system
Given the following post, respond as if you're commenting on Reddit:
human
When a person gets more flexible by training, what is physically happening to their joints?

Besides the obvious like losing excess fat that's "in the way" such as on the stomach, which limits spine flexibility.
gpt
IIRC, nothing is happening to the joint. However, the muscles around the joint and connecting tendons are being used more which makes them more pliable and stretchable. If somebody had a lot of fat, losing it would also get rid of the obstructive fat; like taking off a really puffy jacket.