This limited existing evidence-base demonstrates the potential for use of IC to characterise physical rehabilitation in critically ill patients, but is restricted to particular types of activities and fails to capture sequential assessments of patients as their rehabilitation progresses through increasingly functional levels. Acknowledging the potential clinical limitations of applying the IC measurement technique to all critically ill patients, the aim of this study is to examine the feasibility of measuring energy expenditure of critically ill patients completing a range of physical rehabilitation activities (as characterised on the ICU Mobility Scale) performed sequentially during their ICU or ventilator weaning unit (Lane Fox Respiratory Unit, LFU) admission using indirect calorimetry.