Is it a fitness boom - or an unhealthy addiction? Doctors warn there's a fine line between a healthy exercise regimen and addictive behavior that can actually harm the body.
"My goal was to try to come under 21 minutes for a 5k by the end of the summer," says Chris Lear, a self-described recovering exercise addict.
Lear, 53, admits he was never a runner, but when he decided to join a racing series last year, he says the thrill of competition became addictive.
"Once you started winning, it's just very contagious."
So he started training harder, running longer distances and adding boot camp classes. But it was taking a toll on his body.
"I just hit a point where I couldn't walk anymore."
Physical therapist Scott Epsley says he's seeing more and more patients like Lear, who are over-exercising and taking their workouts to dangerous extremes.
"It's very common in runners in particular to see this over exercising mentality. There seems to be that psychological…that type A personality," says Epsley.
In the last 12 years, the number of people finishing marathons has increased 50 percent. And it's not just runners. The number of yoga practitioners has exploded from 4 to 20 million in the last decade.
"They're afraid they're going to be fat, fear of losing control,” says psychiatrist Dr. Antonia Baum. “Fear of losing structure in their day to day life, not staying in shape, not staying desirable."
Dr. Baum says over-exercisers are often similar to people with eating disorders. They use exercise as a way to control their lives or as an escape.
It becomes a problem when it gets in the way of social activities or relationships, and of course, when it starts to affect their bodies physically.
"Unfortunately, the way most people know is because they start to get pain,” says Epsley.
Things like overuse injuries, bone stress, shin splints and tendonitis. Epsley says over exercisers will also feel extreme fatigue and joint stiffness.
Chris Lear suffered a stress fracture and had to take a break from running. After a few months of rest, he's now back on track.
"It's a bonding thing, it's a competitive thing and it's an eternal drive just to keep going," says Lear, who plans to keep on running.