In the body, it's the ligaments that hold bones together. In the knee Joint Pain Hack Review for instance there are several ligaments that attach the femur to the tibia. These ligaments stabilize the knee when we stress it mechanically, for instance when we're running. The muscles around the knee also work to stabilize it and give it mechanical stability, but right now I'll just talk about the ligaments. When there is too much stress to a ligament, it tears. This is what we call a sprain.
If the stress to the ligament isn't too much, the injury heals and afterward the knee moves just like it should--the joint is still mechanically stable. But if the mechanical stress to the ligaments was too much, the ligament will over stretch so much that it remains overstretched after the injury heals.