RESUMO
Objective. To investigate the effects of different training methods on cardiovascular autonomic regulation under bedrest. Method. 15 healthy male volunteers aged 19-22 participated tests in head-down tilt (HDT) -6 degrees bedrest in order to observe the changes of cardiovascular system under simulated weightlessness. They were divided into control (5 men), hypoxia training (5 men) and Fangsong training (5 men) groups. 24 h dynamic ECG were recorded on the 2nd day of pre-bedrest, on the 3rd, 14th and 18th day of bedrest and on the 7th day of the post- bedrest. All spectra were estimated from entire 24 h HRV, before, during and after Fangsong and hypoxia training by autoregressive (AR) modeling method. Normalized low-frequency (LF%) was a quantitative marker of cardiac sympathetic activity, normalized high-frequency (HF%) reflected the changes in cardiac vagal activity, and LF/HF was considered to be related to sympathovagal balance or sympathetic activity. Result. In control group, LF% and HF% were all significantly reduced (P<0.05), LF/HL showed no significant changed during bedrest. In Fangsong group, HF% increased markedly (P< 0.05), while in hypoxia group, LF% increased markedly (P< 0.05). Conclusion. Fangsong training counteracted markedly the reduction in vagal activity, while hypoxia training counteracted markedly the decrease in sympathetic activity. It was possible that HRV indices could be used to evaluate the efficiency of countermeasures counteracting the adverse effects of weightlessness.