Hirofumi Tanaka, an exercise physiologist at the University of Texas at Austin, bristles when he sees dirt paths carved out of the grass along paved bicycling or running routes. The paths are created by runners who think softer ground protects them from injuries.

Dr. Tanaka, a runner, once tried it himself. He was recovering from a knee injury, and an orthopedist told him to stay away from hard surfaces, like asphalt roads, and run instead on softer surfaces, like grass or dirt. So he ran on a dirt path runners had beaten into the grass along an asphalt bike path.

The result? “I twisted my ankle and aggravated my injury while running on the softer and irregular surface,” he said.

In the aftermath of his accident, Dr. Tanaka said he could not find any scientific evidence that a softer surface is beneficial to runners, nor could other experts he asked. In fact, it makes just as much sense to reason that runners are more likely to get injured on soft surfaces, which often are irregular, than on smooth, hard ones, he said.