A motion segment is the vertebra, the disc, and the next vertebra. When you lose disc height, the segment can sheer. Sheer in the lumbar spine is not a physiological movement. sheer produces a change in the location of the axis of motion, and that leads to muscle guarding and pain. It is fair to say that sheer in a motion segment equals instability. an unstable motion segment can make the pain tricky to resolve or pin down.
For instance, I saw a patient this past week who experiences pain at night – which is very unusual for mechanical lower back pain, but she sleeps on a soft bed. As she relaxes into her sleep, the pain increases because the segment sheers, creating pressure on pain sensitive structures.When she is up and around, she has no pain, and is even able to engage in activities such as horseback riding. But the pain at night wakes her from sleep. Usually pain in a non-weight-bearing posture would be a red flag for requiring further investigation, but in the absence of findings, it is most likely that her pain originates in an unstable segment, and this was confirmed by physical examination.
Nevertheless, it is quite challenging to stabilize an unstable segment though, because the instability is mechanical, while the stability has to come from muscles. The deep paraspinal muscles multifidus and rotatores for example, are responsible for providing stability to motion segments, and can be over trained allowing dynamic stability to reduce mechanical instability. In addition, this person has to restore normal functional length to her hamstrings which are tight, and her thoracic spine which is restricted. Taking the pressure off the lumbar spine for motion reduces the tendency to sheer. Coupling that new flexibility with core stability drills and in particular, deep rotator muscle drills will sort this problem out.