The occurrence of an acute respiratory failure necessitates mostly admission to ICU and mechanical ventilation (MV). Rapid and safe discontinuation of MV should be the objective for the majority of patients. Many reasons may contribute to weaning , extubation failure and prolongation of MV. Critical illness myopathy, induced by immobilisation and prolonged MV, may represent a main factor and early rehabilitation may reverse these conditions and improve the success of weaning from MV.The objective of this study is to evaluate the effect of an early chair sitting (while the patient is awake but still mechanically ventilated) on weaning from mechanical ventilation and ICU mortality.