If our bodies move a great deal, our muscles become larger, stronger and more functional. If we sit all day and don't move much, our muscles become smaller, weaker and less functional. This would hardly be noticed except under unusual circumstances. The only way you could detect such a fine variation would be by some means of comparing individual muscles, rather than an entire limb or body part, and realize that the collective function of the individual muscles makes up the whole. This same principal applies to the back muscles.
All back muscles attach to the central spine and consequently the individual vertebra.
The spine is flexible, and the very thing that makes the spine flexible is the range of motion of each vertebra. One would think that the complexity would stop there, but there is one more very important factor to consider in order to complete the complex paradox at hand. Paradox Element No. Equilibrium is the balancing mechanism of the human body. It is a complex system with the semi-circular canals of the inner ear and hair cells reporting to the brain our head location.