- The pathophysiology of impaired exercise tolerance in patients with cystic fibrosis (CF) is not completely understood. The objective of this study was to compare exercise ability (at clinical and cellular levels) and resting energy expenditure in female athletes with CF compared with matched control subjects. Sixteen subjects and matched control subjects participated in the study. The girls with CF not only had a significantly greater resting energy expenditure (7.6% higher; p<0.05), their habitual daily activity was also significantly greater than that of control subjects (15% greater; p<0.01). Peak aerobic capacity was similar in both groups. However, peak anaerobic power was 20% less (p<0.05) in girls with CF. The 31P magnetic resonance spectroscopy studies demonstrated that there were no differences between the groups at rest, but at 25% total work output the girls with CF were less acidotic (CF, pH 6.99 [0.06]; control subjects, 6.90 [0.05]) and had a significantly lower inorganic phosphorus-to-phosphocreatine ratio (CF, 0.34 [0.07]; control subjects, 0.41 [0.08]). These differences continued to increase to maximal exercise. This study demonstrates that in spite of normal lung function and good nutritional status, females athletes with CF still had significant deficiencies in some measures of fitness and muscle metabolism compared with healthy athletes.