Acute respiratory failure (ARF) is an extremely variable and heterogeneous syndrome that can be defined as an acute cardiopulmonary dysfunction requiring emergent artificial ventilation support. For patients with ARF, the mortality rate is higher than the general ICU mortality, with estimates ranging from 15-40% depending on the subgroups evaluated. In the largest, most recent report on ARF, the in-ICU mortality was 31% and the overall in-hospital mortality rate was 37%. Muscle weakness is an independent risk factor for mortality from ARF and contributes to long-term reductions in physical function in survivors. Muscle ultrasounds are non-invasive, quick and reproducible. They provide the opportunity of ICU staff to create a patient-specific exercise prescription based on both functional assessments and importantly the degree of muscle architecture abnormality detected by ultrasound. Serial examination of muscle ultrasound images obtained in mechanically ventilated acute respiratory failure patients will allow for better understanding of the time course of specific muscles' architectural abnormalities and the resolution of such abnormalities within hospital survivors. 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. Muscle sampling used in conjunction with ultrasound in patients with ARF will allow for further characterization of the extent of and mechanisms underlying ARF induced muscle dysfunction in these patients. Furthermore, ultrasound guided muscle sampling allows this procedure to be done in a more efficient and effective manner. Study participants will consist of 30 patients with ARF admitted to the Intensive Care Units of Wake Forest University Baptist Medical Center. Muscle ultrasounds will occur at study enrollment and weekly thereafter, and will also include one imaging session as close to hospital discharge as possible. Muscle sampling will occur within 5 days of consent and approximately 2 months later. Handgrip strength, dynamometer, and physical performance tests will also be administered at ICU & hospital discharge and at 2 months post enrollment. The primary outcome will be to investigate the possibility of a relationship between the ultrasound data, muscle sample data, and functional measurements (length of stay, grip strength, SPPB, dynamometer, MRC, simultaneously accounting for severity of illness [APACHE III]).