Like all joints if you fail to take the joints in your back through their full range of motion on a regular basis you will eventually lose that range of motion. Like the saying "if you don't use it, you lose it." You can observe this in someone that has to spend weeks in a cast due to a fracture. While the fracture heals usually without issue, the joints that have been immobilized have become stiff because soft tissues have shortened, muscles have become weak, and cartilage has become softer. If we maintain our joints in a range that allows the surrounding soft tissues to shorten then eventually they will shorten and lead to stiffening.