Midlife crises aren't what they used to be. Serious running is in decline. One reason? Completing that marathon at the age of 40 may no longer confer the bragging rights it once did.

Researchers have in recent years noted a downward trend in the number of people participating in events like marathons and community 5-km races. In a 2018 paper, Heather Kennedy from Temple University and her collaborators dubbed the phenomenon a "running recession."

Obviously, not everyone signs up for these races, but they are one of the few objective measures of interest in the activity. And, as Kennedy and her colleagues point out, events are, for many, a powerful motivation to keep training. At their peak, they represented a $1.4 billion industry that fed even bigger ones, from shoes and apparel to wearable devices.