SOME exercise folklore, such as the runner’s high, is eventually proved. But far more common are the persistent myths, including the following two.

If you’re not sore the day after a workout, you didn’t push yourself hard enough.

Not true, said James Pivarnik, a kinesiology and epidemiology professor at Michigan State University. “Soreness usually comes when you’ve laid off for a while, or you’re trying something different,” he said. Consider this: Only a week after finishing a long-distance triathlon quite comfortably, Dr. Pivarnik, then a postdoctorate fellow, played a round of golf. “The next day I was dying,” he said. “I was more sore after hitting the golf balls than after any training sessions or the race.”

Fit people accustomed to a sport might not be sore, even after a tough session, said Dr. Vonda Wright, an orthopedics professor at the University of Pittsburgh, but “their heart still gets a workout and they improve.”

Running causes arthritis or damages knee cartilage.

“This is so crazy,” said Dr. Tim Church, director of preventative medicine at Pennington Biomedical Research Center in Baton Rouge, La. “The data clearly shows that people who are physically active, whether running or walking, have much better joints as they age.”

And yet, people often liken their knee cartilage to Michelins, telling Dr. Church, “It’s like tires, you only have so many miles to go.”

In fact, Dr. Wright explained, the only research that ever looked at running and cartilage found that elite runners who ignore injuries are the ones whose miles saddle their knees with wear and tear. Not so with average runners. “That’s not to say that runners don’t get arthritis or wear and tear,” she said. “But continuing to run has never been proven to make it worse.” CATHERINE SAINT LOUIS