Age, height, and body mass variables were analyzed individually for their ability to predict 6MWD.
GMFCS I. The total variance in 6MWD explained by age, height, and body mass was 26.2% [R2 = 0.26; F(3, 73) = 8.27; P < .001]. Height was the only independent variable (β value = 0.62) that contributed significantly (P < .05) to explaining 6MWD. The part correlation coefficients indicated that height uniquely explained 6% of the variance in 6MWD.
GMFCS II. The total variance in 6MWD explained by age, height, and body mass was 35% [R2 = 0.35; F(3, 52) = 9.00; P < .001]. Age was the only independent variable (β value = −1.13) that contributed significantly (P = .001) to explaining 6MWD. The part correlation coefficients indicated that age uniquely explained 15.9% of the variance in 6MWD.
GMFCS III. The total variance in 6MWD explained by age, height, and body mass was 43% [R2 = 0.43; F(3, 17) = 3.53; P < .05]. Age (β value = −0.23), height (β value = 0.98), or body mass (β value = −0.11) did not independently contribute significantly (P > .05) to explaining 6MWD.
TD. The total variance in 6MWD explained by age, height, and body mass was 37.1% [R2 = 0.37; F(3, 136) = 26.16; P < .01]. Age was the only independent variable (β value = 0.54) that contributed significantly (P < .01) to explaining 6MWD. The part correlation coefficients indicated that age uniquely explained 3% of the variance in 6MWD.
The focus of this research was to quantify the ambulatory capabilities of children with CP on a flat surface compared with their peers that were TD using the 6MWT. The results showed that the 6MWT can differentiate between walking ability across the 4 different test groups of children classified in GMFCS levels I, II, and III and TD. These reference datasets will be useful in clinical and research practice to allow comparison of both, individuals and groups of children with CP. They may also be used to track changes over time in response to growth and potential clinical interventions.
Children with spastic CP functioning at GMFCS level III walked an average distance of 305.28 m (SD = 66.95 m) in 6 minutes, approximately 223 m less than the average distance walked by their peer group that was TD, who covered a mean distance of 528.42 m (SD = 67.77 m) in 6 minutes. On average, children functioning at GMFCS level II varied from their peers that were TD by 142 m, walking a mean distance of 386.74 m (SD = 66.47 m). Of interest, children with spastic CP functioning at GMFCS level I still walked an average of 89 m less than their peers that were TD, covering a mean distance of 439.57 m (SD = 49.81 m). This indicates that CP as a pathology still affects the functional walking ability of the highest functioning child with CP when compared with a peer group that is TD.
Regression analysis of the data assessing the ability of age, height, or body mass to predict 6MWT scores indicated that height contributed significantly to predicting the 6MWT score in children functioning at GMFCS I and that age predicted 6MWT scores in children functioning at both GMFSC II and TD. However, part correlation coefficients in this regression analysis ranged from 0% to 16%, limiting the predictability and therefore clinical usefulness of these findings.
Previous Research in Children With CP
Only 1 previous study, by Thompson and colleagues,7 outlined the 6MWT scores in children with CP classified across the GMFCS levels I to III as shown in Table 1. In their study, they examined the test-retest reliability of 31 children with spastic CP, and therefore, 2 6MWT trials were performed with a retest interval of 1 to 2 weeks.7 Comparison of results shows the current study (Table 4) has lower scores for children functioning at GMFCS level I, but higher scores for those classified in GMFCS levels II and III. There are methodological differences limiting the comparability of results, and 1 factor that might have contributed to the variance in 6MWD may have been the differing course shape and length. We used a 70-m straight corridor compared with a 20-m × 45-m rectangular course used by Thompson et al.7 Therefore, a lower frequency of turning events was used in the current study, whereas the test protocol of Thompson et al7 did not require a full 180° turn. The rectangular course may have helped the higher functioning GMFCS level I group maintain momentum at the 90° corners, but this repeated change of direction was perhaps more difficult for children classified in GMFCS levels II and III.
In the study by Chong et al,10 an oval-shaped course was used. This type of course may reduce the difficulties that participants may have when making 180° turns, as is necessary when performing the 6MWT on a straight course. An oval-shaped course may make the test easier to perform, particularly for participants who use assistive devices such as walkers. This is reflected by a larger mean 6MWD of 455.4 m in the study by Chong et al10 (GMFCS levels I-IV and age range 94.4-776.2) compared with the mean values found in our study.
Results of the current study also showed a significant relationship between participant height and 6MWT scores in children with CP classified in GMFCS level I, and in children classified in GMFCS level II a significant relationship between age and 6MWT scores was found. For children classified in GMFCS level III, no significant predictor of 6MWT scores was found. Perhaps at higher levels of disability, age or height is not as important in determining differences in walking distance. None of the previous 4 studies assessing 6MWT scores examined the relationship of score with age, height, or body mass.1,6,7,10 However, in the current study, these relationships showed low predictability levels limiting their clinical usefulness, and further research is warranted to assess the implications of these varying anthropometric and demographic associations.
Previous Research in Children That Were TD
When results of the group that was TD are compared with previous studies, our mean score of 530 m falls below all studies outlined in Table 2, except for the study by Lammers et al14 in the United Kingdom who reported mean scores of 471 m. A comparison of height profiles shows that all other studies were conducted on a cohort with a higher mean height with the exception of the Brazilian study by D'Silva et al12 who, along with Lammers et al,14 reported on children with a lower mean height. Li et al15 have previously shown a strong correlation between height and 6MWT scores.
Additional factors that may affect 6MWT scores reported in the literature include motivation levels and differences in the interpretation of instructions by participants. Studies by Geiger et al13 and Tonklang et al19 used modified protocols, which included measuring wheels as an incentive or rubber bands as “collecting tokens” given to each participant on completion of a lap (Table 2). These modified protocols create an incentive for participants to walk as far as possible. In the current study, 1 observation was that motivation and interpretation of how the test should be performed varied among participants. Participants were told that “the object of the test is to walk as far as possible in 6 minutes.” Some participants interpreted this by walking quickly, whereas others seemed to walk at a slower, more natural pace. It was also noted that many younger children seemed to lose interest in the object of the test. These factors were not formally evaluated in this study.
Regression analysis of our group that was TD showed that age was the only significant predictor of 6MWT scores, but again, the predictability level was low. When previous research reports of similar studies were evaluated, no consistent trend was seen. Our findings agreed with the results of the study by Ulrich et al20 conducted in Switzerland with 496 children aged 4 to 16 years, reporting that age was the best predictor of 6MWT scores. Lammers et al14 in the UK showed that height, weight, and age all correlated with the test scores in 328 children of 4 to 11 years. Li et al15 in Hong Kong, who assessed 1445 children that were TD from ages 7 to 16 years, showed height to have the greatest correlation with 6MWT scores in children that are TD. Although difficult to infer the implications of these varying results, they might suggest that race or country of origin could be a factor.
Study Limitations and Future Research
Limitations of this research include the fact that no assessment of intellectual ability was conducted during testing. Some participants with CP with cognitive involvement might have reduced understanding of instructions and ability to follow directions during testing, which could have potentially negatively affected their test scores. Sex differences were not assessed in the data analysis, which may have shown additional differences. The cohort of children with CP was a sample of convenience from a gait laboratory database. A population-based study, generated from a registry of children with CP, may be more representative of this patient population. In addition, there may be a limitation in terms of sample size, particularly in the group that was TD, when compared with Li and colleagues15 who reported on 1445 children.
Future research in the area may be directed at including additional physiological monitoring, such as the assessment of heart rate, to evaluate energy expenditure. It may also be useful to use activity monitoring sensors over a series of days to assess whether higher 6MWD correlates with higher activity levels in the daily activities in children with CP. Finally, an unmeasured observation of the study during testing was that some children may have benefited from more instruction. It may be useful to test a modified protocol for children, with instructions given every 30 seconds rather than every minute and assess the results for possible score changes.
This study outlines a reference range of 6MWD across children with spastic CP who are ambulatory and functioning at GMFCS levels I to III and their peers that were TD and shows significant differences among all groups. Our findings also show that height may be a predictor of 6MWD in children with spastic CP classified in GMFCS level I and that age may be a predictor in children classified in GMFCS level II; however, further research is warranted to explore the significance of these findings.
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Keywords:Copyright © 2016 Academy of Pediatric Physical Therapy of the American Physical Therapy Association
activities of daily living; cerebral palsy; child; exercise test; female; male; physical endurance; reference values