In the small study published in the journal Psychology of Sport and Exercise, researchers had 24 participants of both sexes—all of whom were experienced recreational runners—smile or frown throughout their workouts without knowing why. Although frowning worked for a few people—basically because it’s a “game face” of sorts—smiling led to a superior performance as a group.
While smiling seemed to be the key to success, there’s a catch: Lead study author Noel Brick, PhD, told The New York Times once your grin becomes unnatural, it loses its touch. Natural smiles can reduce muscular tension and increase your performance, but faking it can yield opposite results, he said. That’s why it’s best to save your smile until the end of a workout, when you really need it—a trick Kenyan marathoner Eliud Kipchoge used to get his fastest race time this past May, the Times reported.