On Earth we stand up, and the weight of our head and arms squashes our spines throughout the day- we are actually shorter by about 1 cm at bedtime in the evening than we are when we wake up! When we go to sleep our spines get longer again.
In space this daily squashing and lengthening of the spine doesn’t happen- astronauts get taller and taller- sometimes by as much as 5 cm!
Between the bones of our backs ( vertebrae) we have jelly-like discs which allow us to move and bend. We think it’s these discs filling with extra fluid, which doesn’t get squashed back out, that makes astronauts taller in space. Although it might be fun to be a bit taller, being constantly stretched makes lots of astronauts have back pain, and when they return they can injure their backs very easily.
When the the astronauts float about, instead of standing nice and straight, they bend forward most of the time- this changes how the muscles in their spines and hips work. Some muscles ( called the hip flexors) work hard and get stronger but the muscles in the back get longer and weaker. This isn’t a problem in space, but back on Earth again the back muscles has a job to do against gravity so keeping them nice and strong is important.