Electromyography (EMG) signals were recorded synchronously
from the upper trapezius muscle with a bipolar
surface EMG configuration (Neuroline 720 01-K, Medicotest
A/S, Ølstykke, Denmark) and an interelectrode distance
of 2 cm [35]. The electrodes were positioned according to
SENIAM guidelines [36]. The skin was abraded prior to
applying the electrodes to ensure an impedance of less than
10 kΩ (typically the impedance was 1-2 kΩ). If the impedance
was higher than 10 kΩ the procedure was repeated untilimpedance was less than 10 kΩ. The EMG electrodes were
connected directly to small preamplifiers located near the
recording site. The raw analogue EMG signals were led
through shielded wires to instrumental differentiation amplifiers,
with a bandwidth of 10–400 Hz and a common mode
rejection ratio better than 100 dB. Force and EMG signals
were sampled synchronously at 1000 Hz using a 16-bit A/D
converter (DAQ Card-Al-16XE-50, National Instruments,
USA) and stored on a laptop for further analysis.