IN her prime, Joan Benoit Samuelson, one of the best female distance runners, should have been faster than Alberto Salazar, one of the best male distance runners.

Ms. Samuelson’s running was beautifully smooth. Mr. Salazar’s was not.

“He looked terrible,” said Jack Daniels, an exercise physiologist at the Center for High Altitude Training at Northern Arizona University, who studied both runners in the 1980’s. “She looked great.”

Not only that but Ms. Samuelson also had an amazing ability to use oxygen to fuel her body, Dr. Daniels said. Even though women’s maximum oxygen consumption, or VO2 max, is typically lower than that of men, hers was as high as Mr. Salazar’s. Maximum oxygen consumption was often considered one of the best predictors of performance in distance events.

But Mr. Salazar always ran faster than Ms. Samuelson. The difference between them turned out to be one of the least understood and most mythologized aspects of performance: economy of motion. It’s the relationship between how much energy you expend and how fast you go.