How Do Our Spines Curve?
With all the constant reminders to “sit up straight,” it can be easy to forget that our spines are meant to have curves. The adult spine is made up of 3 main regions that form an “S” curve: Cervical, Thoracic, Lumbar. With the sacral acting as a curved piece of bone at the back of the pelvis, the spine ultimately resembles a structure with alternating curves.
The curves maintain enable even weight-distribution of the body
The curves in our spine help to distribute the weight of our body evenly and let our centre of gravity sit on top of the pelvis when standing. This centre of gravity is stable as it aligns vertically to our feet, and as a result minimizes the amount of work our back muscles have to do to keep our body upright. So what happens if we have straight spines like our distant gorilla cousins? This may result in a center of gravity so far forward that it’ll easier to be on all fours instead of on our feet. Additionally, we can think of our heads and the upper body as independent parts that have to be kept upright. The cervical curve in the neck and lumbar curve in the lower back are both necessary to distribute the weight in these parts evenly, enabling them to be held upright with minimal effort.
The curves facilitate absorption of forces and shocks on the spine
Due to gravity, our weight is exerting a downward force on us at all times. When our spine is curved, each vertebra bone isn’t directly vertically stacked on top of each other, and as a result, doesn’t bear the full weight of the bone above it. If the spine is fully straight on the other hand, the weight pressing on each vertebra bone becomes the total weight of all the bones stacked on top of it. The weight borne by each bone will increase exponentially down the spine, with the lumbar region bearing the majority of the total weight.
When we run or jump, an upward force is exerted through our legs as we make contact with the ground. This causes a shock to run up our spine. Luckily for us, the alternating curves in our spine can compress and curve further to absorb the shock before regaining its original shape, much like a spring. If the spine is fully straight, the semi-elastic discs between the vertebrae bones have to absorb more shock, resulting in faster wear-and-tear. A straight spine can also cause this shock to shoot up to your brain directly if the discs cannot absorb it sufficiently.
The curves make our spine more flexible
When we perform a wide range of motions, the muscles around the spine exert forces on it in many different directions. For example, rotating our torso results in torsion forces, leaning to the side results in shear forces, and heavy lifting results in compression forces. As compared to a straight spine, a curved one has more room to be compressed, stretched, and unfurled. This means that the individual vertebra bones do not have to be pulled as extensively when the spine is being extended. As a result, we can make a range of motions without causing substantial wear-and-tear to the spine.