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Muscles of the spine help to support it against gravity. For example, the multifidi interweave throughout the vertebrae and, as I understand, sort of reinforce the spine. So let's suppose muscles help to make the spine more stiff and rigid and this somehow helps it fight gravity.

What portion of support comes from the muscles? Would a spine by itself be able to support itself? In other words, to what extent is our ability to support our spine to remain upright due to muscles and how exactly do they help keep the spine erect in this capacity?

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A biomechanical analysis can be done to determine how much support comes from the muscles vs other (skeletal) support structures. Assume the body is standing upright, in static equilibrium. Consider this (over simplified) free body diagram of the upper torso where the sum of the forces equals zero in static equilibrium.

Free body diagram of upper body in upright position.

Here the diagram over simplifies the analysis with one general equation: Fsupport = sum of all the weights. Since we are considering only how much weight muscle supports, we deduct Wbones from Fsupport.

A spine by itself would not be able to support itself because the sum of the forces do not equal zero in static equilibrium. (Wgravity + Wspine) > Fsupport.

In reality, each muscle, bone, and organ has many more forces with specific magnitudes in different directions that altogether equal the weight of the specific region of mass it supports. I imagine these forces as many vectors over an area/field. A more detailed analysis would require a program like MATLAB that uses triple integrals in calculus to visualize these forces in 3D. This kind of visualization can help us color code only the muscle vectors which answers your question, "to what extent is our ability to support our spine to remain upright due to muscles," as a percentage of the sum of all the forces input into the program. How the muscles keep the spine erect can be determined from such visualizations as descriptions of directional forces.

For an example of how this kind of analysis is done, see this paper on lumbar spinal support. http://www.uvm.edu/~istokes/pdfs/stab-model.pdf

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  • $\begingroup$ I disagree on the interpretation, see my answer. $\endgroup$ – Joce Oct 1 '15 at 15:16
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Spine is the only element between thorax and pelvis that can support compressive stress, and thus the weight of the upper body. So the spine needs to support all that.

However, the spine is not rigit enough, and would buckle without muscles. Muscles stabilize it by pulling in a coordinated manner, just as ropes can help stabilize an erected mast. The paper cited by Anna Lu, http://www.uvm.edu/~istokes/pdfs/stab-model.pdf, illustrates this, and states that muscles have both an active role (their relative tension can be tuned) and a passive role (they behave as elastic ropes).

Note however, that this is done at the price of exerting even more compressive stress on the spine!

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