Older runners who are in excellent shape, with years of marathons under their collective and not very long belts, can still be cut down in mid-stride by a cardiac arrest. That’s the lesson suggested by the autopsy of famed 58-year-old endurance runner Micah True (AKA Caballo Blanco), who reportedly died from an undiagnosed enlarged left ventricle, or cardiomyopathy.

But the fact is even tougher to accept: any runner can have the condition but not realize it. A time-bomb under your sternum.

Runners World has a brief, instructive report that distinguishes between two kinds of enlargement. The first is exercise-related, or left ventricular hypertrophy, which is “a common non-pathological finding in athletes, wherein the heart tissue thickens and strengthens due to the conditioning of exercise. This is a thickening of normal, healthy cardiac tissue that results in the heart pumping blood more strongly and efficiently.” The second, which killed True, is “obstructive cardiomyopathy, also called hypertrophic obstructive cardiomyopathy (HCM). This is usually congenital and inherited. Often, patients have no physical complaints. In fact, HCM is often only discovered post-mortem.”

That last part, about not realizing you have the condition until it kills you, might make any runner—old or young—think twice about pushing past that first hard draw of breath. But it’s worth remembering that recent research “found the odds of a long distance runner suffering a cardiac arrest or dying are extremely low” (although it adds this caveat: “unless you have a pre-existing medical condition”).

The study of 11 million runners who participated in marathons or half-marathons between 2000 and 2010 was published in January in the New England Journal of Medicine. It found one cardiac arrest per 184,000 participants and one death per 259,000 participants. “Those numbers are low compared to other athletic activities, as shown by prior studies of deaths in college athletes, triathlon participants and previously healthy middle-aged joggers.”

Of the 59 documented cardiac arrests, researchers could trace 31 to mostly unrecognized pre-existing conditions like cardiomyopathy. The rate of cardiac arrests climbed during the period from 2005-2010, which might be a statistical anomaly (you’re talking small numbers among big numbers) or because too many older men (51 of the 59 heart attacks were men) now have the impression that anyone can run a marathon. (They cannot.)

Undeterred by headline events like True’s passing, or by any fancy research (or by the necessity of trudging off to work at deadly, sit-down jobs) old people have become a running-mad horde.

  • From the U.K., this profile of a 68-year-old who has run his 409th marathon, which (the Sun calculates) is equivalent to racing from London to Sydney. (Favorite quote: “His grueling hobby began after he made a bet in a pub in 1982 that he and a group of friends would run the local marathon — but David was the only one to turn up on the day.”)
  • From Ohio, a report that the “Cleveland Marathon has seen a 260 percent increase in runners over the age of 50 in the past nine years, nearly keeping pace with the 300-percent rise in runners, overall, in that time span.” Among those are 70-year-old Mary Dunbar, who “began running marathons three years ago.”

Image: screen shot from The Runner, a movie about Marco Olmo, who won the punishing 166 km North Face Ultra-Trail du Mont Blanc in 2006 and 2007, at age 60.