It starts as a persistent and irritating pain in the foot or lower leg, then it gets more intense, maybe with swelling, and soon a runner knows she’s being sidelined by one of the most common running injuries: a stress fracture. These tiny cracks in the bone can halt training for months or even end a sports season. A segment of the multibillion-dollar wearables industry aims to save potential victims from this fate, but an engineering professor found a major problem: the devices are measuring the wrong thing.