The Influence of Lean Body Mass and Leg Muscle Strength on 6-Minute Walk Test Performance in Children with Cystic Fibrosis

Abstract

Purpose: Exercise capacity, measured by the 6-minute walk test (6MWT) is impaired in patients with cystic fibrosis (CF) and is related to their survival and quality of life. 6MWT performance may be influenced or explained by lung function, lean body mass (LBM), or muscle strength. Therefore, the purpose of this study was to determine the relationships between leg muscle strength and 6MWT as well as LBM and 6MWT in children with CF as compared to healthy peers. Methods: Subjects with CF (n=21, mean age 10.5 yrs, mean FEV, 65% predicted) and age- and sexmatched healthy controls were tested for LBM and lower extremity muscle strength. Measurements of skin folds were used to determine LBM. Strength of bilateral hip flexors, knee extensors, and ankle dorsiflexors was determined via hand-held dynamometry. Subjects then completed the 6MWT. A Mann-Whitney U test was used to make between group comparisons for height, weight, LBM, muscle strength, and 6MWT distance. A Kendall's Tau was used to assess within group correlations between the strength of each muscle tested and both LBM and 6MWT distance. Results: Subjects with CF had significantly lower LBM (23.7 ± 8.5 kg vs. 31.4 ± 12.0 kg) and 6MWT (490 ± 77 m vs. 557 ± 94 m) than controls. Hip flexion and ankle dorsiflexion strength was also significantly lower in CF than controls. The 6MWT distance correlated significantly with bilateral hip flexion (r = 0.37-0.39) and knee extension strength (r = 0.31-0.37) in CF. There was no relationship between any strength measure and 6MWT distance in controls. Strength correlated moderately with LBM in both groups. LBM did not correlate with 6MWT in either group. Conclusions: Exercise capacity as measured by 6MWT is impaired in children with CF and is partially related to decreased leg muscle strength. These findings suggest that leg muscle strength, not LBM, is related to 6MWT performance in children with CF.