## 2 Materials and Methods
### sEMG and ENG Recordings  
This section outlines the methodological principles of bilateral surface electromyography (sEMG) recordings used to evaluate muscle tension at rest (rsEMG) and motor unit recruitment during the attempt of a 5-s maximal contraction (mcsEMG), as depicted in Figure 1. The selection of specific muscles for surface electromyography (sEMG) recordings, considering the level of spinal cord injury, relied on the findings described by [PERSON] et al. [32,33]. To conduct the sEMG recordings, we utilised the KeyPoint Diagnostic System (Medtronic A/S, Skovlunde, Denmark) while patients were in a supine position. These tests took place in a temperature-controlled room maintained at an average of 22 \({}^{\circ}\)C. For sEMG measurements, we employed standard disposable Ag/AgCl surface recording electrodes with an active surface area of 5 mm\({}^{2}\). The active electrode was positioned on the muscle belly, the reference electrode on the distal tendon of the same muscle, and the ground electrode on the distal part of the examined muscle, following the Guidelines of the International Federation of Clinical Neurophysiology--European Chapter [34,35]. The recording system was configured with upper and lower filters set at 10 kHz and 20 Hz, respectively. The sEMG examination involved two stages. Initially, patients were instructed to lie down comfortably and endeavour to relax the muscles being examined (see Figure 1A(a-d). Subsequently, patients were guided to contract the targeted muscles and, at the examiner's command, exert the maximum possible contraction, sustaining it for 5 s (see Figure 1B(a-d). Each time, three attempts were made, with a 1-min rest period between attempts. The examiner selected the optimal attempt for analysis based on the highest mean amplitude measured peak-to-peak relative to the isoelectric line. The recorded output included the amplitude measured in microvolts (\(\upmu\)V) and the frequency of muscle motor unit action potential recruitment measured in Hertz (Hz). A frequency index (ranging from 3 to 0) was assigned according to the calculations of motor unit action potential recruitment during maximal contraction in the sEMG recording: 3 = 95-70 Hz (normal); 2 = 65-40 Hz (moderate abnormality); 1 = 35-10 Hz (severe abnormality); 0 = no contraction. Both control subjects and patients underwent sEMG recordings using a base time of 80 ms/D and an amplification range of 20-1000 \(\upmu\)V/D (see Figure 1C-F).  
In this particular study, bilateral electroneurography (ENG) was conducted to evaluate the transmission of neural impulses in the peripheral motor fibres of the median and per-oneal nerves. The objective was to determine if any notable differences in nerve conduction existed that could potentially impact the assessment of muscle function. The procedure involved administering rectangular pulses lasting 0.2 ms at a frequency of 1 Hz, with an intensity ranging from 0 to 80 mA. Bipolar stimulating electrodes were positioned on the skin along the anatomical pathways of the nerves. Compound muscle action potentials (CMAP potentials) were then recorded from the abductor pollicis brevis (APB) and extensor digitorum brevis (EDB) muscles. To record the evoked potentials, surface electrode pairs were employed, using the same type of electrodes used for the sEMG recordings. The measurements were performed with an amplification range of 5-5000 \(\upmu\)V and a time base of 2-10 ms. Normative values obtained from healthy volunteers were subsequently compared with the test results from the patients. The recorded outcomes comprised the amplitudes (in \(\upmu\)V) and latencies (in ms) of the M-wave recordings. The electroneurographical assessment of impulse transmission in motor fibres was expressed using a three-gradescore: 0--indicating no transmission, 1--representing severe abnormalities, 2--signifying moderate abnormalities, and 3--denoting normal transmission.