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Although critical illness neuromyopathy might interfere with weaning from mechanical ventilation, its respiratory component has not been investigated. We designed a study to assess the level of respiratory muscle weakness emerging during the intensive care unit stay in mechanically ventilated patients and to examine the correlation between respiratory and limb muscle strength and the specific contribution of respiratory weakness to delayed weaning.Prospective observational study.Two medical, one surgical, and one medicosurgical intensive care units in two university hospitals and one university- affiliated hospital.A total of 116 consecutive patients were enrolled after ≥7 days of mechanical ventilation.None.Maximal inspiratory and expiratory pressures and vital capacity were measured via the tracheal tube on the first day of return to normal consciousness. Muscle strength was measured using the Medical Research Council score. After standardized weaning, successful extubation was defined as the day from which mechanical ventilatory support was no longer required within the next 15 days. The median value (interquartile range) of maximal inspiratory pressure was 30 (20–40) cm H2O, maximal expiratory pressure was 30 (20–50) cm H2O, and vital capacity was 11.1 (6.3–19.8) mL/kg. Maximal inspiratory pressure, maximal expiratory pressure, and vital capacity were significantly correlated with the Medical Research Council score. The median time (interquartile range) from awakening to successful extubation was 6 (1–17) days. Low maximal inspiratory pressure (hazard ratio, 1.86; 95% confidence interval, 1.07–3.23), maximal expiratory pressure (hazard ratio, 2.18; 95% confidence interval, 1.44–3.84), and Medical Research Council score (hazard ratio, 1.96; 95% confidence interval, 1.27–3.02) were independent predictors of delayed extubation. Septic shock before awakening was significantly associated with respiratory weakness (odds ratio, 3.17; 95% confidence interval, 1.17–8.58).Respiratory and limb muscle strength are both altered after 1 wk of mechanical ventilation. Respiratory muscle weakness is associated with delayed extubation and prolonged ventilation. In our study, septic shock is a contributor to respiratory weakness.