Laura Peeters

78 | Chapter 4 Joint torque and surface electromyography Muscle activity was measured with the use of surface electromyography (sEMG) (Zerowire EMG, Aurion, Italy) and was recorded with a sample frequency of 1000 Hz. Electrodes were placed on the following muscles on both sides of the body: iliocostalis (6 cm from spinous processes of the 1 st lumbar vertebrae), longisimus (3 cm from spinous processes of the 3 rd lumbar vertebrae), external oblique (3 cm from axillae midline at height of umbilicus), trapezius descendens (1/2 on the line from the acromion to the spinous processus of the 7 th cervical vertebrae) and medial deltoid (1/3 on the line from acromion to lateral epicondyle of the elbow) [14]. The trapezius and deltoid muscles were included to get an estimate of shoulder muscle effort when performing tasks. Electrodes on the iliocostalis muscle were not placed on the smaller participants (n=9), due to space limitations on the back. Maximum force was measured using a static frame myometer. The frame consisted of a KAP-E Force Transducer (range 0.2 - 2000 N) (Angewandte System Technik, Dresden, Germany) and a height and position adjustable frame (custom made at the VU medical centre, Amsterdam, the Netherlands). The force signal was filtered with a bi-directional 4 th order low-pass filter of 30 Hz. Afterwards the measured maximum force signal was converted to joint torque by multiplying the force with the segment length (i.e. moment arm) and additionally resulting torques were also corrected for body weight. Maximal voluntary isometric contractions (MVICs) were performed to determine maximal joint torques and corresponding sEMG amplitudes. Participant’s positions for MVIC measurements were adapted to seated positions so all participants with DMD could perform the measurements. Participants performed two MVIC tasks for each of the following movements: trunk flexion, trunk extension, lateral bending trunk (left and right), shoulder elevation (left and right) and shoulder abduction (left and right). Participants were encouraged to push as hard as they could for 3 seconds. When the maximum force of the MVIC tasks varied more than 10% between the two trials, an additional trial was recorded. A 4 th order Butterworth filter (20-450 Hz) was used to filter the sEMG signals, followed by rectification and low-pass filtering (3 Hz) of the signals to obtain the linear envelopes. The maximum sEMG amplitude for each trunk muscle was taken as the highest amplitude from the four MVIC tasks of the trunk, for the trapezius as the highest amplitude from the shoulder elevation task and deltoid as the highest amplitude from the shoulder abduction task. Normalized sEMG amplitudes were used to describe the percentage of muscle capacity used during task performance. These were calculated by dividing the sEMG amplitudes during task performance, by the corresponding maximum