Primary outcome measures
bipolar
The primary outcome measures were the physical workload of the surgeons, including postures and angular velocities of the head, trunk, and upper arms, and the muscular activity of the cervical erector spinae (CES), upper trapezius (UT), and lumbar erector spinae (LES).Postural data of the head, trunk, and upper arms were recorded by four inertial measurement units (IMUs) (AX6, Axivity Ltd., Newcastle, UK) at 25 Hz in the same positions as a previous study (The muscle activities were recorded bilaterally by surface electromyography (EMG) using bipolar electrodes with a gel (Ag/AgCl electrodes, N-00-S/25, Ambu A/S, Copenhagen, Denmark) and a logger (Mobi8, from TMSi, Oldenzaal, The Netherlands) with a sampling rate of 1,024 Hz per channel and a 24-bit AD-convertor. The electrodes for the CES were placed bilaterally, with one at the C2–C3 level on the upper part of the trapezius and the other 3 cm caudally (Positions of EMG electrodes on the cervical erector spinae, upper trapezius, and lumbar erector spinae.To obtain muscle activity, the EMG data were first computed as root mean square (RMS) values of every 1/8 s epoch after a digital bandpass filter (30–400 Hz) (