Cerebral palsy (CP) is caused by damage to the immature brain, often causing primary and secondary impairments, such as increased muscle tone, muscle contractures, muscle weakness, decreased joint range of motion (ROM), loss of selective motor comparison, and incomplete balance.1,2 The secondary musculoskeletal problems increase with increasing age.3 Muscle weakness and progressive muscle contractures contribute to reduced active and passive joint ROM, and consequently to deterioration of gait and motor function.1,4,5 Initiatives to prevent contractures and improve muscle strength are assumed to contribute to optimizing gait and participation in everyday activities.1,5 Children with CP have shorter hamstring muscles in comparison to peers who are developing typically.3,6 The hamstring muscle shortens by age and functional level,3,6,7 as classified by the Gross Motor Function Classification System (GMFCS).8 A short and spastic hamstring muscle rotates the pelvis posteriorly and contributes to gait deviations.4
The validity of the passive popliteal angle (PPA) has been questioned in the literature9; however, it is used for evaluating hamstring length in clinical practice.9 The PPA guides decision-making when a surgical hamstring-lengthening procedure is considered.1 Lengthening of the hamstrings is often part of multilevel surgery, and long-term follow-up studies have shown that the procedure may lead to unwanted effects such as hyperextension of the knee, anterior tilt of the pelvis, and increased lumbar lordosis.10 The pendulum of nonsurgical treatment has shifted toward functional and activity-based interventions, specific modalities, such as muscle stretching and strengthening, and remains approaches in the treatment of children with CP.11
Muscle stretching is used based on the assumption that stretching maintains or increases ROM.12–14 Documentation on stretching as the only treatment modality is scarce, and the effect size is small showing no evident effect on altering muscle length and limited evidence of increased ROM.15,16 The evidence is limited and further research is needed to explore the continued use of this intervention.12–14
Children with CP have reduced muscle strength compared with peers who are developing typically,17 and progressive resistance exercise (PRE) is assumed to be important in maintaining and increasing muscle strength and physical performance.18 Children with CP, performing regular strength training, increase their muscle strength, but not their spasticity. Recent randomized controlled trials and meta-analyses report no positive effect on gait function following strength training.19–21
Physical therapy includes different components, recognizing that complex functional problems need complex interventions.22 Optimal joint function is dependent on both optimal joint ROM and adequate muscle strength.1 We hypothesized that by combining stretching of the hamstrings muscles and specific muscle strength exercises, focusing on the terminal knee extension, the popliteal angle may decrease and improve knee joint function.
The main purpose of the present study was to evaluate the effects of a 16-week combined muscle stretching and PRE program on the popliteal angle and muscle strength in children with spastic bilateral CP. The second purpose was to evaluate the effects of a 16-week maintenance program.
The study was a single-blind block randomized controlled trial, approved by the Regional Committee for Medical and Health Research Ethics, section South East, the Commissioner for the Protection of Privacy in Research, and registered in Clinical trials.gov (NCT02917330).
The participants were recruited from the Norwegian Cerebral Palsy Follow-up program, and the patient register in the Motion Laboratory at Oslo University Hospital. One hundred six children between 7 and 15 years, with spastic bilateral CP, were identified and requested to attend the study by a written invitation. Thirty-seven children accepted to participate and attended the baseline evaluation (T0) (Figure 1). Inclusion criteria were (1) bilateral spastic CP, (2) GMFCS levels I to III, and (3) PPA 35° or more in the most affected leg. Exclusion criteria were (1) surgical procedure on hamstrings or bilateral lengthening of the triceps surae, (2) other surgical procedure in the lower limbs less than 1 year prior to inclusion, (3) botulinum toxin A injections in the lower limbs the last 6 months before inclusion, (4) less than 0° dorsal flexion in the ankle joint, (5) less than 5° external rotation in the hips, and (6) not able to cooperate or understand instructions.
The children who consented to participate were tested in the Biomechanical and Motion Laboratory at Oslo University Hospital (33 children) and in the Motion Laboratory at Haukeland University Hospital in Bergen (4 children). At Haukeland University Hospital, they do not have isokinetic testing equipment; hence, muscle strength measurements were not performed at this location. All children participated in the clinical tests (PPA, active popliteal angle [APA], and hamstrings catch) at the 3 test points (T0, T1, and T2); however, 5 children did not perform the isokinetic muscle strength tests. For 3 of the children this was due to no access to testing equipment, and 2 of the children had problems keeping up with the speed necessary to activate the dynamometer. In 1 child it was not possible to detect a hamstring catch.
After the baseline test, the children were block randomized by an office manager not engaged in the project, who randomly picked an envelope containing prepared blocks of 4 numbers. They were randomized into 2 groups: intervention (n = 17) or comparison (n = 20). Three assessments were completed: baseline (T0), after 16 (T1), and 32 weeks (T2).
The assessors in the laboratory were senior therapists with more than 8 years of experience in testing children with CP. They were masked to the intervention, and children and parents were informed not to reveal group. The assessors were trained prior to the project, following a standardized test protocol, and the same assessor performed the same tasks during all tests.
The intervention period was 16 weeks, the number of weeks recently recommended for meaningful improvements following strength training.23 The physical therapists treating the children in the comparison group were by written information asked to continue their physical therapy program and to not introduce new treatments including specific stretching of hamstring or quadriceps strength training during the project period (32 weeks).
The children in the intervention group received a specific intervention program, which focused on passive and active stretching of the hamstrings muscles and PRE of the muscles extending the lower extremities (Figure 2). There were 16 local physical therapists treating the 16 children in the intervention group, following a one-to-one standardized treatment protocol. An instructional video demonstrating how to perform all exercises was distributed to the physical therapists in the intervention group. The intervention included 3 sessions per week: 2 individual sessions together with the physical therapist and 1 home-based session. The physical therapy sessions followed a detailed protocol, individually tailored and guided by a physical therapist, including a 5-minute warm-up either by walking on a treadmill or cycling on a stationary bicycle. Two stretching exercises (Figures 2a and 2b)—active knee extension for 5 seconds (Figure 2a) followed by 40 seconds of passive stretching (Figure 2b), supported by the physical therapist—exercise seated hamstrings stretch (Figure 2c), both stretching exercises performed for 45 seconds and repeated 5 times bilaterally. Strength exercises were performed bilaterally. Exercises depicted in Figures 2d, 2e, and 2f are multijoint PRE performed with a loaded backpack. Exercise depicted in Figure 2g is a single-joint exercise with the knee over a bolster and with manual resistance on the distal leg. The children who needed balance support held on to a handrail or banister. During the first week the back pack was unloaded. The number of repetitions progressed from 2 series of 12 repetitions to 3 series of 8 repetitions (after 8 weeks), increasing the load in the backpack as the child became stronger. This was based on an 8-repetition maximum test24 performed every third week, and the weight load was recorded. To familiarize the children with the home exercises (Figure 2, exercise depicted in Figures 2c and 2e), the children were instructed by the physical therapist for the first 3 weeks. During the following weeks, the exercises were performed at home without guidance. A short maintenance program, identical to the home exercise program, was performed once a week from week 17 to 32; however, no weight load progression instructions were given. Both groups received care as usual during this period.
The project manager was an experienced physical therapist who was not masked to the interventions. After randomization she contacted the child's local physical therapist to inform about group affiliation, distributed the project protocol and the instructional video, and gave detailed information about the intervention. She contacted the physical therapists during the intervention and was available for questions. All physical therapy and home sessions in the intervention group and the care as usual in the comparison group were registered in a diary. The physical therapists were responsible for the registration and it was returned to the project manager by e-mail.
The primary outcome variable was PPA,25 and the secondary outcome variables were APA,26 hamstrings catch,25 and isokinetic muscle strength (Cybex-Lumex Inc, Ronkonkoma, New York). The ROM tests and spasticity test were performed in a silent room with the child in the supine position, the hip was passively flexed 90° by one of the testers, and the contralateral hip and knee extended and fixed by the other tester. PPA measures were registered as maximum passive extension of the measured knee. The measurement was completed with a hand-held plastic goniometer with 1° increments. The APA was tested in the same position, but with the child actively stretching the knee.
To assess whether strength training increased spasticity, hamstring spasticity was measured using the Modified Tardieu Scale25 with 90° flexion of the hip. The knee was rapidly extended and a catch (R1) was measured with a goniometer. To avoid extra tension and spasticity, the test was performed only once and prior to other tests.
Goniometric measurement is the most used method for evaluating ROM in children with CP, but there is variability in the PPA.27–29 For this reason, a small reliability study including 11 children was conducted in our laboratory prior to this study. We found an interrater and intrarater variability of ±10° (unpublished data), which corresponds to other reliability studies.27–29 To reduce the known variability, the errors were minimized by standardizing the test situation. Two therapists performed the testing: one therapist stabilized and extended the knee and the other therapist measured the angle with a goniometer. Three measures were performed and averaged both for the PPA and APA. Isokinetic quadriceps and hamstring strength was tested using a Cybex 6000 (Cybex-Lumex Inc, Ronkonkoma, New York). The children performed 5 repetitions at 60°/second bilaterally, measuring muscle strength expressed as peak torque (Nm).
Compliance with the intervention program was recorded as the number of exercise sessions attended, and progression of the loading in the backpacks was recorded in kilograms.
Descriptive values are presented as means ± standard deviations, and a paired-samples t test was used to calculate mean differences and 95% confidence interval between T0 and T1 and T0 to T2, for group comparison (Table 1). To compare baseline variables between the groups, a t test was used for continuous normally distributed data and a χ2 test for categorical variables. Two-tailed value of P < .05 was considered statistically significant. To evaluate mean differences between the groups at T1 and T2, a linear regression analysis with covariates correcting for baseline (ANCOVA) was performed. Due to a wide age span, age was added to the model as covariate. To account for the multiple comparisons, a Hochberg adjustment was performed. Electronic equipment error caused 2.3% missing values in the muscle strength dataset, which was handled by single imputation by group mean for each variable. This is a pragmatic method for substituting missing values when the missing values are few and are missing by random as in the present study.30
TABLE 1 -
Variables Over Time
||T0 Mean (SD)
||T1 Mean (SD)
||Mean Difference (SD)
||T2 Mean (SD)
||Mean Difference (SD)
||T0 Mean (SD)
||T1 Mean (SD)
||Mean Difference (SD)
||T2 Mean (SD)
||Mean Difference (SD)
|Quadriceps PT L, Nm
|Quadriceps PT R, Nm
|Hamstrings PT L, Nm
|Hamstrings PT R, Nm
|Hamstrings catch L,°
|Hamstrings catch R,°
Abbreviations: APA, active popliteal angle; L, left; PPA, passive popliteal angle; PT, peak torque; R, right; SD, standard deviation; T0, baseline; T1, 16 weeks' follow-up; T2, 32 weeks' follow-up.
Sample Size Calculation
The power calculation was based on PPA. Based on the results from McDowell et al27 and a small reliability study (unpublished data) performed prior to the present study, we assumed that the errors of measurement for the change of PPA should be 10° or more. When comparing PPA between groups, a 2-sided independent-samples t test was used. A minimal detectable change in the PPA was considered 10° or more. In order to achieve 80% power, at least 16 children in each group had to attend the T1.
Thirty-seven children were eligible for inclusion: 17 children were allocated to the intervention group and 20 to the comparison group. At baseline there were no significant differences between the groups regarding gender, age, GMFCS level, or body mass index (Table 2). Three children dropped out after T0 and were excluded from the analysis (Figure 1). Thirty-four children completed all clinical tests (PPA, APA, hamstrings catch) at the 3 test points (T0, T1, and T2) and 29 children performed the isokinetic muscle strength tests.
TABLE 2 -
||Intervention Group (n = 17)
||Comparison Group (n = 20)
|Age, mean (SD), y
|Height, mean (SD), cm
|Body weight, mean (SD), kg
|BMI, mean (SD), kg/m2
|Most affected leg, left/right
|Backpack load at 2 wk, mean (SD), kg
Abbreviations: BMI, body mass index; GMFCS, Gross Motor Function Classification System; SD, standard deviation.
The 16-week intervention program included 48 training sessions, 32 physical therapy sessions, and 16 home-based sessions. The compliance registration form, which was answered by 81% of the physical therapists, recorded a compliance rate of 73% (37 sessions ± 5.5, range 28-46), for the total number of sessions. For the physical therapy sessions and the home exercise sessions, the compliance rates were 79% (25 sessions ± 3.5, range 20-30) and 76% (12 sessions ± 5, range 3-16), respectively. Compliance with the maintenance program, 16 sessions in total (weeks 17-32), was 72% (13 sessions ± 3.5, range 5-16). Reasons for absence from training sessions were vacations, illness, and conflicting appointments. The mean progression of weight load during the intervention period (weeks 2-16) was 6.6 kg (±3.8, range 1-13); the start load at week 2 was 4.6 kg (±3.1, range 0-12). Physical therapy sessions in the comparison group indicated that 60% were treated by the physical therapist 1 to 2 sessions per week and the intervention varied extensively. Ten children received functional training or water training once a week; 5 children received specific stretching and/or specific strength training 2 to 10 sessions during the first 16 weeks.
Descriptive values for outcome measures at T0, T1, and T2 and the differences within the groups are in Table 1. When correcting for multiple comparisons (Hochberg correction) (Table 3), there were no significant mean differences between the intervention group and the comparison group for any of the test variables from T0 to T1. There were small changes in ROM between the 2 groups in favor of the intervention group. The mean difference between the groups for PPA was 3.5° and 4.3° for the left and right sides, respectively. And for the APA the mean differences between the 2 groups were 8.6°and 7.5° for the left and right sides, respectively (Table 3). For muscle strength and hamstrings spasticity, there were no significant mean differences from T0 to T1 between the groups (Table 3). At T2 there were no significant mean differences between the groups for any of the test variables (Table 3).
TABLE 3 -
Mean Differences and Confidence Intervals Over Timea
||Intervention Group Test T0-T1
||Mean Difference (95% CI)
||Hochberg Adjusted P Value
||Mean Difference (95% CI)
||Hochberg Adjusted P Value
||3.5 (−0.2 to 7.0)
||3.3 (−0.3 to 6.9)
||4.3 (0.3 to 8.3)
||0.4 (−3.5 to 4.4)
||8.6 (2.0 to 15.2)
||6.6 (−0.4 to 13.1)
||7.5 (1.5 to 13.5)
||4.5 (−1.7 to 10.7)
|Quadriceps L PT, Nm
||−4.1 (−10.2 to 1.9)
||−7.1 (−14.2 to 0.09)
|Quadriceps R PT, Nm
||−2.6 (−9.8 to 4.6)
||−1.5 (−8.6 to 5.2)
|Hamstrings L PT, Nm
||−4.9 (−9.6 to −0.3)
||−4.6 (−0.2 to −0.2)
|Hamstrings R PT, Nm
||−4.4 (−8.3 to −0.5)
||−6.2 (−10.6 to 0.3)
|Hamstrings Catch L,°
||−0.8 (−6.0 to 4.3)
||3.6 (−2.5 to 9.6)
|Hamstrings Catch R,°
||−1.2 (−9.9 to 4.5)
||0.5 (−2.4 to 8.8)
Abbreviations: ANCOVA, analysis of covariance; APA, active popliteal angle; CI, confidence interval; L, left; PPA, passive popliteal angle; PT, peak torque; R, right; SD, standard deviation; T0, baseline; T1, 16 weeks' follow-up; T2, 32 weeks' follow-up.
aAdjusted for baseline and age in the linear regression model (ANCOVA).
There were no significant between-group differences for PPA and APA, but the intervention group improved compared with the comparison group (Table 3). The differences, however, were small and may not have clinical relevance. The change in PPA (Table 3) was less than 10°, which was less than the minimal detectable difference. There was a positive trend for the intervention group, which, put in the context of natural loss of range,3,6 might be important over time. There are few well-designed studies evaluating the effect of stretching on flexibility in children with CP,15,16 and 3 recently published reviews all conclude that the results are inconclusive and that the effects of stretching on joint ROM are of minor value.12–14 The results from the present study showing minor differences in the PPA (Table 3) are in line with these previous published studies. However, the results indicate that the program introduced in this study might have effect on preventing or slowing down the expected increase in PPA during childhood and adolescence. Despite studies showing that the PPA, by age, will increase significantly in CP,3,6 the value of efforts to maintain ROM has not been a focus. There is skepticism concerning time and effort spent on treatment, which may achieve only minor changes.11 However, several studies provide evidence that contracted hamstring muscle influences gait and function negatively and in multiple cases hamstring tenotomy is necessary.1,10 Children receiving surgery at a young age are more frequently at risk of having additional surgical procedures than those who are older at the index operation.31 Maintaining ROM by only a few degrees might therefore be of clinical importance.
In the present study, the children performed a combined exercise program including hamstring stretching and PRE 3 times per week. There is little consistency in the literature concerning frequency of stretching interventions; however, in most studies the stretching is performed 3 to 5 times per week.12,13 To achieve an acceptable compliance, we choose a frequency of 3 sessions per week. Despite a relatively low number of interventions per week, positive changes were seen in PPA and APA. The strength exercise program was, as far as possible, developed based on evidence-based recommended guidelines for PRE in youths in general,32 and for children with CP in particular.18 Based on the recommendations18,32 to cover for periods of sick leave, short vacations, and the fact that PRE requires an acclimatization period, the intervention lasted for 16 weeks. Despite a relatively long intervention period, there were only small differences between the groups. The intervention group was weaker than the comparison group at baseline, and after the intervention period, both at T1 and T2, the intervention group was as strong as the comparison group. Baseline values and age, which is a confounder when measuring strength in children,33 were corrected for in the statistical model. There were minor group differences in quadriceps strength at T1, which is in contrast to other shorter lasting PRE studies in children with CP showing more extensive muscle strength effects.19,21 A possible explanation could be that the loaded backpack may have forced the children to flex their hips to achieve stability, using hamstrings more than the quadriceps for extension when rising up.2 Another contributor to knee extension when the foot is fixed on the floor is the gastrocnemius, and the plantar flexion/knee extension couple, producing an extension moment on the knee with less activation of quadriceps.1,2 A relatively low progress in weight load gain of 6.6 kg (±3.0, range 1-13 kg) from week 2 to week 16 may be due to the use of the loaded backpack, which might have become too heavy to carry on the shoulders. A leg extension bench would have been more stable, more suitable for further weight load progression, and probably more specific for isolated quadriceps activation. For practical reasons this was not possible to implement, and the positions chosen for the strength exercises in an upright position were considered more functional also activating ankle plantar flexors and hip extensors. Despite a detailed treatment protocol, it is possible that 16 different physical therapists made different considerations regarding progression of load. An instructional seminar for the physical therapists prior the PRE intervention program would have been preferable; however, it was difficult to implement in practice. The small differences in strength gain between the intervention group and the comparison group could also be explained by the care as usual in the comparison group. The frequency, intensity, and loading varied and were not systematic, but these interventions may have influenced the results.
The isokinetic quadriceps strength measurements showed no significant change after the intervention; however, the APA test, which mainly is quadriceps strength dependent, showed increased active terminal knee extension bilaterally. The difference between passive and active knee extension in terminal knee extension is described as a possible contributor to crouch in spastic bilateral CP.2 After the intervention period, the changes in APA were larger than the changes in PPA resulting in reduced extension deficit in the intervention group. This may be explained by the combined focus on active full knee extension in all the intervention exercises.
At T2, the improvements were not maintained. The results indicate that a maintenance program including only 2 exercises once a week does not maintain the achieved results. The compliance with the maintenance program was 72%, which is acceptable, but the quality of the exercises performed at home is unknown.
There were a wide age range and GMFCS levels among the included children. This was necessary to include the required number of participants estimated by the power calculation. The exclusion of children not able to walk without support for 10 m resulted in only 1 child with GMFCS level III; hence, the result is only valid for GMFCS levels I and II. Another limitation was that 5 children did not perform the strength tests, making the strength measurement results less conclusive. The physical therapy sessions were provided by the patients' local physical therapists, which mean that 16 different therapists guided the included children. This may have resulted in a less consistent exercise program and suboptimal weight load progression. However, the physical therapists got a detailed written description and video demonstrations of the program prior to the treatment period in addition to information from an experienced project manager. The physical therapists also gave one-to-one instructions based on their physical therapy skills and clinical experience with the child. This was one reason for a relatively high compliance with the intervention.
A 16-week intervention program including stretching of the hamstring muscles and PRE targeting the extending muscles in the lower limbs did not result in a significant difference in change in PPA and APA between the intervention group and the comparison group. If the goal is to maintain hamstring length and to increase active knee extension, the present results indicate that a combination of hamstring stretching and PRE might be effective. The minor effects were not maintained after 32 weeks, indicating that a maintenance program should include more than 2 exercises and a frequency of more than 1 session a week. To evaluate whether a combined stretching exercise program and PRE may prevent deterioration of ROM, and function and consequently prevent or delay surgery, further research including longer follow-up periods is needed.
We are grateful to the children, their parents, and the local physical therapists for their enthusiasm and contribution making this project possible. We are also grateful for the time-consuming engagement from Ingrid Skaaret, Marie Johansson, Leif Andre Viken, and Merete Malt contributing to the testing, Kjersti Ramstad for appreciated feedback, and statistician Prof Are Hugo Pripp for valuable statistical support.
1. Gage JR, Schwartz MH, Koop SE, Novacheck TF. The Identification and Treatment of Gait Problems in Cerebral Palsy
. 2nd ed. Cambridge, England: Mac Keith Press; 2009.
2. Gage JR, Novacheck TF. An update on the treatment of gait problems in cerebral palsy
. J Pediatr Orthop B. 2001;10(4):265–274.
3. Nordmark E, Hagglund G, Lauge-Pedersen H, Wagner P, Westbom L. Development of lower limb range of motion from early childhood to adolescence in cerebral palsy
: a population-based study. BMC Med. 2009;7:65.
4. Rodda JM, Graham HK, Carson L, Galea MP, Wolfe R. Sagittal gait patterns in spastic diplegia. J Bone Joint Surg Br. 2004;86(2):251–258.
5. Bartlett DJ, Chiarello LA, McCoy SW, et al. Determinants of gross motor function of young children with cerebral palsy
: a prospective cohort study. Dev Med Child Neurol. 2014;56(3):275–282.
6. Bell KJ, Ounpuu S, DeLuca PA, Romness MJ. Natural progression of gait in children with cerebral palsy
. J Pediatr Orthop. 2002;22(5):677–682.
7. McDowell BC, Salazar-Torres JJ, Kerr C, Cosgrove AP. Passive range of motion in a population-based sample of children with spastic cerebral palsy
who walk. Phys Occup Ther Pediatr. 2012;32(2):139–150.
8. Rosenbaum PL, Palisano RJ, Bartlett DJ, Galuppi BE, Russell DJ. Development of the Gross Motor Function Classification System for cerebral palsy
. Dev Med Child Neurol. 2008;50(4):249–253.
9. Thompson NS, Baker RJ, Cosgrove AP, Saunders JL, Taylor TC. Relevance of the popliteal angle
to hamstring length in cerebral palsy
crouch gait. J Pediatr Orthop. 2001;21(3):383–387.
10. Dreher T, Vegvari D, Wolf SI, et al. Development of knee function after hamstring lengthening as a part of multilevel surgery in children with spastic diplegia: a long-term outcome study. J Bone Joint Surg Am. 2012;94(2):121–130.
11. Novak I, McIntyre S, Morgan C, et al. A systematic review of interventions for children with cerebral palsy
: state of the evidence. Dev Med Child Neurol. 2013;55(10):885–910.
12. Pin T, Dyke P, Chan M. The effectiveness of passive stretching
in children with cerebral palsy
. Dev Med Child Neurol. 2006;48(10):855–862.
13. Wiart L, Darrah J, Kembhavi G. Stretching
with children with cerebral palsy
: what do we know and where are we going? Pediatr Phys Ther. 2008;20(2):173–178.
14. Harvey LA, Katalinic OM, Herbert RD, Moseley AM, Lannin NA, Schurr K. Stretch for the treatment and prevention of contractures. Cochrane Database Syst Rev. 2017;1:CD007455.
15. Theis N, Korff T, Mohagheghi AA. Does long-term passive stretching
alter muscle-tendon unit mechanics in children with spastic cerebral palsy
? Clin Biomech (Bristol, Avon). 2015;30(10):1071–1076.
16. Theis N, Korff T, Kairon H, Mohagheghi AA. Does acute passive stretching
increase muscle length in children with cerebral palsy
? Clin Biomech (Bristol Avon). 2013;28(9/10):1061–1067.
17. Wiley ME, Damiano DL. Lower-extremity strength profiles in spastic cerebral palsy
. Dev Med Child Neurol. 1998;40(2):100–107.
18. Verschuren O, Ada L, Maltais DB, Gorter JW, Scianni A, Ketelaar M. Muscle strengthening in children and adolescents with spastic cerebral palsy
: considerations for future resistance training protocols. Phys Ther. 2011;91(7):1130–1139.
19. Taylor NF, Dodd KJ, Baker RJ, Willoughby K, Thomason P, Graham HK. Progressive resistance training and mobility-related function in young people with cerebral palsy
: a randomized controlled trial. Dev Med Child Neurol. 2013;55(9):806–812.
20. Moreau NG, Bodkin AW, Bjornson K, Hobbs A, Soileau M, Lahasky K. Effectiveness of rehabilitation interventions to improve gait speed in children with cerebral palsy
: systematic review and meta-analysis. Phys Ther. 2016;96(12):1938–1954.
21. Scholtes VA, Becher JG, Janssen-Potten YJ, Dekkers H, Smallenbroek L, Dallmeijer AJ. Effectiveness of functional progressive resistance exercise
training on walking ability in children with cerebral palsy
: a randomized controlled trial. Res Dev Disabil. 2012;33(1):181–188.
22. Damiano DL, Arnold AS, Steele KM, Delp SL. Can strength training predictably improve gait kinematics? A pilot study on the effects of hip and knee extensor strengthening on lower-extremity alignment in cerebral palsy
. Phys Ther. 2010;90(2):269–279.
23. Verschuren O, Peterson MD, Balemans AC, Hurvitz EA. Exercise and physical activity recommendations for people with cerebral palsy
. Dev Med Child Neurol. 2016;58(8):798–808.
24. Scholtes VA, Dallmeijer AJ, Rameckers EA, et al. Lower limb strength training in children with cerebral palsy
—a randomized controlled trial protocol for functional strength training based on progressive resistance exercise
principles. BMC Pediatr. 2008;8:41.
25. Boyd RN, Graham HK. Objective measurement of clinical findings in the use of botolinium toxin type A for the management of children with cerebral palsy
. Eur J Neurol. 1999;6:23–35.
26. Hamid MSA, Ali MRM, Yusof A. Interrater and intrarater reliability of the active knee extension (AKE) test among healthy adults. J Phys Ther Sci. 2013;25:957–961.
27. McDowell BC, Hewitt V, Nurse A, Weston T, Baker R. The variability of goniometric measurements in ambulatory children with spastic cerebral palsy
. Gait Posture. 2000;12(2):114–121.
28. McWhirk LB, Glanzman AM. Within-session inter-rater reliability of goniometric measures in patients with spastic cerebral palsy
. Pediatr Phys Ther. 2006;18(4):262–265.
29. Ten Berge SR, Halbertsma JP, Maathuis PG, Verheij NP, Dijkstra PU, Maathuis KG. Reliability of popliteal angle
measurement: a study in cerebral palsy
patients and healthy controls. J Pediatr Orthop. 2007;27(6):648–652.
30. Bennett DA. How can I deal with missing data in my study? Aust N Z J Public Health. 2001;25(5):464–469.
31. Terjesen T, Lofterod B, Skaaret I. Gait improvement surgery in ambulatory children with diplegic cerebral palsy
. Acta Orthop. 2015;86(4):511–517.
32. Faigenbaum AD, Kraemer WJ, Blimkie CJ, et al. Youth resistance training: updated position statement paper from the national strength and conditioning association. J Strength Cond Res. 2009;23(5 suppl):S60–S79.
33. Holm I, Fredriksen P, Fosdahl M, Vollestad N. A normative sample of isotonic and isokinetic muscle strength measurements in children 7 to 12 years of age. Acta Paediatr. 2008;97(5):602–607.