Peripheral muscle weakness and exercise capacity in children with cystic fibrosis

Exercise intolerance in cystic fibrosis (CF) is attributed to diminished nu tritional and pulmonary function. We studied the pathophysiology of such in tolerance in relation to muscle force and fat-free mass (FFM), in 15 childr en with moderately severe symptoms of CF (FEV1 < 80% predicted and/or weigh t for age < -1 SD of reference median), 13 children with mild symptoms of C F (FEV, and weight, above these thresholds), and 13 healthy controls. Cycle maximal workload (Wmax) and V (over dot)(O2)max were assessed. Maximal per ipheral muscle force was measured, and FFM was calculated from skinfolds, P atients with mild CF, as compared with matched controls, had lower values o f Wmax per kilogram of FFM (3.9 +/- 0.5 versus 4.6 +/- 0.3 W/kg [mean +/- S D], respectively; difference = 0.7 [95% CI = 0.4 to 1.1]), and diminished m aximal muscle force (2.7 +/- 0.4 kN versus 3.1 +/- 0.7 kN; difference = 0.4 4 [95% CI = 0.03 to 0.87]), but similar V (over dot)(O2)max. Patients with moderate CF had lower FFM, muscle force, and exercise tolerance than did th e other groups. Oxygen cost of work was elevated in both groups of CF patie nts. Muscle force showed a strong positive correlation with Wmax in patient s and controls, with disproportionately lower regression slopes in the CF p atients. In children with CF, muscle force is decreased and associated with diminished maximal work load, even in the absence of diminished pulmonary or nutritional status.