Mapping the role of exercise on microgravity-induced cardiorespiratory deconditioning among astronauts- A scoping review

Deconditioning of the cardiovascular system is caused by the effects of microgravity through reduced mechanical loading, which results in a reduction in baroreflex function, plasma volume, stroke volume, and orthostatic tolerance. Countermeasures for deconditioning include physical activity, but the nature and effectiveness of this countermeasure are inconsistent. Therefore, the purpose of the review is to provide an overview of the evidence base related to physical activity interventions to prevent or reduce the effects of cardiorespiratory deconditioning resulting from real or simulated microgravity environments. The review employed the Arksey and O’Malley methodology for conducting a scoping review and followed the PRISMA-ScR (Preferred Reporting Items for Systematic Reviews and Meta-Analyses Extension for Scoping Reviews) reporting guideline. Literature searches using databases (PubMed, Scopus, CINAHL, EMBASE, and Web of Science) were conducted comprehensively to identify relevant studies. Only those studies involving individuals exposed to microgravity (astronauts or human microgravity analogues such as head-down bed rest, parabolic flights), who had been provided with exercise-based countermeasures, and measured their effects on the cardiovascular system, were selected for inclusion. Twelve studies met the inclusion criteria. For the most part, multimodal exercise (both aerobic and resistance training) interventions demonstrated better preservation of cardiovascular function than single modality exercise, especially if they involved mechanical loading. Other countermeasures, such as LBNP (Lower Body Negative Pressure) and AG (Artificial Gravity) were found to improve both hemodynamics and orthostatic tolerance. Significant variability existed among the studies in terms of exercise duration, intensity, and population characteristics, making it difficult to determine the best dose of exercise. Exercise training appears to be the most effective countermeasure for microgravity induced cardio-respiratory deconditioning. However, additional studies are required to establish the most effective training parameters and to evaluate the feasibility of these interventions for long-duration spaceflight as well as terrestrial unloading conditions.