Muscles and ligaments require regular movement, otherwise they will become stiff and inflexible. As you age, your discs become thinner, bringing the intravertebral joints closer together. The ligaments that support the spine become more slack and joints become looser. It is like a worn machine with a loose drive belt or pulley. Any undue strain may throw something out of gear. Throughout your life you obtain strains by bending the spine too far, placing too much load on the spine while in the bent position, or bending repeatedly.
Ligaments have a limited blood supply and thus do not heal easily. Sometimes they fail to heal completely, thus leaving scar tissue. Scar tissue is not as strong or flexible as regular ligaments or muscles and leaves the area stiffer and weaker. This is how your back ages and becomes less flexible. Without regular loading and movement, ligaments can become stiff, which may increase the risk of injury.
Unless appropriate exercises are performed to restore normal flexibility, the unhealed tissue may produce a continuous source of back pain and/or stiffness. Simple, gentle loading and movement can help scar tissue become more flexible and more strong, or absorbed and replaced by more normal ligament and muscle tissue. This strengthening can help to prevent a re-injury. Although there are exercises to stretch parts of your back separately, inversion helps to gently load and move all the ligaments encasing your spinal column at once.
Summary