Children and adolescents with cerebral palsy (CP) have decreased levels of physical activity (PA) compared with their peers without CP.1 The ability to sustain PA at the intensity and duration necessary for participation is an important outcome of intervention. Young children with CP may be at risk for reduced PA and/or ability to sustain PA secondary to impairments in muscle performance, limitations in mobility, high calorie demands for growth, and decreased aerobic capacity.2–4 The National Association for Sports and Physical Education has published PA guidelines for infants and children to promote the development of movement skill and health-related fitness.5 The recommendations for daily PA and opportunities to explore movement are 30 minutes of structured and 30 minutes of unstructured PA for toddlers, and at least 60 minutes each of structured and unstructured PA for preschoolers. We recommend incorporating these guidelines into goals and interventions for young children with CP with the long-term goal of improving health and participation in life activities.
PA is defined as bodily movement of any type including play, recreation, fitness, and sports.6 Endurance for PA is defined by the International Classification of Functioning, Disability and Health7 as “related to the general level of tolerance of physical exercise or stamina.” Both Adamo and colleagues8 and Pate et al9 have published reviews of PA measures in young children. These reviews include a discussion of the benefits and limitations of both direct and indirect measures of PA. However, they do not include studies involving children with disabilities.
Several direct and indirect measures of endurance for PA have been developed.8–19 Direct measures include the use of doubly labeled water, calorimetry, physiologic markers (heart rate and respiratory rate), motion sensors (accelerometers, pedometers), direct observation, and observations of timed activity such as the 6-minute walk test (6MWT).8–11 Indirect measures include parent- and/or child-completed questionnaires, surveys, diaries, logs, and interviews that can imply endurance via reporting estimates of PA.8,9,12–18 Many direct measurements of endurance for PA can be time-consuming, expensive, and hard to complete in young children, particularly in the home environment. Some direct observation measures, such as the Children's Activity Rating Scale,10 allow for use in the home, school, or daycare but require extensive hours of observation with frequent reliability testing. Indirect measures that imply endurance via measurement of PA, such as the Activities Scale for Kids,12 Functional Mobility Scale,16 Pediatrics Outcome Data Collection Instrument,17 and Child Health Questionnaire,18 may be less intrusive and cost-effective but focus on health-related quality of life, activity, and participation, and not specifically on endurance for PA in children younger than 5 years.
Measurement of endurance for PA (including frequency, intensity, and duration of activity) is greatly understudied.19 Children younger than 5 years pose unique challenges for measurement of endurance, particularly children with disabilities, as PA covers a broad range of activity types. For children who may not be walking, structured and unstructured play constitutes a large portion of PA. From a cognitive and behavioral development perspective, children are often not able to easily respond and adhere to structured testing processes or sustain activity for the period of time needed to measure endurance.
To begin to fill this gap, our research team developed the Early Activity Scale for Endurance (EASE), a parent-completed measure for children with CP who are 18 months to 5 years old. It was designed to estimate endurance for activity via reports of the frequency, intensity, and duration of PA of young children within their typical environments. Ease of administration and low burden on the family were high priorities. The purpose of this study was to examine the validity and reliability of the EASE with respect to its use with children with CP. The primary research questions and hypotheses were as follows: (1) does the EASE demonstrate construct validity? Specifically, (a) do scores differ among children with CP and without CP across functional motor ability levels classified using the Gross Motor Function Classification System20 (GMFCS) levels? We hypothesized that EASE scores would differ on the basis of children's GMFCS level; children with higher motor function would be reported to have greater endurance for PA. And (b) do scores differ among children with and without CP by age and gender? We hypothesized that older children would have greater endurance for PA than younger children, but no differences would be found between boys and girls; (2) does the EASE demonstrate convergent validity with the 6MWT in children with CP, 3 to 5 years of age, who walk independently without assistive devices? We hypothesized correlations of fair to moderate magnitude21 (r = 0.40−0.60) between EASE and 6MWT scores, given that the EASE reports on overall activity, not just walking; and (3) does the EASE demonstrate test-retest reliability for children with CP? We hypothesized an intraclass correlation coefficient of greater than 0.80, supporting sufficient test-retest reliability for clinical and research discriminatory testing.
The EASE was developed as part of a larger, observational study to identify determinants of basic motor abilities, self-care, and play in young children with CP (titled Move & PLAY: Movement and Participation in Life Activities of Young Children).22,23 To evaluate construct validity of the EASE, a known-groups design was used. For convergent validity and test-retest reliability, a correlation design was employed.
Convenience samples of parents or caregivers and their children with CP and without CP (children who were developing typically), aged 1.5 to 5 years old, were recruited through therapist referral and word of mouth across several sites in the United States and Canada. Because at least 96% of the participants were the children's mothers or fathers, they will be referred to as parents. The study was approved by Institutional Review Boards at participating institutions. All parents provided informed consent for their children's participation in the reliability and validity studies. Parents of children without CP provided implied consent by completing and returning the anonymous EASE questionnaire.
The characteristics of children who participated in each component of the study are summarized in Table 1. The characteristics of the parents are presented in Table 2. No parental data were collected for the children without CP who participated in the convergent validity study. Using independent t tests and chi-square as appropriate for interval and nominal data, respectively, no significant differences were found in children with and without CP on age and gender; however, a significant difference was found in race/ethnicity (P = .02); a greater percentage of children without CP were white and a greater percentage of children with CP were biracial. No significant differences were found in gender and race/ethnicity between parents of the children with and without CP. Parents of children without CP were slightly older (mean difference 1.5 years, P = .01) and had a significantly higher education level (P < .001).
This convenience sample included children with and without CP who participated in the Move & PLAY study. They were recruited from 4 regions across the United States (greater Seattle, Washington; greater Philadelphia, Pennsylvania; greater Oklahoma City, Oklahoma; greater Atlanta, Georgia) and 9 locations across Canada (Victoria and Vancouver, British Columbia; Regina, Saskatchewan; Winnipeg, Manitoba; Hamilton, Toronto, and Peterborough, Ontario; Halifax, Nova Scotia; St John's, Newfoundland, and Labrador). The children with CP included 232 boys and 182 girls (mean age = 38 months, SD = 11 months); 10 children with monoplegia, 101 with hemiplegia, 95 with diplegia, 25 with triplegia, 181 with quadriplegia, and 2 unclassified. The 106 children without CP included 50 boys and 56 girls. For age comparisons, the children were divided into 3 age groups: <31 months; 31 to 42 months; >42 months to create 3 relatively equal groups of approximately 2-year-olds, approximately 3-year-olds, and approximately 4–5 year-olds.
Fourteen children with CP (recruited from the Move & PLAY sample) and 14 children without CP (a convenience sample from greater Seattle, Washington, region) participated in the convergent validity testing. Only children with CP classified at GMFCS levels I or II participated in this component of the study as it was determined that the 6MWT was not valid for children who do not walk or walk using an assistive mobility device. This decision was made after pilot testing several children with CP at GMFCS level III. The children were just learning to walk with assistive devices and their walking movements were slow and labored as compared with their ability to move on the floor. Therefore, the 6MWT distance was not thought to be a valid representation of their endurance for activity. It was also decided to exclude children younger than 3 years in this component of the study, as they would likely have difficulty following the directions for a valid 6MWT.
The 32 parents of children with CP who participated in the test-retest reliability component were recruited from the Move & PLAY sample within the greater Seattle, Washington, and Philadelphia, Pennsylvania, regions.
Gross Motor Function Classification System
The GMFCS is a 5-level system used to classify children with CP on present abilities and limitations in motor function with emphasis on sitting and walking.24,25 There are specific rating scales for children before their second birthday, between the second and fourth birthdays and between the fourth and sixth birthdays. A child who is classified at level I sits, crawls, and eventually walks with small limitations, whereas a child at level V requires assistance for all movements and eventually is transported in a wheelchair. The GMFCS has evidence of reliability and validity20,24 and has been used in many studies.26 Prior to data collection, assessors demonstrated 80% or greater agreement with criterion scores from videotapes of children at all GMFCS levels. To allow comparison of EASE scores between children with and without CP, children without CP were classified as GMFCS level 0.
Early Activity Scale for Endurance
The EASE was the product of a collaborative effort by 4 experienced pediatric physical therapists who were investigators in the Move & PLAY study (S. M., D. B., L. C., R. P.). After feedback from the entire Move & PLAY team, which included 2 parents of children with CP, an 11-item questionnaire, including 10 questions about frequency, intensity, duration, and type of PAs a child performs and 1 item directing whether the parent completes the second part of the questionnaire, was produced. The EASE is completed by parents who rate their children's levels of energy and fatigue as well as both the frequency and need for rest and the average length of time their children can engage in PA without rest (Table 3). The first 6 questions are completed for all children. The seventh question is a “yes/no” question that asks whether the child is independent (defined as the ability to move himself or herself without assistance from others) in any form of mobility. If true, the parent is to continue the questionnaire and answer the last 4 items; if false, then the parent stops and the last 4 items receive a score of 0. Items 1 to 5 and 8 to 10 are scored on a scale of 1 to 5 (1 = never, 2 = rarely, 3 = sometimes, 4 = often, and 5 = always). For items 6 and 11, the parent rates the typical length of time the child plays indoor or outdoor on a scale of 1 to 5, with 1 representing 0 to 5 minutes to 5 representing greater than 20 to 30 minutes. A total score is calculated by summing the scores for each item, except Question 7. To create a total score where a higher score is better endurance for PA, the item score for the 2 items on fatigue (4 and 10) is reversed so that a higher score represents less fatigue. The maximum total score is 50. The EASE is available in both the 11-item form described above and used in this study, and a shorter 4-item form that is available on the Move & PLAY Web site (http://www.canchild.ca/en/ourresearch/moveplay.asp).
Six-Minute Walk Test
The 6MWT was chosen to compare with the EASE because it is a simple, submaximal test of functional exercise that has evidence of reliability and validity.11,27,28,29 The distance walked under controlled conditions in 6 minutes is measured. Lammers et al30 published normative values of the 6MWT for children aged 4 to 11 years but did not report on reliability. Test-retest reliability has been previously established and excellent intraclass correlation coefficients have been reported in children with (intra-class correlation [ICC] = 0.98)29 and without CP (ICC = 0.94).28 The test was also found to be reliable between trials within the same testing session in older children and adolescents with CP (11–17 years old).27 The 6MWT was selected because, in our opinion, it was the best simple direct measure of endurance for PA that, we thought, could be completed in 3- to 5-year-old children within the home environment.
Parents of children with CP completed the EASE as part of the first Move & PLAY assessment. Parents of children without CP completed the EASE after receiving the questionnaire in person or by mail and then returned it to investigators by mail. Data were collected from participants across all months of the year; therefore, all seasons and weather situations were reflected.
In 1 test session, children performed the 6MWT and parents completed the EASE. The walk was completed in the child's home or outdoors nearby the family's home. Two examiners completed the testing. When possible, a long, continuous path was chosen for walking, avoiding turning around as often as possible. One examiner, and sometimes the parent, walked with the child giving encouragement to continue moving if he or she stopped, or occasionally holding the child's hand, if required, to encourage him or her to keep moving. Adults holding a child's hand were instructed to let the child set the speed of walking. The examiner walking with the child also monitored the time on a stopwatch. A second examiner walked behind the child with a surveyor measuring wheel to record the distance the child walked in 6 minutes.
Within 2 weeks after completing the EASE for the Move & PLAY study, the family was mailed another EASE and asked to complete it a second time and return the questionnaire by mail. Up to 2 reminder calls were made to the families encouraging them to return the questionnaire. For 6 families, the EASE was completed the second time during a telephone interview. The actual median interval from test to retest was 20 days (range, 5–70). Eight parents returned their second EASE at greater than 1 month (38–70 days) from the first EASE.
The Statistical Package for the Social Sciences (PASW, Version 18; IBM Corporation, Somers, NY) was used for all data analyses. The total EASE test score (sum of all items, with a higher score indicating better endurance for PA) was used for all data analyses. Data were examined descriptively and nonparametric statistics were selected because of the ordinal nature of the EASE questions. The alpha value used was .05 with adjustments for multiple comparisons made as described later, dependent on the component of the study.
Construct validity was analyzed using the known-groups method. For this method of validity testing, groups of individuals for whom there is an expectation that the construct measured will be different are compared. Descriptive data and comparison tests were used to examine construct validity. A Kruskal-Wallis analysis of variance (ANOVA) was used to determine whether EASE scores differed between GMFCS levels across children with and without CP. Post hoc comparisons between GMFCS levels were performed using Mann-Whitney U tests with a Bonferroni correction applied for repeated testing (P = .05/15 = .003). Differences between the EASE scores across age categories and between girls and boys were analyzed separately for the children with and without CP. A Kruskal-Wallis ANOVA, with post hoc Mann-Whitney U tests and Bonferroni correction applied for repeated testing, was used to analyze differences between the 3 age groups (P = .05/3 = .017). Mann-Whitney U tests were used to examine differences between girls and boys.
Descriptive data for the EASE and the 6MWT in the convergent validity component were skewed. Spearman's correlation coefficients, therefore, were used to analyze the association between 6MWT and EASE total scores.
Intraclass correlation (2,1) was used to analyze test-retest reliability. Internal consistency of items was analyzed using the Cronbach alpha coefficient. The standard error of measurement (SEM) and minimal detectable difference (MDD) were computed to estimate the error associated with individual scores and the smallest EASE score that represents actual change that cannot be attributed to measurement error. The equation used was
SEM = SD × square root of (1 − test-retest reliability).
The SD was computed for the 414 EASE scores reported by parents of children with CP. The MDD for 95% confidence interval was calculated as follows:
MDD = 1.96 × SEM × square root of 2.
The SEM was the value obtained from the previous calculation.
Descriptive data on the EASE for children with and without CP, grouped by motor ability (GMFCS level), are presented in Table 4 and Figure 1. Kruskal-Wallis ANOVA results were statistically significant (χ2 = 316.4, df = 5, P < .001). Post hoc analysis indicated that median EASE scores differed significantly between children in all GMFCS levels (P < .001), except between children in levels II and III (P = .62). Children without CP had the highest median EASE score, whereas children with CP in level V had the lowest median score.
Descriptive data on the EASE for children with and without CP grouped by age are presented in Table 5 and for girls and boys in Table 6. No significant effects were found for children with CP among age groups (χ2 = 4.63, P = .10) and between girls and boys (U = 19 899.5, P = .33). A significant effect of age group for children without CP was found (χ2 = 6.93, P = .03). However, when pairwise comparisons were made, only a significant difference between children aged 31 to 42 months and 43 to 60 months was found (P = .01). Children without CP who were 43 to 60 months old had a median score that was 2 points higher than the children who were 31 to 42 months old. No significant differences in EASE scores were found between girls and boys without CP (U = 1385, P = .92).
For the sample of 28 children with CP and children with typical development, the Spearman correlation between EASE and 6MWT scores was 0.57 (P = .001).
Test-retest reliability was high for the EASE (ICC (2,1) = 0.95, 95% CI: 0.90–0.98). The mean difference was small (mean total score difference = 0.8, SD = 3.7). The Cronbach alpha was 0.93, indicating good internal consistency among the items. The standard error of the measurement was 2.9, indicating that, for a 68% confidence interval, the true EASE score for an individual child is within 3 points of the reported score. The MDD was 8.0, indicating that, for a 95% confidence interval, a change of 8 points or less could be attributed to measurement error.
The results provide evidence of construct validity for the EASE. Children without CP had the highest median scores (greater endurance for PA) and children with CP in level V had the lowest. The finding that median EASE scores did not differ between children with CP in levels II and III suggests similarity in tolerance for PA among children in these 2 levels but may reflect the fact that there were fewer children in level III within our sample.
The hypothesis that the older children would have higher EASE scores than the younger children was not supported. Although we anticipated that EASE scores would be highest in children older than 42 months as a consequence of increased motor function and cardiopulmonary and physical capacity, no differences were found among the children with CP. For children without CP, we found that the median EASE score of children 42 to 60 months was only 2 points higher than that of children 32 to 42 months, even though this difference was statistically significant. Collectively, the findings suggest that for young children, there are not meaningful age-related differences in EASE scores. The EASE is a parent-report ordinal rating scale and is not as precise in measuring small differences that might be attributed to age as is an interval measure such as the 6MWT.
Whereas differences between the activity levels of boys and girls have been reported in older children with CP31 and children without CP,32 we did not expect to find a difference between the young boys and girls on the EASE in our study. The results support our hypothesis; however, we do note that the descriptive data for girls and boys with CP in levels IV and V suggest a trend for difference, so further research is recommended.
The correlation between EASE and 6MWT scores is of moderate magnitude21 (rs = 0.57), supporting convergent validity. This level of association was anticipated because scores for a measure of general daily PA (EASE) were compared with scores for a single PA (walking). It has also been documented that children tend to engage in a variety of different PAs in a time frame (run, walk, jump, tumble, etc) rather than one sustained activity.8,13 The EASE, with questions regarding a child's level of overall daily activity, may capture the intensity and frequency of this pattern more than a single task such as a 6-minute walk. Our findings are also consistent with a recent report of a moderate to good relationship21 (rs = 0.70) between parent ratings of perceptions of walking performance and 6MWT scores of children with CP, aged 7 to 18 years in GMFCS levels I to III.33 We suggest, however, that our results should be taken with caution. Although we considered the 6MWT the best available, and most feasible, direct test of endurance for activity that could be completed by young children within the home environment, reliability has not been determined for young children or for our modifications of the test procedures. The challenges we found with testing young children in their home setting included keeping their attention on the task, finding relatively flat continuous paths to walk on for 6 minutes, and testing under varying outside weather conditions.
The EASE was shown to have excellent test-retest reliability for children with CP. It should be noted, however, that no children with CP at GMFCS level IV took part in component of the study. The MDD 95% CI estimate for children with CP was large (8 points or 16% of the maximum score) primarily because of the high variability of EASE scores across GMFCS levels. This estimate would likely become lower if calculated separately for children at each GMFCS level. We chose not to make these calculations with our data because of the differences in numbers of children within each level.
Benefits and Limitations of the EASE
Based on our findings, the EASE provides an easy, inexpensive, and low-burden way to obtain an estimate of endurance for PA in young children with CP at all GMFCS levels. Given our test-retest reliability and construct validity results, we recommend the use of the measure within both clinical practice and research studies as an indirect estimate of endurance for PA in young children with CP. To our knowledge, there are no similar tools for children with CP in this age range that are as clinically feasible. It can be completed within 10 minutes, in person, by phone, or by mail, making the burden of administration low for therapists, researchers, and parents. It includes a broad and comprehensive definition of endurance for PA with questions regarding the frequency, intensity, and duration of PA. Results from the EASE may help facilitate a health and fitness discussion when children are young and assist in prevention of future problems that are currently reported in older children with CP.34
The EASE questions are focused on a child's endurance for activity within their own environment, versus a clinic, making it more likely to accurately reflect ability within the child's home and community. It also inquires about a child's PA level throughout the entire day. Although overestimation of activity is a reported limitation with parent-completed questionnaires,8,13,14 it has been suggested that validity is improved in questionnaires that reflect a child's overall activity level rather than focusing on specific activities.13,14 As with any type of questionnaire, the questions themselves can be a limitation as they are open for interpretation by each individual reading them. Correlations between questions were analyzed for both groups of children. Correlations were generally higher in the group of children with CP (0.21–0.89, P < .001, with CP; 0.21–0.68, P < .03, without CP). Among the children without CP, 6 of the 10 nonsignificant correlations involved question number 3 (“My child does enough activity so that he or she is breathing quickly or gets flushing in his or her face at least one time each day”). Further research is recommended to understand why interitem correlations for this question, in particular, are low before suggesting modifications.
For further consideration of issues related to activity levels in young children, we examined differences between racial/ethnic groups and groups based on parental education. In the children with CP, there were significant, but subtle, differences in scores due to both race (white/not Hispanic vs all other races and/or Hispanic) and parent education levels (high school or less vs community college/associate degree vs bachelor's degree or higher). Whereas the median scores between groups were significantly different, the range of EASE scores was similar within all of these groups. There are likely many explanations for these differences, ranging from differences in environment to differences in understanding the questionnaire. Further research would be required to make definitive conclusions. No differences were found between racial/ethnic categories or parent education levels in the group of children without CP.
The use of the EASE with families of children with CP younger than 6 years provides therapists with information that assists in evaluating endurance for PA compared with expectations for children without CP. If a child with CP has a score less than 46 (all children without CP had scores ≥ 46), then the therapist and parents might discuss health and aerobic fitness and identify opportunities for sustained PA. The therapist can assist the family in finding a mode of PA appropriate for their child. We have not determined whether the EASE is sensitive to the effects of intervention; similarly, responsiveness (ie, the ability to detect meaningful change) has not yet been established. Further research is recommended to explore the potential ceiling effect of the EASE. Our results indicate that when testing children in GMFCS level 1 (with a median score of 44 and MDD of 8), the EASE questions may not be responsive enough for their level of activity.
Recommendations for future studies of the EASE include using larger samples of children with CP across the GMFCS levels, exploring the findings of a potential ceiling effect with a larger sample of children in GMFCS level I, developing validity studies using other direct activity measures such as pedometers or heart rate monitors, and examining the responsiveness of the EASE in children with CP after interventions focusing on improving endurance. The measure may also need additional items reflecting greater and more complex questions about endurance for activity to be valid in children with typical development or conditioned children in GMFCS level I.
The EASE is a feasible parent-completed questionnaire for estimating endurance for PA for children with CP 2 to 6 years of age. The results provide evidence of construct validity and test-retest reliability for children with CP across all GMFCS levels. Further research is needed to examine convergent validity and validity for children with other disabilities and children with typical development. Examination of sensitivity-to-change and responsiveness is needed to support the use of the EASE as an evaluative measure in practice and research.
The authors thank all the children and their families who participated in this study. They also thank Eli Zygmuntowicz, Ben Kevan, and Elisa Katsel, who assisted with part of the study while they were DPT students at University of Puget Sound and all the rest of the Move & PLAY research team, including Peter Rosenbaum, MD; Alyssa Fiss, PT, PhD; Barbara Stoskopf, RN, MHSc; Audrey Wood, PT, MS, PCS; Barbara Sieck Taylor, parent consultant; Tina Hjorngaard, parent consultant; and all the therapist assessors and interviewers from the US and Canadian test sites.
1. Van den Berg-Emons HJG, Saris WHM, de Barbanson DC, Westerterp KR, Huson A, van Baak MA. Daily physical activity of schoolchildren with spastic diplegia and of healthy control subjects. J Pediatr. 1995;127:578–584.
2. Samson-Fang L, Fung E, Stallings VA, et al. of the North American Growth in Cerebral Palsy Project. Relationship of nutritional status to health and societal participation in children with cerebral palsy. J Pediatr. 2002;141:637–643.
3. Stevenson RD, Conaway M, Chumlea C, et al. Growth and health in children with moderate-to-severe cerebral palsy. Pediatrics. 2006;118:1010–1019.
4. Parker DF, Carriere L, Hebestreit H, Bar-Or O. Anaerobic endurance and peak muscle power in children with spastic cerebral palsy. Am J Dis Child. 1992;146:1069–1073.
5. National Association for Sports and Physical Activity. Active start: a statement of physical activity guidelines for children birth to age 5, 2nd edition. http://www.aahperd.org/naspe/standards/nationalGuidelines/ActiveStart.cfm
. Accessed December 10, 2010.
6. National Association for Sports and Physical Activity. Is it physical education or physical activity? http://www.aahperd.org/naspe/publications/teachingTools/PAvsPE.cfm
. Accessed June 8, 2011.
7. World Health Organization. International Classification of Functioning, Disability and Health. Short Version. Geneva, Switzerland: World Health Organization; 2001. http://apps.who.int/classifications/icfbrowser/
. Accessed December 10, 2010.
8. Adamo KB, Prince SA, Tricco AC, Connor-Gorber S, Tremblay M. A comparison of indirect versus direct measures for assessing physical activity in the pediatric population: a systematic review. Int J Pediatr Obes. 2009;4:2–27.
9. Pate RR, O'Neill JR, Mitchell J. Measurement of physical activity in preschool children. Med Sci Sports Exerc. 2010;42:508–512.
10. Puhl J, Greaves K, Hoyt M, Baranowski T. Children's Activity Rating Scale (CARS): description and calibration. Res Q Exerc Sport. 1990;61:26–36.
11. Li AM, Yin J, Yu J CCW, et al. The six-minute walk test in healthy children: reliability and validity
. Eur Respir J. 2005;25:1057–1060.
12. Plint AC, Gaboury I, Owen J, Young NL. Activities scale for kids: an analysis of normals. J Pediatr Orthop. 2003;23:788–790.
13. Harro M. Validation of a questionnaire to assess physical activity of children ages 4–8 years. Res Q Exerc Sport. 1997;68:259–268.
14. Sallis JF, Saelens BE. Assessment of physical activity by self-report: status, limitations, and future directions. Res Q Exerc Sport. 2000;71:1–14.
15. Morris C, Kurinczuk JJ, Fitzpatrick R. Child or family assessed measures of activity performance and participation for children with cerebral palsy: a structured review. Child Care Health Dev. 2005;31:397–407.
16. Graham HK, Harvey A, Rodda J, Nattrass GR, Pirpiris M. The functional mobility scale (FMS). J Pediatr Orthop. 2004;24:514–520.
17. Daltroy LH, Liang MH, Fossel AH, Goldberg MJ. The POSNA Pediatric Musculoskeletal Functional Health Questionnaire: report on reliability, validity, and sensitivity to change. J Pediatr Orthop. 1998;18:561–571.
18. Landgraf JM, Abetz L, Ware JEJ. Child Health Questionnaire (CHQ): A User's Manual. Boston, MA: Health Institute NEMC; 1996.
19. Oliver M, Schofield GM, Kolt GS. Physical activity in preschoolers: understanding prevalence and measurement issues. Sports Med. 2007;37:1045–1070.
20. Palisano RJ, Rosenbaum P, Bartlett D, Livingston MH. Content validity of the expanded and revised Gross Motor Function Classification System. Dev Med Child Neurol. 2008;50:744–750.
21. Portney LG, Watkins MP. Foundations of Clinical Research: Applications to Practice. 3rd ed. Upper Saddle River, NJ: Pearson Education Inc; 2009:525.
22. Bartlett D, Chiarello L, Palisano R, McCoy SW, Jeffries L, Fiss A. The Move & PLAY study: an example of comprehensive rehabilitation outcomes research. Phys Ther. 2010; 90(11):1660–1672.
23. Chiarello L, Bartlett D, Palisano R, McCoy SW, Jeffries L, Fiss A. A multivariate model of determinants of change in motor abilities and engagement in self care and play of young children with cerebral palsy. Phys Occup Ther Pediatr. 2011;31:150–168.
24. Palisano R, Rosenbaum P, Walter S, Russell D, Wood E, Galuppi B. Development and reliability of a system to classify gross motor function in children with cerebral palsy. Dev Med Child Neurol. 1997;39:214–223.
25. CanChild Centre for Childhood Disability Research. McMaster University. GMFCS Expanded and Revised (2007). http://motorgrowth.canchild.ca/en/GMFCS/expandedandrevised.asp
. Accessed November 1, 2011.
26. Gray L, Ng H, Bartlett DJ. The Gross Motor Function Classification System: an update on impact and clinical utility. Pediatr Phys Ther. 2010;22:315–320.
27. Maher CA, Williams MT, Olds TS. The Six-Minute Walk Test for children with cerebral palsy. Int J Rehabil Res. 2008;31:185–188.
28. Li AM, Yin J, Au JT, et al. Standard reference for the Six-Minute-Walk Test in healthy children aged 7 to 16 years. Am J Respir Crit Care Med. 2007;176:174–180.
29. Thompson P, Beath T, Bell J, et al. Test-retest reliability of the 10-metre fast walk test and 6-minute walk test in ambulatory school-aged children with cerebral palsy. Dev Med Child Neurol. 2008;50:370–376.
30. Lammers AE, Hislop AA, Flynn Y, Haworth SG. The 6-Minute Walk Test: normal values for children of 4–11 years of age. Arch Dis Child. 2008;93:464–468.
31. Van Eck M, Dallmeijer AJ, Beckerman H, Vanden Hoven PA, Voorman JM, Becher JG. Physical activity level and related factors in adolescents with cerebral palsy. Pediatr Exerc Sci. 2008;20:95–106.
32. Dencker M, Thorsson O, Karlsson MK, et al. Gender differences and determinants of aerobic fitness in children aged 8–11 years. Eur J Appl Physiol. 2007;99:19–26. [Erratum: Eur J Appl Physiol.
33. Chong J, Mackey AH, Broadbent E, Stott S. Relationship between walk test and parental reports of walking abilities in children with cerebral palsy. Arch Phys Med Rehabil. 2011;92:265–270.
34. Fowler EG, Kolobe TH, Damiano DL, et al. Section on Pediatrics Research Summit Participants; Section on Pediatrics Research Committee Task Force. Promotion of physical fitness and prevention of secondary conditions for children with cerebral palsy: Section on Pediatrics research summit proceedings. Phys Ther. 2007;87:1495–1510.