At least 8 out of 10 Australians will suffer from lower back pain in their lifetime. So the chances are that at some point, you will experience a bout of lower back pain yourself.
There are two main forms of back pain that physiotherapists see and treat. There is acute back pain that has a specific mechanism of injury, such a lifting injury, or there is more chronic non-specific back pain, which typically presents as a general ache or discomfort and results from a gradual build up of pain with no obvious mechanism of injury.
Excessive sitting can be linked to both of these types of back pain. The more common relationship is between sitting and the second type of back pain mentioned above, that of non-specific back pain.
Following are some reasons why prolonged sitting causes lower back pain:
Constant Lumbar Flexion
When you sit, your lower back is placed in a flexed position. It is just the reverse of what you would usually associate with flexion - when you're standing and bend forwards, you take your torso towards your thighs, whilst when you're sitting, you bring your thighs towards your torso. Nevertheless, the strain on the lumbar spine is similar.
When the lumbar spine is constantly flexed (as in sitting), there is a build up of pressure on the lumbar discs. Over time, this pressure can wear away the integrity of the disc itself, the surrounding ligaments, as well as simply irritating the intervertebral joints (the joints between each level of the spine) . This is likely to result in a sensation of pain.
More pressure is placed on the lumbar spine in sitting than in standing.
If you sit with bad posture, the load that is transferred through your lumbar spine is 2.75 times greater than standing. Even sitting with perfect erect posture still transfers a load 1.5 times greater than when standing.
Over time, this increased pressure is likely to wear down the structures in the lumbr spine, causing pain and discomfort, not to mention increasing your chances of a more serious injury.
When you sit, the body's natural posture is to slightly round at the shoulders and slouch forwards. This automatically brings the whole upper body forwards. Now, you can resist this posture, by making sure your chair is set up correctly and that you lean back into your backrest. However, most people who experience back pain do not do this - hence why they have back pain.
If the body is constantly leaning forwards, even slightly, then gravity wants to pull you down even further - until your face hits the desk! To resist this, you have to activate your lower back muscles, to hold you upright. This constant contraction of the lower back muscles is likely to result in fatigue over the course of the day, as well as causing a sensation of pain.
Excessive sitting results in weight gain when compared with switching between sitting and standing throughout the day. See this blog post.
Weight gain around the midsection places much more stress on the lumbar spine. Think of your lumbar spine as a fulcrum or pivot point. The further away from the fulcrum that your stomach is, the longer the lever is, and the harder it is to produce the force required to support that weight.
I'm sure everyone has heard of the saying, "use it or lose it". Well, it applies in this instance. If you're sitting all day, then your postural muscles, or the muscles responsible for keeping you upright, don't have to do any work. Over time, these muscles will deteriorate.
If you lack strength through these muscles, your lumbr spine has less muscular support to protect it, making it more vulnerable to injury.