2.4.3. Electromyography
muscle fatigue
CONTRACTION
Directly before the blepharoplasty and 2, 6, and 12 months postoperatively, an electromyography of the frontalis muscles and orbicularis oculi muscles was performed. All the electromyography signals were recorded (unfiltered) with the BrainRT system with a 1 kHz reach, and a Duo 44 US EEG-PSG amplifier (Natus Europe GmbH, Planegg, Germany), without a notch filter. First, the skin of the upper face was cleaned with alcohol and abrasive gauze before attaching the surface electrodes. The Ag/AgCl-electrodes were rectangular (22 × 32 mm; 3 M Red Dot™ 3 M Center, St. Paul Minnesota, MI, USA) and contained conductive adhesive. The reference electrodes were attached to the skin covering the temporalis muscles (both sides; In order to assess whether the muscle fatigue was caused by constantly raising the eyebrows preoperatively, multiple aspects of the EMGs were evaluated. During isometric contraction, muscle fatigue causes a decrease in the motor unit firing rate and the power density shift to lower frequencies. Then, additional fibres are recruited to maintain the muscle contraction which results in increased EMG amplitude and RMS (root-mean-square) values [During the Matlab processing, filters were applied, i.e., a high-pass filter of 20 Hz as recommended by Van Boxtel et al. [Then, a proportional index was provided by RMS/maximal amplitude of the maximal contraction to normalise the frontalis and orbicularis oculi activity values among the individuals.