| After evolution for more than 1 million year on earth, human beings are already acclimatized to the 1G environment. Consequently, loss of hydrostatic pressure, lack of gravitation stress and the miss of sense information in individuals exposed to microgravity may result in a series of adaptive changes in cardiovascular system, skeletal musculature, vestibular nervous system, hematological system and immune system, etc. These alterations will not only influence the performance of astronauts, but also result in decrease in orthostatic tolerance and exercise capacity when the astronauts returned to 1 G environment. In order to ensure the health, safety, and working efficiency of astronauts, it is essential to carry out researches on the development of countermeasures against prolonged microgravity.There are four hypotheses for cardiovascular deconditioning after microgravity exposure: 1) hypovolemia due to centralization of body fluid and diuresis, 2) cardiac hypofunction, 3) reduced responsiveness of resistant vessels to sympathetic stimulation due to lack of shear stress, 4) alterations in the baroreflex. Present countermeasures include: exercise, penguin suits, lower body negative pressure, saline loading, anti-G suits, nutrition, drugs, muscle stimulation and adaptive training in pre-flight, etc. Despite the wide range of countermeasures used, orthostatic intolerance upon return to Earth's gravity environment and bone calcium metabolism are still unsolved problems faced by many astronauts. Artificial gravity (AG) training on short-arm centrifuge (SAC) may be a promising countermeasure due to its profound effects on various physiological systems.Previous studies have shown that aerobic exercise training helped maintain plasma volume and work capacity during bed rest. On the other hand, AG training improved orthostatic tolerance following simulated microgravity. Human-powered centrifuge training includes an aerobic exercise component as well as an AG component. However, up to now, no definite results are established regarding to the effects of the combination of these two countermeasures and the contribution of each component to the maintenance of orthostatic tolerance. With adjustable resistance trig, subjects can be trained with fixed work loads on an electrically driven SAC designed by our research group. This SAC device is especially developed to carry out researches to understand the contribution of two different countermeasures and the effects of their combination.This study was designed to elucidate heart rate (HR) and respiratory responses when human subjects performed different training protocols including AG and exercise training. Furthermore, the effects of intermittent AG combined with exercise training on cardiovascular function as well as autonomic regulation function were analytically evaluated. We hope that present results would provide support for further studies on AG regimen selection.The main results and findings of this study are as follows:1. Heart Rate and Respiratory Responses When Exposed to Short Arm Centrifuge with Exercise. We observed and compared HR and respiratory responses when human subjects performed different protocols of SAC induced AG with exercise training. Ten healthy male subjects were exposed to different +Gz (1G, 2G, 3G) with work loads (0W, 30W, 60W) intermittently, and HR and respiratory rate (RR) were monitored continually and analyzed subsequently. The results showed that HR increased significantly when exposed to +Gz acceleration compared with supine baseline. 2G or 3G exposure combined with 30W or 60W work load exercise induced significant increase of HR (P<0.05), compared with the same work load exercise. As compared with simple +Gz acceleration exposure, HR and RR increased significantly when centrifuging combined with 30W work load exercise, and reached to higher level with 60W work load exercise. Under the condition of 30W or 60W work load, HR and RR increased with G levels. It was concluded that AG via SAC with exercise training could induce higher HR and respiratory response compared to simple +Gz exposure.2. Effects of 3 weeks of Intermittent Centrifuge Training with Exercise on Cardiac Function in humans. We observed and compared the changes of human cardiac systolic and pumping function during 3 weeks of intermittent graded G centrifuge training with exercise. During 3 weeks experiment, 8 healthy ambulatory men were exposed to graded G centrifugation with 30W exercise for 30min everyday. Cardiac systolic and pumping function was measured using electromechanical cardiograph and impedance rheogram. The results showed that as compared with baseline, left ventricular ejection time (LVET) and ejection fraction (EF) increased significantly after 2 weeks training, while HR and the ratio of pre-ejection period (PEP) to LVET (PEP/LVET) decreased significantly. LVET, EF, HR and PEP/LVET changed further after 3 weeks training. At the same time, PEP declined significantly compared with baseline. Stroke volume (SV) and total peripheral resistance (TPR) increased significantly only after 3 weeks training. Our results suggested that three weeks centrifuge training with exercise could improve human cardiac systolic and pumping function.3. Changes of Cardiovascular Autonomic Regulation Function after Intermittent Centrifuge Training with Exercise. In this study we evaluated the changes of cardiovascular autonomic regulation function during 3 weeks of intermittent SAC training with exercise. During 3 weeks experiment, 8 healthy ambulatory men were exposed to graded G centrifugation with 30W exercise for 30min everyday. The ECG and beat by beat blood pressure were non-invasively recorded. Heart rate variability and systolic blood pressure variability were analyzed using auto regression method. A combination of head up tilt test (HUT) and lower body negative pressure (LBNP) was used as an orthostatic stress to evaluate cardiovascular function. During supine position, heart rate spectral analysis showed that the power ratio of low to high frequency components was significantly declined after 3 weeks training, while systolic blood pressure variability analysis showed that the low frequency components increased significantly. During HUT and LBNP, HR after training decreased significantly while TPR increased, when compared with those of before training. SV had a tendency to increase. The results of the present study suggest that three weeks centrifuge training with 30W exercise might induce an increase in cardiac vagal modulation and vascular sympathetic responsiveness, and enhance blood pressure regulation capacity and improve the potential of cardiac function during orthostatic stress.In conclusion, we observed HR and respiratory responses when human subjects were exposed to different G protocols with exercise on SAC. It is concluded that AG via SAC with exercise training could induce higher HR and respiratory response compared to simple +Gz exposure. Additionally, present results have shown that three weeks of graded G load centrifuge training with 30W exercise might induce an increase in cardiac function, cardiac vagal modulation and vascular sympathetic responsiveness, enhance blood pressure regulation capacity and improve the potential of cardiac function during orthostatic stress. |
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