Longer journeys away from Earth’s gravity have been known since the dawn of human Spaceflight to harm an astronaut’s body. In the same way that a patient’s muscles might atrophy if they spend too much time lying in bed, floating weightless results in drastically diminished muscular mass. An astronaut’s legs, back, and neck muscles will deteriorate in as little as a week if they are not continually fighting gravity.
While this may not be an issue during Spaceflight, astronauts who return to Earth in this physiologically compromised state are more likely to be injured. Fortunately, rigorous exercise can help reduce this problem. Any orbiting vessel large enough to accommodate human passengers for weeks or months will have enough interior room to include primary exercise equipment like a treadmill or resistance machine.
Of course, there’s a problem with this: every hour spent exercising in space is an hour that could be spent undertaking research or conducting spaceship maintenance. Given the enormous cost of sending a human into orbit and sustaining them there for an extended period, time is money. This brings me back to my original point: astronauts spending two or more hours each day on the International Space Station’s various exercise equipment to avoid muscle loss makes it the world’s most expensive gym membership.