Inclusion Criteria:  healthy university students  Exclusion Criteria:  Suffering or having suffered aortic aneurysm; pneumothorax; rib wall or pulmonary fistulas; recent trauma or surgery to the upper airway or thoracoabdominal; recent myocardial infarction or angina; retinal detachment, glaucoma, or recent eye surgery; increased intracranial pressure (tonsillar wedge, meningocele, hydrocephalus); abdominal hernias and acute middle ear problems.