Klepper, Susan E. PT, PhD; Muir, Nancy DPT
INTRODUCTION AND PURPOSE
The self-paced 6-minute walk test (6MWT) is increasingly used as a measure of functional exercise capacity in children. The test requires minimal equipment, training, and time to administer and can be conducted in schools or other pediatric clinical settings. When administered according to the American Thoracic Society (ATS) guidelines,1 the 6MWT has been found to be a reliable and valid measure of submaximal exercise capacity in children who are healthy. Li et al2 found excellent test-retest reliability (ICC = 0.94) for the 6MWT and also reported a moderate, and significant, correlation (r = 0.44, P < .0001) between 6-minute walk distance (6MWD) and VO2peak in 78 Chinese children who were healthy, aged 12 to 16 years. Several studies also report on the reliability, validity, and usefulness of the 6MWT to assess functional exercise capacity in children with chronic disease or neuromuscular disorders, including juvenile idiopathic arthritis,3–5 cerebral palsy,6,7 end-stage renal disease,8 hemophilia,3 spina bifida,3 cardiopulmonary disease,9–12 and children awaiting heart and lung transplants.13
A review of the literature found 6 studies that report measured values or prediction equations for the 6MWT in children who are healthy.14–19 Table 1 shows the sample characteristics, testing procedures, and factors influencing 6MWD in each sample. The influence of sex, age, and anthropometric attributes on the 6MWD varied among these studies although all 6 studies found height to correlate significantly with the 6MWD, whereas 5 studies found age and weight to correlate significantly with the 6MWD.14–18 Two studies found body mass index (BMI)15,17 and 2 found leg length15,19 to be significantly associated with 6MWD. Four provided prediction equations for the 6MWD for their population, with height being the most consistent predictor of walk distance, followed by age.14,15,17,18
Each of these studies was conducted in a country other than the United States, and each established reference values on the 6MWT for their specific population. Li et al14 studied Chinese children living in Hong Kong; Limsuwan et al19 studied children living in Thailand; Geiger et al15 studied Caucasian children living in Austria; and Saad et al18 studied Tunsian, Arab, and Berber children living in North Africa. The samples of Lammers et al16 in England and Priesnitz et al17 in South America were more diverse; however, the majority of their participants were Caucasian. The only study conducted in North America restricted the sample to third-grade children and deviated greatly from the ATS administration guidelines, testing children in groups on an outdoor rectangular grass field.20 The authors followed the guidelines for the 9-minute run test, a measure of maximal exercise capacity in children.21 It is unclear whether these measured values or prediction equations for the 6MWD developed for children living in other countries can be applied to children living in the United States. A study by Poh et al22 found that the equations developed for Caucasian adults did not reliably predict the 6MWD of healthy Singaporean adults. Saad et al18 also reported that measured values on the 6MWT in their sample of North African children were significantly underestimated by the prediction equation developed for Austrian children,15 measured values for English children, and significantly overestimated by the prediction equation for Chinese children.14
The 6MWT could be a useful measure to screen children for impairments in functional exercise capacity, monitor changes over time, and evaluate the effects of interventions to improve functional mobility. This requires reference values and information regarding the influence of age and anthropometric variables on 6MWD that are relevant to children living in the United States. Therefore, the purposes of this study were to (1) investigate the 6MWD of children who are healthy and typically developing, aged 7 through 11 years, living in a metropolitan area of the United States and (2) examine the relationship between 6MWD, age, sex, and anthropometric variables.
Children were recruited from public and private schools and an after-school program in the New York City metropolitan area as well as through family and friends of the investigators. Inclusion criteria were children between the ages of 7 and 11 years who were healthy and developing typically for the past 11 months. Children were excluded if they reported a recent illness, injury, or chronic health disorder, with the exception of mild asthma controlled by medication. Parents received a brochure explaining the study and inviting their child's participation. The study was explained orally to each child. Each participant was offered an incentive of $10 in cash or a $10 bookstore gift card for completing a single test session consisting of two 6MWTs and measures of height, weight, and leg length. The study received approval by Columbia University's Institutional Review Board and the New York Department of Education Office of Research. All children signed informed assent and a parent or legal guardian gave written consent.
Testing for 74 subjects was performed over a period from 2006 to 2009 by a group of 5 physical therapist students. A single student performed all tests for an individual subject. Intertester agreement on the anthropometric measures was established during a training session in which testers measured height, weight, and leg length on 8 classmates. All intraclass correlation coefficients (ICC: 2,1) were 0.94 or more. During data collection inter- and intratester agreements were assessed for 14 children, aged 7.25 to 11.25 years, who were randomly selected as subjects entered the study and all testers were available. The ICC (3,1) values for intratester agreement ranged from 0.88 to 0.99 (leg length), 0.95 to 1.00 (height), and 0.99 to 1.00 (weight). The ICC (2,1) values and 95% confidence interval for intertester agreement for height and weight were both 1.00 (0.99-1.00) at each test session. The ICC value for leg length was 0.98 (0.95-0.99) for test 1 and 0.97 (0.93-0.98) for test 2. The ICC (2,1) value for test-retest stability on the 6MWD in these subjects was 0.93.
The final sample of 100 children included 26 children who completed the 6MWT as part of a study to compare gross motor efficiency and functional capacity in children who are healthy and children with juvenile idiopathic arthritis. Reliability data for this study have been reported previously.23 All intertester ICC values were 0.98 or more. The coefficient of variation for test-retest on the 6MWD performed 2 to 4 weeks apart was 8%. Testing procedures for the 6MWT were the same in both studies.
Data collection tools included a Seca 214 portable stadiometer (Hanover, MD) to measure height, a Taylor lithium electronic scale (Las Cruces, NM) to measure weight, a 150-cm plastic-coated tape measure (Dynatronics, Salt Lake City, UT) to measure leg length, and a digital stop watch to measure walk time. Three testing locations were used: the gymnasium in two elementary schools in New York City and a straight hallway in the academic physical therapy building. Track length was dictated by the space available in each setting. The track measured 15 m in each school setting and 25 m in the physical therapy department. In each setting a 100-foot vinyl-measuring tape was placed next to the walking track and secured to the floor using painter's tape to demarcate the track and facilitate measuring the distance subjects walked. A 1-yard strip of tape was placed at either end of the walking track to indicate the beginning and end points.
Height, weight, and leg length measurements were obtained from each child and recorded to 2 decimal places. Height and weight without shoes were measured and recorded in meter and kilograms, respectively. Body mass index was calculated using the formula: weight/height2. Right leg length was measured with the child lying supine, both legs extended and in neutral rotation. Leg length was measured in centimeters from the apex of the right anterior superior iliac spine to the distal border of the right medial malleolus. The mean of 3 measurements was used for analysis.
The 6MWT was conducted in accordance with ATS guidelines.1 Prior to testing, each child walked along the length of the track with the tester and was shown the beginning and end of the course. Demonstration and instructions were given at both ends of the course to “touch the long strip of tape with your foot, turn around and walk back to the other end.” Participants were informed that the purpose of the test was to find out how far children walk in 6 minutes. They were told to walk like they were trying to get somewhere they really wanted to go, but hopping, skipping, running, and jumping were not allowed. Unlike the original ATS guidelines developed for patients with cardiac or pulmonary disease, our subjects were not told they could stop and rest during the test.
The child stood behind the starting line and began walking on the command “Go.” One complete lap consisted of walking from the starting line to the end of the track, turning, and walking back to the starting line. Standard phrases of encouragement (“You are doing well,” “Things are going well,” and “Keep going”) were given at 30-second intervals, and participants were informed of the remaining time at each minute mark, for example “You have 5 minutes left.”1 The tester stood at the starting line throughout the test and made a tick mark on the score sheet to indicate a completed lap each time the child passed the starting line. At the end of 6 minutes, the tester told the child “Stop and do not move until I come to you.” The tester walked to the child and placed an index card on the floor in line with the front of the child's most forward foot. An arrow on the card pointed in the direction the child was walking. A straight edge was placed from the end of the card to the measuring tape, and the tester recorded the distance walked in any final partial lap. The total 6MWD was calculated by multiplying the number of complete laps with the length of a lap (50m or 30 m) and adding the distance covered in any partial lap. Results were recorded in meters rounded to 2 decimal places. Subjects performed a second 6MWT after a 15-minute rest break.
Descriptive statistics were calculated on all variables for males and females separately and combined in the full sample and within each age group (7/8, 9, 10, and 11 years). All data were assessed for normality using the Shapiro-Wilk test and inspection of normality plots. Normally distributed data were reported as the mean and standard deviation (SD). Data with a significantly skewed distribution were reported as median and interquartile range.
Independent t tests were used to examine differences in the 6MWD between test 1 and test 2, between males and females, and between groups based on track length. Separate one-way analyses of variance with pairwise comparisons were used to examine differences among age groups on the 6MWD and anthropometric variables.
Univariate correlation analyses were performed to examine the relationship of age, sex, and anthropometric variables to the 6MWD. The Benjamin and Hochberg False Discovery Rate24 method was used to correct for multiple tests. Partial correlations were performed to examine the unique relationship of each variable with the 6MWD while controlling for other variables. Analyse-it™ and SPSS version 16 (SPSS Inc, Chicago, Illinois) software programs were used to analyze the data. An alpha level of less than .05 was considered significant.
The minimal detectable change (MDC) on the 6MWD was calculated for the group of children who completed the test twice in 1 session. The MDC is considered the minimal amount of change over and above measurement error of 2 repeated measures at a specified level of confidence.25 We chose the 90% level of confidence because that appears to be the level most often reported in the literature as an acceptable confidence level for clinical application to individuals.25,26 The formula was MDC90 = SEM (standard error of measurement) × 1.65 × √2.26 The SEM is calculated as the product of the pooled SD from 2 repeated trials and the square root of 1 minus the test-retest reliability coefficient.
One hundred children, 57 females and 43 males, with a mean age of 9.66 ± 1.08 years (range = 7.08-11.58 years) completed the 6MWT. The sample was culturally diverse; 35% were Caucasian, 4% Asian, 21% African-American, 38% Hispanic, and 2% mixed. Nine subjects completed only one 6MWT, however, because there was no significant difference in mean 6MWD between test 1 and test 2 in the full sample (t = −0.312, df = 90, P = 0.76); the single 6MWD for each of these subjects was used in the analysis. Table 2 shows descriptive data on all variables for males and females separately and combined within each age group. Data were normally distributed with the exception of weight and BMI in the full sample, weight in females, and BMI in males. Mean 6MWD (m) ± SD (95% confidence interval) was 518.5 ± 72.56 (504.1-532.9) for the full sample. The MDC90 was 48.34 m for children who performed two 6MWTs within the same testing session. Males and females did not differ significantly on 6MWD or any personal attribute. Mean 6MWD was 518.73 ± 72.61 (496.39-541.08) for males, and 518.32 ± 73.16 (498.91-537.74) for females.
Among the age groups, 6MWD did not differ significantly (F = 1.61, df = 3, 96, P = .19); however, there were significant differences in height (F = 21.33, df = 3, 76, P ≤ .0001), weight (F = 10.71, df = 3, 75, P ≤ .0001), leg length (F = 14.70, df = 3, 68, P ≤ .0001), and BMI (F = 4.25, df = 3, 75, P = .008) after correcting for multiple tests.23 Pairwise comparisons indicated no significant differences in any variables between the 7/8- and 9-year-old groups. All anthropometric variables increased significantly from 9 to 10 years, with the largest increase seen in weight and BMI in the 10-year-old children. On the basis of sex and age-referenced BMI percentile data available for 79 subjects (21 with missing data), 18% (14/79) were classified as overweight (BMI ≥ 85th but < 95th percentile) and 28% (22/79) as obese (BMI ≥ 95th percentile). The 10-year-old group had the highest prevalence of overweight (32%) and obesity (39%) as well as the lowest 6MWD (497.15 ± 66.81).
Independent t tests showed a significant difference in 6MWD based on track length (t = 3.47, df = 98, P = .002), with those walking the 25-m track covering 50 m more than those walking the 15-m track (Table 3). After correcting for multiple tests, the analysis indicated that the 2 groups also differed significantly in age and all anthropometric variables with the exception of leg length. Examining 6MWD by age group, we found a significant difference only in the 10-year-old group, with those walking the 25-m track covering more distance (t = 2.42, df = 32.6, P = .02).
Table 4 shows the univariate correlations between 6MWD and other variables in the full sample. Only BMI was significantly related to 6MWD (rS = −0.23, t = −2.11, df = 77, P = .04); however, the correlation was low and not significant (P = 0.20) after correcting for multiple tests. Partial correlation analysis to examine the unique relationship between 6MWD and BMI, removing the influence of all other variables, confirmed that the relationship was not significant at P = .49. Univariate analyses between 6MWD and other variables by sex showed no significant relationships in females; however, BMI correlated significantly to 6MWD in males after correcting for multiple tests (r = −0.49, t = −2.95, df = 27, P = .007).
6-Minute Walk Distance
This is the first study to examine the performance of elementary school-age children living in the United States on the 6MWT. A direct comparison of our findings to reference values reported for children living in other countries is difficult because each study organized its data in a different way. Lammers et al16 and Priesnitz et al17 reported values separately for each age group, but combined data for males and females. Geiger et al15 and Saad et al18 provided separate values for males and females, but combined data across several age groups. Li et a14 reported mean height rather than 6MWD by age and constructed height-referenced percentile curves for males and females on the 6MWD. Table 5 compares our findings to those of previous studies in 3 ways. First, our measured 6MWD for each age group is compared with those of other studies reporting the actual 6MWD for their participants in a similar age range. To compare our findings with those of Li et al,14 we estimated the actual 6MWD for their subjects by plotting the mean height that the authors reported for each age group on the appropriate reference curve at the 50th percentile. Second, our data is presented as the 6MWD predicted for each age group in our sample by the reference equations of Saad et al18 and Geiger et al15 and the height-referenced percentile curves reported by Li et al.14 Finally, the actual 6MWD for our subjects is presented as a percent of the measured or predicted values reported by previous studies.
Our measured 6MWD values were lower at every age group compared with values reported or predicted by previous studies, with the exception of Lammers et al.16 Across all age groups our subjects walked, on average, 77% and 82% of the distance were predicted by the reference equations of Saad et al18 and Geiger et al,15 respectively. On average our subjects walked 80% of the distance predicted by the height-referenced percentile curves of Li et al,14 90% of measured values reported by Priesnitz et al,17 and 88% of the measured values reported by Limsuwan et al.19 In contrast, our subjects walked, on average, 105% of the actual distance reported by Lammers et al16 with the exception of our 10-year-old group who had the lowest 6MWD of the sample. We were not able to use the reference equations of Li et al14 or Priesnitz et al,17 because we did not measure heart rate (HR), a variable included in their equations. These differences raise the question of whether prediction equations for the test developed for children in 1 country are directly applicable to other populations. Saad et al18 found that 6MWD in their North African children could not be reliably predicted by references equations developed for children living in other countries. Limsuwan et al19 reported that 6MWD in their sample of children living in Thailand were lower than the values reported by Li et al14 for children living in Hong Kong, despite the fact that the children in both studies were Asian. This suggests that there may be differences in the performance of children on the 6MWT even within a single racial or ethnic group.
Discrepancies in reported 6MWD among these studies may be due to in part to cultural differences among countries regarding physical activity (PA) and fitness. Because the 6MWT is a measure of submaximal exercise capacity, a sedentary lifestyle and low aerobic fitness may negatively affect a child's performance.5 Vincent et al,27 who examined PA levels and BMI in children living in 3 different countries, found that American children were less active and heavier than the children in Australia or Sweden. Environmental factors might have contributed to different PA levels among their groups. Children in Sweden, who had the highest activity level and lowest BMI, lived in moderate-sized communities with many walking and biking paths that encouraged daily exercise. Children living in North Africa who reported being active outside of school walked an average of 57 m farther on the 6MWT than those who were inactive.18 Although we did not collect data on socioeconomic status in this study, 63% of our participants were recruited from 3 sources—a public school, a parochial school, and an after-school program—located in a low income neighborhood in the inner city. These children may have had fewer opportunities for PA or organized sports outside of school than children living in suburban communities.
The percentage (46%) of children who were overweight or obese in our sample was higher than that on the national level, 35.5% (32.4, 38.7), reported for 6- to 11-year-old children living in the United States in 2007-2008.28 Several studies support the view that children who are obese may be less active and less motivated to perform well on the 6MWT. Vincent et al27 found that BMI significantly and inversely correlated to PA in American children who were overweight. Morinder et al29 found that overall 6MWD and exercise HR were significantly lower in children and adolescents who were obese than those with a normal weight, and a recent study by Maggio et al30 found that the PA level and aerobic fitness were lower in children who were obese compared with controls who are healthy. These findings suggest that performance during the 6MWT may be influenced by motivation and attitudes toward exercise as well as the child's aerobic fitness. Geiger et al15 commented that some children in their sample appeared to be highly motivated, but others were disinterested. Although we did not monitor exercise intensity or effort in our participants, testers commented that some children who were overweight walked more slowly than children of normal weight and often asked how much time was left in the test.
Differences in testing procedures among current studies may also contribute to variability among test results. We followed ATS administration guidelines with the exception of using child-friendly directions for the test. Four of the previous studies followed these guidelines.14,17–19 However Geiger et al15 required children to push a measuring wheel as a motivator while they walked, and Lammers et al16 had the tester follow the child to record HR and VO2 each minute from a portable pulse oximeter attached to the child's wrist and a finger.16 These deviations might alter the child's walking speed and thus the score on the test.
The influence of track length on a child's 6MWD is also unclear. Our analysis showed that subjects tested on the 25-m track walked significantly farther than those tested on the 15-m track. This may indicate that children lose speed with each turn on a shorter track, resulting in lower 6MWD. However, these data must be viewed with caution because the 2 groups also differed significantly on all other variables, with the 25-m group being younger, taller, and lighter. The prevalence of overweight or obesity was 63% in the 15-m group compared with 18% in the 25-m group suggesting children who were heavier walked more slowly during the test.29 Testing conditions also varied among the 3 sites. The 25-m group was tested in the physical therapy department where we were able to control conditions during the test. In contrast, subjects in the 15-m group were tested in their school gymnasium during a physical education class, where there were many distractions that could have affected their performance. Although the ATS guidelines for adults recommend a 30-m straight track1 a multicenter study conducted by Weiss31 found that there was no significant difference in 6MWD among straight tracks ranging from 15 to 50 m. Track length in current studies of children who are healthy fall within this range; however, some studies of children with chronic health conditions used an 8-m track.4,5 Further research, comparing 6MWD in children under similar conditions but on different track lengths would help to clarify this issue.
This is the first study to report the MDC on the 6MWT in healthy, typically developing children. We calculated the MDC90 only for the group who performed 2 walk tests in a single session because we believed that differences in 6MWD in the group who performed the test twice within a 2- to 4-week interval might reflect actual change in their performance rather than just measurement error. The MDC90 of 48.34 m in this sample should be useful to clinicians to determine whether the change in a child's 6MWD over time or with intervention exceeds measurement error. However, it is important to keep in mind that this MDC is based on the performance of children who are healthy and typically developing and may not directly apply to children with specific diagnoses, particularly those with impaired neuromotor function that affect gait pattern or speed.
Factors Influencing 6MWD
The second purpose of our study was to examine the relationship between 6MWD, sex, and age and anthropometric variables. Although most previous studies reported that age correlated significantly to 6MWD, we did not find this association. In the study by Lammers et al,16 walk distance increased significantly in the intervals from 4 to 7 years and 7 to 11 years; year-to-year increases were significant in the younger group, but not in the older group, suggesting 6MWD may stabilize during this period. This supports our finding of no significant year-to-year differences in 6MWD in our sample. Our failure to find a significant increase in 6MWD between the 7-year and 11-year groups may be due to the small number of children in the oldest group where one would expect walking speed to increase.18
We, like several previous groups,16,17,19 found no significant difference in 6MWD based on sex. In contrast Li et al14 reported that 6MWD was higher in males at every age. Saad et al18 also found that pubescent males walked an average of 135 m further than prepubescent males. The lack of association between sex and 6MWD in our participants may reflect their younger age in comparison to studies that included adolescents.14,15 Krahenbuhl et al32 reported that sex differences in maximum aerobic capacity (VO2max) are small before the age of 12 years; however, after this age, VO2max continues to increase in males while it plateaus in females, resulting in a sex-based difference of more than 50% by the age of 16 years. In the study by Geiger et al,15 6MWD reached a plateau in females at about 11 years of age but continued to increase in males up to the age of 18 years. Considering these potential differences in 6MWD between males and females after puberty, it seems wise to use separate, sex-based reference values on the 6MWT for adolescents. The extent of differences in prepubertal children is still unclear.
Four studies provided prediction equations for the 6MWD on the basis of height and some combination of the other variables. Geiger et al15 found that height plus age explained 49% and 50% of the variability in 6MWD in boys and girls, respectively. Li et al14 reported that height and HR difference before and after the test were predictive of 6MWD, and Lammers at al16 found height combined with age and weight accounted for 44% of the variation in the 6MWD. Priesnitz et al17 reported that height, in combination with age, weight, and HR difference, accounted for 37% of the variation in 6MWD, and in the North African sample of Saad et al18, height combined with age and weight explained 60% of the variance in 6MWD. It is possible that our study lacked adequate power to detect significant correlations between 6MWD and these variables; however, our actual correlations were much smaller (range, 0.03-0.19) than those reported by other studies (range, 0.20-0.65).14–18
Only 2 previous studies reported a significant association between 6MWD and BMI, despite the reported influence of both height and weight on test results. Although BMI did not correlate to 6MWD in our full sample, it was significantly and negatively related to 6MWD in males, accounting for 24% of the variation in walk distance. This finding is most likely due to the high level of overweight and obesity in our male participants and may reflect the cultural differences regarding PA among countries previously discussed.
In summary, the 6MWD values in our participants on average ranged from 79% to 105% of the measured values and 77% to 82% of predicted values reported previously for children in other countries. The information in Table 5 should be useful to clinicians when evaluating the 6MWD of a child or group of children with impaired mobility. Values below the lower end of the reported range for a child's age suggest low functional exercise capacity and indicate the need for monitoring and possibly intervention. This is especially important for children who are overweight or obese and physically inactive. Pediatric physical therapists can help children, parents, and school personnel to set appropriate goals for the child to improve his or her performance on the 6MWT. The MDC90 of 48.34 m, calculated from the current sample of children, can be used to determine whether the change in the child's 6MWD following intervention is statistically significant. One should keep in mind that the real goal is to improve health-related fitness. To this end, therapists can help children choose recreational physical activities that match the child's interests and physical abilities and develop the motor skills necessary to safely and successfully participate in those activities.
Limitations of the Study
This study had several limitations including a relatively small sample size, especially in the 7- and 11-year-old groups. We also did not examine other factors that may have contributed to subjects' 6MWD, including variations in the time of day the test was administered, choice of footwear, habitual PA level, exercise intensity, motivation, and attitudes toward the test. Finally, we used 2 different track lengths and were not able to control conditions across the 3 testing sites that may have affected a child's performance.
This is the first study to provide reference values on the 6MWT, performed according to ATS guidelines, for elementary school children living in the United States. Our findings support the belief that reference values or prediction equations for performance on the 6MWT developed for children living in one country may not be applicable to those in other countries. This is also the first study to report the MDC on the 6MWT for school-age children, information that should be useful to pediatric therapists working with children who have low functional exercise capacity because of impaired mobility.
We thank Ilanit Hersh and Jody Naiburg who, as phy-sical therapist students, assisted with the reliability phase of this study, and Aileen McKernan who assisted with subject recruitment and assessment and data analysis.
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age factors; body mass index; child; exercise test; reference values; sex factors; United States
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