CHICAGO – The most dangerous time for amateur athletes may not be during the heat of the game or even in rigorous practices. A total of 21 college football players have collapsed and died during conditioning workouts since 2000 — many on the first few days, when even the fittest players are often pushed too hard.

There's little regulation of these sessions, and coaches "just run willy-nilly" trying to make men out of boys, said athletic trainer Douglas Casa. "A lot of them are not focused on health and safety issues."

Conditioning sessions typically include running sprints, lifting weights, and endurance exercises. Games and practices have more oversight and safeguards. These include heat acclimatization rules limiting equipment worn, intensity and number of sessions for summer practices. Between 2000 and 2011, there were no deaths among top-level college football players in practices or games.