Exercised: The Science of Physical Activity, Rest and Health

by Daniel E. Lieberman

I’ve been focusing here on the energetic disadvantages of being bipedal, but being upright causes a host of other problems, one of which is that our heads jiggle dreadfully. A human runner’s head bounces up and down, but the head of a galloping dog or horse stays impressively still even though the rest of its body is moving. The quadruped’s head looks like a missile mounted on its body. This missile-like stability is crucial for running because the reflexes that stabilize the eyeballs to keep an animal’s vision from blurring are not fast enough to overcome rapid displacements. Experiments show that when the head rotates too fast, animals (including humans) can no longer effectively focus on whatever they are looking at including obstacles in their way. Overly bouncy heads are thus potentially perilous for runners. Quadrupeds solve this problem by having a relatively horizontal neck that runs from the front of the thorax to the back of the head. Thanks to this cantilevered configuration, when the animal’s body falls or rises, it raises or lowers its neck to keep its head still. Some animals have gone a step further and added a highly elastic structure (the nuchal ligament) that passively stabilizes the head without requiring muscles to do much work. Humans, however, run like pogo sticks because we have short, little, vertical necks that attach to the middle of the base of the skull. Although we have evolved some special mechanisms to keep our heads from pitching too much, t...