## 2 Materials and Methods
### Neurophysiological Recordings
This study verified the correct conduction of the motor pathways of the spinal cord during bilateral procedures of corrective instrumentation implantation **(c)**. In each subject, bilateral electroneurography (ENG) recordings were performed (Figure 2B) to assess the peripheral conductivity of neural impulses in the motor fibers of the peroneal nerves. The test aimed to assess if the abnormal muscle function or the efferent transmission was caused by pathologies in the L5 ventral root fibers' neural conduction and/or the consequence of peripheral neuropathies. Applying the electrical stimulation with rectangular pulses of a 0.2 ms duration at a frequency of 1 Hz and an intensity ranging from 0 to 80 mA using bipolar stimulating electrodes placed over the skin along the anatomical passages of the nerve in the ankle evoked the compound muscle action potentials (M-wave CMAPs). The simultaneously recorded F-waves evoked potentials in the extensor digitorum brevis muscle (EXT) were assumed to verify the transmission of neuronal impulses in the motor fibers, peripherally and within the L5 ventral spinal roots, respectively. The ENG recordings were acquired with an amplification of 100-5000 \(\upmu\)V/D and a 2-10 ms/D time base adjusted in the recorder settings. The outcome measures of the ENG were the amplitude (in \(\upmu\)V) and latency (in ms) parameters of the M-waves, the inter-latencies of the recorded M-F waves (in ms), and the frequencies of the F-waves (usually not less than 14 while evoking 20 positive, successive recordings of M-waves). The test results obtained in patients were compared with the normative values recorded in healthy volunteer subjects. Other papers present further details regarding the methodology of the acquisition and interpretation of the ENG studies [32; 33]. The motor-evoked potentials (MEPs) were induced with a magnetic circular coil (C-100; 12 cm in diameter) placed over the scalp in the area of the M1 motor cortex. The coil released a single, biphasic, 5 ms lasting magnetic stimulus (TMS; Figure 2(Ca)). A MagPro X100 magnetic stimulator (Medtronic A/S, Skovlunde, Denmark) was used for the stimulus generation. The excitation was targeted mainly toward the cells of origin of the fibers of the corticospinal tract for the innervation of the lower extremity muscles and the whole corona radiata's excitation. All neural structures up to 3-5 cm deep were excited via the magnetic field stream delivered at a strength of 70-80% of the resting motor threshold (RMT; 0.84-0.96 T). Under such conditions, the cells of origin of the rubrospinal tract in the midbrain were probably excited. The MEPs were bilaterally recorded with surface electrodes on the TA muscles. The latency and amplitude parameters were the outcome measures to assess the primary motor cortex output and evaluate the global efferent transmission of neural impulses to effectors via the spinal cord descending tracts. The location of an optimal stimulation (a hot spot in the area where TMS elicited the largest recorded MEP amplitude; Figure 2(Cb,Cc)) was searched following the consecutive tracking distance of 5 mm from each other. The accurate photographic documentation of hot spots marked at similar locations of the transcranial stimulation aimed to ensure the reproducibility of the MEP recordings in T0-T2. The MEP amplitude was measured from peak to peak in the recording. The latency from the stimulus application marked by the artifact in the recording to the onset of the positive inflection of the potential was analyzed. Subjects did not report the stimulation as painful. During MEP acquisition, the low-pass filter of the recorder was set to 20 Hz, the high-pass filter to 10 kHz, the time base to 10 ms/D, and the amplification of signals to between 200 and 5000 \(\upmu\)V. A bandwidth of 10 Hz to 1000 Hz, a digitalization rate of 2000 samples per second, and channels were used during the recordings. The methodology of the MEP recordings has been described in detail elsewhere [34; 35].