## 2 Materials and Methods
### Neurophysiological Testing  
All tests were performed once with four-channel Keypoint System (Medtronic A/S, Skovlunde, Denmark) in the prone position (Figure 2A-C) according to the guidelines of European Chapter of The International Federation of Clinical Neurophysiology. Figure 2: Photographs of experimental setup applied during neurophysiological recordings. (**A**) Sites of bilateral placement of bipolar surface electrodes over muscles in proximal (a) and distal (b) parts of upper extremities when sEMG recordings at rest and during maximal contraction were performed. (**B**) Sites of bilateral electrical stimulation of motor nerve fibers in the median and ulnar nerves at ErbB’s point (a), arm, cubital fossa or ulnar sulcus (not shown), and wrist (b) during ENG studies of M-wave recordings of hand muscles. The methodology of sensory conduction studies (SCV) (c) on the median and ulnar nerves (not shown) following stimulation of fingertips with recordings of prewrist areas. (**C**) Principles of multiple motor-evoked potentials (MEP) recordings from upper extremity muscles (a) following transvertebral magnetic stimulation of motor centers at C5–C8 spinal neuromores (b). Abbreviations: r—recording—and s—stimulation. Surface electromyography (sEMG, Figure 2A) was performed bilaterally at rest (rEMG) and during maximal muscle contraction (mcEMG) on the deltoideus (DEL), biceps brachii (BB), abductor digiti minimi (ADM), and abductor pollicis brevis (APB) for 5 s. Parameters of amplitudes (in \(\upmu\)V) in rEMG and mcEMG recordings and, additionally, frequencies (in Hz) of recruiting action potentials of motor units were analyzed and ranked from normal (3) to severe pathology (1). Standard disposable Ag/AgCl surface electrodes with an active surface of 5 mm\({}^{2}\) were used. The active electrodes were placed over the belly of the examined muscle, while the reference electrodes were placed on the distal part of the tendon. The ground electrode was placed close to the recording pair. Upper 10 kHz and lower 20 Hz filters of the recorder were used. In the first stage of the study, the patient was asked to completely relax the examined muscles and then to perform a maximum contraction for 5 s during which simultaneous recording took place. Subjects were instructed to contract the tested muscle as hard and fast as possible until the neurophysiologist asked to end the test. The test was performed three times with a one-minute rest period between each muscle contraction. The recording with the highest amplitude (in \(\upmu\)V) and frequency (in Hz) parameters was selected for the final analysis. sEMG recordings were made with a time base of 80 ms/D and an amplification of 20-1000 \(\upmu\)V. The methodical and analytical principles of sEMG have been described elsewhere [7; 8; 10; 16]. Electroneurography (ENG, Figure 2B) included the assessment of peripheral neural transmission in the motor and sensory fibers of the median and ulnar nerves bilaterally after stimulation with electrical impulses over the anatomical passage in the prewrist, cubital fossa or ulnar sulcus, shoulder, and Erb's point [8; 9; 10; 13]. To establish neural transmission peripherally or at ventral roots in the cervical spine, M-wave- and F-wave-evoked potential recordings were recorded on the APB and ADM muscles, respectively. Parameters of amplitudes (\(\upmu\)V) and latencies (ms) of M-wave potentials were compared between the results obtained from shooters and healthy volunteers. In the case of F-wave recordings, a frequency of no less than 14 to 20 M responses after applying electrical stimuli was analyzed and was considered proper [17]. All patients underwent sensory conduction studies (SCV) (Figure 2B) on the median and ulnar nerves after stimulation of the fingertips or the fifth finger with recordings of the prewrist areas. Pairs of surface electrodes recorded evoked potentials; the same types of surface electrodes were also used for sEMG recording.