Training-phase-dependent dynamics of 15 heart rate variability parameters in elite cross-country skiers during clinostasis

To indirectly demonstrate non-neuronal acetylcholine (NN-ACh) synthesis, we conducted a comparative analysis of 15 heart rate variability (HRV) indicators recorded during clinostasis (clino-HRV) in elite skiers. The study included: (1) 8 members of the men’s Republic of Tatarstan elite ski team (ESRT) evaluated during preparatory and competitive phases, with one athlete (K. D.) additionally assessed during the transition phase; and (2) 11 members of the Republic’s youth ski team (YSRT; preparatory phase only). Results were compared with literature data on HRV parameters in skiers of varying skill levels, other athletes, and non-athletes. Key findings revealed that median clino-HRV indices reflecting parasympathetic division (PD) activity of the autonomic nervous system (ANS) were highest in elite skiers, particularly during the preparatory phase when training loads peaked (178 min/day vs. 131 min/day in competitive and 99 min/day in transition phases). These results suggest that endurance training enhances PD activity, potentially through NN-ACh synthesis. Our findings support the hypothesis that sports vagotonia results from increased PD/ANS activity, mediated in part by NN-ACh production as a component of cardiac anti-apoptotic mechanisms.