50 consecutive patients programmed for elective ACSS that could meet inclusion criteria will be monitored with IONM using transcranial electric motor evoked potentials (tceMEPs), somatosensory evoked potentials (SSEPs) and spontaneous electromyography (EMG) after total endovenous anesthesia. On the operating table, a basal lateral x-ray picture and a first determination of IONM will be performed and used for comparison. Five maneuvers will be sequentially performed: 1) Feet compression, 2) Arm traction, 3) Arm traction plus feet compression combined, 4) Shoulder taping and 5) Shoulder taping plus feet compression.