Astronauts have to work out for at least two hours per day or they can experience muscle or bone density loss. I can only assume not working out for a few days means that their bodies will quit on them in the exact way that Rigby’s did in that episode of Regular Show where he refused to eat a salad.
While the ISS has space for larger bits of workout gear, the Orion capsule set to eventually take a crew to Mars won’t. This required a new workout device to be developed. It’s called the MED-2 and it weighs only 65 pounds. Despite its light weight, it can offer both aerobic and resistive workouts with up to 350 pounds of resistance.
The MED-2 is self-powered using motion from aerobic exercise to generate the power needed for resistive workouts. They should also consider using it to power the laptop they’ve got connected to Pornhub on long duration missions.