Muscle weakness, particularly impairment of the respiratory muscles, is a frequent abnormality in ICU patients. This is more relevant in some clinical situations--for example, in weaning patients from mechanical ventilation. Intensive care procedures that are designed to "rest" respiratory muscles, such as mechanical ventilation, may also contribute to impaired muscle function. Pharmacologic administration of glucocorticoids, several antibiotics, NMB agents, and so on has the potential to cause untoward effects. The development of myopathy and prolonged paresis has been increasingly recognized after prolonged use of these drugs in the ICU. Pathophysiologic changes in the nerve, muscle, or neuromuscular junction associated with the patient's underlying condition may also play a role in the development of impaired function. The assessment of muscle function is difficult and inaccurate. The techniques developed have a poor predictive value because of the difficulty in making the measurements in uncooperative patients and the lack of standardization. Furthermore, it is likely that some voluntary maneuvers underestimate muscle strength. Invasive procedures such as phrenic nerve stimulation or EMG recording are also of limited value.