This is an excerpt from Yoga Anatomy-3rd Edition by Leslie Kaminoff & Amy Matthews.
Nature’s ability to reconcile conflicting demands is gloriously apparent in the human spine because, as the only true bipeds on the planet, we also seem to be earth’s least mechanically stable creatures. From an engineering perspective, we have the smallest base of support, the highest center of gravity, and the brainiest cranium (proportional to our total body weight2) of any other mammal. Fortunately, the disadvantage of having a head as heavy as a bowling ball (10-11 pounds or 4.5-5 kilograms) balanced on top of our bodies is offset by the advantage of having that big brain; it can figure out how to make it all work efficiently, and yoga practice can help us sense when our spine and breath are fully supporting our head. Why is this so important? It has been estimated that for every inch (2.5 centimeters) the weight of our skull moves forward of our center line of gravity, an extra 10 pounds of force are loaded into the muscles required to hold it.
Our human form, particularly our spine, exhibits an extraordinary resolution between the contradictory requirements of rigidity and plasticity. The structural balancing of forces of sthira and sukha in our living bodies relates to the principle of intrinsic equilibrium, a deep source of indwelling support that can be uncovered through yoga practice.
2. More refined than brain–body size ratios, encephalization quotient (EQ) is a relative measure of brain size that is defined as the ratio between the observed to predicted brain mass for an animal of a given size. Among mammals, EQ puts humans in first place, with dolphins, orcas, and chimpanzees right behind. Ravens score quite high, with hippos dead last (Pontarotti 2016).