## 2 Material and Methods
This apparatus complies with the requirements of the 93/42/EEC Medical Devices Directive for class I products. It has the appropriate CE mark and corresponds to the definition of Class I devices and a product of the standard. Safety: IEC 60601-1 (1988) and EMC: IEC 60601-1-2 [17]. Pre-gel electrodes with a 3-cm diameter were used for the tests. The skin of the subjects was cleaned with abrasive liquid at the points where the electrodes were applied, so as to obtain resistance between the skin and the electrode below 2 k\(\Omega\)[17]. The electrodes were located parallel to the direction of the tested muscle fibers. The distance between them was about 2 centimeters. The erector spinae muscle was examined in the thoracic and lumbar segments, both on the left and right side. Each of the trials lasted 10 s.  
After connecting the Noraxon device to the computer and the electrodes to the erector spinae muscle, each of the tests was preceded by entering the data for the examined person. Then, via the Noraxon TeleMyo DTS programme, the erector spinae was selected for analysis. Furthermore, a raw signal lasting about 15 s appeared on the computer screen, preceding the target recording of the muscle potential. Then, the measurement of the raw signal was recorded and analyzed. The researcher chose a signal modification programme consisting of cleaning the raw record from extreme, maximum, and minimum deflections. The examiner chose the option of processing the raw signal in order to calculate the average SEMG amplitude. Following, the processed mean amplitude of the SEMG of the extensor muscle of the spine in millivolts (mV) was recorded. The results of the test took the scale of the voltage intensity in the time interval into account, which was 100 ms. The study used the continuous trace recording mode. Functional potentials were recorded from the erector spinae in the thoracic and lumbar sections at the top of the curvature arches:  
1. in habitual standing position (Frankfurt plane),
2. in resting position: in supine position (lower limbs straightened in the knee joints, upper limbs placed along the trunk),
3. under isometric contraction conditions: - in supine position (lower limbs straightened in the knee joints, upper limbs placed along the trunk, pelvis stabilized), the subject lifts the trunk within the limits of the mobility of the spine, and then maintains this position for 10 s, - in supine position, with the upper trunk stabilized (shoulders and chest, limbs positioned as before), the subject lifts both lower limbs to the maximum possible height and maintains this position for 10 s.  
The SEMG amplitudes of the erector spinae were analyzed. This variable is used to assess the degree of activity and muscle tone. SEMG measurements were in line with SENIAM (Surface ElectroMyoGraphy for the Non-Invasive Assessment of Muscles) recommendation, a European research programme containing a number of guidelines regarding the selection of electrode types, their location, anatomy and muscle function, muscle group tests, signal processing, and hardware conditions [17]. Both the examination of body posture and spine as well as SEMG amplitude analysis were painless and non-invasive. In order to determine the normality of the distribution of variables, the Kolmogorov-Smirnov test was carried out. In order to assess the differences in the size of the curvature of the spine, between girls and boys in scoliosis, scoliotic posture and the norm groups, SEMG amplitudes of the erector spinae in different study positions, one-way Analysis of Variance (ANOVA) was used. To determine the dependence and predictors between dependent variables, which were the types of postures and scoliosis angle, and the independent variable, that is the SEMG amplitude of the erector spinae, multiple stepwise and progressive regression models were used. Prior to regression analysis, k-fold cross validation was performed.