Bed rest is frequently prescribed for critically ill patients because it is assumed to be beneficial for preventing complications, for conserving scarce metabolic resources, and for providing patient comfort. Furthermore, higher levels of physical activity in critically ill patients have been assumed to be impractical or not feasible. Bed rest has been prescribed in the past for several other clinical conditions including acute flares of rheumatoid arthritis, cavitary tuberculosis, acute myocardial infarction, and acute low back pain. However, randomized, controlled, clinical trials failed to demonstrate beneficial effects of bed rest in most of these conditions. Bed rest can cause several complications that may delay or prevent recovery from critical illnesses including disuse muscle atrophy, joint contractures, thromboembolic disease, and insulin resistance. Recent studies demonstrated the feasibility and safety of physical medicine programs in critically ill patients including those with acute respiratory failure requiring mechanical ventilation. Other physical medicine tools, such as neuromuscular electrical stimulation and passive stretching of muscles, may also reduce some complications of bed rest.