There is a lot of discussion in yoga about what constitutes safe practice in regards to disc problems, particularly of the lumbar spine. To help make sense of the diverse opinions on the subject it is worth reviewing the anatomy and physiology of the disc, and also the history of the ideas around the causation of disc herniation and prolapse.
Most yoga practitioners are familiar with the basic anatomy of the disc, which consists of a collagenous outer layer called the annulus fibrosus, and an inner more fluid/gel centre called the nucleus pulposus. The tough collagen fibres of the annulus are arranged in layers and at alternating angles. This makes the annulus resilient in regards to both internal pressure and external forces acting upon it. The internal pressure comes from the fact that the inner nucleus is hygroscopic in nature, meaning that it absorbs fluid from the surrounding area. This keeps discs plumped up and resistant to the compressive forces of gravity.
The blood supply to the nucleus and annulus is almost non-existent. However, the bone of the adjacent vertebra – up to and including the endplates (the part of the vertebra that contacts the disc) – are vascularised, as shown in the illustration below left. Discs gain much of their nutrition through fluids pulled into the pulposus by osmosis, as well as through the movement of the spine itself. In a healthy disc the strength of the outer annulus is enormous, and high forces moving through the spine will damage the vertebral bone before damaging the disc itself – a fact worth remembering.
The pathology of disc disease is not well understood and many authorities simply put it down to ageing. However, when we take a wide-scale look at those suffering from disc prolapse (when the nucleus starts to push through a damaged annulus) we can see that there is little correlation with age. In fact, some authorities note that disc prolapse becomes less common past the age of 60, so it is clearly not simply a degenerative process. Men are also twice as likely as women to suffer from a prolapsed disc, perhaps because of the higher numbers of men doing heavy manual work – the most common cause. (Some also think that extended periods of sitting are another risk factor, though there is not much data to back this up.)
In the first half of the 20th century it was almost ubiquitously believed that compression was the primary culprit in the cause of disc prolapse. This is an understandable point of view; the pull of gravity acting on our bodyweight – the result of our upright stance – has an undeniably compressive effect on discs. Lower discs receive the most compression by virtue of their position, and lower lumbar discs are most at risk from prolapse.
The main proponent of this ‘compression’ perspective was a Swedish man called Alf Nachemson. He and many others argued in the 1950s and ‘60s that when the lumbar spine flexes, compression forces can build up to 13 Kilo-Newtons (13kN) – enough, they argued, to damage the annulus. If this idea turned out to be true, flexion would indeed be detrimental to the disc, and lifting and bending forward with a straight spine would be the preferred option.
There was, however, a dissenting voice in those days. This was Harry Farfan, a spinal surgeon and meticulous researcher. He studied the prolapsed discs of more than 6,000 cadavers and came to the conclusion that there were two different types of degenerative patterns that lead to disc failure, compression and torsional injuries. He was suspicious of the compression hypothesis for two main reasons. Firstly, in forward flexion the fibres of the annuluses were extended by only a small percentage of their resting length, so they were not being tensioned to any serious extent – certainly not enough to cause stretch damage to their tough collagen fibres. He also noticed that in the discs of the cadavers he studied, the diagonal fibres in one direction were the fibres that appeared to be ruptured. The only movement that could cause oblique tears in these fibres was a torsional, or twisting, one. Secondly, it had been shown by many researchers that if compression reached values of over 2.2kN, rupture of the endplates of the adjacent vertebrae would follow rather than damage to the annulus. This begged another interesting question: if 2.2kN was enough to damage the endplates of the vertebrae, and forward flexion could increase the pressures to 13kN, why weren’t people destroying their spines every time they bent forward? This inconvenient fact had been studiously ignored by the compression theorists.
Further petrol was thrown on this fire when, in the early 1980s, Serge Gracovetsky – a brilliant mathematician – homed in on this bit of blindsight, arguing that the compression theorists needed to completely reappraise their understanding of the way the lumbar spine functions. Though neither a clinician nor an anatomist, Gracovetsky exposed the fallacy at the centre of the debate. After much discussion with other experts in the field, he came to the conclusion that other, previously unconsidered factors must act on the lumbar spine – with his main focus the lumbodorsal fascia, a large tract of collagenous fibres that span the whole of the lumber spine from the twelfth thoracic vertebra down to the tip of the sacrum. When we bend forward, this structure is tensioned, taking much of the weight of the upper body, thus reducing the compression forces within the disc to tolerable levels. In this model, flexion of the lumbar spine is essential in tensioning the lumbodorsal fascia) – the opposite action suggested by the compression theorists. This perhaps is the nub of the problem for yoga practitioners. If you believe that bending the spine increases the load on the discs to dangerous levels, you will tend to want to keep the spine straight, in line with the thinking of compression theorists. If, on the other hand, you take the view that the lumbodorsal fascia takes the strain, as proposed by Gracovetsky, then bending the spine makes more sense.
It is interesting to speculate why such observations were not made earlier. Perhaps it is something to do with the way we have traditionally looked at the body in terms of separate bits. Prior to the 1980s, interest in fascia was almost non-existent; it was something to be stripped away on dissection to reveal the much more interesting muscles. It appeared to have no intrinsic value… it didn’t ‘do’ anything and was consequently ignored. These days we know better. Fascia has been studied extensively, and its role in mediating forces, reducing the spread of infections and informing the brain about the feel of the body are better understood. Evolution is rarely wasteful, and to ignore any aspect of the body will almost certainly make you look foolish at some point in the future.
How does this discussion affect us as yoga teachers? Well, it is worth pointing out that this is still a live debate. Critics of Gracovetsky’s work argue that he underestimated the size and power of the spinal extensor muscles, and that the cadavers that Farfan used were elderly and not representative of the healthy population. Amongst his chief critics is fellow Canadian and professor of spine biomechanics Stuart McGill, who has argued that repeated flexion and extension of the lumbar spine stresses the annulus enough to cause degradation of the fibres, leading to eventual rupture. He is particularly critical of flexing the lumbar spine under heavy load, or doing repeated flexion exercises. He does differentiate between slender and stocky people, arguing that the shape of the discs in thick-set people makes them more liable to disc problems when flexing the spine under load.
Biomechanists and bodyworkers seem divided into these two camps with an almost religious fervour, and the research papers on both sides can seem convincing to the uninitiated. For me, the following question alludes to one other compelling piece of information: what do people who know nothing of this argument do when involved in tasks that require bending? Potato pickers in Lancashire, rice pickers in China, seed planters in Africa… people who have to bend a lot for their work and who are reliant on their nervous systems to make their decisions. These people almost ubiquitously bend their spines. It would be strange if nature had given us a nervous system that selected the hazardous over the safe in terms of movement. We also see children bend to pick things up using both the spine and the knees in a very natural way. It seems to me that the most important lesson is to listen to our bodies, what ‘feels’ right when bending or lifting. If we are bending and reaching forward our spines will tend to straighten. If, on the other hand, we are bending to put our trousers on, our spine is likely to round. We need a spine that can do either, and we need to select our movements not on the basis of the intellect but on the basis of how it feels.