such insightful responses
obviously the further you are hunched over, as well as the more time you spend in this type of position, will increase the risk for permanent back pain and injury, most likely from damage and swelling in between the discs of your (lower)back from stress
however what most riders experience would be soreness due to increased use of certain muscles that help stabilize the spine, this obviously usually goes away in time as their muscles strengthen to accommodate for the change in posture, I'm pretty sure there are at least two muscles in the abdominal region (which support the lower back) that can be strengthened with certain exercises if you are really concerned about it as an issue lol
2cents