The Musculoskeletal Framework: Tensegrity or Compression?
We have seen that, to produce upright support, muscles pull on bones, but bones also pull on muscles, creating tensegrity support against gravity. But is the human musculoskeletal system a true tensegrity structure, or is it a compression structure in which parts are stacked one on top of the other? The upper spinal column, for instance, is clearly designed to support the weight of the head, and the spine as a whole, which forms a column designed to support the weight of the trunk, sits on the pelvis, which in turn sits on the heads of the femurs, which in turn sit on the platforms of the tibia, which sit on the ankle bones. Viewed in this way, we are more compression than tensegrity.
But to describe the spine, or the skeleton as a whole, as a compression structure is somewhat misleading. The skull, for instance, sits on the atlas, but the neck vertebrae that support the head would be sorely tested if the weight of the head (and whatever other weight we place on it!) were simply sitting on the atlas. In fact, the upper spinal column is supported by a network of ligaments and muscles, as well the larger neck muscles, which attach to the occiput and mastoid processes. These ligaments and muscles function as a complex rigging, or tensegrity structure, that distributes weight so that, in reality, the actual pressure of the head on the atlas is relatively small. This makes it possible for the women in this image to carry up to 1/5 of their body weight on their heads, without compressing the vertebrae.
Consider also what happens when you position yourself with knees bent and torso inclining forward:
In this case, the legs are buckling at the ankles, hips, and knees; and the spine, far from supporting weight, must itself be supported in very complex ways. Four-footed animals are always in a position rather like this, with the legs in a kind of buckled position and the spine maintained in the horizontal, with lots of muscles and tendons and connective tissue bearing and distributing the forces required to maintain postural support:
From this point of view, our upright, vertical, weight-bearing spine is a constantly-moving variation of a more primitive animal design—one that preserves the basic tensegrity principles found in other animals, but in a complex and refined form that enables us, when walking and standing, to stack parts vertically.