Diaphragm electromyographic activity (Edi, obtained from the NAVA catheter), airway pressure and flow are acquired at 100 Hz from the ventilator via an interface connected to a computer using commercially available software (Maquet Critical Care, Solna, Sweden). The auxiliary surface EMG will be recorded with a dedicated device (Dipha16, InBiolab, Groningen, The Netherlands) at the diaphragm, and intercostal and rectus abdominis muscles. All data will be stored and later analysed.