The assessment of the work of the respiratory muscles is a fundamental clinical data in intensive care, especially to guide the management of patients requiring ventilatory support. This data is difficult to access in current practice. The reference technique to estimate the work of breathing (transdiaphragmatic pressure) is not feasible in clinical routine and evaluates only the respiratory work of the diaphragm, not that of the accessory muscles. The ultrasound technique of speckle tracking allows a fine and multidimensional analysis of the deformation of the respiratory muscles during the respiratory cycle.

The investigators hypothesize that the analysis of the multidimensional deformation of the diaphragm at the level of the apposition zone can produce a robust and reproducible index, which is correlated with the work of breathing. The performance of this index will be compared to that of the thickening fraction of the diaphragm. On the other hand, the investigators will evaluate the feasibility of measuring the thickening of the accessory respiratory muscles (scalene and intercostal).

Fifteen healthy subjects will be equipped with a double-balloon esophagogastric tube and subjected to increasing and gradual loading of the respiratory system by a negative-pressure inspiration device. The respiratory work will be recorded continuously by measuring the transdiaphragmatic pressure (reference technique).

The recording of the deformation of the diaphragmatic apposition zone by speckle tracking technique will make it possible to define a multi-dimensional deformation index of this muscular portion.

The reproducibility and the repeatability of this index will be evaluated then this index will be compared to the reference technique, as well as to the fraction of thickening. The deformation of the accessory muscles will also be evaluated in a second step.

Inclusion Criteria:

• Healthy volunteer (male or female)
• Free and informed consent

Exclusion Criteria:

• known pregnancy, parturient or nursing mother
• Under 18 years old
• Age> 45 years
• Incapable major
• Non-affiliation to the social security system
• Any history of respiratory pathology
• Any history of coagulation disorder or platelet function

The subjects and participants will be contacted individually by the investigators. The principles of the study will be explained and explained to the volunteer.