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Lung compliance has been found to be low in patients with chronic diaphragmatic weakness or paralysis but has not been well-studied in patients with amyotrophic lateral sclerosis (ALS). Noninvasive positive-pressure ventilation (NPPV) prolongs survival in ALS patients but may also have additional beneficial effects.This study evaluated static expiratory lung compliance (CL) in subjects with ALS and determined the effect of lung inflation with supramaximal inflation on CL.This was a prospective trial comparing CL before and after supramaximal lung inflation via mouthpiece-delivered positive pressure.A single university medical center with an multidisciplinary ALS center.Fourteen subjects with ALS were compared to 4 healthy volunteers.Subjects underwent a battery of pulmonary function tests including for CL. Then they used positive pressure administered via a mouthpiece set to 10 cm H2O above their maximal static recoil pressure for 5 min. The CL measurement was then repeated.The mean (± SD) baseline CL was reduced (164.1 ± 82.1 mL/cm H2O) in subjects with ALS and was significantly lower than that in healthy volunteers (237.5 mL/cm H2O; p = 0.04). CL increased significantly in subjects with evidence of diaphragm weakness (change in CL, 11.3 ± 16.7 mL/cm H2O; p = 0.03). Healthy volunteers did not have an increase in CL.Patients with ALS and diaphragmatic weakness have reduced CL, and brief supramaximal inflation increases CL. These findings suggest that atelectasis or increased alveolar surface forces are present in ALS patients and that these patients will have increased work of breathing. Some of the beneficial effects demonstrated with NPPV therapy may be through its effects on CL and the work of breathing.