 Intensive Care Unit-acquired weakness ICU-AW is a well-recognized, important and preventable sequelae of critical illness, affecting up to 60 of adult ICU patient. ICU-AW is associated with increased mortality and length of stay, and negatively impacts long-term functional outcomes and quality of life in affected patients and their caregivers. While delayed mobilization adversely affects clinical outcomes, early rehabilitation in the critically ill adult population is safe, feasible, cost effective, results in more ventilator free-days and better functional outcomes at hospital discharge. In contrast, there is a paucity of this research in pediatrics. Our research suggests that immobilization is common in critically ill children, and rehabilitation is delayed particularly in the sickest children who are arguably at highest risk of morbidity. It is unclear however, whether delayed rehabilitation leads to adverse outcomes in critically ill children, as has been demonstrated in adults. Our objectives of this study are to evaluate if immobilization and delayed rehabilitation negatively impacts short-term clinical outcomes and the time to functional recovery in critically ill children. The investigators hypothesize that the following factors may influence functional recovery and morbidity in critically ill children: 