Experiment set-up. Left: continuous positive airway pressure via a full face mask and ventilation (FmCPAP). A carbon dioxide (CO2)/flow sensor (Novametrix Medical Systems Inc., Wallingford, CT, USA) was placed between the mask and the breathing circuit. The sensor was connected to a non-invasive cardiac output (NICO) monitor (model 7300; Respironics, Murrysville, PA, USA). This sensor allows continual monitoring and recording of the following data: flow waveform, airway pressure, respiratory rate, tidal volume, and exhaled CO2 waveform. With the respiratory inductance plethysmograph (Respitrace Calibrator; Ambulatory Monitoring, Inc., Ardsley, NY, USA), the rib cage band was placed at the level of the nipples and the abdominal band at the level of the upper abdomen. Right: continuous positive airway pressure via a nasal mask and ventilation (nCPAP). The equipment is the same as with FmCPAP, except for the nasal mask.