* pregnant (positive pregnancy test during screening) or breastfeeding women * patient on long-term oxygen therapy or continuous positive airway pressure (cpap) or non-invasive ventilation (niv) at home * chronic obstructive pulmonary disease (copd) patient stage 3 or 4 * patient with known chronic diffuse interstitial lung disease * patient with neuromuscular pathology * contraindication to the pp (recent thoracic trauma, pneumothorax, orthopaedic fracture preventing mobilization, ...) * deep vein thrombosis of the lower limbs or pulmonary embolism with effective anticoagulation for less than 48 hours * hemodynamic instability (map \< 65 mm hg) persisting for more than 1 hour * respiratory rate greater than 40 cycles per minute * excessive use of accessory respiratory muscles (as judged by the clinician) * indication for curative niv (acute pulmonary edema or acute hypercapnic respiratory failure) * intestinal occlusive syndrome * patient unable to protect upper airway * inability to understand french or to follow instructions for the prone position. * person under guardianship * protected majors * not affiliated to french social security * decision not to forgo life sustaining therapy * patient discharged from an intensive care unit and has been treated by invasive or non-invasive mechanical ventilation at 2 pressure levels during the resuscitation stay.

* pregnant (positive pregnancy test during screening) or breastfeeding women * patient on long-term oxygen therapy or continuous positive airway pressure (cpap) or non-invasive ventilation (niv) at home * chronic obstructive pulmonary disease (copd) patient stage 3 or 4 * patient with known chronic diffuse interstitial lung disease * patient with neuromuscular pathology * contraindication to the pp (recent thoracic trauma, pneumothorax, orthopaedic fracture preventing mobilization, ...) * deep vein thrombosis of the lower limbs or pulmonary embolism with effective anticoagulation for less than 48 hours * hemodynamic instability (map \< 65 mm hg) persisting for more than 1 hour * respiratory rate greater than 40 cycles per minute * excessive use of accessory respiratory muscles (as judged by the clinician) * indication for curative niv (acute pulmonary edema or acute hypercapnic respiratory failure) * intestinal occlusive syndrome * patient unable to protect upper airway * inability to understand french or to follow instructions for the prone position. * person under guardianship * protected majors * not affiliated to french social security * decision not to forgo life sustaining therapy * patient discharged from an intensive care unit and has been treated by invasive or non-invasive mechanical ventilation at 2 pressure levels during the resuscitation stay.

- pregnant (positive pregnancy test during screening) or breastfeeding women - patient on long-term oxygen therapy or continuous positive airway pressure (cpap) or non-invasive ventilation (niv) at home - chronic obstructive pulmonary disease (copd) patient stage 3 or 4 - patient with known chronic diffuse interstitial lung disease - patient with neuromuscular pathology - contraindication to the pp (recent thoracic trauma, pneumothorax, orthopaedic fracture preventing mobilization, ...) - deep vein thrombosis of the lower limbs or pulmonary embolism with effective anticoagulation for less than 48 hours - hemodynamic instability (map < 65 mm hg) persisting for more than 1 hour - respiratory rate greater than 40 cycles per minute - excessive use of accessory respiratory muscles (as judged by the clinician) - indication for curative niv (acute pulmonary edema or acute hypercapnic respiratory failure) - intestinal occlusive syndrome - patient unable to protect upper airway - inability to understand french or to follow instructions for the prone position. - person under guardianship - protected majors - not affiliated to french social security - decision not to forgo life sustaining therapy - patient discharged from an intensive care unit and has been treated by invasive or non-invasive mechanical ventilation at 2 pressure levels during the resuscitation stay.

- pregnant (positive pregnancy test during screening) or breastfeeding women - patient on long-term oxygen therapy or continuous positive airway pressure (cpap) or non-invasive ventilation (niv) at home - chronic obstructive pulmonary disease (copd) patient stage 3 or 4 - patient with known chronic diffuse interstitial lung disease - patient with neuromuscular pathology - contraindication to the pp (recent thoracic trauma, pneumothorax, orthopaedic fracture preventing mobilization, ...) - deep vein thrombosis of the lower limbs or pulmonary embolism with effective anticoagulation for less than 48 hours - hemodynamic instability (map < 65 mm hg) persisting for more than 1 hour - respiratory rate greater than 40 cycles per minute - excessive use of accessory respiratory muscles (as judged by the clinician) - indication for curative niv (acute pulmonary edema or acute hypercapnic respiratory failure) - intestinal occlusive syndrome - patient unable to protect upper airway - inability to understand french or to follow instructions for the prone position. - person under guardianship - protected majors - not affiliated to french social security - decision not to forgo life sustaining therapy - patient discharged from an intensive care unit and has been treated by invasive or non-invasive mechanical ventilation at 2 pressure levels during the resuscitation stay.

- pregnant (positive pregnancy test during screening) or breastfeeding women - patient on long-term oxygen therapy or continuous positive airway pressure (cpap) or non-invasive ventilation (niv) at home - chronic obstructive pulmonary disease (copd) patient stage 3 or 4 - patient with known chronic diffuse interstitial lung disease - patient with neuromuscular pathology - contraindication to the pp (recent thoracic trauma, pneumothorax, orthopaedic fracture preventing mobilization, ...) - deep vein thrombosis of the lower limbs with effective anticoagulation for less than 48 hours - hemodynamic instability (map < 65 mm hg) persisting for more than 1 hour - respiratory rate greater than 40 cycles per minute - excessive use of accessory respiratory muscles (as judged by the clinician) - indication for curative niv (acute pulmonary edema or acute hypercapnic respiratory failure) - intestinal occlusive syndrome - patient unable to protect upper airway - inability to understand french or to follow instructions for the prone position. - person under guardianship - protected majors - not affiliated to french social security - decision not to forgo life sustaining therapy - patient discharged from an intensive care unit and has been treated by invasive or non-invasive mechanical ventilation at 2 pressure levels during the resuscitation stay.

- pregnant (positive pregnancy test during screening) or breastfeeding women - patient on long-term oxygen therapy or continuous positive airway pressure (cpap) or non-invasive ventilation (niv) at home - chronic obstructive pulmonary disease (copd) patient stage 3 or 4 - patient with known chronic diffuse interstitial lung disease - patient with neuromuscular pathology - contraindication to the pp (recent thoracic trauma, pneumothorax, orthopaedic fracture preventing mobilization, ...) - deep vein thrombosis of the lower limbs with effective anticoagulation for less than 48 hours - hemodynamic instability (map < 65 mm hg) persisting for more than 1 hour - respiratory rate greater than 40 cycles per minute - excessive use of accessory respiratory muscles (as judged by the clinician) - indication for curative niv (acute pulmonary edema or acute hypercapnic respiratory failure) - intestinal occlusive syndrome - patient unable to protect upper airway - inability to understand french or to follow instructions for the prone position. - person under guardianship - protected majors - not affiliated to french social security - decision not to forgo life sustaining therapy - patient discharged from an intensive care unit and has been treated by invasive or non-invasive mechanical ventilation at 2 pressure levels during the resuscitation stay.