He was uncertain about how to test that theory, though, until his colleague Andrew Yegian, a college fellow in the department of human evolutionary biology at Harvard, suggested weighting runners’ arms and heads. Add mass there, he said, watch how the muscles respond, and you would be able to tell if the arms and shoulders were working together to steady the head or not.
So, for the new study, which was published last month in the American Journal of Physical Anthropology, Dr. Lieberman, Dr. Yegian and their colleagues fitted 13 men and women with sensors on their upper bodies that track muscle activity and asked them to, first, walk and then run on a treadmill while the researchers filmed them with motion-capture technology. Then the scientists handed the volunteers light hand weights and asked them to run again. Finally, they strapped weighted masks to the volunteers’ faces and had them run once more, before comparing how everyone’s muscles had responded to each of these interventions.
It turned out that not much of interest happened while the study volunteers walked; the muscles in their forearms and shoulders showed no evidence of coordinated activity. But those same muscles snapped into synchronized action when the volunteers started to run; the muscles began firing at the same time and with about the same amount of force.
That synchrony grew during the weighted runs. When the volunteers carried weights and their forearm muscles fired with extra force to compensate, the muscles in their shoulders did the same. Similarly, when their weighted faces prompted the runners’ shoulder muscles to fire more forcefully, their arm muscles did likewise.
The study does not explain how these widely separate muscles communicate with one another, though. Nor can the findings pinpoint when, in our existence as a species, they may have started to work together in this way. It also does not prove that all of us are natural born runners; plenty of people do not enjoy the sport.
Still, the results do tell us more than we knew before about our bodies, Dr. Lieberman says, and underscore that running molded us as a species. “If we didn’t have to run” in our early days as humans, he says, “we wouldn’t have this system” of muscular interplay today.