceps stretching exercises in the patients with PFP who
had inflexible quadriceps.
Previous studies reported that while static stretching is more
effective than dynamic stretching for increasing muscle
flexibility,4,45 dynamic stretching is more effective than static
stretching for improving quadriceps strength.39,68 A possible
explanation may be related to the difference in the holding time
of the stretching exercises. In the present study, both the static
and dynamic stretching exercises were repeated 3 times, with a
holding time of 15 seconds, whereas in the aforementioned
studies, they were repeated 3 times, with a holding time of 30
seconds. In a study by Bandy et al,4 static stretching with a
holding time of >30 seconds was more effective for muscle
flexibility than dynamic stretching; however, no significant
difference was found in the holding time of 15 seconds3,4
because the stiffness of a musculotendinous unit may change in
static stretching with a holding time of >30 seconds.29,43 On the
other hand, muscle strength may decrease in static stretching
with a holding time of >30 seconds.24 Herda et al24 reported that
muscle strength decreased in static stretching with a holding
time of 30 seconds when compared with that in dynamic
stretching. In addition, Siatras et al61 investigated quadriceps
strength after static stretching with different holding times,
including 10, 20, 30, and 60 seconds and found that quadriceps
strength significantly decreased in static stretching with a
holding time of >30 seconds but did not change when the
holding time was <20 seconds. This is because the compliant
musculotendinous unit from the stretched muscle fibers may be
insufficient to produce muscle force after static stretching with a
holding time of >30 seconds.11,61,66 Therefore, in the present
study, static and dynamic stretching exercises with a holding
time of 15 seconds may not be any different in terms of
quadriceps flexibility and strength in patients with PFP who
have inflexible quadriceps. Further study with stretching holding
Figure 2. Quadriceps flexibility (degrees), strength (N·m·kg−1 × 100), muscle activation time (ms), and patient-reported outcomes
(points) of the involved knees before and after quadriceps stretching in the static (A) and dynamic (B) stretching groups. AKPS,
anterior knee pain scale; VAS, visual analog pain scale.
SPORTS HEALTHvol. XX • no. X
7
times of >15 seconds may be needed to identify the difference
in quadriceps flexibility and strength between the 2 groups.
Several previous studies reported reduced neuromuscular
control of the quadriceps muscle in patients with PFP,10,52
particularly during static stretching exercises.6,9 This study found
no significant difference in muscle activation time between the
static and dynamic stretching groups among the PFP patients
with inflexible quadriceps. Although the reasons for these results
are unclear, they may be due to activation of the motor unit
(MU) improved by the eccentric contraction of the quadriceps
muscle from the hip-strengthening exercises performed in the 2
groups. The activation of a MU is associated with an
improvement in neuromuscular control.59 In the present study,
hip-strengthening exercises were performed in the 2 groups.
Hip-strengthening exercises are a combination of hip external
rotation and extension, which causes an eccentric contraction of
the quadriceps muscles.28,37 Several previous studies reported
that eccentric contraction is more effective in the recruitment of
fast-twitch MU than concentric contraction23,42 because eccentric
contraction can affect the regulation of the central descending
pathways through the Golgi tendon organ reflex, such as the
inhibitory pathways23; that is, the inexcitability of motor neurons
caused by the Golgi tendon organ reflex may be prevented.16
Therefore, the eccentric contraction of the quadriceps muscle
from hip-strengthening exercises may enhance the activation of
the MU, which may lead to an improvement in neuromuscular
control.36 Some researchers have suggested that eccentric
contraction is more effective than other methods for improving
pain and function in patients with anterior knee pain, such as
those with PFPS or chondromalacia.22,27
In the present study, no statistically significant differences in
patient-reported outcomes were found between the static and
dynamic stretching groups. The VAS and AKPS scores improved
after each stretching exercise by more than the MCID results
presented in a previous study by Crossley et al12 because
patient-reported outcomes may be affected by the recovery of
neuromuscular control.54 Another possible reason why no
significant differences in muscle activation time and patient-
reported outcomes were found between the groups may be
related to the improved hip muscle strength. The hip muscle
strengthening program was included in the conservative
rehabilitation protocol for both groups. According to a
systematic review by Barton et al,5 the hip muscles can affect
the biomechanics of the knee joint in patients with PFPS, which
can be associated with knee joint pain and function.5,57 Hip
muscle strength plays an important role in improving the
neuromuscular control of the hip and knee joints in patients
with knee injuries.8,18,65 The reason why no significant
differences in muscle activation time and patient-reported
outcomes were found between the static and dynamic stretching
groups may be related to the improvement of the quadriceps
MU and enhancement of the gluteal muscles strength by the
hip-strengthening exercises.
The present study has several limitations. Hip muscle strength,
which plays an important role in the biomechanics of the lower
extremity, was not measured.50 Another important limitation was
the absence of an electromyographic device to assess
neuromuscular control. Finally, there was not a healthy control
group in this study, which limits the interpretation of these
results.
conclusion
Quadriceps flexibility and strength, muscle activation time, and
patient-reported outcomes in patients with PFP who had
inflexible quadriceps showed no significant differences between
the static and dynamic quadriceps stretching exercise groups.
Therefore, both static and dynamic stretching exercises may be
effective for improving pain and function in patients with PFP
who have inflexible quadriceps.
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