Muscle relaxation during surgery will be performed using intraoperative boli intermediate duration non-depolarizing NMBA, rocuronium (0.3 mg/kg) until TOF 0 at the hand is achieved. TOF measurements will be performed at baseline 5 minutes after induction. Moreover, TOF measurements will be performed 5 minutes after each rocuronium bolus. When TOF 0 at the hand is achieved the MEPs of the paraspinal musculature will be measured. When there is remaining muscle tonus of the paraspinal musculature another bolus of rocuronium is administered until no MEPs can be measured for the paraspinal musculature.