Effect of Habitual Activity, Lung Function, and Nutritional Measurements on 3-Minute Step Test Performance in Children with Cystic Fibrosis

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Children with cystic fibrosis (CF) often have impaired exercise capacity that has been associated with progressive lung disease. However, other factors may play a role in determining exercise ability. This study examined the influence of nutritional factors and habitual physical activity levels on pulse change (ΔPR) during the 3-minute step test (3MST) in children with CF compared to healthy peers.


Thirty-one children with CF (age 11.9 ± 4.0 yrs, mean FEV, 78 ± 25% predicted value) and 24 healthy age- and gender-matched volunteered for this study. Measurements included body mass index (BMI), triceps and subscapular skinfolds, mid-arm circumference (MAC), and self-reported habitual activity in the past year (duration [hours/week]; intensity [MET hours/week]). All subjects performed the 3MST. Differences between the CF and control group in nutritional factors, lung function, and habitual activity scores were determined using independent t-tests. Within each group, a correlational analysis was used to evaluate the relationship between ΔPR during the 3MST and measures of nutritional factors, lung function, and habitual activity using Spearman rho. Regression analysis was performed to attempt to predict ΔPR from these same variables in the CF and the control groups.


Compared to the control group, the CF group had lower BMI percentile (33 vs. 59%), but did not differ in skinfolds, MAC, ΔPR, or duration of habitual activity (hours/week). However, the mean intensity of habitual activity was significantly lower for the CF group (31.4 vs. 52.0 MET hours/week). Habitual activity intensity correlated most strongly with ΔPR for CF group (r = - 0.39 MET hours/week; rs = -0.41 hours/week). Activity duration and BMI percentile, but not lung function, significantly predicted exercise capacity in the CF group (R2= 0.25).


Children with CF had lower BMI percentile and participated in less vigorous habitual activities than their peers. These factors, but not lung function, predicted ΔPR on the 3MST. This suggests that increasing physical activity, particularly vigorous activities, might improve 3MST performance in children with mild to moderate CF lung disease.

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