(CNN) Rick Murphy, a real estate appraiser in Atlanta, had no idea he had a bad heart.
When Murphy turned 50, he decided it was time to get in shape. It wasn't long before he progressed from running races to an Ironman triathlon.
A fellow triathlete recommended he get checked out by a sports cardiologist, so he went to see Dr. Jonathan Kim at Emory Healthcare in July 2015.
"The next thing you know, Dr. Kim is saying, 'I think we need to do a heart catheterization on you.' "
One of the heart's main arteries was over 95% blocked. Murphy was in the operating room within a few days.
"I was obviously surprised," said Kim. "He could've been that unfortunate terrible story where somebody drops dead."
"My only symptom was a little extra fatigue," said Murphy, now 55. "I chalked it up to working too much and not getting enough sleep."
Murphy wouldn't be running another Ironman anytime soon, but he was eager to get back to his normal routine. And Kim prescribed just that: more exercise. Though the benefits of exercise are clear for those with an injured heart, Kim said, patients often want to know how best to exercise, how much and how hard.
Increasingly, experts are pushing patients with heart problems harder than what was considered useful -- or perhaps even safe -- in the past.
Let's get physical
The idea that exercise can help hearts recover is a relatively modern one.
Until the 1950s, doctors often told cardiac patients to avoid any physical activity at all. In 1952, the recommendation that heart attack patients get out of the hospital bed and into an armchair was seen as controversial. It wasn't until the late '50s that exercise guidelines emerged for these patients.
Nowadays, aerobic exercise is seen as a key to recovery, said Kim, who runs an exercise physiology lab.
"One of the tenets of what we do is that exercise is medicine," he said.
Aerobic exercises like swimming, jogging and cycling raise the heart rate. Over time, the heart becomes more efficient, allowing it to pump more blood with less effort. Exercise can also reverse some of the effects of heart disease, like the narrowing of arteries.
"The goal is to raise and sustain that elevated heart rate in what we call a training heart rate zone," said Dr. Jonathan Whiteson, medical director for cardiac rehabilitation at NYU Langone Medical Center.
A typical target zone for aerobic exercise might be 70 to 80% of your maximum heart rate, said Whiteson, though many aim higher or lower.
Most people can calculate a ballpark maximum heart rate by subtracting their age from 220, said Whiteson. At 55, Murphy said his heart rate follows that trend to a T, peaking around 165. However, many heart attack survivors fall quite a bit lower, said Whiteson.
Exercise specialists like Whiteson and Kim use a battery of tests and gadgets to find each patient's new normal -- from heartbeat monitors to a mask that measures oxygen use. Once their patients are stable, it's back to work -- though slowly, at first.