Examination of serial muscle ultrasounds and muscle sampling within the population of ICU patients who require mechanical ventilation for acute respiratory failure, will lead to the ability of investigators to link specific baseline comorbidities, drugs, or fluid administrations, to the onset and duration of architectural changes within muscle and correlate ultimately with muscle function. With this study, we will be better able to understand the relationships between the pattern of resolution of the muscle architectural abnormalities within the context of multiple other clinical abnormalities and therapies present and rendered to ICU patients.