“Being upright has another disadvantage: when running, we lost the use of our spines as stride-extending springs. Watch a slow-motion video of a greyhound or a cheetah galloping. When it lands on its back legs, its hind paws land below the shoulders as its long, flexible spine curves like a powerful bow, storing elastic energy. Then as the animal’s hind limbs push off, the spine rapidly unbends, releasing elastic energy to help catapult it into the air and increase its stride length.11 Our short, little upright spines do nothing to help us run faster, but instead struggle to keep our inherently tippy upper bodies stable while also dampening the shock wave that travels from the foot up to the head every time we hit the ground.12”