During anesthesia, neuromuscular blocking agents (NMBA) are routinely used for relaxation of muscles necessary for the conduction of the surgical procedure. Train-of-four (TOF) test is based on supramaximal stimulation of peripheral nerve resulting in four twitches: T1 to T4. The assessment of the NMBA blockade is performed routinely by measurement of the amplitude of compound muscle action potential (CMAP) and calculation of percentage of CMAP decrement from T1 to T4.

Train-of-four monitoring is routinely performed during spine surgery by stimulation of the ulnar nerve. Furthermore motor evoked potentials (MEPs) are routinely used in intraoperative neuromonitoring to assess the whole motor pathway from the cortical level down to the distal muscle. During anesthesia MEPs are routinely evoked by transcranial electrical stimulation with single or short train stimuli. In clinical practice even though full muscle relaxation of the hand by NMBA can be observed, utilizing the TOF test, remaining muscle tonus can be observed at the paraspinal musculature during spine surgery.

The goals of this study are to determine (1) if any differences between muscle relaxation of the hand and foot (measured by TOF test and MEPs) and MEPs of the paraspinal musculature occur; (2) how much more NMBA must be administered to achieve full muscle relaxation of the paraspinal musculature in comparison to the hand or foot.

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.

Inclusion Criteria:

• All patients that will undergo spinal surgery with intraoperative neurophysiological Monitoring
• Age: 14 - 99 years

Exclusion Criteria:

• No intraoperative neurophysiological Monitoring
• Age <14 year
• prior neurological diseases or deficits that may affect safety of surgery/Intraoperative neuromonitoring
• No informed consent