Many patients who are admitted to the intensive care unit (ICU) experience muscle weakness. This muscle weakness occurs whilst patients are unconscious, immobile in bed and on a breathing machine (ventilator). It can develop very quickly, as soon as they become unwell. The investigators know that this weakness can make it harder for patients to regain their normal level of functional ability, such as standing and walking independently. Physical rehabilitation, delivered by physiotherapists, is important for patients as they recover from their critical illness to help them regain strength and to practice the ability to perform physical activities. These activities include sitting on the edge of the bed, standing, stepping on the spot and walking.