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Knee muscle strength at varying angular velocities and
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associations with gross motor function in ambulatory children
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with cerebral palsy
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Wei-Hsien Hong,a PhD; Hseih-Ching Chen,b PhD; I-Hsuan Shen,c PhD; Chung-Yao
Chen,d,e MD; Chia-Ling Chen,f,g MD, PhD; Chia-Ying Chung,e,f MD
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Technology, 1, Sec. 3, Chung-Hsiao E. Rd, Taipei, 10608, Taiwan.
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Department of Occupational Therapy, Chang Gung University, 259 Wen-Hwa 1st Rd,
Kwei-Shan, Tao-Yuan 333, Taiwan.
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Department of Physical Medicine and Rehabilitation, Chang Gung Memorial Hospital,
Keelung, 222 Maijin Rd, Keelung 204, Taiwan.
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School of Medicine, College of Medicine, Chang Gung University, 259 Wen-Hwa 1st Rd,
Kwei-Shan, Tao-Yuan 333, Taiwan.
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Physical Medicine and Rehabilitation, Chang Gung Memorial hospital, 5 Fu-Hsing St.
Kwei-Shan, Tao-Yuan 333, Taiwan.
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Department of Sports Medicine, China Medical University, 91 Hsueh-Shih Road,
Taichung 40402, Taiwan.
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Graduate Institute of Early Intervention, Chang Gung University, 259 Wen-Hwa 1st Rd,
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Department of Industrial Engineering and Management, National Taipei University of
Kwei-Shan, Tao-Yuan 333, Taiwan.
*
Address correspondence to:
Chia-ling Chen, MD, PhD
Department of Physical Medicine & Rehabilitation,
Chang Gung Memorial Hospital,
5 Fu-Hsing St. Kwei-Shan, Tao-Yuan 333, Taiwan
E-mail: clingchen@gmail.com
Phone number: +886-3-3281200 ext 3846
Fax number: +886-3-3281320
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Running title: Muscle strength & gross motor function in CP
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Word count of the text: 4257
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The number of figures and tables in the article: one figure and 4 tables
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FINANCIAL DISCLOSURE/CONFLICT OF INTEREST
All funding sources supporting the work and all institutional or corporate affiliations of
mine are acknowledged in an acknowledgement section.
I assert that there are no conflicts of interest (both personal and institutional) regarding
specific financial interests that are relevant to the work conducted or reported in this
manuscript. There are no potential conflicts of interest related to individual authors'
commitments, no potential conflicts of interest related to project support, and no potential
conflicts of interest related to commitments of editors, journal staff, or reviewers.
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Abstract
The aim of this study was to evaluate the relationships of muscle strength at different
angular velocities and gross motor functions in ambulatory children with cerebral palsy
(CP). Thirty-three ambulatory children with spastic CP aged 6–15 years who were
categorized by the Gross Motor Function Classification System (GMFCS) as having either
with level I (n=17) or level II (n=16) and 15 children with normal development. All
children underwent cur-up test and isokinetic tests of the knee extensor and flexor muscle.
Children with CP underwent the gross motor function assessments, including the Gross
Motor Function Measure (GMFM-66) and the gross motor subtests of Bruininks-Oseretsky
Test of Motor Proficiency (BOTMP). The hamstring-quadriceps ratio (HQ ratio) was
calculated as 100% × (isokinetic peak torque of hamstring (knee flexor)/isokinetic peak
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torque of quadriceps (knee extensor)). Children with GMFCS level II had lower BOTMP
and GMFM-66 scores, curl-up scores, HQ ratio, and knee muscle strength, especially knee
flexor, compared to those with GMFCS level I. The regression analysis showed that knee
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flexor torques at 60 and 90/s are mainly related to balance (r2=0.167, p=0.011) and strength
(r2=0.243, p=0.002) while knee flexor torques at 120/s mainly contribute to running speed
and agility (r2=0.372, p<0.001). These findings suggest that children with CP had knee
strength deficits, especially knee flexor. Postural muscle (knee flexor) strength dominated
gross motor function than antigravity muscle strength (knee extensor). The knee flexor
strength at different angular velocities was associated with various gross motor tasks. The
HQ ratio may be used as a potential biomarker to probe the therapeutic effectiveness for
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muscle strengthening in these children. These data may allow clinician for formulating
effective muscle strengthening strategies for these children.
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Keywords: cerebral palsy; postural muscle; gross motor function; muscle strength;
isokinetic strength.
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1. Introduction
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(Elder et al., 2003; Wiley & Damiano, 1998). The muscle weakness showed a stronger
association with mobility limitations in children with CP than spasticity (Ross & Engsberg,
2007). Muscle weakness can cause further loss of function and further limitation of
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participation in daily life (Givon, 2009).
Children with CP exhibit weakness not only in limb muscles (Larsson, Karlsson, &
Gerdle, 2008; Chen, Lin, et al., 2012a), but also in trunk muscles (Prosser, Lee, VanSant,
Barbe, & Lauer, 2010; Rosenbaum et al., 2007). Muscle strength and endurance must be
accurately measured for effective treatment of children with CP who show muscle weakness.
Limb muscles are typically measured by manual muscle testing and isokinetic testing
(Chmielewski, Mizner, Padamonsky, & Snyder-Mackler, 2003). Although manual muscle
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testing is relatively easier to administer, its results are less objective in subjects associated
with spasticity, such as CP. In contrast, isokinetic testing is an objective measurement of
muscle strength (Nicholas, 1989; Esselman, de Lateur, Alquist, Questad, & Giaconi, 1991).
Previous studies showed that the knee extensor and flexor muscle strength were decreased
with increasing angular velocity in normal children (Chan, Maffulli, Korkia, & Li, 1996;
Kannus & Beynnon, 1993; Alangari & Al-Hazzaa, 2004). The studies by Engsberg, Olree,
Ross, & Park (1998) showed children with CP were not only weaker than normal children at
slow isokinetic velocities but had increasingly greater decrements in torque with increasing
velocity. The isokinetic strength tests of the knee flexor and extensor are reportedly highly
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reliable for different angular velocities (Perrin, 1993). Muscle endurance, can be defined as
the capability of a muscle group to execute repeated contractions sufficient to cause muscular
fatigue within a given time or as its capability to maintain a maximal voluntary contraction
for a prolonged period (Ruiz et al., 2006). The curl-up test is widely used to assess trunk
muscle endurance (Ruiz et al., 2006). Thus, the curl-up test and isokinetic measurements were
used to measure the trunk muscular endurance and knee muscle strength in this study. The
Gross Motor Function Measure (GMFM) is a standardized and well-validated observational
instrument for measuring change in gross motor function in children with CP (Russell,
The reported incidence of cerebral palsy (CP), the most common motor disability in
childhood, is 1.5–2.5 per 1000 live born children (Himmelman, Hagberg, Beckung, Hagberg,
& Uvebrant, 2005). The term CP describes a group of movement and posture disorders that
limit activity and participation (Rosenbaum, et al., 2007). The typical motor manifestations
include various neuromuscular and musculoskeletal problems such as spasticity, dystonia,
contractures, abnormal bone growth, poor balance, loss of selective motor control, and muscle
weakness (Giuliani, 1991; Gormley, 2001). Muscle weakness, which is a common
impairment in children with CP (Damiano, Vaughan, & Abel, 1995; Wiley & Damiano, 1998),
is attributable to incomplete recruitment or decreased motor unit discharge rate (Elder et al.,
2003; Rose & McGill, 2005) and to inappropriate coactivation of antagonist muscle groups
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Rosenbaum, Avery, & Lane, 2002). Reports of moderate to high correlations between muscle
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strength and GMFM scores (Kramer & MacPhail ,1994; Damiano, Martellotta, Quinlivan, &
Abel, 2001; Eek & Beckung, 2008) indicate that muscle strength has an important role in
gross motor abilities. The GMFM-66 has superior scoring, interpretation, and overall clinical
and research utility compared to the original GMFM-88 (Avery, Russell, Raina, Walter, &
Rosenbaum, 2003). The GMFM-66 is more responsive than the GMFM-88 in terms of
consistency with the subjective clinical judgment of the therapist (Wang & Yang, 2006).
However, the GMFM-66 scores may achieve the ceiling levels for CP children with high
motor ability. Therefore, the Bruininks–Oseretsky Test of Motor Proficiency (BOTMP)
(Bruininks, 1978), another well-validated measure for evaluating motor coordination in
children with CP (Chen et al., 2011; Gordon, Schneider, Chinnan, & Charles, 2007), was also
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used to measure gross motor function and coordination in this study.
To date, however, no studies have examined relationships between muscle strength and
muscle endurance and gross motor function during different motor tasks in ambulatory
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children with CP. This study tested three hypotheses: muscle strength and endurance are
associated with gross motor functions in children with CP; the knee flexor (postural muscle)
has a stronger association with gross motor functions compared to the knee extensor
(antigravity muscle); and isokinetic knee strength at different angular velocities is associated
with different motor tasks. This work explores how gross motor function during different
motor tasks is affected by muscle strength of the knee extensor and flexor at varying angular
velocities in these children. Muscle strength, muscular endurance and gross motor functions
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were also compared in children with varying severity of motor impairment. More importantly,
this study attempts to identify a potential biomarker to probe the therapeutic effectiveness for
muscle strengthening in these children.
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2. Material and Methods
2.1. Participants
Children with spastic CP from the Physical Medicine and Rehabilitation Department of a
tertiary hospital were recruited for this study. The inclusion criteria were comprised a
diagnosis of CP with spastic diplegia, spastic hemiplegia, or mild spastic quadriplegia, an age
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of 6–15 years, ability to walk independently, ability to undergo a motor function and
isokinetic muscle test, and ability to comprehend commands and cooperate during an
examination. Exclusion criteria were as follows: 1) children with recognized chromosomal
abnormalities; 2) children with a progressive neurological disorder or severe concurrent
illness or disease that is not typically associated with CP; 3) children with active medical
conditions such as pneumonia; 4) children who had undergone any major surgery or nerve
block in the preceding 3 months; 5) children with hormonal disturbance; and, 6) children with
poor tolerance for performing the isokinetic test or a poor ability to cooperate during
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assessment. Ultimately, 33 children with spastic CP enrolled in this study were categorized
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into two groups according to the Gross Motor Function Classification System (GMFCS)
(Palisano, et al., 1997): level I (n=17) and level II (n=16) groups. An additional 15 age- and
gender-matched children with normal development (ND) were selected as the control group.
The exclusion criteria for the ND children included any diagnosis of developmental delay,
growth failure, or neurological disorder; or any active medical problems (e.g., congenital
heart disease) or other physical disabilities. The Institutional Review Board for Human
Studies at Chang Gung Memorial Hospital approved this protocol, and caregivers of all
participants or participants gave informed consent.
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All participants underwent a series of examinations, including characteristics, and knee
muscle strength assessments. The gross motor function (including GMFM-66 and BOTMP
assessments) were performed only in children with CP. A physical therapist was trained to
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use an isokinetic dynamometer and gross motor function assessments as a precondition of
study participation. Motor severities, GMFCS scores, were graded by the same physiatrist.
Participant characteristics, including demographic, growth, and clinical data were also
recorded.
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weight, and body mass index (BMI). Weight was assessed with an ACME seated weight scale
(Model ACSMIN). Height was defined as the maximum distance from the feet to the highest
point on the head with the subject standing with the heels, knees, buttocks, and back in
contact with a wall with the help of assistants. The BMI was calculated by dividing weight by
height squared (kg/m2). Abdominal muscle endurance was measured by a curl-up test. With
the subject lying on the floor with knees flexed, the test required the participant to “curl up”
the trunk and then lower it to the floor as many times as possible within 1 minute. The curl-up
score was calculated as the maximum number of curl-ups correctly performed in one minute.
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2.2.2. Gross motor function
In both clinical practice and international rehabilitation research, the GMFM is the
recognized gold standard for evaluating quantitative changes in gross motor function. The
GMFM scores are known to remain stable over a 2-year period in children aged 9 to 15 years
(Voorman, Dallmeijer, Knol, Lankhorst, & Becher, 2007). The 66 items in the GMFM-66
subset have been graded for difficulty using Rasch analysis, with a maximum score of 100.
(Russell, et al., 2002). The GMFM-66 score was obtained by using Gross Motor Ability
Estimator software (Russell, et al., 2002). Each item on the GMFM is graded on a 4-point
2.2. Procedures
2.2.1. Demographic and growth data
Collection of demographic and growth data included age, gender, body height, body
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scale (0= child unable to initiate the task, 1= child initiates the task, 2= child partially
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completes the task, 3= child completes the task); these scores are then converted into a total
score. The GMFM-66 is best suited for children who can walk and has shown good validity
and reliability (Russell, et al. 2002).
The BOTMP (Bruininks, 1978) is an individually administered test that assesses the motor
function of children. The complete battery consisting of 46 items grouped into eight subtests
provides a comprehensive index of motor proficiency along with separate measures of gross
and fine motor skills. The four gross motor subtests of the BOTMP are running speed and
agility (RSA), balance (BAL), bilateral coordination (BCO), and strength (STR). The standard
score for the gross motor composite (GMC) is calculated by summing the standard scores for
the four subtests. The raw score for each subtest is calculated after administering the test. By
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using the scale provided, the raw score can be converted into a point score for each subtest.
By comparisons with the norm, the point score can then be converted to a standard score for
each subtest (where a higher standard score indicates a better subtest performance). In this
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study, the standard scores for the gross motor subtests and gross motor composite were used
for comparison. The content and construct validity and the test-retest reliability of the
BOTMP have been confirmed previously (Bruininks, 1978).
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NORM®, Humac, CA, USA). Participants were positioned on the Cybex testing chair with a
trunk-to-thigh angle of approximately 95°. The dynamometer input shaft was aligned with the
knee and the dynamometer lever arm was strapped just above the malleoli of the tested leg.
Straps were used to stabilize each participant’s involved thigh, pelvis, and trunk. Each
participant performed warm-up contractions and practiced concentric knee extension and
flexion twice before the test. After resting for 10 seconds, a participant performed five
consecutive cycles of concentric knee extension and flexion. Knee extension-flexion
comprised a maximal voluntary knee extension, followed immediately by a maximal
voluntary knee flexion. Testing velocity was set to 60°/s, 90°/s, and 120°/s, and range of
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motion was 70°, starting with the knee flexed at 80° and ending in an extension at -10°.
Measured variables were isokinetic peak torque of the knee extensor and knee flexor. High
intraclass correlation coefficient (ICC) values (range, 0.93–0.98) existed for peak torque of
absolute isokinetic muscle strength of the knee extensor and flexor at different angular
velocities using Cybex (Impellizzeri, Bizzini, Rampinini, Cereda, & Maffiuletti, 2008). The
isokinetic peak torque of the knee extensor and knee flexor was normalized by body weight
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(Nm/kg). The hamstring-quadriceps ratio (HQ ratio) was calculated as 100%  (isokinetic
peak torque of hamstring (knee flexor)/ isokinetic peak torque of quadriceps (knee extensor)).
2.2.3. Muscle strength
The knee extension and flexion torque of the more-affected lower limb that was generated
during repeated extension-flexion was measured using an isokinetic dynamometer (Cybex
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2.3. Statistical Analyses
Statistical analysis was performed using SPSS 12.0 (SPSS, Inc., Chicago, IL, USA).
Group differences in gender were determined by Fisher exact test. Group comparisons of
demographic data (i.e., age, body height, body weight, and BMI) and curl-up score were
performed by a one-way analysis of variance (ANOVA) with post hoc Tukey multiple
comparisons. Student t-test was used to determine the gross motor function scores
(GMFM-66, the subtests and gross motor composite of the BOTMP) differences between CP
groups. Repeated measure ANOVA was used to determine the knee muscle strength with
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group denoting the between factor and angular velocity (60, 90, and 120/s) denoting the
within factor. First, Pearson correlation was used to choose the related variables by testing
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demographic data, muscle strength and curl-up scores for relationships with gross motor
function scores. Multiple stepwise linear regression analysis was then performed to
characterize the relationship of gross motor function scores with related variables. In
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correlation testing, an r of 0.9 to 1.0 was considered very high, 0.7 to 0.89 was considered
high, 0.5 to 0.69 was considered moderate, 0.26 to 0.49 was considered mild, and 0.0 to 0.25
was considered negligible (Munro, 2005). A p < 0.05 was considered statistically significant.
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3. Results
3.1. Demographic, muscle endurance, and gross motor function
Comparisons among the three groups showed no significant differences in age, gender,
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height, weight, and BMI (Table 1). Comparisons of gross motor function showed that
children with GMFCS level I had higher scores for the GMFM-66, the four BOTMP
subtests, and the GMC compared to those with GMFCS level II (p < 0.05) (Table 1).
In muscle endurance, ANOVA results showed that curl-up scores significantly differed
among the three groups (p < 0.001). Post hoc comparisons showed that the curl-up score
was highest in the ND children, followed by children with GMFCS level I , and lowest with
GMFCS level II (Table 2). (本來和前一段一起, 分段後 table1 改成 table 2)
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3.2. Muscle strength
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There were significant differences in peak torques of knee extensor and flexor within
varying angular velocities (extensor: p < 0.001; flexors: p = 0.045) and among groups
(extensor: p = 0.047; flexors: p < 0.001). Significant interaction between groups and velocities
in knee flexor strength (p < 0.001) were noticed (Table 2). Post hoc comparisons showed that
the children with GMFCS level II had lower knee extensor strength than ND children.
Additionally, knee flexor strength was highest in the ND children, followed by children with
GMFCS level I, and lowest with GMFCS level II. Velocity comparisons showed that knee
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extensor strength was greatest at 60/s followed by 90/s and lowest at 120/s for children
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with CP and ND children. Knee flexor was greater at 60/s than at 120/s. Tests of interacting
effects showed that knee flexor strength in ND children significantly decreased with
increasing velocities; conversely, it significantly increased with increasing velocities in
children at GMFCS level II (Table 2). Children at GMFCS level II had 67–77% of normal
knee extensor torque and only 24–39% of normal knee flexor torque as compared with ND
children. Children with GMFCS level I had 84.8–97.8% of normal knee extensor torque and
only 66.2–76.6% of normal knee flexor torque as compared with ND children. There were
significant differences in HQ ratio within angular velocities (p < 0.001) and among groups (p
< 0.001) (Table 2). Post hoc comparisons showed that the ND children had the highest HQ
ratio followed by children with GMFCS level I, and lowest with GMFCS level II. Velocity
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comparisons showed the HQ ratios at 60/s and 90/s were significantly lower than those at
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120/s for all three groups (Table 2).
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Figure 1 showed that the knee flexor strength was significantly associated with the
GMFM-66, and the BOTMP GMC, RSA, and STR sub-scores. Age correlated negatively
with BOTMP BCO and STR sub-scores (r = -.731 and -.353, respectively; p < 0.05, Table
3). The GMFM-66 scores had stronger correlations with knee flexor strength and with
curl-up score (r = 0.643–0.723, p < 0.01) than with knee extensor strength (r = 0.373–0.416,
p < 0.05). Except for BCO, all BOTMP sub-scores correlated with curl-up scores (r =
0.361–0.634, p < 0.05) and peak torques of knee flexor at all angular velocities (r =
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0.388–0.630, p < 0.05) but correlated with peak torques of knee extensor only at 120/s (r =
0.363–0.466, p < 0.05).
The regression analysis results showed the different variables related to motor function
during various motor tasks (adjusted r2 = 0.167–0.581, p < 0.05, Table 4). The knee flexor
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strength at 60/s was positively associated with the GMFM-66 score and with the BOTMP
STR sub-score (Table 4). The knee flexor strength at 120/s was positively related to the
BOTMP GMC scores and RSA sub-scores. Additionally, the knee flexor strength at 90/s
was positively related to the BOTMP BAL sub-scores. The curl-up scores were positively
related to the GMFM-66 score and BOTMP RSA sub-score. Knee flexor strength was the
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main contributor to the GMFM-66 score (i.e., 50.7% of the variance). Knee flexor strength
was also the main contributor to the BOTMP GMC and all sub-scores other than BCO
sub-score (i.e., 16.7–37.2% of the variance).
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4. Discussion
This study is the first to investigate the relationships between muscle strength at varying
angular velocities and gross motor functions during different motor tasks in children with CP.
This research demonstrates children with CP had lower trunk muscular endurance (measured
3.3. Muscle strength and gross motor functions
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by curl-up scores), HQ ratio and lower limb muscle strength (measured by isokinetic peak
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torques of knee muscle), especially knee flexor, than ND children. Children with GMFCS
level I had greater gross motor function (measured by BOTMP and GMFM-66), trunk
muscular endurance, HQ ratio, and knee muscle strength, especially knee flexor, compared to
those with GMFCS level II. The regression analysis showed that knee flexor muscle strength
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dominated gross motor function. Knee flexor torques at low velocity (60–90/s) are mainly
related to static gross motor function such as balance and strength while knee flexor torques at
high velocity may contribute to dynamic gross motor function such as running agility. These
findings suggest that, in children with CP, postural muscle (knee flexor) strength is lower than
antigravity muscle (knee extensor) strength. Postural muscle strength had a stronger
correlation with gross motor function than with antigravity muscle strength in ambulatory
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children with CP. The knee flexor strength at different angular velocities was associated with
various gross motor tasks. More importantly, the HQ ratio may be used as a potential
biomarker to probe the therapeutic effectiveness for muscle strengthening in these children.
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These data may allow clinician for formulating effective muscle strengthening strategies for
these children.
Compared to antigravity muscle strength, postural muscle strength of the lower limbs is
more important in determining gross motor function, balance and strength in ambulatory
children with CP. Additionally, muscular endurance also contributes to the gross motor
function and running agility. Stepwise linear regression showed that knee flexor strength
was associated with GMFM-66 score and all BOTMP sub-scores except BCO by 17-51% of
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variance. The curl-up score was associated with GMFM-66 score and BOTMP RSA
sub-score by 7% of variance. Previous studies also showed that peak torque of knee flexor
and extensor was moderately related to the GMFM-88 (Damiano et al., 2001), and knee
flexor were more correlated with GMFM-66 for children with ambulatory CP than knee
flexor (Chen et al., 2012a). The core muscles, i.e., the abdominal muscle group and the back
extensors, are considered important postural muscles for trunk control (Hrysomallis &
Goodman, 2001). Studies show that the core stability of trunk control is highly related to
lower extremity function in normal subjects (Willson, Dougherty, Ireland, & Davis, 2005).
The poor control of trunk postural muscles is also a major impairment in CP (Prosser et al.,
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2010; Rosenbaum et al., 2007). These findings suggest that the knee flexor muscle should
be prioritized in treatment for enhancing gross motor function in children with CP.
Muscular endurance training can also modulate gross motor function, especially running
agility, in children with CP.
Compared to muscular endurance and knee muscle strength, age had a stronger
association with BOTMP BCO sub-score. The reason may be that bilateral coordination
involves not only the lower limb, but also the upper limb coordination. Furthermore, the
bilateral coordination could not catch up the expected developmental with increasing age in
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children with CP. The knee flexor strength at different angular velocities was associated
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with various gross motor tasks. Regression analysis showed that knee flexor torques of
different angular velocities were related to GMFM-66 score and to some of BOTMP
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sub-scores. For example, knee flexor torques at low velocity (60–90/s) were mainly
associated with GFMF-66 score and with the strength and balance sub-scores of the
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BOTMP while knee flexor torques at high velocity (120/s) were mainly associated with
BOTMP RSA sub-score. These findings suggest that knee flexor torques at low velocity
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(60–90/s) are mainly related to static gross motor function such as balance and strength
while knee flexor torques at high velocity may contribute to dynamic gross motor function
such as running agility.
Children with CP had lower knee flexor strength than knee extensor strength. This result
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is consistent with previous studies (Chen et al., 2012a; Larsson et al., 2008; Fowler, et al.,
2010). Furthermore, the reduced knee flexor strength is exaggerated between children with
GMFCS level II and those with GMFCS level I. The reason may be that tradition
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rehabilitation programs focus more on the strengthening antigravity muscles (e.g. knee
extensors) than postural muscles (e.g. knee flexors). For example, a strength increase of
more than 50% in quadriceps with no significant change in hamstring was found after a
quadriceps strengthening program in children with CP (Damiano et al., 1995). The results
strongly suggested the hamstring strengthening was needed at the same time when the
quadriceps was developed (Damiano et al., 1995). A randomized controlled trial showed a
novel home-based virtual cycling training program induced larger gains in the knee flexor
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than in the knee extensor (Chen, Hong, et al., 2012b). These findings suggest that cycling
programs may be a choice for postural muscle strengthening for children with CP,
especially for those with severe motor impairment.
Knee flexor peak torques remained unchanged or even increased with increasing
velocity in children with CP, especially in those with GMFCS level II. However, the knee
extensor decreased with increasing angular velocity. In ND children, strength in the knee
extensor and flexor muscle was decreased with increasing angular velocity, which was
consistent with previous studies (Kannus & Beynnon, 1993; Chan et al., 1996; Alangari &
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Al-Hazzaa, 2004). The knee flexor peak torque increased from 60/s to 120/s in children
with GMFCS level II. These findings imply that muscle strengthening at high angular
velocity may be included in the training programs under safety condition when designing
interventions targeting the knee flexor in children with CP.
The HQ ratio was reduced in children with CP, especially in those with severe motor
impairment. The HQ ratio was also increased with increasing angular velocities, which was
consistent with the literature (Chen et al., 2012b). In normal children, the HQ ratio correlated
with angular velocities, which concurs with previous works (Gaul, 1996; Hewett, Myer, &
Zazulak, 2008). In this study, the HQ ratios in children with CP were only 20-49%, which was
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markedly lower than the 50-80% reported for healthy subjects in Chan et al. (1996). The
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reason is that the knee flexor strength is more obvious affected by CP as compared with knee
extensor. A previous work also revealed strength deficit between the knee flexor and knee
extensor joint moment is larger in children with CP than in ND children (Damiano et al.,
1995). Therefore, the findings suggest that strengthening knee flexor muscles in individuals
with CP should be a treatment priority. Moreover, the HQ ratio may be used as a potential
biomarker for muscle strengthening effectiveness in children with CP.
Muscle strength and motor function were associated with GMFCS levels in children
with CP. In this study, children with GMFCS level II had lower gross motor function, trunk
muscular endurance, and knee muscle strength, and especially knee flexor, compared to
those with GMFCS level I. These results were compatible with previous studies (Chen et al.,
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2012a; Chen et al., 2012b). The muscle weakness associated with CP may be the direct
result of an upper motor neuron lesion impairing neural output and neuromotor control
(Rose & McGill, 1998; Moreau, Jolicoeur, & Peretz, 2009). Muscle contraction force is
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generally determined by both firing rate and the motor unit recruitment (De Luca & Erim,
1994). Children with CP have hamstring and quadriceps impairments (i.e., co-contraction,
spasticity, weakness) that can impair motor unit recruitment and rate modulation (Rose &
McGill, 2005; Stackhouse, Binder-Macleod, & Lee, 2005). These findings suggested that
muscle strength and muscular endurance training should be included in treatment strategies
even for children with mild motor severities (GMFCS levels I-II).
The findings of this study are limited by its design. This work selectively analyzed
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characteristics of interest in the participants. The focus was on independent ambulatory
children with CP; those with limited ambulatory capability were excluded. Only the knee
muscle strength was assessed, the other lower limb muscle strength, such as hip or ankle
muscle strengths, was not measured in this study. Hence, the study results cannot be
generalized to all CP cases or to all lower limb muscles. Despite this limitation, this study
established the relationships of muscle strength and gross motor functions in ambulatory
children with CP.
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5. Conclusions
This study showed that gross motor function, trunk muscular endurance and HQ ratio
were lower in children with GMFCS level II than in those with GMFCS level I. Postural
muscle strength (knee flexor) was also lower than antigravity muscle strength (knee extensor)
and had a relatively stronger correlation with gross motor functions in ambulatory children
with CP. Moreover, knee flexor strength at different angular velocities was associated with
various gross motor tasks. These findings suggest that muscle training for children with CP
should target postural muscles, especially at high angular velocity. The HQ ratio may be used
as a potential biomarker to probe the therapeutic effectiveness for muscle strengthening in
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children with CP. More importantly, these data may help to formulate effective muscle
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strength training strategies for maximizing gross motor capacity and for enhancing daily
activities and participation in children with CP.
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Acknowledgements
The authors would like to thank the National Science Council, Taiwan, for financially
supporting this research under Contract No. NSC93-2314-B-182A-201 and
NSC96-2314-B-182A-044-MY2. Ted Knoy is appreciated for his editorial assistance.
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Figure Legend
Fig. 1. Scatter-plot showing relationships of peak torque of knee flexor to (a) GMFM-66,
BOTMP sub-scores: (b) strength, (c) gross motor function composite score, and (d) running
speed and agility (RSA).
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