Dr.Biol. O.V. Lanskaya
Dr.Biol., Professor E.Yu. Andriyanova
Applicant E.V. Lanskaya
Velikie Luki State Academy of Physical Culture and Sport, Velikie Luki

Keywords: motor plasticity of spinal cord, transcutaneous electrical stimulation of spinal cord, provoked motor responses of muscles, representatives of various sports.

Introduction. For the moment, it is obvious that, as well as being a rather conventional structure, the spinal cord (SC) is characterized by considerable plasticity, which manifests itself even in adults. Plastic changes, that depend on motor activity, occur locally in the SC structures, and are expressed in "top-down" control. This leads to changes in the spinal circuit functioning, thus making it possible to improve movements as required in conditions of sports training. Despite the meaningful progress in understanding of the SC functioning, the spinal mechanisms of improvement of motor skills in adulthood, peculiar to the internal organization, remain unclear in many respects. Nevertheless, it is practically assured that during regular sports activity certain changes occur in the SC neural circuits, and their orientation depends on the duration, power and structure of physical exercises.

The purpose of the study was to identify the characteristics of plasticity of the spinal neuronal networks as a result of multi-purpose long-term motor activity.

Materials and methods. During the study we used the transcutaneous electrical stimulation of the spinal cord (TESSC), placing it down the line on the acanthas at the level of the C2 to C7 and T11 to L3 vertebrae, in order to receive provoked motor responses (PMR) from the bilateral muscles of the upper (biceps and triceps brachii, brachioradialis muscles, finger extensors) and lower (biceps femoris, medial gastrocnemius and soleus muscles, flexor digitorum brevis) extremities, respectively. We were based on the technique of registration of posterior root-muscles (PRMs) reflexes, caused by TESSC, which was proposed, described and implemented by a group of authors [6]. According to the authors of this technique, as well as the in-house studies, PRMs have the same neurophysiological characteristics as H-reflex: we observed the repression of reflexes when applying the conditioning stimulus prior to the test one and the vibration depression of PMR.

These facts suggest that TESSC causes motor responses by activating the monosynaptic neuronal network associating afferents with motor neurons. Currently, TESSC has become widely used and is being implemented when examining healthy people [6], those with neurological disorders, [2], athletes [2, 5] and as a non-invasive method of activation of step movements generators and for rehabilitation after SC trauma [1]. This technique can be used to record a variety of muscle reflexes all at once and is a fairly new and informative tool for the study of the signs of functional plasticity of spinal motor structures both in normal conditions and in case of pathology.

PMRs of the muscles of the upper and lower extremities were recorded used the eight-channel "Mini-electromyograph" (ANO "Vozvrashcheniye", St. Petersburg, 2003). The impulses generated by the stimulator "Mini-electrostimulator" (ANO "Vozvrashcheniye", St. Petersburg, 2003) served as the stimuli. We studied the electroneuromyographic (ENMG) parameters: PMR thresholds, maximum amplitude of PMR, PMR latency. At the same time, we identified the optimal positions, at the level of which the minimum threshold values and maximum amplitude of PMR of the studied muscles were registered during electrical stimulation (ES), which may testify to the activation of the spinal segments of the cervical and lumbosacral regions with higher α-motor neuron excitability (α-MN), that innervate the examined muscle groups, compared with the rest of the ES points. Team (13 basketball and 13 volleyball players) and cyclic (13 middle-distance track athletes and 13 racing skiers) athletes 18-22 years of age, who, at the time of the study, had the I senior degree, were involved in the study.

Results and discussion. During the study, no statistically significant differences in the threshold values, the maximum amplitude and PMR latency of the muscles of the volleyball and basketball players were registered by ES at the levels of C2-C7 and T11-L3, which can be explained by relatively the same number of muscle groups of both the upper and lower extremities involved in primarily acyclic work of variable power, performed during these play activities. Among the general signs of plasticity of the spinal neural networks is also the fact that in the team sports representatives the spinal-projection area (SPA) with the highest reflex excitability of α-MN of the muscles of the upper extremities corresponded to the levels of the C4 to C7 vertebrae, and for the muscles of the lower extremities - to the level of the T11 vertebra.

The studies have shown that when using ES at the levels of C2-C7 in skiers there were cases of significantly lower threshold and latency values, as well as the highest amplitude of PMR of the muscles of the upper extremities, compared with runners, which indicates higher reflex excitability of the low- and high-threshold α-MN cervical segments of SC and reduction of the time of manifestation of the reflex responses of the shoulder and forearm muscles in skiers compared to runners. This fact can be explained by varying degrees of influence of the shoulder muscle activity on sports results. Thus, the effects of skiing are largely determined by the possibility to protractedly and effectively activate the upper extremity muscle groups, which is necessary for poling, unlike track races, where arm movements are not so important. However, when using ES at the levels of T11-L3 the values of PMR of the hip, calf and plantaris muscles did not differ significantly between the representatives of cyclic sports, which can be explained by consistent endurance requirements to the locomotor muscles of the lower extremities when these athletes are covering the competitive distance. Moreover, in both of the groups of athletes, SPA with a higher level of α-MN excitability of the upper extremity muscles corresponds to the levels of the C4-C7 vertebrae, and for the hip, calf and plantaris muscles - to the levels of the T11-T12 vertebrae. Then, we compared the parameters of athletes adapted to perform multi-purpose physical exercises, particularly, of skiers and basketball players. It was established that the threshold values ​​and PMR latency of the arm and leg muscles of skiers were significantly lower, and the amplitude of these responses - significantly higher compared with the group of basketball players. These data testify to a higher level of α-MN excitability and conductivity of neural structures of the cervical and lumbosacral regions of SC in skiers compared to basketball players, that can be explained by different number of impulses sent by the nervous system in the process of motor activity. In accordance with the classification standards specified by the unified all-Russian sports classification in cross-country skiing (2011-2014), a I Class skier covers the competitive distance of 15 km in 47:46.5-50:20.2 min:sec. With each push off, he generates force equal to 60-70% of the maximal voluntary contraction (MVC) [4]. A skier performs about 60-70 cycles a minute, each consisting of 2 gliding steps with an average length of 6 m. Therefore, while covering the competitive distance, he performs over 5,000 movements (steps) with his upper and lower extremities. In basketball running time equals 40 minutes, and while playing, a player runs about 4 kilometers, and performs about 150 maneuvers (runs, passes, shots), most of which are performed in a jump. The force a basketball player gains when jumping is comparable to that of a skier when pushing off (60-70% of MVC), as for a basketball player it is "muscle capacity to quickly display the necessary maximum of dynamic strength", rather than, for instance, agility, that plays a decisive role in vertical jumps [3]. Thus, over the similar in duration competitive period, a basketball player performs far less movements, and therefore has a much lower α-MN impulse frequency. Accordingly, the SC of a competitive skier, as well as the proprioceptive afferents of his/her working muscles, are characterized by higher impulse activity. This may explain the difference in the electrophysiological properties of the α-motor neuron pool in the representatives of the examined sports.

Conclusions. The study has found that the pronouncedness of the signs of functional plasticity of the spinal-motoneuron pools of the upper and lower extremities, typical for long-term adaptation to physical loads, is stipulated by the specific nature of sports activities.

References

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Corresponding author: lanskaya2012@yandex.ru