WHEN BRONNY JAMES collapsed in July, the shock of what happened was perhaps outstripped only by the surprise of the circumstances surrounding the incident. At just 18 years old, James—the son of undoubtedly one of the best professional hoopers of all time, LeBron James—went into cardiac arrest during a routine practice. A fit guy in prime shape—a budding superstar getting ready to play for a Division I basketball program, no less—was rushed to the hospital and placed in intensive care.

Such a turn of events prompts the obvious question: Why did a young athlete in top form, with access to elite medical care, suddenly find his heart malfunctioning? James, though, is not the first young athlete who’s experienced cardiac arrest, and he will not be the last. Which prompts the next obvious question: How do young athletes even have heart problems? The answer comes down to the nature of the muscle itself and the role that exercise sometimes plays in triggering potentially deadly events.

Cardiac arrest taking down young, competitive athletes is actually not a new phenomenon. Recent data from the Sports Institute at the University of Washington puts