Much of this research came from the mind and laboratory of Daniel Lieberman, a professor of human evolutionary anatomy at Harvard University and author of the new book “Exercised,” which delves into exercise and evolution. At first, most of his and other scientists’ work related to evolution and running centered on lower bodies, since legs play such an obvious part in how we get from one place to another.
But Dr. Lieberman also was interested in runners’ upper bodies and, especially, their heads. As a longtime marathon runner himself, he knew that a stable head is critical for successful running, but not necessarily a simple thing to achieve. Running is propulsive. You push off, rise and then brake forcefully against the ground with every stride, placing forces on your head that could make it flop uncontrollably, like that bobbing ponytail.
How we manage to keep our heads stable, however, has not been altogether clear. Like most cursorial species, or animals that run, including dogs and horses, we have a well-developed nuchal ligament, a tissue that connects the skull and neck. That is not the case in species that aren’t natural runners, like apes or swine.
When he was a young scientist, Dr. Lieberman recalled, he enticed pigs — who are inelegant runners — onto treadmills to study their biomechanics. Their heads jiggled like bobbleheads when they were forced to run, prompting Dr. Lieberman and his colleagues to conclude they lacked a nuchal ligament, a finding borne out by anatomical studies.
But we humans also have the challenge of being upright, on two legs. Presumably to balance ourselves while running, we began, at some point, to swing our arms. Dr. Lieberman guessed that the arm swing helped to stabilize our heads. But, if so, there would have to be coordination between the muscles in our forearms and shoulders, he thought, even though these muscles do not physically connect. They would need to fire together and with comparable force during running, if they were to be successful in stabilizing our heads.