The detriments of sitting on spinal health.
What we know from scientific studies:
- That sustained axial loading/compression of our spine (eg sitting) reduces the metabolic activity within our spinal discs which speeds up the rate of disc degeneration and cellular death. So this performed day in day out has serious implications to the long term health of our discs.
- Axial loading day in/day out —> reduced metabolic activity within the disc —> less production of proteoglycans within the disc —> reduces the osmotic pull of fluid into the disc = reduced rehydration of the disc —> increased load transference onto the outer disc walls (instead of a hydrated disc nucleus) —> buckling/tearing of the outer disc walls (annular tears) = can be painful —> eventual complete rupture of disc wall —>leaking out of toxic disc material (sequestrated disc) —>loss of disc height/multitude of spinal problems
- Mechanical loading of a spinal disc results in the production of proteoglycans within the disc (studies in cow discs have shown that the concentration of proteoglycans can be doubled within 2 hours of inducing pressure changes in the disc). The axial loading needs to be less than 20 kgs in load (greater than 20 kgs appeared to inhibit the production of proteoglycans). Good quality animal studies have shown that mechanical loading of discs;
- reduces the number of dead cells within a disc
- the cellular changes within the disc happen within hours/days
- helps regenerate a ‘sick’ disc
- Pressure change therapy appears to be important in maintaining/regaining the stature and health of the IV discs to allow the disc to function within the spinal column and allow normal spinal movement/function.
- Mechanical loading also allows the disc to act simply like a sponge being rinsed. As you increase the mechanical load on a disc it ‘squeezes’ out the old fluid/waste products from within the disc, and then when you decrease/release this mechanical load the fluid rushes back into the disc to replenish its water and nutrient content.
- It is also proposed that mechanical loading/pressure changes induce healing within the articular cartilage (which lines the facet joint surfaces). A study in 2004 showed that 6 mins of mechanical loading every 2nd day improved the quality of the tissue grown in vitro.
- Need to adopt the use it or loose it approach – the more we use our backs day to day the stronger and healthier they will remain. Measured normal compressive loads of our lumbar spine are;
lying = 150-250 Newtons
standing upright = 500-800 N
sitting upright = 700-1000 N
stooped to lift 10 kgs = 1900 N
(Nachemson AL, Disc pressure measurements. Spine 1981 6 : pp93-97)
- We lose 20% of the fluid from our discs in the first 2 hours of being upright and then the loss of fluid is static. At the beginning of the day we should be 2cm taller than we are at the end of the day. This only occurs in a healthy normal spine in which the discs have been rehydrated overnight whilst supine sleeping and where a high concentration of proteoglycan synthesis has occurred the previous day through regular bouts of spinal movement i.e. this will not occur in the spines of those who sit all day!
- Disc degeneration occurs slowly over time so there can be no warning signs until the process has been going on for a while.
- An older disc already has a reduced cellular activity due to the natural ageing process so that to reverse the changes in an older disc associated with excessive axial loading it takes a lot longer!
- Sustained slumped sitting not only induces sustained axial loading throughout the discs but can also induce shortening of the anterior longitudinal ligament which then emphasises a stiff spinal segment and local compressive forces through the disc (cyclic effect). This ligamentous shortening can then mean the spinal segment remains in this position even when the person stands/changes position with the segment unable to adopt a better position = further impedes the ability to generate movement at this spinal level —> breakdown of disc and facet joints
- Sustained overly erect sitting results in an increased load through the facet joins and over activity of the large back muscles which results in increased compressive loads through the disc and joints.
- The spine needs to be able to move easily and freely to absorb/accommodate the loads we put through it (like a slinky/spring). eg when we run each spinal level needs to be able to load and unload each time our heel hits the ground. If it is rigid/stiff then the disc can not absorb the shock and the shock gets transferred to the facet joints. Its like trying to use a pogo stick that has rusted up, it just can’t bounce (facet joints are designed to allow and control movement, not absorb shock. The joint surfaces of the facet joints are lined with cartilage that can be damaged by excessive/repeated compressive loads).
- Scoliosis – can be a fixed structural anomaly that we are only able to manage and minimise the impact it has on our spine, but sometimes a scoliosis can be the result of a leg length discrepancy or a pelvic rotation induced by other musculoskeletal factors which can be identified and treated by a Physiotherapist.
- Disc re-hydration occurs through
- convention – pressure changes throughout the day pumps fluid in and out of the discs
- diffusion – increased protein concentration within the disc draws fluid in through osmosis
- Why do we have our “S” bends in our spine?
– this allows the head to balance over our feet easily
– it allows the vertical load to go through the vertebral body and not the facet joints
– allows spine stocking is sitting
– absorbs load with impact by being able to act like a spring e.g. with running, jumping and walking
Interesting articles to read about the effects of sitting on not just the spine, but the whole body;
Is sitting a lethal activity? http://www.nytimes.com/2011/04/17/magazine/mag-17sitting-t.html?_r=0
What are the risks of sitting too much? http://www.mayoclinic.org/sitting/expert-answers/faq-20058005
Do you have ‘sitting disease? ‘http://www.mayoclinic.org/healthy-living/nutrition-and-healthy-eating/expert-blog/sitting-disease/BGP-20056238
Sitting All Day: Worse For You Than You Might Think http://www.npr.org/2011/04/25/135575490/sitting-all-day-worse-for-you-than-you-might-think