The question has occurred to many endurance athletes, and it seems so basic: Will cross-training — doing a second sport, or lifting weights on days when you aren’t running or cycling or swimming — improve your performance in your primary sport? And at first glance, the answer might seem to be an obvious no. If you want to be a better runner, you have to run — regularly, consistently, and with a training plan that forces you to gradually increase your distance and speed. If you want to be a better cyclist, you have to ride and train according to the same principles. Same goes for swimming or any other endurance sport.

But there also is a body of opinion that says cross-training is necessary and important if you want to improve your performance and avoid injury.

The science, though, is not nearly so definitive. And the answer as to what, if anything, cross-training can accomplish depends on your goal.

The American Orthopedic Society for Sports Medicine encourages cross-training, saying that it can provide a “ ‘total body tune-up,’ something you won’t get if you concentrate on just one type of activity,” and that “you may experience fewer overuse injuries.”

The American College of Sports Medicine’s guidelines for most Americans advise doing some of everything: exercises that increase your heart rate, weight lifting, stretching and balance exercises.

But the purpose of its recommendations is overall health, not performance. If that is your goal, researchers say, it is not so clear that cross-training in an alternate sport will help.

Hirofumi Tanaka, an exercise physiologist at the University of Texas in Austin, came to that conclusion more than a decade ago in a review of published papers. Studies comparing athletes, both trained and untrained, had found that only one factor mattered if performance was the goal: training in that sport.

Since then, he said, there have been numerous small studies, asking the same question and coming to the same conclusion. For example, two subsequent recent studies — one involving moderately fit runners and the other trained runners — found that adding cycling to a running program did not improve running performance.

The results make sense, Dr. Tanaka said. Each sport uses highly specific muscles and nerves. Using an elliptical cross-trainer may feel as if it is exercising your running muscles, but it is not giving you the same kind of training that running does. Nor does it train the muscles you need for cycling.

“You can maintain your cardiovascular capacity by cross-training, but it is extremely difficult to maintain your performance when you rely on cross-training,” Dr. Tanaka said. “This is because you are violating the principle of the specificity of training.”

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