That biomechanical distinction is important, says Dr. Cronin, who is now a researcher at the University of Jyvaskyla in Finland. “Several studies have shown that optimal muscle-tendon efficiency” while walking “occurs when the muscle stays approximately the same length while the tendon lengthens. When the tendon lengthens, it stores elastic energy and later returns it when the foot pushes off the ground. Tendons are more effective springs than muscles,” he continues. So by stretching and straining their already shortened calf muscles, the heel wearers walk less efficiently with or without heels, he says, requiring more energy to cover the same amount of ground as people in flats and probably causing muscle fatigue.