Troy Calhoun is willing to take measures to slow down college offenses, but only if he sees hard evidence that defending an up-tempo offense creates genuine health risks.

Six days after asserting a rule change would be made "to enhance student-athlete safety by guaranteeing a small window for both teams to substitute," the Air Force coach and NCAA Football Rules Committee chairman backtracked in a conference call with reporters, saying he has seen no such data.

"The key is this: I think the only way that it can or it should become a rule is if it is indeed a safety concern. And that can't be something that's a speculation or a possibility," Calhoun said Tuesday afternoon. "I think there's got to be something empirical there where you realize, 'Yep, this truly is a health matter' in terms of not being able to get a defensive player off the field."