Editorial: Gravitational Physiology, Aging and Medicine

Division of Physiology, Otto Loewi Research Center for Vascular Biology, Immunology and Inflammation, Medical University of Graz, Graz, Austria, Department of Health Sciences, Alma Mater Europaea Maribor, Maribor, Slovenia, Department Oral and Maxillofacial Surgery/Pathology, Amsterdam UMC, Vrije Universiteit Medical Center, Academic Centre for Dentistry Amsterdam (ACTA), Amsterdam, Netherlands, Department of Biomedical Physiology and Kinesiology, Simon Fraser University, Burnaby, BC, Canada, 5 INSERM UMR1093-CAPS, Université Bourgogne Franche-Comté, UFR des Sciences du Sport, Dijon, France

Physiological deconditioning changes induced by spaceflight are similar to those that occur in aging, thus leading to, for example, a greater incidence of syncope and falls and to a decrease in quality of life. The Research Topic "Gravitational Physiology, Aging and Medicine" examines effects of gravitational changes on human physiology, with applications to geriatrics and clinical medicine. The Research Topic also aimed at promoting national and international networks that include activities used to stimulate student-oriented learning.
The presence of gravity is known to influence human physiology and behavior. For instance, gravitational loads are required to maintain a healthy musculoskeletal system. Indeed, upon standing, ∼500 mL of blood move into the lower limbs within seconds. This can compromise venous return, and hence blood pressure, leading to falls. Gravity also influences the way we interact with the environment because it provides a strong reference to align multimodal information such as vision and proprioception, critical for balance. Fortunately, the body, through neural mechanisms, compensates rapidly to postural changes, acting to maintain mean arterial pressure and prevent orthostatic intolerance.
The microgravity environment of spaceflight causes cardiovascular, neurovestibular and musculoskeletal changes, including bone loss and muscle atrophy. The observed physiological changes lead to deconditioning and impaired responses to gravitational stress on return to Earth. These changes share important common features with the deconditioning and impaired functions due to the aging process. Such changes that are seen in astronauts as well as in older persons lead to greater incidence of orthostatic intolerance. In elderly persons, this can lead to falls which are associated with head injuries and/or bone fractures. Such injuries and fractures are often associated with long-term immobilization and, consequently, further deconditioning, a vicious cycle from which patients may not be able to recover. Several papers in this Research Topic specifically address the parallel mechanisms between spaceflight deconditioning and aging (Goswami; Goswami et al.; Strollo et al.; Siamwala et al.).
Furthermore, studies aimed at understanding spaceflightinduced deconditioning often use ground-based analogs such as bedrest confinement and wet and dry water immersion (Tomilovskaya et al.). Such studies provide unique insights into the role of short and long term bedrest as well as immersion on physiological responses ( Bed confinement is a paramount problem in older persons. Hence, data from bedrest studies are useful for understanding the deconditioning effects of this kind of immobilization in this population, which is a rapidly growing segment of the overall population. Bedrest can lead to postural control deficiencies and orthostatic hypotension which are major contributors to falls in the elderly (see Goswami). Naturally, it can be expected that integrating information inherited from space environments and ground-based models of deconditioning will provide novel perspectives and innovative approaches for expanding knowledge in both space physiology and aging medicine. For example, scientific insights and methodologies developed in space science research of orthostatic intolerance can be exploited to study cardiovascular, cerebrovascular, and postural sensory motor control systems in males and females (reviewed in Evans et al.). This Research Topic is a perfect example of translational and multidisciplinary research.

TOPICS COVERED
This Research Topic received 45 papers for peer-review. Following rigorous reviewing of abstracts and full papers, and after careful screening of revised manuscripts, 36 were selected for publication. The areas covered include aspects related to physiology of gravitational (un-)loading, effects of bedrest confinement, dry immersion, as well as aging-related physiological deconditioning. In addition, this Research Topic has examined the ways in which knowledge of life in spaceand the countermeasures developed in space to overcome spaceflight-induced effects-can help life on Earth. Like the possible application of large radius artificial gravity for research, possible treatment and training (van Loon et al., 2012). While most of the contributions were based on studies of responses in human participants, some used animal models to investigate more fundamental mechanisms (Gambara et al.; Giuliani et al.; Ling et al.). This Research Topic included both original research articles and reviews. The collection of articles, which also examine sex-based diiferences in physiological responses

AUTHOR CONTRIBUTIONS
NG, JL, AR, AB, and OW were responsible for this Research Topic design, invitation to potential authors, reviews of abstracts, and submitted manuscripts and finally, in preparation of the editorial.