Hip and Knee Strength Is Not Affected in 12-16 Year Old Adolescents with Patellofemoral Pain - A Cross-Sectional Population-Based Study
et al. (2013) Hip and Knee Strength Is Not Affected in 12-16 Year Old Adolescents with
Patellofemoral Pain - A Cross-Sectional Population-Based Study. PLoS ONE 8(11): e79153. doi:10.1371/journal.pone.0079153
Hip and Knee Strength Is Not Affected in 12-16 Year Old Adolescents with Patellofemoral Pain - A Cross-Sectional Population-Based Study
Camilla Rams Rathleff 0
William Neill Baird 0
Jens Lykkegaard Olesen 0
Ewa Maria Roos 0
Sten Rasmussen 0
Michael Skovdal Rathleff 0
Francois Hug, The University of Queensland, Australia
0 1 Orthopaedic Surgery Research Unit, Aalborg University Hospital , Aalborg , Denmark , 2 Department of Rheumatology, Aalborg University Hospital, Denmark , Aalborg , Denmark , 3 Research Unit for Musculoskeletal Function and Physiotherapy, Institute of Sports Science and Clinical Biomechanics, University of Southern Denmark , Odense, Denmark, 4 HEALTH , Aarhus University , Aarhus , Denmark
Background: One of the rationales behind using strength training in the treatment of adolescents with Patellofemoral Pain (PFP) is that reduced strength of the lower extremity is a risk factor for PFP and a common deficit. This rationale is based on research conducted on adolescents .15 years of age but has never been investigated among young adolescents with PFP. Objectives: To compare isometric muscle strength of the lower extremity among adolescents with PFP compared to ageand gender-matched pain-free adolescents. Methods: In 2011 a population-based cohort (APA2011-cohort) consisting of 768 adolescents aged 12-15 years from 8 local schools was formed. In September 2012, all adolescents who reported knee pain in September 2011 were offered a clinical examination if they still had knee pain. From these, 20 adolescents (16 females) were diagnosed with PFP. Pain-free adolescents from the APA2011-cohort (n = 20) were recruited on random basis as age- and gender-matched pairs. Primary outcome was isometric knee extension strength normalized to body weight (%BW) and blinded towards subject information. Secondary outcomes included knee flexion, hip abduction/adduction and hip internal/external rotation strength. Demographic data included Knee Injury and Osteoarthritis Outcome Score (KOOS) and symptom duration. Results: Adolescents with PFP reported long symptom duration and significantly worse KOOS scores compared to pain-free adolescents. There were no significant differences in isometric knee extension strength (D0.3% BW, p = 0.97), isometric knee flexion strength (D0.4% BW, p = 0.84) or different measures of hip strength (D0.4 to 1.1% BW, p.0.35). Conclusion: Young symptomatic adolescents with PFP between 12 and 16 years of age did not have decreased isometric muscle strength of the knee and hip. These results question the rationale of targeting strength deficits in the treatment of adolescents with PFP. However, strength training may still be an effective treatment for those individuals with PFP suffering from strength deficits.
Knee pain is common during adolescence and up to 25% of
adolescents report having knee pain [1,2]. One of the most
frequent knee conditions among adolescents is Patellofemoral Pain
(PFP) [1,3]. Population-based studies have shown that the
prevalence of PFP among adolescents aged 1519 years of age is
between 6 and 7% [1,3]. These adolescents report long-standing
knee pain with average pain duration of more than three years,
which suggest that PFP may exist among even younger
adolescents. In addition to long standing pain, adolescents with
PFP report severe reductions in function and health-related quality
of life (HRQoL) .
We recently published data showing reduced isometric knee
extension strength among adolescents with PFP aged 1519 years
compared to gender and age-matched adolescents without knee
pain . On average the adolescents reported knee pain for 3
years. The reduction in knee extension strength was expected as
reduced muscle strength is a common deficit among adults (.18
years of age) with PFP . Supporting this, Duvigneaud et al. and
Boling et al. discovered that military recruits who later developed
PFP had ,10% lower isokinetic knee extensor and knee flexor
peak torque than the recruits who did not develop PFP during
basic military training [6,7]. These results indicate that even small
reductions in knee extension and knee flexion strength may
constitute a risk factor for developing PFP among adults. Strength
deficits is not only located around the knee but also hip abduction,
adduction, external and internal rotation strength are reduced
The latest review covering treatment of patients with PFP
advocate for strength training as a keystone in the treatment of
PFP  as strength training offers superior effect compared to
no-treatment control . One of the most important rationales
behind this treatment is that reduced strength of the lower
extremity appears to be both a risk factor and a common deficit.
However this is exclusively based on research conducted on
adolescents above 15 years of age, adults and military recruits.
Adolescents with PFP younger than 15 years of age may be
different than adolescents aged 1519 years because of younger
age, a larger proportion being prior to puberty, and shorter pain
duration. Furthermore, no randomised trials have been conducted
among young adolescents with PFP (,15 years of age) nor have
strength deficits been investigated among young adolescents with
PFP. Therefor the rationale for treating young adolescents with
strength training has never been tested [14,15].
Given the lack of studies specifically investigating young
adolescents with PFP the purposes of this study were to investigate
isometric muscle strength around the knee and hip. The primary
hypothesis was that adolescents with PFP would have reduced
isometric knee extension strength. Secondary hypothesises were
that adolescents with PFP would have reduced isometric muscle
strength in knee flexion and hip abduction/adduction and hip
internal and external rotation.
This cross-sectional study compared 20 adolescents diagnosed
with PFP to 20 age- and gender matched pain-free adolescents.
Both groups were recruited from the same population-based
cohort (Adolescent Pain in Aalborg 2011, the APA2011-cohort)
. The study was approved by the Ethics committee of North
Denmark Region (N-20110020) and the Danish Data Protection
Agency. All participants were required to give written informed
consent accompanied by their parents consent. The study was
conducted according to the Declaration of Helsinki. The reporting
of the study complies with the Strengthening the Reporting of
Observational studies in Epidemiology (STROBE) statement
In September 2011 eight lower secondary schools in the
community of Aalborg were invited to answer an online
questionnaire and to be part of the APA2011-cohort. A total of
768 students aged 1215 years answered the online questionnaire
and 215 students (28%) reported knee pain. In September 2012,
all adolescents who reported knee pain in September 2011 were
contacted by telephone within a time period of two weeks,
Figure 1. Those who still had knee pain were offered a
standardised clinical examination by an experienced
rheumatologist if they fulfilled the following criteria: pain for more than 6
weeks; pain felt anteriorly around the patella or diffusely around
the knee; no treatment within the previous 12 months, no previous
knee surgery and a history of insidious onset of knee pain.
The pain-free adolescents were recruited on random basis as
age- and gender matched pairs from the same cohort that the
adolescents with PFP were recruited from (the APA2011-cohort).
Recruitment took place in the same time period as recruitment of
adolescents with PFP. The inclusion criteria for the pain-free
adolescents were: no current self-reported musculoskeletal pain; no
self-reported prior surgery in the lower extremity; no self-reported
neurological or medical conditions.
In- and Exclusion Criteria during the Clinical Examination
During the clinical examination, the students were diagnosed
with PFP if they met the following criteria :
Insidious onset of anterior or retro-patellar knee pain for more
than 6 weeks and provoked by at least two of the following
positions or functions: prolonged sitting or kneeling, squatting,
running, hopping or stair walking
Tenderness on palpation of the patella, or pain with stepping
down or double leg squatting; and
Worst pain experienced during the previous week should be
reported to be more than 30 mm on a 100 mm Visual
Analogue Scale (VAS).
Exclusion criteria were concomitant injury or pain from the hip,
lumbar spine, or other structures of the knee; i.e. tendinopathy or
Mb Osgood Schlatters disease; previous knee surgery;
patellofemoral instability; knee joint effusion; or weekly use of
antiinflammatory drugs. Adolescents who had been treated for PFP
using physiotherapy during the last year were excluded.
Treatment of PFP would most likely include strength training why
inclusion of these adolescents could cause a differential selection
bias among adolescents with PFP.
Growing pains (GP) may be mistaken for PFP among young
adolescents with PFP. However there are a few important
differences between the two pain conditions that was used during
clinical examination to distinguish the two pain conditions.
Growing Pain is usually non-articular and located to the shins,
calves, thighs or popliteal fossa . The pain usually appears late
in the day or is nocturnal, often awaking the child. Another
distinct difference between the two pain syndromes is the pain
debut. Adolescent PFP usually debuts when the adolescent is 11
13 years of age, while GP debut much earlier, at around 612
years of age [3,19].
Primary outcome were isometric knee extension strength using
the best of three consecutive measurements normalized to body
weight (%BW). Secondary outcomes included knee flexion, hip
abduction/adduction and hip internal/external rotation.
Outcome measurements were collected from the most pain-full knee in
adolescents with PFP and on an identical proportion of right and
left knees among pain-free adolescents. All measurements were
done in September 2012 by a rater with previous experience in
muscle strength testing. The rater was a physiotherapist and
blinded to which of the 40 adolescents were diagnosed with PFP.
Isometric Muscle Strength
The testing setup included a portable dynamometer and an
examination table. Muscle strength was tested with the Mecmesin
AFG2500 dynamometer that was attached to the wall through a
fixed bolted connection to ensure fixation. All strength tests were
isometric strength tests. The test positions were chosen based on
procedures that are often applied in clinical settings. To make sure
the test procedure was reliable, a pilot study was conducted two
weeks prior to the testing of the adolescents (see later header
Reliability). A total of six movement directions around the knee
and hip were tested; knee flexion and extension; hip abduction and
adduction; hip internal and external rotation.
Figure 1. Flow-chart. Flowchart showing recruitment of adolescents with PFPS and gender and age-matched pain free adolescents without knee
pain. *Telephone screening: pain for more than 6 weeks; pain felt anteriorly around the patella or diffusely around the knee; no treatment within the
previous 12 months, no previous knee surgery and a history of insidious onset of knee pain.
During all strength tests the participants were told to stabilize
themselves by holding on to the sides of the examination table. A
cotton cloth was placed between their lower legs and the strap
from the dynamometer to allow for both standardization of the
dynamometer placement and pain reduction from the pressure
created by the dynamometer.
After receiving instruction about the procedure, the participants
were asked to perform one isometric sub-maximal trial. Then an
additional practice trial was applied. Afterwards the individual test
was administered three times to reduce a possible learning effect.
The highest value of three consecutive measurements are
presented. A 1-minute rest period was given after each trial. The
standardized command given by the examiner was: Go
aheadpush-push-push-push and relax. To allow for comparison to the
latest review covering strength deficits in patients with PFP we
reported strength normalized to bodyweight (BW) .
During knee extension and knee flexion, the strap from the
dynamometer was positioned perpendicular to the anterior or
posterior aspect of the tibia, 5 cm proximal to the medial
malleolus (Figure 2, a & b). Knee extension was tested in a fixed
position at 60 degrees of knee flexion while knee flexion was tested
during 90 degrees of knee flexion. A pilot study revealed that 90
degrees of knee flexion during knee extension induced knee pain
why the adolescents reported they could not exert maximal force
due to pain. Therefore 60 degrees of knee flexion was chosen.
During hip abduction and adduction the participant was lying
supine on the examination table (Figure 2, C & F). The strap from
the dynamometer was positioned perpendicular to the medial or
lateral aspect of the tibia, 5 cm proximal to the medial malleolus.
The leg was placed in 0 degrees flexion and 0 degrees abduction.
Hip external and internal rotation was tested with the
participant sitting on one side of the examination table with the
hip and knee flexed at 90 degrees (Figure 2, D & E). The strap
from the dynamometer was positioned perpendicular to the lower
leg 5 cm proximal to the medial malleolus on either the lateral or
medial part of the lower leg. The subject was positioned in an
upright position, sitting on the edge of the examination table, with
90 degrees hip and knee flexion.
Secondary outcomes included the patient-reported
questionnaire Knee Injury and Osteoarthritis Outcome Score (KOOS)
 which contains five separate subscales (Pain, Symptoms,
Activity in Daily Living (ADL), Function in Sport and Recreation
Figure 2. Isometric muscle strength test-positions. The figure shows the test-positions of the six movement directions during isometric knee
and hip strength. A: knee flexion, B: knee extension, C: hip abduction, D: hip internal rotation, E: hip external rotation, F: hip adduction.
(Sport/Rec), knee-related quality of life (QoL)) that assess the
patients opinion about their knee and associated problems. This
questionnaire was chosen as it has previously been used in young
adolescents with knee pain . Further, the Pain Catastrophizing
Scale (PCS) was used to assess the participants response to pain
. The categories of questions can be divided into: rumination,
helplessness and magnification. Health related quality of life was
measured by the youth version of the European Quality of Life 5
dimensions (EQ-5D) .
Physical activity level was measured with The Physical Activity
Scale (PAS) . PAS consists of 9 different activities, where the
participant has to fill out 24 hours of work, leisure time and sports
on an average weekday. The answers were then transformed into a
Metabolic Equivalents (MET). A MET is defined as the oxygen
uptake in ml/kg/min with one MET being equal to the oxygen
cost of sitting quietly, equivalent to 3.5 ml/kg/min. Age, height,
weight, Body Mass Index (BMI) and pain duration were included
Before data collection, a test-retest intra-rater reliability study
was performed to investigate the reliability and agreement of the
isometric strength testing for the best of three measurements. The
study included a convenience sample of 17 young adults who were
tested twice, with 30 minutes between test and retest. A two-way
random effects model (2.1), single measures, absolute agreement,
intraclass correlation coefficients (ICC) were used to express
intrarater reliability. Limits of Agreements (LoA) were used to express
the agreement between test and retest . Agreement was
presented as LoA divided by the mean and multiplied by 100 to
represent the maximal difference in percentage in 95% of the
measurements (LoA%). ICC for all six movement directions were
above.0.92 and LoA% was below 29.2% for all six movement
directions. The lowest reliability and agreement was found in hip
The sample-size was based on detecting a difference between
groups of at least 20% on the normalized isometric quadriceps
strength. This difference was based on quadriceps torque data
from our previous study comparing adolescents aged 1519 years
with a similar methodology . Using a common standard
deviation of 0.5 Nm/Kg, power of 80%, and an alpha level of 5%,
at least 17 adolescents were needed in each group to detect a 20%
difference between groups. The number of adolescents diagnosed
with PFP one year after inclusion in the APA2011-cohort
determined the final sample-size. Therefore 20 adolescents with
PFP and 20 pain-free adolescents were included.
All data were visually inspected using a Q-Q plot. Mean values
6SD are reported if data were normally distributed. If data were
non-normally distributed they were presented as median and
interquartile range (IQR). Paired samples t-test was used to test the
difference in isometric strength between matched pairs. All
calculations were performed using Stata version 11 (StataCorp,
College Station, Texas, USA).
Average pain duration among adolescents with PFP was 28.5
months (Table 1). Self-reported outcome measures showed higher
pain levels and significantly worse KOOS scores across all five
domains among adolescents with PFP (Figure 3). On average, the
adolescents with PFP had 1943 points lower KOOS scores. The
HRQoL, measured with the EQ-5D youth version, and the
participants response to pain measured with the Pain
Catastrophysing Scale, were significantly worse among adolescents with
PFP compared to pain-free adolescents, Table 2. Physical Activity
Scale showed no difference in activity level among adolescents
with PFP and pain-free adolescents.
Primary and Secondary Outcomes
Mean difference between groups in primary outcome, isometric
knee extension strength, was 0.3% BW, p = 0.97. The difference in
isometric knee flexion strength was 0.4% BW while the differences
in hip strength ranged from 0.4 to 1.1% BW, p.0.35 (Table 3).
Converting the isometric strength measurements from %BW to
torque (Nm) or Nm/kg using leg length or length of the lower leg
did not change the results. Likewise, adjusting for age, BMI or
physical activity level did not change the magnitude of difference
This study is the first to compare isometric knee and hip muscle
strength among young adolescents with PFP and compare them to
age- and gender matched pain-free adolescents from the same
population-based cohort. We hypothesized that adolescents with
PFP would have significantly lower isometric muscle strength of
the knee and hip. Despite self-report of functional limitations and
long-lasting severe pain, adolescents with PFP did not have
decreased isometric muscle strength of hip and knee compared to
age- and gender matched pain-free adolescents.
We have previously reported that adolescents aged 1519 years
showed a significantly lower isometric knee extension strength .
However these findings were not reproduced in the current
younger cohort, even though the same methodology was used and
both study groups were recruited from the APA2011-cohort. On
average, the adolescents with PFP between 15 and 19 years of age
were 3 years older and reported a 1 year longer symptom duration
than the younger adolescents in the current study but reported
similar KOOS scores. This could suggest that decreased isometric
muscle strength of the knee in adolescents aged 1519 years may
be a consequence of longstanding PFP. We hypothesize that the
decreased muscle strength found among 1519 year olds may be a
result of a decreased activity level. Previous studies do indeed
suggest that patients with longstanding PFP decrease their activity
level [26,27,28]. A reduced activity level may not be enough to
stimulate the same increases in muscle strength during a period of
rapid growth and weight increase as their gender and age-matched
peers . Other studies have suggested that PFP may develop
after an excessive degree of sports participation and a high activity
level [30,31]. The results showed a trend towards a 3 MET lower
physical activity in adolescents with PFP compared to pain-free
adolescents. The adolescents with PFP in the current study may
already have started to decrease their physical activity level as a
consequence of PFP [28,32,33]. However the cross-sectional
design of the current study does not allow us to infer if adolescents
with PFP have altered their physical activity level after they
Strengths and Limitations
Our sample of adolescents with PFP and the pain-free
adolescents were both recruited from the same well-defined
population-based cohort. In addition knee pain was confirmed at
two time points in the group with PFP, one year apart. The
Adolescents with PFP N = 20
Pain-free adolescents N = 20
Gender (number of females)
Dominant lower extremity (number who replied right leg)
Most symptomatic knee (number who replied right knee)
Figure 3. Knee injury and Osteoarthritis Outcome Score: Outcome profile. Mean KOOS subscales are presented and reported as an
outcome profile for the adolescents with patellofemoral pain (PFP) versus the age- and gender matched pain-free adolescents. KOOS subscales: Pain,
Symptoms, Activity in Daily Living (ADL), Sport and Recreation (Sport&recr) and Knee-related Quality of Life (QoL). Error bars represent 95%
recruitment of a previously untreated population-based sample
suggests the results may be generalizable to the broader adolescent
population. The few previous studies on adolescents with PFP are
all patient-based studies. We recruited adolescents from a closed
population-based cohort, which may suggest that the adolescents
with PFP have shorter symptom duration and lower pain intensity
compared with patients who have already consulted their general
practitioner. Only a small number of the adolescents with
selfreported knee pain were excluded because they were already
receiving treatment (n = 16, 20%) (see figure 1). Worst pain during
the previous week and pain duration indicate that our sample of
young adolescents with PFP is comparable to patient-based studies
with regard to pain levels [10,15,34,35]. The comparison of
symptom duration between adolescents in the current study and
our previous study of adolescents aged 1519 years of age should
be interpreted with care. Symptom duration may be heavily
influenced by recall bias and it is unknown if the knee pain started
as part of PFP or was associated with a different knee condition.
The sample-size calculation was based on the results of our
previous study . The data used in the sample-size calculation
did not hold true for adolescents between 12 and 16 years. The
current study could be in risk of being underpowered, but looking
at the mean difference between groups (0.3 to 1.1% of BW), none
of the current methods for strength measurements would have
been able to significantly detect such a small difference. Also, one
could argue that a difference around 1% of BW would not be
clinically relevant. In some movement directions, adolescents with
PFP had slightly higher isometric muscle strength while in other
movement directions it was slightly lower. This difference in
directions strengthens the assumption that adolescents with PFP
do not have lower isometric muscle strength around the hip and
knee compared to pain free adolescents. This study investigated
isometric strength around the knee and hip. Other aspects of
muscle function such as isokinetic strength and endurance could
still be impaired. Furthermore, recent evidence suggests that hip
extension may be impaired and future studies should investigate
hip extension among young adolescents with PFP.
EQ-5D, visual analog score*
Physical Activity Scale (Metabolic equivalent)
20.23 (20.28; 20.20)
Abreviations: EQ-5D: European Quality of Life 5 Dimensions. PAS: Physical Activity Scale. PCS: Pain Catastrophizing Scale (PCS).
*EQ-5D, PAS and PCS are reported as median and interquartile range. Mean differences are presented together with a 95% Confidence Interval (95%CI).
PFP Mean (SD)
The table show the isometric muscle strength among adolescents with PFP and pain-free adolescents. Strength was normalized to bodyweight (%BW) and best out
three trials is reported.
*Data are presented as mean and standard deviation (SD). The mean differences are reported as a 95% Confidence Interval (95% CI) with a corresponding p-value.
Different subgroups may respond differently to treatment. The
different subgroups of patients with PFP have been discussed
earlier but primarily in relation to the efficacy of foot orthoses
[36,37,38]. Subgrouping is often based on anthropometric
characteristics and physical deficits. Based on the results of this
study one might speculate if it would also be relevant to subgroup
patients with PFP based on age, because the underlying aetiology
might be different from that of older adolescents and adults. Based
on symptom duration of adolescent PFP in the APA2011-cohort,
their knee pain developed when the adolescent was between 11
and 13 years of age . van Linschoten et al reported that almost
70% of their patients had a symptom duration between 2 and 6
months, while the median symptom duration reported by Collins
et al. was 28 months with only 25% having symptom duration
below 12 months [39,40]. Even though the stated symptom
duration may be influenced by recall bias, they suggest that PFP
may develop during adolescence or later in life. This may suggest
that adolescent PFP and adult PFP represent two distinct
pathologies and should be investigated separately or that
adolescent and adult PFP represent the same knee condition at
two different stages of disease. Future studies on the development
of adolescent PFP and aetiology are highly warranted.
Despite self-report of functional limitations, long-lasting severe
pain and decreased quality of life, 1216 year old adolescents with
Patellofemoral Pain do not have decreased isometric muscle
strength of the knee and hip compared to age and gender matched
pain-free adolescents. These results question the rationale of
targeting strength deficits in the treatment of adolescents with PFP.
However, strength training may still be an effective treatment for
those individuals with PFP suffering from strength deficits.
Conceived and designed the experiments: CRR MSR JLO WNB.
Performed the experiments: CRR MSR JLO WNB. Analyzed the data:
CRR MSR EMR. Contributed reagents/materials/analysis tools: JLO
MSR WNB SR. Wrote the paper: CRR WNB JLO EMR SR MSR.
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