Laura Peeters

98 | Chapter 5 Joint torque and surface electromyography Surface electromyography (sEMG) (Zerowire EMG, Aurion, Italy) was used to measure muscle activity at a sample rate of 1000 samples/s. Electrodes were placed on the following muscles on both sides of the body: iliocostalis (6 cm from spinous processes of L1), longisimus (3 cm from spinous processes of L3), external oblique (3 cm from axillae midline at height of umbilicus) and medial deltoid (1/3 on the line from acromion to lateral epicondyle of the elbow) [16, 17]. The deltoid muscles were included to get an estimate for shoulder muscle effort when performing tasks. Electrodes on the iliocostalis muscle were not placed in two smaller participants with SMA, due to space limitations on the back. Maximum force was measured using an adjustable static frame myometer with a KAP-E Force Transducer (range 0.2 - 2000 N) (Angewandte System Technik, Dresden, Germany). The force signal was sampled at 1000 samples/s and filtered with a bi- directional 4 th order low-pass filter of 30 Hz. Afterwards the maximum joint torque was calculated by multiplying the measured force with the segment length (i.e., moment arm). Maximal voluntary isometric contractions (MVICs) were performed to determine maximal joint torques and corresponding sEMG amplitudes. Participants’ positions for MVIC measurements were adapted to seated positions so all participants with SMA could perform the measurements. Two MVIC efforts were performed for 3 seconds by the participants for each of the following directions: trunk flexion, trunk extension, lateral bending trunk (left and right) and shoulder abduction (left and right) (Figure 1). When the maximum force of the MVIC task varied more than 10% between the two trials, an additional trial was recorded. Because patients with SMA are easily fatigued, it was not feasible to perform many MVIC trials. 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 and maximum deltoid sEMG amplitude as the highest amplitude from the shoulder abduction task. Normalized sEMG amplitudes (maximum sEMG amplitude during task divided by the maximum MVIC amplitude for that muscle) were used to describe the percentage of muscle capacity used during maximum active range of motion and daily tasks. Subsequently, average normalized muscle activity of the back muscles (i.e. longissimus and iliocostalis both sides) and average normalized activity of the abdominal muscles (i.e. external oblique both sides) were calculated. If more than two values were missing, due to inability of the participant to perform the task or due to technical errors such as missing signals as result of lose electrodes, the average normalized muscle activity