The search for the ultimate exercise countermeasure to preserve crew health and ensure mission success in long-duration spaceflight.

The current understanding of crew health maintenance is founded upon decades of physiological research conducted in terrestrial spaceflight analogues and in low Earth orbit, particularly on the International Space Station. However, as we progress towards the Lunar Gateway and interplanetary missions, it is imperative that the tools employed to maintain crew health are redefined, including the utilisation of exercise countermeasures. The successful implementation of exercise countermeasures for deep space missions must address a number of challenges, including those posed by new environments with elevated levels of cosmic radiation and solar particle events, extended mission durations and constrained space availability. In this Topical Review, the authors address points that are important (and sometimes critical), but often ignored, in order to define future exercise countermeasures for long-duration space missions. Multi-organ countermeasures, countermeasure enjoyment, time-dependent load variability, the relationship between nutrition and the success of exercise countermeasures, and the individual variability in response to a given countermeasure are presented and discussed. The aim of this article is to raise awareness of important aspects that can profoundly influence the efficacy of exercise countermeasures, thereby affecting the health of the crew and the success of the mission during prolonged spaceflight.