Throughout, the volunteers wore masks that tracked their oxygen consumption, which is one measure of efficiency.
They turned out to be most efficient in the 4% shoe, even when compared to the skinny track spikes and even after the researchers used lead pellets to add weight to the 4% shoe so that its mass equaled that of the other, slightly heavier marathon shoe.
The men and women had benefited equally from the 4% shoe and their efficiency gains had been unrelated, it seems, to that shoe’s featherweight.
But which elements of the 4% shoe did, then, most matter was still unclear.
So for the other new study, which was published in November in Sports Medicine, the same researchers from the University of Colorado at Boulder who had conducted the original study of the shoe invited 10 fast, male runners to their lab, fitted them with motion-capture sensors, and filmed them as they wore the 4% shoe, a different Nike marathon shoe, and a similar Adidas model. (The work was paid for through the lab.)
They also employed specialized equipment to bend and manipulate the shoes, to see how they responded to forces and which portions of the shoes were most affected.
Finally, they used the motion-capture data and complicated mathematical formulas to determine that the 4% shoe had slightly changed how the men ran, reducing the amount of muscular activity around their ankles and within their feet, lessening the amount of energy they burned with each step and making them more efficient.
But those benefits were not due primarily to the carbon-fiber plate, their calculations showed. It stiffened and supported parts of the foot, allowing runners to push off hard with less muscular effort, but did not provide much thrust of its own. In effect, it acted like a lever, not a spring, says Rodger Kram, an emeritus professor at the University of Colorado who conducted the study with a research associate, Wouter Hoogkamer, and others.