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.