A challenging problem facing the clinician is the management of critically ill patients in whom repeated attempts at weaning from mechanical ventilation are unsuccessful. The difficulty weaning these patients from the ventilator can be augmented in the presence of detraining of respiratory muscles, malnutrition, stress and the metabolic response to injury, and/or immobilization. An understanding of basic concepts of established tenets of exercise physiology and of nutritional support will allow their application to respiratory muscle function with the goal of facilitating the endurance and strength of respiratory muscles needed to sustain spontaneous ventilation.