Triathlons -- races that combine swimming, cycling and running -- test athletes' arms, legs, lungs and minds.

But without a strong heart, none of those units can do their jobs. As the sport grows, some athletes find that they give them a way to get back to activity even after serious medical conditions.

One is Dr. Syndi Keats, an emergency room physician. When she was 43, she began to experience shortness of breath and found that she could exercise less and less. Eventually, she couldn't even walk up stairs.

After a year of pushing for answers -- including ruling out metabolic and endocrine problems -- fellow doctors determined that her sinus node had stopped working properly.

The sinus node controls how fast the heart beats. So she needed an artificial pacemaker.

Before her heart trouble, she says she was a casual athlete.

"I didn't push myself too hard," Keats says.

But now that she has her pacemaker she has done several triathlons, including a half-Ironman distance race -- totaling 70.3 miles. In part, she credits modern implants. Her pacemaker knows to adjust her heart rate if it detects from her breathing that she is exercising. It can also be tweaked to change the maximum and minimum rates allowed, reflecting normal exercise patterns.

Mark Usher, an independent investment adviser, also found his love of triathlon after major surgery. But he knew it was coming someday.

Usher was diagnosed with a leaky valve -- a heart murmur -- when he was 5 years old. He always knew it would need surgery some day, but he played football, ran track, wrestled and swam in high school. As an adult, he focused on lifting weights.

But that changed when he had to have the valve replaced at age 47 after experiencing fatigue, shortness of breath and fluid in his lungs. His doctors told him he needed more aerobic activity after the surgery. He started with some treadmills and elliptical machines, along with jogging around his neighborhood.

Three years later, on a whim, he entered his first triathlon with just two weeks to train.

"I hadn't swam for years, but figured with my background in swimming 20-plus years ago it would come back to me. I got my 30-year-old Schwinn out and rode it around the block a couple of times to prepare," he says.

He didn't finish dead last -- but it was close, he says. Still, he was hooked on the sport. He has done several sprint distance races and hopes to break through to the Olympic distance races in 2010.

Besides finding a new sport, he says that his health scare changed his type-A personality a bit.

"Since my surgery, many of my friends have commented to me that I'm much calmer and laid-back. Little things don't seem to bother me as much anymore. I think I'm a more caring, compassionate person, not a bad thing," Usher says.

Keats, the emergency room doctor with the pacemaker, says that heart patients should work with doctors, but shouldn't let their conditions ruin their lives.

"After you get to the bottom of it and get it fixed, don't sit back and be a cardiac cripple," she says. Get out there and be better than you were."