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Function of Anatomy: the SPINE

July 29, 2014

During cadaveric examination of the spine, as we move from the examination of more superficial tissues to deeper spinal structures there is an obvious increase in the amount and density of the surrounding connective tissues. This tissue makes it more difficult to distinguish between named muscles as the boarders between them become more and more difficult to identify (they become progressively less pedunculated). At the level of the spinal column itself, named muscles are visualized as muscular fibers that seem to grow within an encapsulating connective tissue continuum.

This anatomical appearance is best understood when considering movement of the spine. From an evolutionary perspective, muscle cells likely developed in conjunction with movement requirements. When particular movements were naturally selected for, so to were the necessary contractile elements needed to create such movement. At the level of the spine, there is very little relative tissue motion that occurs between each spinal segment (motion segment). The creation of gross spinal movement occurs via the summation of small segmental movements across larger spinal sections. Production of such movement is the job of the more superficial muscle groups that cross several articular segments. For example, when forward flexing the lower spine, movement production is achieved by activation of the larger, stronger, superficial muscles, and the gross movement should be distributed across several lower lumbar motion segments (as well as a significant contribution from the hip articulations). In contrast, when a significant amount of movement is achieved at a single segment, tissue damage often results. We commonly refer to this phenomenon as ‘spinal segmental buckling.’

Because of this, it is more accurate to think of the spine as a single, stiff unit that ‘bends’ rather than as a collection of individual articulations that move independently. To further reinforce this concept, consider that in the CNS homunculus, the entire spine takes up very little real estate when compared to say the
hand (see figure 1). Because the hand can produce various finely tuned movements it is afforded more space and is centrally assessed, by way of afferent feedback, with significantly more specificity. In contrast, the spine is ‘thought of’ more like a single unit and is activated (moved) accordingly.


Because the movement capability between segments is small (and becomes smaller with lack of ongoing training), the function of deepest muscles is not movement production. I offer their small size, low force production capability, and poor mechanical advantage as substantial proof.  Rather, their functional importance is with regards to movement assessment and afferent feedback production.   This also makes logical sense, as the deepest tissues across any articulations of the body are the first to be engaged during movement and thus provides the most specific, and up to date feedback information to the CNS regarding the ongoing assessment of movement outcome. These small muscles/muscle fibers thus act to monitor tension in the connective tissue elements encasing the spine.

From a clinical perspective, this information offers some important insights with regards to the assessment and treatment of spinal motion:

  • If we consider that the spine ‘bends’ vs. moves segmentally, then we shouldn’t concern ourselves with the palpation of specific inter-segmental motions. Rather we should be assessing spinal movement across larger spinal sections.
  • When assessing spinal movement, we must do so in the context of the small segmental movement. We can’t confuse gross movements, where a significant portion of movement is via the contribution from the hips, scapulae, etc, with actual spinal movement.
  • When treating spinal dysfunction by way of manipulation/mobilization, rather than rely on gross body movements, we should be more concerned with inputting forces that promote bending of larger spinal sections.
  • This anatomical concept is in agreement with the recent literature concerning the lack of segmental specificity of spinal manipulation.
  • When attempting to palpate deep spinal tissues, we should not have the incorrect preconceived notion (as offered by textbook anatomical drawings) that we will locate well pedunculated specific structures, rather we should expect to feel non-pedunculated ‘bumps.’
  • For the training and promotion of spinal movement, we should employ strategies that allow gross movement to be distributed across a large number of segments. That means that the small amount of inter-segmental motion that is available should be maintained.

Dr. Andreo A. Spina

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