Loading on the back during sitting – The importance of movement.

By Stephen Bowden – Chartered Ergonomist

Stephen Bowden Chartered Ergonomist

Our first nine months of life are spent in the womb during which time gravity, the force that attracts our bodies towards the centre of the earth, is already influencing our growth. This influence becomes more obvious once we are born and the development of our innate ability as humans to stand upright commences.

When we are first born our spine is in the fetal position wherein it is curled up in the shape of the letter C. This overall curve of the spine is called a primary curve.

At first we do not have the muscle strength to hold up our heads but over the first few months after birth, as we begin to interact with the force of gravity, we lift our head to look around and engage with the world around us. As we move through these experiences we are strengthening the muscles in our neck and developing a secondary curve at the top of the spine. This curve is in the opposite direction to the primary curve and is called either a lordosis or lordotic curve.

The final secondary spinal curve to develop is also lordotic and is in the lower part of the back and is called the lumbar curve or lumbar lordosis. The lumbar curve starts to develop when we begin to crawl and is completed once we have mastered gravity and are walking upright.

In the back, the distribution of loading across and between spinal structures depends on posture and how long each posture is held for.

When the spine is entirely unloaded the lumbar lordotic curvature is can be considered the neutral position. Upright standing increases lumbar curvature whereas upright sitting decreases curvature. Neither standing nor sitting, therefore corresponds to the neutral configuration so neither should be considered a natural or more normal posture than the other.

Inclining the spinal column away from the vertical requires muscle activity in the trunk to balance the moment of force generated by gravity acting on the mass of the upper body. Any tendency to lean forwards or backwards therefore increases the spinal loading arising from muscle tension.

Spinal loading is minimised when the spinal column is balanced vertically on the pelvis and or supported by a chair.

There are many causes of low back pain that are beyond the scope of this article, but sitting with the back in a very flexed (slumped) where the lordotic curve is eliminated is the most common problem and causation of discomfort in office workers.

Prolonged slumped sitting (flexed) postures severely prevent the back muscles from protecting spinal structures. In addition, slumped postures cause the protecting structures such as ligaments to weaken through water being squeezed out of them.

On the other hand rigid lordotic postures overload the spinal joints as well – therefore a sitting posture with not too much flexion and not too much lordosis i.e. a flattened lumbar spine should be utilised during sitting .

The biomechanical benefits of moderate flexion or flattened lumbar spine match the experience of the individual postural habits of people.

Sitting with the legs crossed or standing with one foot raised on a bar rail are postures that are known to be comfortable each of which reduce the lordosis move the spine into slight flexion

Standing affords an opportunity to reduce overall loading on the lumbar spine but no single posture can be comfortably maintained for a long period of time due to impact on blood flow on compressed or contracted tissues.

Furthermore the movement from sitting to standing (as discussed in our recent blog post, Active Movement at Work) switches on the anti-gravity muscles of the body that were used during the development of the secondary spinal curves mentioned earlier. When the anti-gravity muscles are switched on they trigger the release of an enzyme that causes fat to be taken from its storage areas to be used in the muscles as energy.

In summary, any “good” sitting or standing postures should be modified with  opportunity – to make regular postural adjustments by standing up from sitting in order to avoid one static posture being replaced by another one.

 

Stephen Bowden BSc (Hons) C.ErgHF MIEHF EurErg. Chartered Ergonomist. 

Stephen is experienced in Ergonomics and Human Factors ensuring an integrated approach between humans, machines and work systems within industrial, office, manufacturing, defense and aerospace. 

Stephen has a specialist interest in ergonomics approach to wellbeing and the association between Ergonomics, wellbeing and productivity.  

For more information visit www.morganmaxwell.co.uk 

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