 ICU-acquired weakness represents a common and often devastating disease process which affects greater than 50 of critically ill patients. This pathogenesis of this acquired disease is multifactorial and results in variable severity, ranging from mild, transient to severe, permanent dysfunction of peripheral nerves in additional to muscle. In affected patients, weakness may persist for months to years after the acute phase of their illness, and has been implicated as a major contributor to decreased functional status and quality of life. Muscle ultrasound has been validated for assessment of muscle size as well as diagnosis of myopathic and neuropathic changes in patients with other known neuromuscular diseases. The use of muscle ultrasound or other imaging modalities for diagnosis or monitoring of ICU-acquired weakness has not been studied, although a single study using muscle ultrasound has shown significant change in muscle size in ICU patients receiving high dose corticosteroids and a prolonged course of paralytic agents. The investigators plan to use multiple modalities to examine skeletal muscle catabolism, function, and structure in patients during critical illness and recovery. The investigators will combine physical exam, hand grip dynamometry, electrophysiologic studies, serum biomarkers, muscle biopsies, and muscle ultrasound to assess a group of critically ill patients during their hospital stay. The investigators will obtain additional data, including neuropsychiatric assessments, severity of illness scores, administration of potentially harmful medications, and pertinent daily laboratory data. This study will last approximately 12 months. 