| Microgravity induces a loss of hydrostatic pressure, which results in a series of adaptive changes in individuals when exposed to low gravitational stress. Such adaptations in all physiological systems in human body will lead to cardiovascular dysfunction. Cardiovascular dysfunction not only influences the astronauts'health and decreases the performance in space, but also results in maladaptation when the astronauts returned to 1 G on earth. Orthostatic intolerance and decreased exercise capacity are involved in such a process, underlying mechanisms and valuable countermeasures remains to be established. Therefore, it is requisite to carry out medium-term or long-term microgravity countermeasures in light of the health, safety and performance of the astronauts.Present countermeasures include: exercise, penguin suits, lower body negative pressure, add water and salt, anti-G suits, nutrition, drugs, muscle stimulation and adaptive training in pre-flight, et al. Notwithstanding of the limitations in the roles of space exercise in human body, its effectiveness is more comprehensive compared with the other countermeasures, which established exercise a key part in present countermeasures. So far, the controversies remain in the exercise programs and their practical effectiveness. Accordingly, screening a exercise program that can best prevent and counteract the harmful effects caused by space microgravity has been a touchy and imminent focus in space medicine.In the present study, head-down tilt (HDT) bed rest was used to simulate microgravity. We observed and compared the effects of lower limb muscular exercise with bicycle ergometric exercise during 30 days of -6°HDT bed rest on the responses of lower limb muscle and cardiovascular systems. Our aims were to investigate whether exercise could counteract harmful effects caused by microgravity stress, screen effective exercise program, and provide experimental data available for establishing our nation's exercise program in manned spaceflight.The main results and findings of this study were as follows:1. Effects of exercise on orthostatic tolerance, exercise capacity and heart rate variability during 30 d HDT bed rest We observed and compared effects of lower limb muscular exercise with bicycle ergometric exercise during 30 d HDT bed rest on the changes of orthostatic tolerance, maximal exercise time, body weight and heart rate variability. The results showed that in the HDT 30th day, the orthostatic tolerance decreased significantly (P<0.01) in control group compared with pre-HDT, but in the limb muscular group and bicycle ergometric group, the orthostatic tolerance decreased insignificantly compared with pre-HDT. In the HDT 30th day, the maximal exercise time in control group and limb muscular group decreased significantly (P<0.05), but in bicycle ergometric group did not reach significance compared with pre-HDT and increased significantly (P<0.05) compared with control group and limb muscular group. Compared with pre-HDT, body weight trended to decrease in control group and increase in limb muscular group, but had no significant changes in bicycle ergometric group during HDT; in the HDT 10th day, body weight increased significantly (P<0.05) in limb muscular group compared with bicycle ergometric group. In control group, LFn and LF/HF trended to increase, HFn trended to decrease in heart rate variability (HRV) analysis compared with pre-HDT, in the other two groups, these indexes of HRV were similar with the control group. These results suggested that the orthostatic tolerance and exercise capacity have significantly decreased, and cardiovascular autonomic regulation of balance was changed. The limb muscular exercise in some extent can increase orthostatic tolerance following simulated microgravity, but the bicycle ergometric exercise can increase orthostatic tolerance and exercise capacity.2. Effects of exercise on limb muscle during 30 d head-down tilt bed rest We observed and compared the effects of lower limb muscular exercise with bicycle ergometric exercise during 30 d HDT bed rest on the changes of perimeter of leg, cross-sectional area (CSA) of leg and mean amplitude (MA) of calf. The results showed that the perimeter of leg significantly decreased (P<0.05), further decreased (P<0.01) in a HDT time-dependent manner compared with pre-HDT; in limb muscular group, the perimeter of leg decreased significantly (P<0.01) during from the HDT 18th day to the 30th day compared with pre-HDT; in bicycle ergometric group, the perimeter of leg decreased significantly (P<0.01) during from the HDT 22nd day to the 30th day ; in the HDT 10th day, the perimeter of leg in bicycle ergometric group increased significantly (P<0.05) as compared to that of control group; in the HDT 18th day, the perimeter of leg in limb muscular and bicycle ergometric group increased significantly (P<0.05) compared with control group. Compared with pre-HDT, in the HDT 30th day and post-HDT 1-week, the CSA of leg decreased significantly (P<0.05) in the three groups. Compared with pre-HDT, the MA of calf decreased significantly (P<0.05) in the HDT 1st and 22nd day in control group; the MA of calf decreased significantly (P<0.05) in the HDT 7th, 22nd 26th, 30th day in limb muscular group; the MA of calf decreased significantly (P<0.05) in the HDT 7th, 10th, 22nd and 30th day in bicycle ergometric group. These results suggested that 30 d HDT bed rest induced severe muscle atrophy. The limb muscular exercise and the bicycle ergometric exercise in some extent can ameliorated the muscle atrophy following simulated microgravity, and played a role in restoring muscle structure and function. 3. Effects of exercise on heart function during 30d head-down tilt bed rest We observed and compared effects of lower limb muscular exercise with bicycle ergometric exercise during 30 d HDT bed rest on the changes of heart systole function and heart pump function. Obtained results showed that LVET decreased significantly (P<0.05) in the HDT 22nd and 26th day, PEP/LVET, ICT and ICT/LVET increased significantly (P<0.05) in the HDT 26th day, EF decreased significantly (P<0.05) in the HDT 26th day, SV and CO decreased significantly (P<0.05) in the HDT 1st day in control group compared with pre-HDT. In limb muscular group, LVET decreased significantly (P<0.05) during HDT, SV decreased significantly (P<0.05) in the all time except for HDT 5th and 7th day, CO decreased significantly (P<0.05) in the HDT 1st and 30th day. In bicycle ergometric group, the indexes of heart systole function has no significant changes, SV and CO decreased significantly (P<0.05) in the HDT 1st day. These results suggested that 30 d HDT bed rest has induced significant reduction in heart systole function and heart pump function. The limb muscular exercise and the bicycle ergometric exercise to some extent can improve the heart systole function; however, the bicycle ergometric exercise was better.4. Effects of exercise on cerebral blood flow during 30 d HDT bed rest We observed and compared effects of lower limb muscular exercise with bicycle ergometric exercise during 30 d HDT bed rest on the changes of cerebral blood flow (CBF) by TCD. The results showed that in the three groups, the cerebral blood velocity has no significant changes compared with that of pre-HDT. In control group, pulsatility index (PI) and peak systolic flow velocity/ end systolic flow velocity (S/D) decreased significantly (P<0.05) in the HDT 14th, 26th and 30th day, resistance index (RI) decreased significantly (P<0.05) in the HDT 26th and 30th day; in the limb muscular group, PI, RI and S/D decreased significantly (P<0.05) in the HDT 26th day; in the bicycle ergometric group, PI, RI and S/D decreased significantly (P<0.05) in the HDT 3th, 22th and 30th day. These results suggested that 30 d HDT bed rest induced adaptive changes in middle cerebral artery. The limb muscular exercise and the bicycle ergometric exercise have no obvious effects on CBF.In conclusion, this study investigated the effects of lower limb muscular exercise with bicycle ergometric exercise during 30 d HDT bed rest on limb muscle and cardiovascular system. We found that limb muscular exercise and the bicycle ergometric exercise in some extent can improve adverse effects following simulated microgravity. The limb muscular exercise is more effective in maintaining lower limb muscle and body weight, while the bicycle ergometric exercise provided more improvement in cardiovascular function. Findings in present study provided a valuable basis in formulating countermeasure program in astronauts when exposed to microgravity.
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