Pilot Study to Evaluate Doses of Mechanical Stimulus on the Effectiveness of Preterm Breathing

The goal of this clinical trial is to compare, in healthy preterm babies, 9 doses of dorsal mechanical stimuli, in order to answer: * if there is a dose of dorsal mechanical stimuli that improves the basal respiration. Participants will be stimulated through a cotton shirt for one minute with each dose, while respiratory measures will be taken through a nasobuccal mask, slightly tightened by the investigator staff. All time, babies are going to be monitored and assessing that they are comfortable.