INDICATORS OF HEART RATE VARIABILITY AT REST AND DURING AN ORTHOSTATIC CHALLENGE AT DIFFERENT RANGES OF MxDMn VALUE AND THEIR CHANGE IN TRACK AND FIELD ATHLETES IN THE TRAINING PROCESS
The aim of the study is to develop the standards of HRV indexes at rest and during an orthostatic challenge, taking into account different ranges of variability of cardiac intervals (MxDMn) of track and field athletes. The other purpose is to identify the features of changes in the standards in the training process in cases of malfunctions of cardioregulatory systems, sinus node functioning and recovery process, overtraining, and decrease in athletic performance. The authors used the results of individual dynamic express-tests of heart rate variability (HRV) in track and field athletes belonging to different specific training orientations to achieve the goals of the study. Research Methods and Organization. We carried out 1740 dynamic HRV measurements at rest and during an orthostatic challenge. The studies involved 56 track and field athletes (sprinters, middle distance runners and stayers) aged 18 to 29 years, belonging to the 1<sup>st</sup> adult category, Candidates Master of Sports and Masters of Sports, in different periods of the training process. We carried out HRV measurements in the laboratory of functional research methods of the Institute of physical culture and sports at Udmurt State University, as well as at training camps in the middle mountains (Kyrgyzstan, Kislovodsk) and on the plain (Elabuga). We examined the test subjects at rest in the morning after the previous training day, using the VARICARD 2.51 device and the ISCIM6 and VARICARD MP programs (Ryazan). We recorded cardio intervals of athletes for 5 minutes in lying position and for 6 minutes in standing position. We applied HRV measurements to one or four athletes simultaneously using the ISCIM6 and VARICARD MP programs. Research results. We assigned special emphasis to the assessment of changes in the variational range of cardiointervals (MxDMn) reflecting the state of cardiac regulation and the sinus node functioning during dynamic HRV studies in the training process of each runner. We identified seven MxDMn variation ranges from <150ms to >650ms. We revealed that each MxDMn variation range corresponds to a certain prevailing type of regulation. We demonstrated that frequent shifts of the MxDMn ranges from one level to another in the training process of runners indicate instability of cardiac regulation. We elaborated standards for the variational range of cardiointervals (MxDMn), taking into account the predominance of the HF and LF power in the HRV power spectrum for runners with different specific training orientations. We found that respiratory (HF) or vasomotor waves (LF) could prevail within the same MxDMn variation ranges, which points to a different autonomic balance. We revealed that at rest, the variational range of cardiointervals (MxDMn) depends more on the state of cardiac regulation and the sinus node functioning and less on the specifics of running. We often detected paradoxical reactions to an orthostatic challenge at low or extremely high MxDMn values in the overtrained runners. We determined that runners of any specific training orientation could increase their fitness, adaptive and reserve capabilities only in the context of the optimal regulation state, stable favorable ranges of MxDMn values, and the absence of paradoxical reactions to an orthostatic challenge. The paper contains tables with standard HRV indicators for different MxDMn variation ranges and consideration of the predominance of HF and LF waves at rest and during an orthostatic challenge, as well as examples of the results of HRV analysis at rest and during an orthostatic challenge in runners with different MxDMn variation ranges in the training process. Conclusion. The research demonstrated that the use of the HRV analysis method at rest and during an orthostatic challenge, taking into account the standards of MxDMn values in the training process of track and field athletes, is aimed at obtaining the necessary information about the stability of autonomic balance, aut