Then, on a separate lab visit, each volunteer completed a series of four six-minute runs, during which they were assigned four approaches: to smile continuously but sincerely, to frown, to relax their upper body by imagining they gently held a potato chip between loose fingers or, as a control, to use their normal get-me-through-this-run mental techniques.

There were variations in the results. A few runners were most economical when they frowned; the researchers speculate that their grimaces, like ferocious “game faces,” increased their determination to outdo their normal performance. But the runners turned out to be most economical when they smiled. As a group, their economy then was as much as 2.78 percent more efficient than during the other runs, a meaningful difference in competitions. Smiling probably aided economy by prompting a “reduction in muscular tension,” says Noel Brick, a lecturer in sport and exercise psychology at Ulster University, who led the study.

Many of the runners found it difficult to smile throughout the six-minute session, though: Their grins became increasingly fixed and unnatural. Such false smiles activate fewer facial muscles than the sincere version, Brick said, and most likely result in less relaxation and performance enhancements. So the key to using a happy smile to make you a better runner, he says, may be to grin sincerely and often near a race’s end, but in 30-second bursts, rather than continuously. “This is what Eliud Kipchoge seems to do,” Brick said.