In this module we will look at the effects of postures on the intervertebral disc and maybe clear up some confusion. We answer the question about posture and if it matters with respect to the intervertebral disc.
In my opinion the question shouldn’t be – does posture matter, it’s which postures are required during your tasks and for how long. And also are there repeated motions involved?
Have you ever worked around the house for awhile, frequently stooping or getting lost in work tasks and then you straighten up and feel that stiffness and soreness in the low back? Well there is a reason for that because there is a study that evaluated how stooped postures affect the muscles and passive supportive structures like ligaments and the disc and relation to back pain.
So in this study the goal was to investigate the effects of short duration, static trunk bending motions on muscle forces and ligament tension. During the experiment, participants maintained a 30 degree or 60 degree flexed trunk posture. Some had to hold a load for 40 seconds. Results showed with increasing flexion or stooping angle, there was a reduction in trunk muscle activity and increased dependence on the ligaments and disc tissue. The ligaments absorbed the load by stretching. Thus requiring less work for the muscles.
However at some point the passive tissues will begin to change after a period of stretching. A tissue phenomenon called ‘tissue creep deformation’ begins when the load sharing shifts more toward the ligaments and discs. Creep deformation causes reduced load sharing capacity among lumbar disc and ligaments. This basically means the ligaments and collagen fibers of the disc begin to elongate and change structurally. Their capacity to share the load decreases, requiring more work from the muscles. This ligament and muscle interplay can vary with changing postures. If the low back muscles become too fatigued, the resulting reliance on ligaments and discs could potentially lead to increased compression of the disc.
So when you stand up or straighten, the muscles have to contract – which have been stretched, also the ligaments, tendons and disc material have to adjust to the new position and restore their normal length and tension.
This study was mostly focused on passive ligaments – not necessarily the disc. But disc material is passive and supportive. So what happens to the disc specifically with posture changes?
There was a study that looked at that. In this study of the effects of functionally loaded positions on the disc nucleus , participants were required to hold the positions for 10 minutes. All were asymptomatic. In the comparisons of flexed and upright sitting positions, flexed sitting induced significantly more posterior migration of the disc nucleus compared to extended sitting. It’s important to note they were only required to maintain the positions for only 10 minutes.
If there was migration of the nucleus in just 10 minutes, it’s reasonable that sitting for hours will produce significant bulging.
Researchers speculated that maintaining lumbar lordosis or the little curve in the low back, when sitting, should reduce the risk of posterior disc derangement.
Just to be clear, posterior migration of the disc while occasionally sitting slumped isn’t cause for concern. However if you have a disc related problem and the disc wall has already been breached or you have nerve root impingement, then I would avoid slumped sitting. If you have been sitting slumped for some time then it’s best to stand up and go for a walk or stand and do some repeated back bends that are comfortable.
Disc nucleus migration is a naturally occurring phenomenon in response to loads and sustained postures and is securely contained within the disc walls under normal, variable postures and loads
The key is variable manageable loads and opposite motions to maintain a healthy.
Disc nucleus migration is a naturally occurring that is also affected by
Loads and repeated motions.
When there is excessive disc nucleus migration, then the results will lead to bulging. Which is normal, however if the disc wall is breached then protrusions and herniations can result.
Prolonged Flexion movement of the spine strains the layers of collagen in the spinal discs normally. Movement is healthy and needed for healthy discs under low loads. However, when the spine loads are higher in magnitude with sustained postures, excessive tissue creep causes the collagen fibers to begin to splinter and separate in a cumulative fashion. Slowly the nucleus of the disc will work through the delaminated walls and create a disc bulge. The greater the load, the greater the repetitions, and the longer the time in certain postures, the faster this will lead to protrusion and herniation.
Here are some key takeaways. This is by far not an exhaustive review of disc mechanics but I think the following could be helpful. Disc nucleus migration is affected by the task, posture, time, load and if the disc wall is intact. Nucleus migration can happen in as little as 10 minutes.Excessive exposure to positions and loads can accelerate disc wall separation and possibly herniation. The best way to offset potential disc herniation or reduce the effects of an existing herniation is to frequently change positions, have a lumbar support when sitting, vary the load and not stay too long in certain positions.