The stories make headlines and strike fear in the hearts of runners who read them. Athletes—often young and seemingly healthy—die suddenly at races, during training runs, or in the off hours between them. Often, the cause is sudden cardiac arrest, which occurs when the heart stops beating. It’s a short-circuit in the electrical impulses that govern your heartbeat. That’s different from a heart attack, typically caused by a blood clot physically blocking blood flow through an artery, though the two can be linked. According to the latest American Heart Association statistics, sudden cardiac arrest isn’t common among runners—about 0.54 per...