When we think of our spines, we imagine a column of bones, curved and strange, from the fat vertebrae at the base of our spine to the delicate ones at the top. Bones, a column of bones, yet we are able to leap and jump, contort and twist, bend down and arch back, turn and do all kinds of movements that would make one think our spines were composed not of bone but of jelly. How does our spine accomplish all this while still protecting our nerves? This is where a closer examination of the combination of discs, ligaments and bone comes in useful.
To best understand how the spine can handle these different forces and directions it helps to imagine the spine as to actually be composed of two separate columns that are set right next to each other. One column is composed of the disks of bone that form the column of the spine, each separated by a disc of cartilage, while the second column is composed of the spiny protrusions amongst which all the ligaments are interlaced.
This setup is designed to deal with the interplay between stability and flexibility, allowing for support of our heavy heads and the ability to twist and jump. Both columns do this by mixing hard and soft, allowing the hardness of the bone to be counteracted by the softness of either the discs between the vertebrae or the ligaments that hold the spiny protrusions together. When weight is pressed down or up on the vertebrae they press down on the discs between them, which, being spongy and strong, press back, resisting the force. The spiny protrusions transmit force to all the ligaments which bind them together, which resist by pulling back in an elastic manner.
The combined effect allows the spine to resist compressive and tensile forces by creating an interplay between hard and soft, elastic and unyielding, so that forces are evenly distributed and absorbed, allowing us to continue running and twisting while the bones support our weight and the discs and ligaments allow us to move and groove without worrying about damage.