A collegue of mine at The Times who is a triathlete had a question: Everyone tells you to listen to your body, but what are you supposed to listen to?

Turns out it's not so obvious.

Deena Kastor, the American record holder for the marathon, interprets the advice selectively.

"Running isn't always comfortable," she said. "I remember running through a lot of discomfort and pain."

And, Ms. Kastor added, she also runs when she does not feel like it.

"So many times the alarm goes off in the morning and you tell yourself you are too tired," she said. "There are times when you are unmotivated, you don't feel your best and most accomplished."

But if you ignore those messages from your body and just go out and run or do your sport, she said, "those are the days when we have the most pride."

"The trick in listening to your body is to know what you can run through," she said. "If you have a sharp pain you should take care of it."

So does listening to your body mean learning to understand the difference between a pain that signals a serious injury and one that can be ignored? And if it does, why do athletes like Ms. Kastor become seriously injured, anyway?

Last year she broke her foot three miles into the marathon at the Beijing Olympics. In that same race, Paula Radcliffe, who holds the world record in the women's marathon, ran less than her best because her training was interrupted by a stress fracture that had set her back for months.

MAYBE the problem is that it is hard to understand what your body is saying.

" 'Listen to your body' is always a tough one," said Keith Hanson, a coach who directs the Hansons-Brooks Distance Project, which recruits talented distance runners and supports them while they train full time.

One of his runners, Brian Sell, was in the Beijing Olympics, and others are internationally competitive.

"There are several aches and pains that you can run through," Mr. Hanson said, "and others that need some down time. I always try to follow one key rule: If you are gimping - altering your gait- after 10 minutes of running, then it is an injury and not just an ache or pain. You should never run through injuries. If you do, they almost always turn into compensation injuries. What started as an ankle pain becomes knee and hip problems."

But sometimes even when you have a bad feeling about sudden pain, it can be hard to stop, especially during a race.

That happened to my friend Rafael Escandon, a researcher at a small biotech company in San Francisco. It was 2002, and he had decided to run the Twin Cities Marathon. He had run a few dozen marathons before, so he was hardly a beginner. He knew that the trick was to keep going during those stretches when you feel bad.

The race started well. Mr. Escandon had been training by running eight-minute miles but now, he said, he was going much faster, and it all seemed effortless. "It was all I could do to maintain a 7:40 pace, which felt like I was crawling," he said.

Then, just after he passed the 17-mile point in the 26.2 mile race, he felt something awful just below his left calf. "It honestly felt like someone had taken a knife and cut my skin," he said. "I hobbled over to a tree and attempted to stretch my calf for 10 minutes or so.

The pain got worse as he stretched, and even though it diminished when he wasn't stretching, he still felt as if he had been cut. But dropping out of the race was not an option: he had never quit a marathon.

So, he said, he limped along for nine miles and finally crossed the finish line. Then he showered, took some ibuprofen and rushed to the airport to fly to Europe for a business trip.

When the plane landed, Mr. Escandon got out of his seat and, he said, was immediately "blinded by pain in my left leg." It hurt so much he could not stand.

He eventually set off, slowly, "whimpering audibly," he said, as he hobbled to his connecting gate.

Sweating, jet-lagged and still whimpering, he pulled up the leg of his jeans to take a look at his injury. "I was shocked at what I saw," he said. "The medial side of my leg was grotesquely streaked in purple-black from the bottom of my calf to my ankle, including the top of my foot."

It turned out that he had torn the muscle under his calf. For weeks afterward, the pain woke him at night. He could not run for three months, and even when he started again the best he could do for six months was a few miles on a treadmill.

"I should have listened to my body," Mr. Escandon said. "It wasn't just talking to me; it was screaming at me."