Dunno if you guys know this or not, but when a person (like an astronaut) spends an extended amount of time in space, their bones begin breaking down and becoming "soft". I'm not sure exactly why or how this happens, but it does, sort of like if a bodybuilder on steroids doesn't work out for a long time, he'll begin losing all his muscle mass at a faster rate than a normal, non-juicing builder would. Our bodies are used to the feel of the Earth, and doesn't like change, so putting it in a hostile, weightless environment like space causes it to break down from the inside. This happens to everyone who stays in space too long. That's why astronauts like the ones in the ISS are rotated in cycles so nobody is prolonged. So all these life-long badass space marines you see in games and movies nowadays would actually be a bunch of soft, weak scrubs in real life who couldn't even lift their gun, let alone save the galaxy. Commander Shephard? Moar liek commander shep-soft! Huh, m i rite? Earthlings would actually be pretty weak space marines given that our environment is equal to a vacation in Hawaii. We'd have a hard time adapting to something more extreme.
Tl;dr: humans who fly into space weaken from being off Earth's gravitational field and start breaking down from the inside, so all the tough badass space soldiers and marines from games like ME and warhammer would never actually exist in real life. --- CM Punk is the motha****in' truth.
But... ME isn't a work of fiction. So you're wrong, sir. --- He also intimidates his own tail into letting him fly. Physics, Logic, and Common Sense don't have the balls to say anything about this either. -shroom_ninja_X
It's not due to being in space, but due to being in zero-gravity. Since there is much less stress on your body, it atrophies much quicker than it would under the usual gravitational pull. So as long as these guys have their feet on the ground somewhere instead of floating above it they should be fine.